Chapter 1. Understanding Modules

1.1.  Describe the Modular JDK

[Note]

JDK was divided into a set of modules that can be combined at compile time, build time, and 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;

  • Configurations roughly equivalent in content to each of the Compact Profiles defined in Java SE 8; and

  • Custom configurations which contain only a specified set of modules possibly augmented by external library and application modules, and the modules transitively required by all of these modules.

The definition of the modular structure should 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 should also distinguish modules that are included 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 of the JDK implements the following principles:

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

  2. All other modules are merely part of the JDK, and 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 transitive. (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., is included 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 list

You can list all JDK modules by running this command:

java --list-modules
					

List of Java 11 modules is shown below:

java.base@11.0.2
java.compiler@11.0.2
java.datatransfer@11.0.2
java.desktop@11.0.2
java.instrument@11.0.2
java.logging@11.0.2
java.management@11.0.2
java.management.rmi@11.0.2
java.naming@11.0.2
java.net.http@11.0.2
java.prefs@11.0.2
java.rmi@11.0.2
java.scripting@11.0.2
java.se@11.0.2
java.security.jgss@11.0.2
java.security.sasl@11.0.2
java.smartcardio@11.0.2
java.sql@11.0.2
java.sql.rowset@11.0.2
java.transaction.xa@11.0.2
java.xml@11.0.2
java.xml.crypto@11.0.2
jdk.accessibility@11.0.2
jdk.aot@11.0.2
jdk.attach@11.0.2
jdk.charsets@11.0.2
jdk.compiler@11.0.2
jdk.crypto.cryptoki@11.0.2
jdk.crypto.ec@11.0.2
jdk.crypto.mscapi@11.0.2
jdk.dynalink@11.0.2
jdk.editpad@11.0.2
jdk.hotspot.agent@11.0.2
jdk.httpserver@11.0.2
jdk.internal.ed@11.0.2
jdk.internal.jvmstat@11.0.2
jdk.internal.le@11.0.2
jdk.internal.opt@11.0.2
jdk.internal.vm.ci@11.0.2
jdk.internal.vm.compiler@11.0.2
jdk.internal.vm.compiler.management@11.0.2
jdk.jartool@11.0.2
jdk.javadoc@11.0.2
jdk.jcmd@11.0.2
jdk.jconsole@11.0.2
jdk.jdeps@11.0.2
jdk.jdi@11.0.2
jdk.jdwp.agent@11.0.2
jdk.jfr@11.0.2
jdk.jlink@11.0.2
jdk.jshell@11.0.2
jdk.jsobject@11.0.2
jdk.jstatd@11.0.2
jdk.localedata@11.0.2
jdk.management@11.0.2
jdk.management.agent@11.0.2
jdk.management.jfr@11.0.2
jdk.naming.dns@11.0.2
jdk.naming.rmi@11.0.2
jdk.net@11.0.2
jdk.pack@11.0.2
jdk.rmic@11.0.2
jdk.scripting.nashorn@11.0.2
jdk.scripting.nashorn.shell@11.0.2
jdk.sctp@11.0.2
jdk.security.auth@11.0.2
jdk.security.jgss@11.0.2
jdk.unsupported@11.0.2
jdk.unsupported.desktop@11.0.2
jdk.xml.dom@11.0.2
jdk.zipfs@11.0.2
					

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 Java SE 11 Module Graph.

Please take a look at appendix for the instruction how you can re-create it: “How to build JDK module graph”

Herewith a guided tour of the Java 11 module graph:

  • Standard Java SE modules (java.) are colored Cyan; non-SE modules (jdk.) are colored Dark Sea Green.

  • If one module depends upon another, 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.

  • Near 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 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 module will contain all of the API packages of the Java SE Platform. A module is included in the Java SE Platform Specification if, and only if, it is a standard module reachable from the java.se module.

  • The non-standard modules (jdk.) include debugging and serviceability tools and APIs, development tools, and various service providers, which are made available to other modules via the existing java.util.ServiceLoader mechanism.

  • The java.smartcardio module is standard but not part of the Java SE Platform Specification, hence its name starts with the string "java." but it is not reachable from the java.se module.

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