JEP 200: The Modular JDK

OwnerMark Reinhold
Created2014/07/22 14:08
Updated2016/08/16 21:51
TypeFeature
StatusIntegrated
ScopeSE
Discussionjigsaw dash dev at openjdk dot java dot net
EffortXL
DurationXL
Priority1
Reviewed byAlan Bateman, Alex Buckley, Paul Sandoz
Endorsed byBrian Goetz
Release9
Issue8051618
DependsJEP 201: Modular Source Code
JEP 220: Modular Run-Time Images
JEP 261: Module System

Summary

Use the Java Platform Module System, specified by JSR 376 and implemented by JEP 261, to modularize the JDK.

Goals

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:

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.

Motivation

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.

Description

Design principles

The modular structure proposed here implements the following principles:

  1. Standard modules, whose specifications are governed by the JCP, must have names starting with the string "java.".

  2. All other modules are merely part of the JDK, and must have names starting with the string "jdk.".

  3. 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.)

  4. 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.

  5. 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.

  6. 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:

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.

Open issues

The current module-graph definition has at least one known flaw, listed below. Fixing this issue will likely result in small changes to the content and structure of the graph.

Testing

The unit and regression tests in the JDK and jtreg, 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.

Dependences

This JEP is the one of several for Project Jigsaw. The other JEPs are: