JEP 200: The Modular JDK
|Discussion||jigsaw dash dev at openjdk dot java dot net|
|Reviewed by||Alan Bateman, Alex Buckley, Paul Sandoz|
|Endorsed by||Brian Goetz|
|Depends||JEP 201: Modular Source Code|
|JEP 220: Modular Run-Time Images|
|JEP 261: Module System|
Divide the JDK into a set of modules that can be combined at compile time, build time, or run time into a variety of configurations including, but not limited to:
Configurations corresponding to the full Java SE Platform, the full JRE, and the full JDK;
Custom configurations which contain only a specified set of modules and the modules transitively required by those modules.
The definition of the modular structure must make a clear distinction between standard modules, whose specifications are governed by the Java Community Process, and modules that are specific to the JDK. It must also distinguish modules that will be proposed for inclusion in the Java SE Platform Specification, and thereby made mandatory in every Platform Implementation, from all other modules.
Project Jigsaw aims to design and implement a standard module system for the Java SE Platform and to apply that system to the Platform itself, and to the JDK. Its primary goals are to make implementations of the Platform more easily scalable down to small devices, improve security and maintainability, enable improved application performance, and provide developers with better tools for programming in the large.
The modular structure proposed here implements the following principles:
Standard modules, whose specifications are governed by the JCP, must have names starting with the string
All other modules are merely part of the JDK, and must have names starting with the string
If a module exports a package that contains a type that contains a public or protected member that, in turn, refers to a type from some other module, then the first module must grant implied readability to the second, via
requires public. (This ensures that method-invocation chaining works in the obvious way.)
A standard module may contain both standard and non-standard API packages. If a standard module exports a standard API package then the export may be qualified; if a standard module exports a non-standard API package then the export must be qualified. In either case, if a standard module exports a package with qualification then the export must be to some subset of the modules in the JDK. If a standard module is a Java SE module, i.e., to be proposed for inclusion in the Java SE Platform Specification, then it must not export any non-SE API packages, at least not without qualification.
A standard module may depend upon one or more non-standard modules. It must not grant implied readability to any non-standard module. If it is a Java SE module then it must not grant implied readability to any non-SE module.
A non-standard module must not export any standard API packages. A non-standard module may grant implied readability to a standard module.
An important consequence of principles 4 and 5 is that code that depends only upon Java SE modules will depend only upon standard Java SE types, and thus be portable to all Implementations of the Java SE Platform.
The module graph
The modular structure of the JDK can be visualized as a graph: Each module is a node, and there is a directed edge from one module to another if the first depends upon the second. The full module graph has too many edges to be displayed easily; here is the transitive reduction of the graph, in which redundant edges are omitted (click to enlarge):
Herewith a guided tour of the module graph:
Standard Java SE modules are colored orange; non-SE modules are colored blue.
If one module depends upon another, and it grants implied readability to that module, then the edge from the first module to the second is solid; otherwise, the edge is dashed.
At the very bottom is the
java.basemodule, which contains essential classes such as
java.lang.String. The base module depends upon no module, and every other module depends upon the base module. Edges to the base module are lighter than other edges.
At the top is the
java.se.eemodule, which gathers together all of the modules that comprise the Java SE Platform, including modules that overlap with the Java EE Platform Specification. This is an example of an aggregator module, which collects and re-exports the content of other modules but adds no content of its own. A run-time system configured to contain the
java.se.eemodule will contain all the API packages of the SE Platform. A module will be proposed for inclusion in the Java SE Platform Specification if, and only if, it is a standard module reachable from the
java.seaggregator module gathers together those parts of the Java SE Platform that do not overlap with Java EE.
java.compact3aggregator modules implement the Java SE Compact Profiles. There is also a non-standard
jdk.compact3module because a
compact3build of the JDK includes some non-SE API packages, while the other Compact Profile builds do not.
The other non-standard modules contain debugging and serviceability tools and APIs (e.g.,
jdk.jconsole), development tools (e.g.,
jdk.xml.bind), and various service providers (e.g.,
jdk.crypto.ec), which are made available to other modules via the existing
java.smartcardiomodule is standard but not part of the Java SE Platform Specification, hence its name starts with the string
"java."but it is colored blue, and it is not reachable from the
The module graph is, in effect, a new kind of API, and will be specified
and evolved as such. The subgraph of the module graph rooted at the
java.se.ee module, with all non-SE modules and corresponding edges
removed, will be proposed for inclusion in the Java SE Platform
Specification; its evolution will thereafter be governed by the JCP. The
evolution of the remainder of the graph will be covered by future JEPs.
In either case, if a module is specified as being available for general
use then it will be subject to the same evolutionary constraints as other
APIs. Removing such a module or changing it incompatibly, in particular,
will require public notice at least one major release in advance.
A tabular summary of all of the modules, including footprint metrics for a Linux/AMD64 build, is available here.
The current module-graph definition has some known flaws, listed below. Fixes for these issues will likely result in small changes to the content and structure of the graph.
java.basemodule exports the
jdkpackage, thereby violating Principle 4 above; a fix for this issue is in progress.
java.managementmodule depends upon
java.rmi, which itself is nontrivial in size and often unnecessary in embedded-device scenarios; we are looking at moving the
javax.management.remote.rmiAPI package into its own module.
The unit and regression tests in the JDK and
the harness used to run them, will be enhanced to allow tests to be
selected on the basis of the modules which they test and upon which they
depend, so that arbitrary configurations of JDK modules can be tested.
The primary functional test of this enhancement will be able to inspect a configured set of modules to ensure that it is a valid combination of the modules defined herein, that each module has the expected content and exports the expected API packages, and that the modules have the expected dependence relationships.
The JCK must be enhanced to test those aspects of the module graph which become part of the Java SE Platform Specification. This includes the names of the SE modules, their exported API packages, and the dependences amongst them that cause SE API packages to be re-exported. The JCK must also be enhanced to test arbitrary configurations of the SE modules present in a Platform Implementation.
Risks and Assumptions
The modular structure defined here could turn out not to address some important use cases. If a critical use case is not supported in the first released implementation of this JEP then we expect to be able address it in a later release by refactoring the module graph.
The Java SE aspects of the module graph defined in this JEP will, as noted, be proposed to an eventual Java SE Platform JSR Expert Group for review and standardization. That review may mandate further changes to the graph.
This JEP is the one of several for Project Jigsaw. The other JEPs are: