This Specification defines version 20 of the Java Platform, Standard Edition (“Java SE 20”). The Reference Implementation of this Specification is the Java Development Kit, version 20 (“JDK 20”).
|Component JSR Specifications
|APIs proposed for removal
|Complete API Specification
|API Specification differences
|Java Language & Virtual Machine Specifications
|JAR File Specification
|Java AWT Native Interface (JAWT) Specification & Guide
|Java Debug Wire Protocol (JDWP)
|Java Native Interface (JNI) Specification
|Java Object Serialization Specification
|Java Remote Method Invocation (RMI) Specification
|Java Security Standard Algorithm Names
|JVM Tool Interface (JVM TI)
This release continues the evolution of the Platform to ensure the
broadest possible success of the core Java technology. It re-previews
four features which have been updated with further refinements based on
experience and feedback. The two re-previewed language features are
record patterns and pattern matching for
switch expressions. This release re-previews APIs
supporting virtual threads which are lightweight threads that
dramatically reduce the effort of writing, maintaining, and observing
high-throughput concurrent applications. Finally, this release
re-previews foreign function and memory APIs which enable Java programs to
safely call native libraries and process native data as an alternative to
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 20 API Specification and Annex 2 is an annotated API specification showing the exact differences relative to Java SE 19. 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 20 Editions of The Java Language Specification and The Java Virtual Machine Specification in Annex 3. The Java SE 20 Editions contain all corrections and clarifications made since the Java SE 19 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 20.
This Specification includes the Java SE 20 versions of additional narrative specifications in Annexes 4 — 11. The Java SE 20 versions contain all corrections and clarifications made since the Java SE 19 versions, as well as additions for new features.
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.
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, which are incorporated here by reference:
Each Component JSR Specification, or revision thereto, may be related to one or more features in the detailed list below.
Work on features in the Java SE 20 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 20 Reference Implementation on a preview basis, to gain exposure before achieving permanent status in a later release of the Java SE Platform.
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.
Enhance the Java programming language with pattern matching for
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. This is a preview language feature.
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. This is a preview language feature.
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:
java.base module, or (2) the
ModuleLayer.Controller.enableNativeAccess, or (3) as
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
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
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
IllegalCallerException to be thrown, rather than a
warning to be issued.)
The restricted methods in Java SE 20 are:
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
Future revisions of this Platform Specification are expected to disable finalization by default and, eventually, to remove finalization from the Java Language Specification.
(The Reference Implementation provides the ability to
enable the entire set of preview features via the command-line option
javac and the
The following non-Java APIs and language-independent protocols were added to the Java SE Platform by this Platform Specification. They are enabled by default and cannot be disabled.
JVM TI Specification
Preview features may be removed in a future release, or upgraded to permanent features of the Java SE Platform.
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:
Compared to Java SE 19, 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):
Here is how to read this visualization of the module graph:
If one module depends upon another, and it grants implied readability to that module via a requires transitive directive, then there is an edge from the first module to the second.
At the very bottom is the java.base module, which contains essential classes such as java.lang.Object and java.lang.String. The base module depends upon no module, and every other module depends upon the base module.
At the top is the java.se module, which gathers together all of the modules that comprise the Java SE Platform. This is an example of an aggregator module, which logically gathers the content of other modules by granting implied readability to them, but adds no content of its own. A run-time system configured to contain the java.se module will contain all of the packages of the Java SE Platform.
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
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.
No APIs were removed from the Java SE Platform by this Platform Specification.
Profiles The Java SE 8 Platform Specification (JSR 337), “Profiles” introduced the concept of a Java SE Profile, a well-defined subset of the Java SE Platform. Profiles allow applications that use just part of the Platform to run on resource-constrained devices. Implementors were free to implement zero, one, or more Profiles. They were also free to implement one or more Profiles without implementing the entire Platform. The Technology Compatibility Kit (TCK) for the Java SE 8 Platform Specification was structured to allow the conformance of an Implementation of any Profile, or of the entire Platform, to be tested. A Reference Implementation (RI) was provided for each Profile, and for the entire Platform.
As of the Java SE 20 Platform Specification, an RI will no longer be provided for any Profiles; an RI is provided only for the entire Platform.
The following APIs were proposed for removal from the Java SE Platform by the Platform Specifications for Java SE 9, Java SE 10, Java SE 13, Java SE 14, Java SE 16, Java SE 17, Java SE 18, and Java SE 19. They are not removed in this release of the Java SE Platform. They continue to be eligible for removal in a future release.
The following APIs are proposed for removal from the Java SE Platform
by this Platform Specification. They are annotated as
forRemoval=true, following the
convention established by
This makes them eligible for removal in a future release.
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.