Java SE 22 ( JSR 397)
Public Review Specification
Iris Clark & Brian Goetz
2024/1/11 20:50 -0800 [db92989476a9]

This Specification defines version 22 of the Java Platform, Standard Edition (“Java SE 22”). The Reference Implementation of this Specification is the Java Development Kit, version 22 (“JDK 22”).

Contents
1Summary
2Structure
3Definitions
4Component JSR Specifications
5Features
6APIs previewed
7Modules
8APIs removed
9APIs proposed for removal
Annexes
A1Complete API Specification
A2API Specification differences
A3Java Language & Virtual Machine Specifications
A4JAR File Specification
A5Java AWT Native Interface (JAWT) Specification & Guide
A6Java Debug Wire Protocol (JDWP)
A7Java Native Interface (JNI) Specification
A8Java Object Serialization Specification
A9Java Remote Method Invocation (RMI) Specification
A10Java Security Standard Algorithm Names
A11JVM Tool Interface (JVM TI)
1
Summary  

This release continues the evolution of the Platform to ensure the broadest possible success of the core Java technology. It standardizes   the language feature, unnamed variables and patterns. It previews a new language feature, statements before super(...) and re-previews two language features: String templates and implicitly declared classes and instance main methods. This release standardizes foreign function and memory APIs which enable Java programs to inter-operate with code and data outside of the Java runtime. It previews stream gatherers which support custom intermediate operatons without directly overloading the Stream interface and a Class-File API which provides a standard API for parsing, generating, and transforming Java class files. Finally, this release re-previews a structured concurrency API, simplifying multithreaded programming, and a scoped value API, enabling sharing of immutable data within and across threads.

2
Structure  

This document directly specifies API features, enhancements, clarifications, and bug fixes. It also specifies features by reference to other Specifications which are revised in Maintenance Releases of existing JSRs. The specifications of these changes are contained in two accompanying documents: Annex 1 is the complete Java SE 22 API Specification and Annex 2 is an annotated API specification showing the exact differences relative to Java SE 21. Informative background for these changes may be found in the list of approved Change Specification Requests for this release.

This Specification includes the Java SE 22 Editions of The Java Language Specification and The Java Virtual Machine Specification in Annex 3. The Java SE 22 Editions contain all corrections and clarifications made since the Java SE 21 Editions, as well as additions for new features. Annex 3 also presents changes to The Java Language Specification and The Java Virtual Machine Specification in connection with preview language features in Java SE 22.

This Specification includes the Java SE 22 versions of additional narrative specifications in Annexes 4 — 11. The Java SE 22 versions contain all corrections and clarifications made since the Java SE 21 versions, as well as additions for new features.

3
Definitions  

Changes to the Java SE Platform Specification are categorized as either features or enhancements. A feature is, roughly speaking, a change of which at least one of the following statements is true:

Any addition that is not a feature is considered an enhancement.

There is, obviously, room for judgment when interpreting this definition. In order to maximize the visibility of Platform revisions we generally tend to consider borderline items to be features rather than enhancements.

4
Component JSR Specifications  

Some Component JSR Specifications previously incorporated into the Platform are still available separately or have significant Specifications themselves. Changes to their Specifications are therefore made in separate Maintenance Releases. The following existing component JSRs may have Maintenance Releases. These will be incorporated by reference before the Final Release of this Specification as necessary:

JSR 269: Pluggable Annotation-Processing API [MR 16]

Each Component JSR Specification, or revision thereto, may be related to one or more features in the detailed list below.

5
Features  

Work on features in the Java SE 22 Reference Implementation is organized in terms of JDK Enhancement Proposals (JEPs). Each feature description gives a link to the corresponding JEP document as a convenience, but that document is not a normative part of this Specification. Some features are included in the Java SE 22 Reference Implementation on a preview basis, to gain exposure before achieving permanent status in a later release of the Java SE Platform.

Summary

LibrariesClass-File API (Preview)
Foreign Function & Memory API
Scoped Values (Second Preview)
Stream Gatherers (Preview)
Structured Concurrency (Second Preview)
LanguageImplicitly Declared Classes and Instance Main Methods (Second Preview)
Statements before super(...) (Preview)
String Templates (Second Preview)
Unnamed Variables & Patterns
Libraries
Class-File API (Preview)

Provide a standard API for parsing, generating, and transforming Java class files. This is a preview API.

Foreign Function & Memory API

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.

Scoped Values (Second Preview)

Introduce scoped values, which enable managed sharing of immutable data both with child frames in the same thread, and with child threads. Scoped values are easier to reason about than thread-local variables and have lower space and time costs, especially when used in combination with Virtual Threads and Structured Concurrency. This is a preview API.

Stream Gatherers (Preview)

Enhance the Stream API to support custom intermediate operations. This will allow stream pipelines to transform data in ways that are not easily achievable with the existing built-in intermediate operations. This is a preview API.

Structured Concurrency (Second 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.

Language
Implicitly Declared Classes and Instance Main Methods (Second Preview)

Evolve the Java programming 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 the language, 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.

Statements before super(...) (Preview)

In constructors in the Java programming language, allow statements that do not reference the instance being created to appear before an explicit constructor invocation. This is a preview language feature.

String Templates (Second 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.

Unnamed Variables & Patterns

Enhance the Java programming language with unnamed variables and unnamed patterns, which can be used when variable declarations or nested patterns are required but never used. Both are denoted by the underscore character, _.

Preview features A preview feature is a new feature of the Java language, Java Virtual Machine, or Java SE API that is fully specified, fully implemented, and yet impermanent. Preview features must possess the following properties:

Preview features may be removed in a future release, or upgraded to permanent features of the Java SE Platform.

The Java SE API consists solely of Java APIs, but the complete Java SE Platform includes non-Java APIs, such as JNI and JVM TI, and language-independent protocols, such as JDWP and Java Object Serialization. A preview feature may include changes to non-Java APIs and language-independent protocols.

Implementations of this Specification must:

Additional details about preview features, including a description of a preview feature’s life cycle and the relationship of preview features in the Java language to preview features in the Java SE API, are available in JEP 12 Preview Features.

This section may be compared to the corresponding section that appeared in Java SE 18.

Restricted methods Various methods in the Java SE API allow Java code to interoperate with resources outside the Java runtime in such a way that the runtime cannot prove correct or safe use of the resources. These methods, which are provided because of their high utility, are specified as having the potential to crash the Java runtime or corrupt memory. They are known as restricted methods.

All methods in the Java SE API that are not restricted are unrestricted. Given the potential danger of restricted methods, developers should use restricted methods only when no suitable functionality is available in unrestricted methods. To encourage developers to seek unrestricted alternatives to restricted methods, the following rule applies:

An Implementation may provide a means to invoke its run-time system with all restricted methods treated as unrestricted when invoked from code identified to the run-time system.

If the run-time system is invoked in this way, then by definition there are no restricted methods for the identified code to invoke, so no warnings are issued during the execution of such code.

If code other than that identified to the run-time system invokes a restricted method, the Implementation may give a signal other than a warning issued on the standard error stream.

(The Reference Implementation provides the ability to treat restricted methods as unrestricted for code in certain modules via the command-line option --enable-native-access=M. M is a comma-separated list of modules whose code is permitted to invoke restricted methods as if they were unrestricted, i.e., without warnings. The special operand
ALL-UNNAMED indicates every unnamed module, which includes code on the class path. When the --enable-native-access option is present, any invocation of restricted methods from code outside the listed modules will cause an IllegalCallerException to be thrown, rather than a warning to be issued.)

The list of restricted methods may be found in the Restricted Methods section of the API Specification.

Compared to Java SE 21, this Specification does not add or remove any restricted methods.

Preparing for removal of finalization An Implementation must support the finalization of objects, as described in The Java Language Specification, section 12.6. However, the Java SE 18 Platform Specification deprecates finalization, for removal. To aid preparations for the removal of finalization, an Implementation may provide a means to invoke its run-time system with finalization disabled. If finalization is disabled, the effect is that the Java Virtual Machine never invokes an object’s finalizer before the storage for the object is reclaimed by the garbage collector. An Implementation must not, by default, disable finalization.

(The Reference Implementation provides the ability to disable finalization via the command-line option --finalization=disabled.)

Future revisions of this Platform Specification are expected to disable finalization by default and, eventually, to remove finalization from the Java Language Specification.

6
APIs previewed  

The list of Java APIs which are preview features may be found in the Preview API section of the API Specification. They are disabled by default at compile time and run time.

(The Reference Implementation provides the ability to enable the entire set of preview features via the command-line option
--enable-preview to javac and the java launcher.)

No non-Java APIs or language-independent protocols were added to the Java SE Platform by this Platform Specification.

Preview features may be removed in a future release, or upgraded to permanent features of the Java SE Platform.

7
Modules  

A module is a named set of packages designed for reuse. A specification governed by the JCP defines standard packages, and may group them into one or more standard modules.

This Specification groups the standard packages of the Java SE Platform into 21 standard modules, which we refer to as the Java SE modules. The name of a Java SE module always starts with the string "java.". The complete list of such modules is:

java.base
java.compiler
java.datatransfer
java.desktop
java.instrument
java.logging
java.management
java.management.rmi
java.naming
java.net.http
java.prefs
java.rmi
java.scripting
java.se (aggregator)
java.security.jgss
java.security.sasl
java.sql
java.sql.rowset
java.transaction.xa
java.xml
java.xml.crypto

Compared to Java SE 21, this Specification does not add or remove any modules.

The module graph The Java SE modules depend upon each other as stated in their specifications, which are part of the overall API Specification. The corresponding complete Java SE module graph has too many edges to be displayed easily in visual form; here is the transitive reduction of the directed acyclic graph, in which redundant edges are omitted (click to enlarge):

java.se Module Graph

Here is how to read this visualization of the module graph:

A module is a Java SE module — that is, considered part of the Java SE Platform Specification — if and only if it is a standard module reachable from the java.se module.

Relaxing strong encapsulation As an aid to migration, previous versions of this Specification permitted an Implementation to provide a means to invoke its run-time system with one or more packages of one or more of its modules open to code in all unnamed modules, i.e., to code on the class path. This is no longer permitted.

(The Reference Implementation provided this capability via the command-line option --illegal-access=permit.)

An Implementation must not relax the strong encapsulation of any of its modules, either by default or upon request. That is, its run-time system must not behave as if some packages in the Implementation’s modules are open when they are not open according to their module declarations. A package, or an entire module, is open to code in all unnamed modules if and only if:

This section may be compared to the corresponding section that appeared in Java SE 16.

Incorporated subsections This Specification incorporates the following subsections by reference from the Java SE 9 Platform Specification (JSR 379), “Modules”:

8
APIs removed  

The following API was removed from the Java SE Platform by this Platform Specification. The number in parentheses indicates the Java SE Platform Specification which first proposed its removal.

Methods

9
APIs proposed for removal  

The following APIs were proposed for removal from the Java SE Platform by the Platform Specifications for Java SE 10, Java SE 13, Java SE 14, Java SE 16, Java SE 17, Java SE 18, Java SE 19, Java SE 20, and Java SE 21. They are not removed in this release of the Java SE Platform. They continue to be eligible for removal in a future release.

Packages 

Classes 

Exceptions 

Methods 

Constructors 

Fields 

Compared to Java SE 21, this Specification does not add or remove any further APIs.

Additional details about deprecations, including potential alternatives, may be found in the Deprecated API section of the API Specification. Migration away from deprecated APIs is strongly encouraged.