Megjelent a Java 21

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Megjelent és letölthetővé vált a Java 21 (OpenJDK 21). A JDK 21 a Java SE platform 21. verziójának nyílt forráskódú referenciaimplementációja, ahogyan azt a JSR 394 a Java közösségi folyamat (Java Community Process) keretében meghatározta. A 2021-es JDK 17 óta az első olyan kiadás, amely hosszú távú támogatást (LTS) nyújt.

A JDK 21 lesz az OpenJDK disztribúciók következő, hosszú távú támogatású (LTS) verziója. Ez azért jelentős, mert sok vállalati felhasználó kizárólag ezeket a verziókat használja a termelésben. Hosszú távon ez jelentős elterjedséget fog eredményezni. A tapasztalatok szerint a legtöbb felhasználó azonban hat-tizenkét hónapot fog várni a bevezetéssel, hogy az új funkciók több valós tesztelésen és hibajavításokkal ellátott frissítésen mehessenek keresztül. A JDK 21 számos új funkciót tartalmaz, amelyek a platform minden aspektusára kiterjednek: a nyelv, a könyvtárak, a JVM és az eszközrendszer.

A JDK 21 talán legjelentősebb újdonsága a virtuális szálak, amelyek az preview státuszból teljes körű funkcióvá válnak. Ezek ideálisak az olyan alkalmazások teljesítményének és skálázhatóságának javítására, amelyek a szál-per-kérés modellt használják, és a szálak jelentős időt töltenek blokkolt IO-val. A Project Loom részeként a fejlesztők már egy ideje türelmetlenül várták ezt a funkciót. A fejlesztők számára különböző nyelvi szintű funkciók állnak rendelkezésre, elsősorban a mintaillesztés körül. Mind a rekordminták, mind a switch kifejezéshez való mintaillesztés teljes értékű funkcióvá válik. Emellett egy kis, de hatékony kiegészítés a meg nem nevezett minták és változók. A Vector API (a numerikusan intenzív számításokhoz) és az idegen nyelvi és memória API (a Project Panama része) továbbra is inkubátor formátumban fejlődik.

A JDK 17 óta beépített új funkciók összesített listáját figyelembe véve a Java ezen új LTS kiadása vonzó lesz a fejlesztők és a végfelhasználók számára.

Újdonságok:

JEP 430 String Templates (Preview)

Enhance the Java programming language with string templates. String templates complement Java's existing string literals and text blocks by coupling literal text with embedded expressions and template processors to produce specialized results. This is a preview language feature and API.

For more information: String Templates (Preview)

JEP 431 Sequenced Collections

Introduce new interfaces to represent collections with a defined encounter order. Each such collection has a well-defined first element, second element, and so forth, up to the last element. It also provides uniform APIs for accessing its first and last elements, and for processing its elements in reverse order.

"Life can only be understood backwards; but it must be lived forwards."

— Kierkegaard

For more information: Sequenced Collections

JEP 439 Generational ZGC

Improve application performance by extending the Z Garbage Collector (ZGC) to maintain separate generations for young and old objects. This will allow ZGC to collect young objects — which tend to die young — more frequently.

For more information: Generational ZGC

JEP 440 Record Patterns

Enhance the Java programming language with record patterns to deconstruct record values. Record patterns and type patterns can be nested to enable a powerful, declarative, and composable form of data navigation and processing.

JEP 441 Pattern Matching for switch

Enhance the Java programming language with pattern matching for switch expressions and statements. Extending pattern matching to switch allows an expression to be tested against a number of patterns, each with a specific action, so that complex data-oriented queries can be expressed concisely and safely.

JEP 442 Foreign Function & Memory API (Third Preview)

Introduce an API by which Java programs can interoperate with code and data outside of the Java runtime. By efficiently invoking foreign functions (i.e., code outside the JVM), and by safely accessing foreign memory (i.e., memory not managed by the JVM), the API enables Java programs to call native libraries and process native data without the brittleness and danger of JNI. This is a preview API.

JEP 443 Unnamed Patterns and Variables (Preview)

Enhance the Java language with unnamed patterns, which match a record component without stating the component's name or type, and unnamed variables, which can be initialized but not used. Both are denoted by an underscore character, _. This is a preview language feature.

JEP 444 Virtual Threads

Introduce virtual threads to the Java Platform. Virtual threads are lightweight threads that dramatically reduce the effort of writing, maintaining, and observing high-throughput concurrent applications.

JEP 445 Unnamed Classes and Instance Main Methods (Preview)

Evolve the Java language so that students can write their first programs without needing to understand language features designed for large programs. Far from using a separate dialect of Java, students can write streamlined declarations for single-class programs and then seamlessly expand their programs to use more advanced features as their skills grow. This is a preview language feature.

For more information: Unnamed Classes and Instance Main Methods (Preview)

JEP 446 Scoped Values (Preview)

Introduce scoped values, values that may be safely and efficiently shared to methods without using method parameters. They are preferred to thread-local variables, especially when using large numbers of virtual threads. This is a preview API.

In effect, a scoped value is an implicit method parameter. It is "as if" every method in a sequence of calls has an additional, invisible, parameter. None of the methods declare this parameter and only the methods that have access to the scoped value object can access its value (the data). Scoped values make it possible to pass data securely from a caller to a faraway callee through a sequence of intermediate methods that do not declare a parameter for the data and have no access to the data.

JEP 448 Vector API (Sixth Incubator)

Introduce an API to express vector computations that reliably compile at runtime to optimal vector instructions on supported CPU architectures, thus achieving performance superior to equivalent scalar computations.

JEP 449 Deprecate the Windows 32-bit x86 Port for Removal

Deprecate the Windows 32-bit x86 port, with the intent to remove it in a future release.

JEP 451 Prepare to Disallow the Dynamic Loading of Agents

Issue warnings when agents are loaded dynamically into a running JVM. These warnings aim to prepare users for a future release which disallows the dynamic loading of agents by default in order to improve integrity by default. Serviceability tools that load agents at startup will not cause warnings to be issued in any release.

For more information: Warning Printed when an Agent Is Loaded into a Running VM

JEP 452 Key Encapsulation Mechanism API

Introduce an API for key encapsulation mechanisms (KEMs), an encryption technique for securing symmetric keys using public key cryptography.

JEP 453 Structured Concurrency (Preview)

Simplify concurrent programming by introducing an API for structured concurrency. Structured concurrency treats groups of related tasks running in different threads as a single unit of work, thereby streamlining error handling and cancellation, improving reliability, and enhancing observability. This is a preview API.

New Features

Generational ZGC ( JEP 439)

hotspot/gc

Applications running with Generational ZGC should enjoy:

  • Lower risks of allocations stalls,
  • Lower required heap memory overhead, and
  • Lower garbage collection CPU overhead.

Enable Generational ZGC with command line options -XX:+UseZGC -XX:+ZGenerational

For further details, see JEP 439.

New System Property to Toggle XML Signature Secure Validation Mode (JDK-8301260)

security-libs/javax.xml.crypto

A new system property named org.jcp.xml.dsig.secureValidation has been added. It can be used to enable or disable the XML Signature secure validation mode. The system property should be set to "true" to enable, or "false" to disable. Any other value for the system property is treated as "false". If the system property is set, it supersedes the XMLCryptoContext property value.

By default, secure validation mode is enabled. Disabling secure validation mode is done at your own risk.

Math.clamp() and StrictMath.clamp() Methods (JDK-8301226)

core-libs/java.lang

The methods Math.clamp() and StrictMath.clamp() are added to conveniently clamp the numeric value between the specified minimum and maximum values. Four overloads are provided in both Math and StrictMath classes for int, long, float, and double types. A clamp(long value, int min, int max) overload can also be used to safely narrow a long value to int.

New String indexOf(int,int,int) and indexOf(String,int,int) Methods to Support a Range of Indices (JDK-8302590)

core-libs/java.lang

Two new methods indexOf(int ch, int beginIndex, int endIndex) and indexOf(String str, int beginIndex, int endIndex) are added to java.lang.String to support forward searches of character ch, and of String str, respectively, and limited to the specified range of indices.

Besides full control on the search range, they are safer to use than indexOf(int ch, int fromIndex) and indexOf(String str, int fromIndex), respectively, because they throw an exception on illegal search ranges.

Method indexOf(int ch, int beginIndex, int endIndex) is covered by JDK-8302590, and method `indexOf(String str, int beginIndex, int endIndex) is covered by JDK-8303648.

New splitWithDelimiters() Methods Added to String and java.util.regex.Pattern (JDK-8305486)

core-libs/java.lang

Unlike the split() methods, these new splitWithDelimiters() methods in java.lang.String and java.util.regex.Pattern return an alternation of strings and matching delimiters, rather than just the strings.

Runtime.exec and ProcessBuilder Logging of Command Arguments (JDK-8303392)

core-libs/java.lang

Processes started by Runtime.exec and ProcessBuilder can be enabled to log the command, arguments, directory, stack trace, and process id. The exposure of this information should be reviewed before implementation. Logging of the information is enabled when the logging level of the System#getLogger(String) named java.lang.ProcessBuilder is System.Logger.Level.DEBUG or Logger.Level.TRACE. When enabled for Level.DEBUG, only the process id, directory, command, and stack trace are logged. When enabled for Level.TRACE, the command arguments are included with the process id, directory, command, and stack trace.

Unicode Emoji Properties (JDK-8303018)

core-libs/java.lang

The following six new methods are added to java.lang.Character for obtaining Emoji character properties, which are defined in the Unicode Emoji Technical Standard (UTS #51) :

   - isEmoji(int codePoint)
   - isEmojiPresentation(int codePoint)
   - isEmojiModifier(int codePoint)
   - isEmojiModifierBase(int codePoint)
   - isEmojiComponent(int codePoint)
   - isExtendedPictographic(int codePoint)

System.exit() and Runtime.exit() Logging (JDK-8301627)

core-libs/java.lang

Calls to java.lang.System.exit() and Runtime.exit() are logged to the logger named java.lang.Runtime with a logging level of System.Logger.DEBUG. When the configuration of the logger allows, the caller can be identified from the stacktrace included in the log.

The java.net.http.HttpClient Is Now AutoCloseable (JDK-8267140)

core-libs/java.net

The following methods have been added to the API:

  • void close(): closes the client gracefully, waiting for submitted requests to complete.
  • void shutdown(): initiates a graceful shutdown, then returns immediately without waiting for the client to terminate.
  • void shutdownNow(): initiates an immediate shutdown, trying to interrupt active operations, and returns immediately without waiting for the client to terminate.
  • boolean awaitTermination(Duration duration), waits for the client to terminate, within the given duration; returns true if the client is terminated, false otherwise.
  • boolean isTerminated(): returns true if the client is terminated.

The instances returned by HttpClient.newHttpClient(), and the instances built from HttpClient.newBuilder(), provide a best effort implementation for these methods. They allow the reclamation of resources allocated by the HttpClient early, without waiting for its garbage collection.

Note that an HttpClient instance typically manages its own pools of connections, which it may then reuse when necessary. Connection pools are typically not shared between HttpClient instances. Creating a new client for each operation, though possible, will usually prevent reusing such connections.

Support for GB18030-2022 (JDK-8301119)

core-libs/java.nio.charsets

China National Standard body (CESI) has recently published GB18030-2022 which is an updated version of the GB18030 standard and brings GB18030 in sync with Unicode version 11.0. The Charset implementation for this new standard has now replaced the prior 2000 standard. However, this new standard has some incompatible changes from the prior implementation. For those who need to use the old mappings, a new system property jdk.charset.GB18030 is introduced. By setting its value to 2000, the previous JDK releases' mappings for the GB18030 Charset are used which are based on the 2000 standard.

New StringBuilder and StringBuffer repeat Methods (JDK-8302323)

core-libs/java.util

The methods public StringBuilder repeat(int codePoint, int count) and public StringBuilder repeat(CharSequence cs, int count) have been added to java.lang.StringBuilder and java.lang.StringBuffer to simplify the appending of multiple copies of characters or strings. For example, sb.repeat('-', 80) will insert 80 hyphens into the value of the java.lang.StringBuilder sb object.

Emoji Related Binary Properties in RegEx (JDK-8305107)

core-libs/java.util.regex

Emoji-related properties introduced in ([JDK-8303018](https://bugs.openjdk.org/browse/JDK-8303018)) can now be used as binary properties in the java.util.regex.Pattern class. One can match characters that have Emoji-related properties with the new p{IsXXX} constructs. For example,

Pattern.compile("\p{IsEmoji}").matcher("***itt egy emodzsi***").matches()

returns true.

Sequenced Collections ( JEP 431)

core-libs/java.util:collections

The Sequenced Collection API introduces several new interfaces into the collections framework, providing enhancements to many existing collections classes. The new API facilitates access to elements at each end of a sequenced collection, and provides the ability to view and iterate such collections in reverse order. See JEP 431 for additional information.

The introduction of new collections interfaces, along with default methods, introduces some compatibility risk, including the possibility of both source and binary incompatibilities. The introduction of default methods in an interface hierarchy may cause conflicts with methods declared on existing classes or interfaces that extend collections interfaces - this could result in either source or binary incompatibilities. The introduction of new interfaces also introduces new types into the system, which can change the results of type inference, leading in turn to source incompatibilities.

For a discussion of potential incompatibilities and possible ways to mitigate them, please see the document JDK 21: Sequenced Collections Incompatibilities.

Warning Printed when an Agent Is Loaded into a Running VM ( JEP 451)

core-svc/tools

The Java Virtual Machine (JVM) now prints a warning to standard error when a JVM Tool Interface (JVM TI) agent or Java Agent is dynamically loaded into a running JVM. The warning is intended to prepare for a future release that disallows, by default, dynamic loading of agent code into a running JVM.

Agents are programs that run in the JVM process and make use of powerful JVM TI or java.lang.instrument APIs. These APIs are designed to support tooling such as profilers and debuggers. Agents are started via a command line option, for example -agentlib or -javaagent, or they can be started into a running VM using the JDK specific com.sun.tools.attach API or the jcmd command. Agents loaded into a running VM will now print a warning. There is no warning for agents that are loaded at startup via command line options.

The HotSpot VM option EnableDynamicAgentLoading controls dynamic loading of agents. This option has existed since JDK 9. The default, since JDK 9, is to allow dynamic loading of agents. Running with -XX:+EnableDynamicAgentLoading on the command line serves as an explicit "opt-in" that allows agent code to be loaded into a running VM and thus suppresses the warning. Running with -XX:-EnableDynamicAgentLoading disallows agent code from being loaded into a running VM and can be used to test possible future behavior.

In addition, the system property jdk.instrument.traceUsage can be used to trace uses of the java.lang.instrument API. Running with -Djdk.instrument.traceUsage or -Djdk.instrument.traceUsage=true causes usages of the API to print a trace message and stack trace. This can be used to identify agents that are dynamically loaded instead of being started on the command line with -javaagent.

More information on this change can be found in JEP 451.

Last Resort G1 Full GC Moves Humongous Objects (JDK-8191565)

hotspot/gc

A full garbage collection (GC) in the Garbage First (G1) collector now moves humongous objects to avoid Out-Of-Memory situations due to a lack of contiguous space in the Java heap when the application allocates humongous objects.

Previously, G1 failed to allocate those humongous objects, reporting an Out-Of-Memory exception in this situation.

This functionality is a last resort measure, causing a second full GC in the same pause after the previous full GC fails to clear out enough contiguous memory for the allocation.

New JFR View Command (JDK-8306703)

hotspot/jfr

A new view command has been added to the JFR tool and jcmd. The command can aggregate and display event data in a tabular form without the need to dump a recording file or open JDK Mission Control. There are 70 predefined views, such as hot-methods, gc-pauses, pinned-threads, allocation-by-site, gc, memory-leaks-by-class, and more. A list of available views can be found through using jcmd <pid> JFR.view or jfr view.

Support for HSS/LMS Signature Verification (JDK-8298127)

security-libs

A new standard signature algorithm named "HSS/LMS" has been introduced. The HSS/LMS algorithm is defined in RFC 8554: Leighton-Micali Hash-Based Signatures and NIST Special Publication 800-208. New KeyFactory and Signature implementations are available for the algorithm. The KeyFactory only operates on public keys and the Signature only covers the verification part.

Enhanced OCSP, Certificate, and CRL Fetch Timeouts (JDK-8179502)

security-libs/java.security

This feature delivers an enhanced syntax for properties related to certificate, CRL, and OCSP connect and read timeouts. The new syntax allows the timeout values to be specified either in seconds or milliseconds. This feature also delivers three new System properties related to connect and read timeouts.

New properties: The existing com.sun.security.ocsp.timeout property will now be paired with the new com.sun.security.ocsp.readtimeout property. The former property will be used to set timeouts for the transport-layer connection while the latter will be used to manage timeouts for reading the data. The new com.sun.security.cert.timeout and com.sun.security.cert.readtimeout properties will be used to control connect and read timeouts, respectively, when following an X.509 certificate's AuthorityInfoAccess extension. For the certificate fetching properties, the com.sun.security.enableAIAcaIssuers property must be set to true in order for fetching to occur and these property timeouts to be enabled.

Enhanced timeout syntax: The new syntax applies to the aforementioned properties, and also to the com.sun.security.crl.timeout and com.sun.security.crl.readtimeout properties as well. The allowed syntax is as follows:

  • A decimal integer will be interpreted in seconds and ensures backward compatibility.
  • A decimal integer ending in "s" (case-insensitive, no space) appended to it. This will also be interpreted in seconds.
  • A decimal integer value with "ms" (case-insensitive, no space) appended to it. This will be interpreted as milliseconds. For example, a value of "2500ms" will be a 2.5 second timeout.
  • Negative, non-numeric, or non-decimal (for example, hexadecimal values prepended by "0x") values will be interpreted as illegal and will default to the 15 second timeout.
  • Whether the value is interpreted in seconds or milliseconds, a value of zero will disable the timeout.

SunJCE Provider Now Supports SHA-512/224 and SHA-512/256 As Digests for the PBES2 Algorithms (JDK-8288050)

security-libs/javax.crypto

The SunJCE provider is enhanced with additional PBES2 Cipher and Mac algorithms, such as those using SHA-512/224 and SHA-512/256 message digests. To be more specific, callers can now use the SunJCE provider for PBEWithHmacSHA512/224AndAES_128, PBEWithHmacSHA512/256AndAES_128, PBEWithHmacSHA512/224AndAES_256, and PBEWithHmacSHA512/256AndAES_256 Ciphers and PBEWithHmacSHA512/224, and PBEWithHmacSHA512/256 Mac.

Support for Password-Based Cryptography in SunPKCS11 (JDK-8301553)

security-libs/javax.crypto:pkcs11

The SunPKCS11 security provider now supports Password-Based Cryptography algorithms for Cipher, Mac, and SecretKeyFactory service types. You will find the list of algorithms in [JDK-8308719](https://bugs.openjdk.org/browse/JDK-8308719). As a result of this enhancement, SunPKCS11 can now be used for privacy and integrity in PKCS #12 key stores.

Update XML Security for Java to 3.0.2 (JDK-8305972)

security-libs/javax.xml.crypto

The XML Signature implementation has been updated to Santuario 3.0.2. The main, new feature is support for EdDSA. One difference is that the JDK still supports the here() function by default. However, we recommend avoiding the use of the here() function in new signatures and replacing existing signatures that use the here() function. Future versions of the JDK will likely disable, and eventually remove, support for this function, as it cannot be supported using the standard Java XPath API. Users can now disable the here() function by setting the security property jdk.xml.dsig.hereFunctionSupported to "false".

Unnamed Classes and Instance Main Methods (Preview) ( JEP 445)

specification/language

Unnamed classes and instance main methods enable students to write streamlined declarations for single-class programs and then seamlessly expand their programs later to use more advanced features as their skills grow. Unnamed classes allow the user to provide class content without the full ceremony of the class declaration. The instance main method feature allows the user to drop the formality of public static void main(String[] args) and simply declare void main().

String Templates (Preview) ( JEP 430)

specification/language

String templates allow text and expressions to be composed without using the + operator. The result is often a string, but can also be an object of another type. Each string template has a template processor that validates the text and expressions before composing them, achieving greater safety than basic 'string interpolation' features in other languages.

Generate "output file clash" Warning when an Output File is Overwritten During Compilation (JDK-8287885)

tools/javac

Prior to JDK 21, the javac compiler was overwriting some output files during compilation. This can occur, for example, on case-insensitive filesystems.

Starting from JDK 21 a new compiler option: -Xlint:output-file-clash has been added to the javac compiler. This new option should provide a way for users experiencing this problem to convert what is currently a runtime error into a compile-time warning (or error with -Werror). This new compiler option enables output file clash detection. The term "output file" covers class files, source files, and native header files.

New javac Warning When Calling Overridable Methods in Constructors (JDK-8015831)

tools/javac

The new lint option, this-escape, has been added to javac to warn about calls to overridable methods in the constructor body which might result in access to a partially constructed object from subclasses.

The new warning can be suppressed using SuppressWarnings("this-escape").

Support Searching for Section Headings in Generated Documentation (JDK-8286470)

tools/javadoc(tool)

API documentation generated by JavaDoc now supports searching for headings of sections within the documentation.

JDK Tool Access in JShell (JDK-8306560)

tools/jshell

The JShell tool for interactive exploration of Java code has been enhanced with a new predefined script, TOOLING. The TOOLING script provides direct access to the JDK's command line tools, such as javac, javadoc, and javap, from within JShell.

Similar to the existing predefined DEFAULT and PRINTING scripts, the TOOLING script can be loaded when JShell starts by running: jshell TOOLING. Alternatively, it can be loaded within a JShell session by using: /open TOOLING. With the TOOLING script loaded, JDK tools can be run by passing a name and arguments to the method run(String name, String... args). The method tools() prints the names of available tools.

The TOOLING script defines convenience methods for the most commonly used tools, such as javac(String... args). Here is an example of running the javap tool that disassembles and prints an overview of a class or interface:

jshell> interface Empty {}
jshell> javap(Empty.class)

-XshowSettings:locale Output Now Includes Tzdata Version (JDK-8305950)

tools/launcher

The -XshowSettings launcher option has been enhanced to print the tzdata version configured with the JDK. The tzdata version is displayed as part of the locale showSettings option.

Example output using -X:showSettings:locale:

.....
Locale settings:
    default locale = English
    default display locale = English
    default format locale = English
    tzdata version = 2023c
    .....

Changes to JAXP Configuration Files (JDK-8303530)

xml/jaxp

The following changes have been made with regard to the JAXP configuration files:

  • Added the jaxp.properties file to the JDK at $JAVA_HOME/conf/jaxp.properties as the default JAXP configuration file. Property settings in the file reflect the current, built-in defaults for the JDK.

  • Added a new System Property, java.xml.config.file, for specifying the location of a custom configuration file. If it is set and the named file exists, the property settings contained in the file override those in the default JAXP configuration file. For more details, see the Configuration section of the module specification.

  • Deprecated the stax.properties file that was defined in the StAX API and used by the StAX factories. It had been made redundant after StAX's integration into JAXP since the function has been fully covered by the JAXP configuration file. It is recommended that applications migrate to the JAXP configuration file as the stax.properties file is deprecated and may no longer be supported in the future.

Removed Features and Options

Removed SECOM Trust System's RootCA1 Root Certificate (JDK-8295894)

security-libs/java.security

The following root certificate from SECOM Trust System has been removed from the cacerts keystore:

+ alias name "secomscrootca1 [jdk]"
  Distinguished Name: OU=Security Communication RootCA1, O=SECOM Trust.net, C=JP

java.io.File's Canonical Path Cache Is Removed (JDK-8300977)

core-libs/java.io

java.io.File has historically cached canonical paths, and part paths, to help the performance of the File::getCanonicalFile and File::getCanonicalPath when running with a SecurityManager set. The cache had correctness issues in environments with symbolic links and has been disabled by default since JDK 12. The cache has been removed in this release, along with the system properties sun.io.useCanonCaches and sun.io.useCanonPrefixCache. Setting these properties no longer has any effect.

Removal of the java.compiler System Property (JDK-8041676)

core-libs/java.lang

The system property java.compiler has been removed from the list of standard system properties.

Running with this system property set on the command line will now print a warning to say that the system property is obsolete; it has no other effect. In previous releases, running with -Djava.compiler or -Djava.compiler=NONE on the command line selected interpreter only execution mode. If needed, the -Xint option can be used to run in interpreter only mode.

The java.lang.Compiler Class Has Been Removed (JDK-8205129)

core-libs/java.lang

The java.lang.Compiler class has been removed. This under-specified API dates from JDK 1.0 and the "Classic VM" used in early JDK releases. Its implementation in the HotSpot VM does nothing but print a warning that it is not supported. The class has been deprecated and marked for removal since JavaSE 9.

ThreadGroup.allowThreadSuspension Is Removed (JDK-8297295)

core-libs/java.lang

The method java.lang.ThreadGroup.allowThreadSuspension(boolean) has been removed in this release. The method was used for low memory handling in JDK 1.1 but was never fully specified. It was deprecated and changed to "do nothing" in JDK 1.2 (1998).

Remove the JAR Index Feature (JDK-8302819)

core-libs/java.util.jar

The "JAR Index" feature has been dropped from the JAR file specification. JAR Index was a legacy optimization in early JDK releases to allow downloading of JAR files to be postponed when loading applets or other classes over the network. The feature has been disabled since JDK 18, meaning the META-INF/INDEX.LIST entry in a JAR file is ignored at run-time.

The system property jdk.net.URLClassPath.enableJarIndex, introduced in JDK 18 to re-enable the feature, has been removed. Setting this property no longer has any effect.

As part of the change, the jar tool will now output a warning if the -i or --generate-index options are used.

javax.management.remote.rmi.RMIIIOPServerImpl Is Removed (JDK-8307244)

core-svc/javax.management

The class javax.management.remote.rmi.RMIIIOPServerImpl has been removed. The IIOP transport was removed from the JMX Remote API in Java 9. This class has been deprecated and its constructor changed to throw UnsupportedOperationException since Java 9.

Removal of G1 Hot Card Cache (JDK-8225409)

hotspot/gc

The G1 Hot Card Cache has been removed. Performance testing has shown that after improvements to the concurrent refinement control, it does not contribute to performance.

Removal reduces the memory footprint of the G1 garbage collector by around 0.2% of the Java heap size.

The associated configuration options G1ConcRSLogCacheSize and G1ConcRSHotCardLimit have been obsoleted. A warning will be issued at startup about these options if they are used.

Obsolete Legacy HotSpot Parallel Class Loading Workaround Option -XX:+EnableWaitForParallelLoad Is Removed (JDK-8298469)

hotspot/runtime

Some older, user-defined class loaders would workaround a deadlock issue by releasing the class loader lock during the loading process. To prevent these loaders from encountering a java.lang.LinkageError: attempted duplicate class definition while loading the same class by parallel threads, the HotSpot Virtual Machine introduced a workaround in JDK 6 that serialized the load attempts, causing the subsequent attempts to wait for the first to complete.

The need for class loaders to work this way was removed in JDK 7 when parallel-capable class loaders were introduced, but the workaround remained in the VM. The workaround was deprecated in JDK 20 and the option -XX:+EnableWaitForParallelLoad was introduced for users who relied on this legacy behavior. The default for this option was off.

In JDK 21, the option -XX:+EnableWaitForParallelLoad, and the code to support it, has been removed.

See CSR JDK-8304056 for more details.

The MetaspaceReclaimPolicy Flag has Been Obsoleted (JDK-8302385)

hotspot/runtime

The option MetaspaceReclaimPolicy existed to fine-tune the memory reclamation behavior of metaspace after class unloading. In practice, this had limited effect and was rarely used.

The option has therefore been obsoleted. It now produces an obsolete warning and is ignored.

Removal of ContentSigner APIs and jarsigner -altsigner and -altsignerpath Options (JDK-8303410)

security-libs/jdk.security

The jarsigner options -altsigner and -altsignerpath have been removed, along with the underlying ContentSigner API in the com.sun.jarsigner package. The mechanism was deprecated in JDK 9 and marked for removal in JDK 15.

Deprecated Features and Options

Deprecate GTK2 for Removal (JDK-8280031)

client-libs/java.awt

Implementation support for AWT/Swing using GTK2 on Linux is now deprecated for removal.

With the announcement of the GTK4 release in December 2020, the GTK 2 toolkit is reaching its end of life. GTK2 support is therefore expected to be removed some time after no JDK supported platform needs it.

GTK3 is the current default and Swing applications which opt-in to using GTK2 on Linux by setting the System Property -Djdk.gtk.version=2 will now see the following warning printed:

WARNING: the GTK 2 library is deprecated and its support will be removed in a future release.

com.sun.nio.file.SensitivityWatchEventModifier Is Deprecated (JDK-8303175)

core-libs/java.nio

com.sun.nio.file.SensitivityWatchEventModifier has been deprecated and is marked for removal in a future release. The constants in this enum were used with the polling based WatchService implementation on macOS to set the interval when polling files for changes. The polling based WatchService has been changed to ignore these modifiers when registering files to be watched.

Emit Warning for Removal of COMPAT Provider (JDK-8304982)

core-libs/java.util:i18n

Users now see a warning message if they specify either COMPAT or JRE locale data with the java.locale.providers system property and call some locale-sensitive operations. COMPAT was provided for migration to the CLDR locale data at the time of JDK 9, where it became the default locale data (JEP 252). JDK 21 retains the legacy locale data of JDK 8 for compatibility, but some of the newer functionalities are not applied. The legacy locale data will be removed in a future release. Users are encouraged to migrate to the CLDR locale data.

Deprecate JMX Subject Delegation and the JMXConnector.getMBeanServerConnection(Subject) Method for Removal (JDK-8298966)

core-svc/javax.management

The JMX Subject Delegation feature is deprecated and marked for removal in a future release. This feature is enabled by the method javax.management.remote.JMXConnector.getMBeanServerConnection(javax.security.auth.Subject) which is deprecated for removal.

If a client application needs to perform operations as, or on behalf of, multiple identities, it will need to make multiple calls to JMXConnectorFactory.connect() and to the getMBeanServerConnection() method on the returned JMXConnector.

Notable Issues Fixed

Error Computing the Amount of Milli- and Microseconds between java.time.Instants (JDK-8307466)

core-libs/java.time

The computation of the time between java.time.Instants using ChronoUnit.MILLIS.between(t1, t2), ChronoUnit.MICROS.between(t1, t2), t1.until(t2, MILLIS), or t1.until(t2, MICROS) has been corrected. The implementation computing the number of units between Instants, as of JDK 18, did not propagate carry and borrow between seconds and nanoseconds when computing milliseconds and microseconds.

Fixed Indefinite jspawnhelper Hangs (JDK-8307990)

core-libs/java.lang

Since JDK 13, executing commands in a sub-process uses the so-called POSIX_SPAWN launching mechanism (that is, -Djdk.lang.Process.launchMechanism=POSIX_SPAWN) by default on Linux. In cases where the parent JVM process terminates abnormally before the handshake between the JVM and the newly created jspawnhelper process has completed, jspawnhelper can hang indefinitely in JDK 13 to JDK 20. This issue is fixed in JDK 21. The issue was especially harmful if the parent process had open sockets, because in that case, the forked jspawnhelper process will inherit them and keep all the corresponding ports open, effectively preventing other processes from binding to them.

This misbehavior has been observed with applications which frequently fork child processes in environments with tight memory constraints. In such cases, the OS can kill the JVM in the middle of the forking process leading to the described issue. Restarting the JVM process after such a crash will be impossible if the new process tries to bind to the same ports as the initial application because they will be blocked by the hanging jspawnhelper child process.

The root cause of this issue is jspawnhelper's omission to close its writing end of the pipe, which is used for the handshake with the parent JVM. It was fixed by closing the writing end of the communication pipe before attempting to read data from the parent process. This way, jspawnhelper will reliably read an EOF event from the communication pipe and terminate once the parent process dies prematurely.

A second variant of this issue could happen because the handshaking code in the JDK didn't handle interrupts to write(2) correctly. This could lead to incomplete messages being sent to the jspawnhelper child process. The result is a deadlock between the parent thread and the child process which manifests itself in a jspawnhelper process being blocked while reading from a pipe and the following stack trace in the corresponding parent Java process:

java.lang.Thread.State: RUNNABLE
  at java.lang.ProcessImpl.forkAndExec(java.base@17.0.7/Native Method)
  at java.lang.ProcessImpl.<init>(java.base@17.0.7/ProcessImpl.java:314)
  at java.lang.ProcessImpl.start(java.base@17.0.7/ProcessImpl.java:244)
  at java.lang.ProcessBuilder.start(java.base@17.0.7/ProcessBuilder.java:1110)
  at java.lang.ProcessBuilder.start(java.base@17.0.7/ProcessBuilder.java:1073)

Enhance Contents (Trusted Certificate Entries) of macOS KeychainStore (JDK-8303465)

security-libs/java.security

The macOS KeychainStore implementation now exposes certificates with proper trust in the user domain, admin domain, or both. Before, only the user domain was considered. Furthermore, if there exists a "deny" entry for a particular purpose in a certificate's trust settings in either domain, the certificate will not be part of the macOS KeychainStore.

Allow Key/Nonce Reuse for DECRYPT_MODE ChaCha20 and ChaCha20-Poly1305 Cipher Objects (JDK-8305091)

security-libs/javax.crypto

The SunJCE implementation for Cipher objects using the ChaCha20 and ChaCha20-Poly1305 algorithms will now allow key/nonce reuse when in DECRYPT_MODE. This change aligns these algorithms with the current SunJCE AES-GCM decrypt mode behavior as it pertains to key/nonce reuse. All ENCRYPT_MODE key/nonce reuse prohibitions remain unchanged from their current behavior.

Disallow Extra Semicolons Between "import" Statements (JDK-8027682)

tools/javac

The Java Language Specification does not allow extra semicolons to appear between import statements, yet the compiler was allowing them; [JDK-8027682](https://bugs.openjdk.org/browse/JDK-8027682) fixed this.

As a result, a program like this, which previously would have compiled successfully:

import java.util.Map;;;;
import java.util.Set;
class Test { }

will now generate an error:

Test.java:1: error: extraneous semicolon
import java.util.Map;;;;
                     ^

For backward compatibility, when compiling source versions prior to 21, a warning is generated instead of an error.

Known Issues

JVM May Crash or Malfunction When Using ZGC and Non-Default ObjectAlignmentInBytes (JDK-8312749)

hotspot/compiler

Running the JVM with -XX:+UseZGC and non-default value of -XX:ObjectAlignmentInBytes may lead to JVM crashes or incorrect execution. The issue is caused by an incorrect JIT compiler optimization of the java.lang.Object.clone() method for this configuration. If using ZGC with a non-default value of ObjectAlignmentInBytes is desired, JIT compilation of java.lang.Object.clone() can be disabled using the command-line options -XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_clone.

Validations on ZIP64 Extra Fields (JDK-8313765)

core-libs/java.util.jar

A (JDK enhancement)[https://bugs.openjdk.org/browse/JDK-8311940] has improved validation of the ZIP64 Extra Fields contained within zip files and jar files. Files which do not satisfy these new validation checks may result in ZipException : Invalid CEN header (invalid zip64 extra data field size).

The following third party tools have released patches to better adhere to the ZIP File Format Specification:

  • Apache Commons Compress fix for Empty CEN Zip64 Extra Headers fixed in Commons Compress release 1.11
  • Apache Ant fix for Empty CEN Zip64 Extra Headers fixed in Ant 1.10.14
  • BND issue with writing invalid Extra Headers fixed in BND 5.3
  • The maven-bundle-plugin 5.1.5 includes the BND 5.3 patch.

If these improved validation checks cause issues for deployed zip or jar files, check how the file was created and whether patches are available from the generating software to resolve the issue. The new validation checks can be disabled by adding -Djdk.util.zip.disableZip64ExtraFieldValidation=true to the runtime launcher arguments.

Further modification of validations on ZIP64 Extra Fields contained within zip and jar files will be made in the upcoming JDK release. See JDK-8313765.

java.util.regex.MatchResult Might Throw StringIndexOutOfBoundsException on Regex Patterns Containing Lookaheads and Lookbehinds (JDK-8132995)

core-libs/java.util.regex

JDK-8132995 introduced an unintended regression when using instances returned by java.util.regex.Matcher.toMatchResult().

This happens on java.util.regex.Patterns containing lookaheads and lookbehinds that, in turn, contain groups. If these are located outside the match, it results in throwing StringIndexOutOfBoundsException when accessing these groups. See JDK-8312976 for an example.

JVM May Hang When Using Generational ZGC if a VM Handshake Stalls on Memory (JDK-8311981)

hotspot/gc

The JVM can hang under an uncommon condition that involves the JVM running out of heap memory, the GC just starting a relocation phase to reclaim memory, and a JVM thread-local Handshake asking to relocate an object.

Other Notes

System Property to Handle HTML ObjectView Creation (JDK-8296832 (not public))

client-libs/javax.swing

Some Swing components, such as JLabels and JButtons, which display application text, will try to interpret that text as HTML, principally to enable styled text. The HTML processing of the text for these components will no longer recognize the <object> tag which allows for subclasses of java.awt.Component to be rendered on the component. To re-enable this, applications must specify -Dswing.html.object=true.

ObjectInputStream::readObject() Should Handle Negative Array Sizes without Throwing NegativeArraySizeExceptions (JDK-8306461)

core-libs/java.io:serialization

ObjectInputStream::readObject() now throws a StreamCorruptedException instead of a NegativeArraySizeException when reading an array with a negative array size from a corrupted object input stream. Collection classes with a custom readObject() method which previously threw a NegativeArraySizeException when the number of their elements read from the deserialization stream was negative will now throw a StreamCorruptedException instead.

Added 4 GTS Root CA Certificates (JDK-8307134)

security-libs/java.security

The following root certificates have been added to the cacerts truststore:

+ Google Trust Services LLC
 + gtsrootcar1
  DN: CN=GTS Root R1, O=Google Trust Services LLC, C=US

+ Google Trust Services LLC
 + gtsrootcar2
  DN: CN=GTS Root R2, O=Google Trust Services LLC, C=US

+ Google Trust Services LLC
 + gtsrootecccar3
  DN: CN=GTS Root R3, O=Google Trust Services LLC, C=US

+ Google Trust Services LLC
 + gtsrootecccar4
  DN: CN=GTS Root R4, O=Google Trust Services LLC, C=US

Added Microsoft Corporation's 2 TLS Root CA Certificates (JDK-8304760)

security-libs/java.security

The following root certificates have been added to the cacerts truststore:

+ Microsoft Corporation
  + microsoftecc2017
    DN: CN=Microsoft ECC Root Certificate Authority 2017, O=Microsoft Corporation, C=US

+ Microsoft Corporation
  + microsoftrsa2017
    DN: CN=Microsoft RSA Root Certificate Authority 2017, O=Microsoft Corporation, C=US

Added TWCA Root CA Certificate (JDK-8305975)

security-libs/java.security

The following root certificate has been added to the cacerts truststore:

+ TWCA
  + twcaglobalrootca
    DN: CN=TWCA Global Root CA, OU=Root CA, O=TAIWAN-CA, C=TW

Added Certigna(Dhimyotis) CA Certificate (JDK-8245654)

security-libs/java.security

The following root certificate has been added to the cacerts truststore:

+ Certigna (Dhimyotis)
  + certignaca
    DN: CN=Certigna, O=Dhimyotis, C=FR

The Default TLS Diffie-Hellman Group Size Has Been Increased from 1024-bit to 2048-bit (JDK-8301700)

security-libs/javax.net.ssl

The JDK implementation of TLS 1.2 now uses a default Diffie Hellman keysize of 2048 bits when a TLS_DHE cipher suite is negotiated and either the client or server does not support FFDHE, which can negotiate a stronger keysize. The JDK TLS implementation supports FFDHE and it is enabled by default.

As a workaround, users can revert to the previous size by setting the jdk.tls.ephemeralDHKeySize system property to 1024 (at their own risk).

This change does not affect TLS 1.3 as the minimum DH group size is already 2048 bits.

File::listRoots Changed to Return All Available Drives on Windows (JDK-8208077)

core-libs/java.io

The behavior of the method java.io.File.listRoots() on Microsoft Windows has changed in this release so that the returned array includes a File object for all available disk drives. This differs from the behavior in JDK 10 to JDK 20, where this method filtered out disk drives that were not accessible or did not have media present. This change avoids performance issues observed in the previous releases and also ensures that the method is consistent with the root directories in the iteration returned by FileSystem.getDefault().getRootDirectories().

Thread.sleep(millis, nanos) Is Now Able to Perform Sub-Millisecond Sleeps (JDK-8305092)

core-libs/java.lang

The Thread.sleep(millis, nanos) method is now able to perform sub-millisecond sleeps on POSIX platforms. Before this change, a non-zero nanos argument would round up to a full millisecond. While the precision is improved on most POSIX systems, the actual sleep duration is still subject to the precision and accuracy of system facilities.

New Network Interface Names on Windows (JDK-8302659)

core-libs/java.net

Maintainers of applications that do network multicasting or use the java.net.NetworkInterface API should note that the names that the JDK assigns to network interfaces on Windows have changed in this release. The JDK historically synthesized names for network interfaces on Windows. This has changed to use the names assigned by the Windows operating system. For example, the JDK may have historically assigned a name such as “eth0” for an ethernet interface and “lo” for the loopback. The equivalent names that Windows assigns may be names such as “ethernet_32768” and “loopback_0".

This change may impact code that does a lookup of network interfaces with the NetworkInterace.getByName(String name) method. It also may also be surprising to code that enumerates all network interfaces with the NetworkInterfaces.networkInterfaces() or NetworkInterface.getNetworkInterfaces() methods as the names of the network interfaces will look different to previous releases. Depending on configuration, it is possible that enumerating all network interfaces will enumerate network interfaces that weren’t previously enumerated because they didn’t have an Internet Protocol address assigned. The display name returned by NetworkInterface::getDisplayName has not changed so this should facilitate the identification of network interfaces when using Windows native tools.

FileChannel.transferFrom Extends File if Called to Transfer Bytes to the File (JDK-8303260)

core-libs/java.nio

FileChannel.transferFrom has changed in this release to support transferring bytes from a source channel to a file at a file position that is beyond the file's current size. FileChannel.transferFrom was previously specified to not transfer any bytes when called with a file position greater than the file's current size.

Clarification of the Default Charset Initialization with file.encoding (JDK-8300916)

core-libs/java.nio.charsets

If the system property file.encoding is set on the command line to the name of a charset that is not in the java.base module, then the JDK will ignore it and default to UTF-8. In JDK 17 and older, the JDK would attempt to locate the charset, even though it was never supported or documented to change the value of this system property in these releases. Since JDK 18, it is possible to set the system property on the command line to the value UTF-8 (the default) or COMPAT. Setting it to any other value is not defined.

Support Unicode Extension for Collation Settings (JDK-8308108)

core-libs/java.text

The BCP 47 Unicode extension for the strength and normalization collation settings are now supported in the java.text.Collator. If the locale passed to the getInstance(Locale) factory method contains ks and/or kk collation settings, the created Collator instance is set to have the strength and the decomposition modes corresponding to the specified strength and normalization settings.

Support Variant Collations (JDK-8307547)

core-libs/java.text

java.text.Collator now supports multiple collations for a locale. The type of collation may be specified with the Unicode collation identifier if the runtime provides an implementation. For example, the Collator instance created with the locale sv-u-co-trad, traditional collation in the Swedish language, may sort strings, treating v and w the same.

Swedish Collation Rules (JDK-8306927)

core-libs/java.text

Swedish collation rules have been modified to reflect the modern sorting for the language. Collation in Swedish now distinguishes 'v' and 'w' as well as sorting alphabetically. For example, {"wb", "va", "vc"} is sorted as {"va", "vc", "wb"} with this change, whereas previously it was sorted as {"va", "wb", "vc"}. In order to specify the old collation, use the co Unicode identifier in the locale. Refer to Support variant collations for more detail.

java.util.Formatter May Return Slightly Different Results on double and float (JDK-8300869)

core-libs/java.util

The implementation of java.util.Formatter for double and float conversions to decimal ('e', 'E', 'f', 'g', 'G') is now aligned with the one in Double.toString(double), which was changed in JDK 19.

As a consequence, in some rare circumstances, the outcomes may slightly differ from the ones in earlier releases.

One example is with double 2e23 and format "%.16e". With this change, the outcome is 2.0000000000000000e+23, while earlier releases produce 1.9999999999999998e+23. Any smaller precision in the format (e.g., "%.15e") on this value will produce outcomes that are equal to each other, though.

Another example is with double 9.9e-324 and format "%.2g". The new outcome is 9.9e-324, but earlier releases generate "1.0e-323".

Improved ZIP64 Extra Field Validation (JDK-8302483 (not public))

core-libs/java.util.jar

java.util.zip.ZipFile has been updated to provide additional validation of ZIP64 extra fields when opening a ZIP file. This validation may be disabled by setting the system property jdk.util.zip.disableZip64ExtraFieldValidation to true.

Support for CLDR Version 43 (JDK-8296248)

core-libs/java.util:i18n

Locale data based on the Unicode Consortium's CLDR has been upgraded to version 43. The JDK locale data now employs coverageLevels.txt, including the 'basic' and above level locale data, in addition to the data already existing in prior JDK releases for compatibility. For detailed locale data changes, please refer to the Unicode Consortium's CLDR release notes.

JVM TI ThreadStart and ThreadEnd Events Not Sent for Virtual Threads (JDK-8307399)

hotspot/jvmti

Maintainers of JVM Tool Interface (JVM TI) agents should note that JVM TI now specifies that the ThreadStart and ThreadEnd events are not sent for virtual threads. This is different from when virtual threads were a preview feature in Java 19 and Java 20. When the feature was in preview, these events were sent for virtual threads even when the can_support_virtual_threads capability was not enabled. Agents that wish to be notified when virtual threads start or terminate need to add the can_support_virtual_threads capability and enable the VirtualThreadStart and VirtualThreadEnd events.

The JNI_GetCreatedJavaVMs Method Will Now Only Return a Fully Initialized VM (JDK-8308341)

hotspot/runtime

In prior releases, JNI_GetCreatedJavaVMs:

jint JNI_GetCreatedJavaVMs(JavaVM **vmBuf, jsize bufLen, jsize *nVMs);

could return a JavaVM, via the vmBuf array, that was still in the process of being initialized and may not be ready for use. This has now changed so that it will only return fully initialized VMs. It is important that the programmer checks that the returned number of VMs, in nVMs, is greater than zero, before trying to use any vmBuf entries.

ASLR Support for CDS Archive (JDK-8294323 (not public))

hotspot/runtime

Starting with the July 2023 CPU, on operating systems where ASLR (Address Space Layout Randomization) is enabled, the CDS archive will be placed at a random address picked by the operating system.

This change may have a minor performance impact: (a) Start-up time may increase because the JVM needs to patch pointers inside the CDS archive; (b) Memory usage may increase because the memory used by the CDS archive is no longer shareable across processes. We expect the impact to be small because such increases should be only a small fraction of the overall application usage.

In the unlikely event that you must disable ASLR for CDS, you can use the JVM flags -XX:+UnlockDiagnosticVMOptions -XX:ArchiveRelocationMode=0. The usage of such flags is not recommended.

Add final Keyword to Some Static Methods (JDK-8302696)

security-libs

Added the final keyword to the static java.security.cert.CertStore::getDefaultType(), javax.net.ssl.KeyManagerFactory::getDefaultAlgorithm() and javax.net.ssl.TrustManagerFactory::getDefaultAlgorithm() methods.

This reverts changes made in JDKs 19 and 20.

New System Property to Control the Maximum Size of Signature Files (JDK-8300596 (not public))

security-libs/java.security

A new system property, jdk.jar.maxSignatureFileSize, has been added to allow applications to control the maximum size of signature files in a signed JAR. The value of the system property is the desired size in bytes. The default value is 8000000 bytes.

keytool -genseckey And -importpass Commands Warn if Weak PBE Algorithms Are Used (JDK-8286907)

security-libs/java.security

The keytool -genseckey and -importpass commands have been updated to warn users when weak password-based encryption algorithms are specified by the -keyalg option.

SunPKCS11 Provider Now Uses the Same DH Private Exponent Length as Other JDK Providers (JDK-8295425)

security-libs/javax.crypto:pkcs11

When initializing the DH KeyPairGenerator implementation of the SunPKCS11 provider with the keysize argument, it looks up the default DH parameters, including the default private exponent length used by other JDK providers, to initialize the underlying native PKCS11 implementation. If the KeyPairGenerator implementation is initialized with the DHParameterSpec object having a negative private exponent length, this invalid negative value will also be overridden with a default value matching the DH prime size.

javac Message If Implicit Annotation Processors Are Being Used (JDK-8310061)

tools/javac

Annotation processing by javac is enabled by default, including when no annotation processing configuration options are present. Implicit annotation processing by default may be disabled in a future release, possibly as early as JDK 22. To alert javac users of this possibility, in JDK 21 javac prints a note if implicit annotation processing is being used. The text of the note is:

     Annotation processing is enabled because one or more processors were
    found on the class path. A future release of javac may disable
    annotation processing unless at least one processor is specified by
    name (-processor), or a search path is specified (--processor-path,
    --processor-module-path), or annotation processing is enabled
    explicitly (-proc:only, -proc:full).
    Use -Xlint:-options to suppress this message.
    Use -proc:none to disable annotation processing.

Good build hygiene includes explicitly configuring annotation processing. To ease the transition to a different default policy in the future, the new-in-JDK-21 -proc:full javac option requests the current default behavior of looking for annotation processors on the class path.

Annotations with No @Target Annotation Should Be Applicable to Type Parameter Declarations (JDK-8303784)

tools/javac

Prior to JDK 21, the javac compiler was not allowing annotations with no @Target annotation to be applied to type parameter declarations.

This is a bug in the javac compiler in versions prior to JDK21. JLS 9.6.4.1 specifies that annotations without an @Target annotation are applicable in 'all declaration contexts', which includes type parameter declarations.

Starting from JDK21, the javac compiler will accept code like:

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)
@interface Anno {}

class C<@Anno T> {}

This change affects compilations targeting -source/--release 14 and higher.

Emit synthetic and mandated Flags for Parameters by Default (JDK-8292275)

tools/javac

Prior to JDK 21, the javac compiler did not always mark method parameters as synthetic or mandated when applicable. Starting with JDK 21, the javac compiler emits the MethodParameters attribute in the class file when applicable. This attribute stores information on whether or not parameters are synthetic or mandated. This change applies to all release and target versions supported by javac since all currently supported releases and targets have the MethodParameters attribute defined.

This change is justified by JLS § 13.1, in particular:

A binary representation for a class or interface must also contain all of the following: 

[...]

11. A construct emitted by a Java compiler must be marked as synthetic if it does not
    correspond to a construct declared explicitly or implicitly in source code, unless
    the emitted construct is a class initialization method (JVMS §2.9).
12. A construct emitted by a Java compiler must be marked as mandated if it corresponds
    to a formal parameter declared implicitly in source code (§8.8.1, §8.8.9, §8.9.3,
    §15.9.5.1).

Detection for Output File Clashes (JDK-8296656)

tools/javac

A new compiler lint flag, output-file-clash, enables detection of output file clashes. An output file clash is when the compiler intends to write two different output files, but due to the behavior of the operating system, these files end up being written to the same underlying file.

This usually happens due to case-insensitive file systems. For example, a class like this would cause two class files to be written to the same file, Test$Inner.class:

public class Test {
    class Inner {
    }
    class INNER {
    }
}

However, this problem can also happen when the file system "normalizes" file names. For example, on macOS, compiling this class will generate such a clash:

public class Test {
    interface Cafeu0301 {
    }
    interface Cafu00e9 {
    }
}

The reason is that u0301 is the Unicode character "Combining Acute Accent" which means "add an accent over the previous character". MacOS normalizes the letter e followed by a u0301 into a Unicode u00e9, that is, é. However, the Java language treats these the two names, Cafeu0301 and Cafu00e9, as distinct.

Compiling the example above on macOS with -Xlint:output-file-clash will now generate a warning like this:

    warning: [output-file-clash] output file written more than once: /home/test/Test$Café.class

Generate "potentially ambiguous overload" Warning for Inherited Methods (JDK-8026369)

tools/javac

Prior to JDK 21, the javac compiler was omitting some "potentially ambiguous overload" warnings enabled by the -Xlint:overloads option.

If the -Xlint:overloads option is enabled, the compiler warns when the methods in a class create a potential ambiguity for method invocations containing an implicit lambda expression parameter like x -> { ... }. An ambiguity can occur if two or more methods could match such a method call, like when one method takes a Consumer<Integer> parameter where the other takes an IntConsumer. For example, the javac compiler should issue a warning for code such as:

    interface I {
        void foo(Consumer<Integer> c);
        void foo(IntConsumer c);
    }

Prior to JDK 21, the warning was only issued for a class if one of the methods was declared in the class. The javac compiler now also warns when neither method is declared in the class. That is, both methods are inherited from supertypes. For example, for code like:

    interface I {
        void foo(Consumer<Integer> c);
    }

    interface J {
        void foo(IntConsumer c);
    }

    interface K extends I, J {}
405: Record Patterns (Preview)
422: Linux/RISC-V Port
424: Foreign Function & Memory API (Preview)
425: Virtual Threads (Preview)
426: Vector API (Fourth Incubator)
427: Pattern Matching for switch (Third Preview)
428: Structured Concurrency (Incubator)

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