Project Jigsaw: Module System Quick-Start Guide

This document provides a few simple examples to get developers started with modules.

The file paths in the examples use forward slashes, and the path separators are colons. Developers on Microsoft Windows should use file paths with back slashes and a semi-colon as the path separator.

Greetings

This first example is a module named com.greetings that simply prints "Greetings!". The module consists of two source files: the module declaration (module-info.java) and the main class.

By convention, the source code for the module is in a directory that is the name of the module.

    src/com.greetings/com/greetings/Main.java
    src/com.greetings/module-info.java

    $ cat src/com.greetings/module-info.java
    module com.greetings { }

    $ cat src/com.greetings/com/greetings/Main.java
    package com.greetings;
    public class Main {
        public static void main(String[] args) {
            System.out.println("Greetings!");
        }
    }
The source code is compiled to the directory mods/com.greetings with the following commands:
    $ mkdir -p mods/com.greetings

    $ javac -d mods/com.greetings \
        src/com.greetings/module-info.java \
        src/com.greetings/com/greetings/Main.java
Now we run the example with the following command:
    $ java -modulepath mods -m com.greetings/com.greetings.Main
-modulepath is the module path, its value is one or more directories that contain modules. The -m option specifies the main module, the value after the slash is the class name of the main class in the module.

Greetings world

This second example updates the module declaration to declare a dependency on module org.astro. Module org.astro exports the API package org.astro.
    src/org.astro/module-info.java
    src/org.astro/org/astro/World.java
    src/com.greetings/com/greetings/Main.java
    src/com.greetings/module-info.java

    $ cat src/org.astro/module-info.java
    module org.astro {
        exports org.astro;
    }

    $ cat src/org.astro/org/astro/World.java
    package org.astro;
    public class World {
        public static String name() {
            return "world";
        }
    }

    $ cat src/com.greetings/module-info.java
    module com.greetings {
        requires org.astro;
    }

    $ cat src/com.greetings/com/greetings/Main.java
    package com.greetings;
    import org.astro.World;
    public class Main {
        public static void main(String[] args) {
            System.out.format("Greetings %s!%n", World.name());
        }
    }
The modules are compiled, one at a time. The javac command to compile module com.greetings specifies a module path so that the reference to module org.astro and the types in its exported packages can be resolved.
    $ mkdir mods/org.astro mods/com.greetings

    $ javac -d mods/org.astro \
        src/org.astro/module-info.java src/org.astro/org/astro/World.java

    $ javac -modulepath mods -d mods/com.greetings \
        src/com.greetings/module-info.java src/com.greetings/com/greetings/Main.java
The example is run in exactly the same way as the first example:
    $ java -modulepath mods -m com.greetings/com.greetings.Main
    Greetings world!

Multi-module compilation

In the previous example then module com.greetings and module org.astro were compiled separately. It is also possible to compile multiple modules with one javac command:
    $ mkdir mods

    $ javac -d mods -modulesourcepath src $(find src -name "*.java")

    $ find mods -type f
    mods/com.greetings/com/greetings/Main.class
    mods/com.greetings/module-info.class
    mods/org.astro/module-info.class
    mods/org.astro/org/astro/World.class

Packaging

In the examples so far then the contents of the compiled modules are exploded on the file system. For transportation and deployment purposes then it is usually more convenient to package a module as a modular JAR. A modular JAR is a regular JAR file that has a module-info.class in its top-level directory. The following example creates org.astro@1.0.jar and com.greetings.jar in directory mlib.
    $ mkdir mlib

    $ jar --create --file=mlib/org.astro@1.0.jar \
        --module-version=1.0 -C mods/org.astro .

    $ jar --create --file=mlib/com.greetings.jar \
        --main-class=com.greetings.Main -C mods/com.greetings .

    $ ls mlib
    com.greetings.jar   org.astro@1.0.jar
In this example, then module org.astro is packaged to indicate that its version is 1.0. Module com.greetings has been packaged to indicate that its main class is com.greetings.Main. We can now execute module com.greetings without needing to specify its main class:
    $ java -mp mlib -m com.greetings
    Greetings world!
The command line is also shortened by using -mp as an alternative to -modulepath.

The jar tool has many new options (see jar -help), one of which is to print the module declaration for a module packaged as a modular JAR.

    $ jar --print-module-descriptor --file=mlib/org.astro@1.0.jar

    Name:
      org.astro@1.0
    Requires:
      java.base [ MANDATED ]
    Exports:
      org.astro

Missing requires or missing exports

Now let's see what happens with the previous example when we mistakenly omit the requires from the com.greetings module declaration:
    $ cat src/com.greetings/module-info.java
    module com.greetings {
        // requires org.astro;
    }

    $ javac -modulepath mods -d mods/com.greetings \
        src/com.greetings/module-info.java src/com.greetings/com/greetings/Main.java
    src/com.greetings/com/greetings/Main.java:2: error: package org.astro does not exist
    import org.astro.World;
                    ^
    src/com.greetings/com/greetings/Main.java:5: error: cannot find symbol
            System.out.format("Greetings %s!%n", World.name());
                                                 ^
      symbol:   variable World
      location: class Main
    2 errors
We now fix this module declaration but introduce a different mistake, this time we omit the exports from the org.astro module declaration:
    $ cat src/com.greetings/module-info.java
    module com.greetings {
        requires org.astro;
    }
    $ cat src/org.astro/module-info.java
    module org.astro {
        // exports org.astro;
    }

    $ javac -modulepath mods -d mods/com.greetings \
       src/com.greetings/module-info.java src/com.greetings/com/greetings/Main.java
    src/com.greetings/com/greetings/Main.java:2: error: package org.astro does not exist
    import org.astro.World;
                    ^
    src/com.greetings/com/greetings/Main.java:5: error: cannot find symbol
            System.out.format("Greetings %s!%n", World.name());
                                                 ^
      symbol:   variable World
      location: class Main
    2 errors

Services

Services allow for loose coupling between service consumers modules and service providers modules.

This example has a service consumer module and a service provider module:

The following is the source code for module com.socket .
    $ cat src/com.socket/module-info.java
    module com.socket {
        exports com.socket;
        exports com.socket.spi;
        uses com.socket.spi.NetworkSocketProvider;
    }

    $ cat src/com.socket/com/socket/NetworkSocket.java
    package com.socket;

    import java.io.Closeable;
    import java.util.Iterator;
    import java.util.ServiceLoader;

    import com.socket.spi.NetworkSocketProvider;

    public abstract class NetworkSocket implements Closeable {
        protected NetworkSocket() { }

        public static NetworkSocket open() {
            ServiceLoader<NetworkSocketProvider> sl
                = ServiceLoader.load(NetworkSocketProvider.class);
            Iterator<NetworkSocketProvider> iter = sl.iterator();
            if (!iter.hasNext())
                throw new RuntimeException("No service providers found!");
            NetworkSocketProvider provider = iter.next();
            return provider.openNetworkSocket();
        }
    }


    $ cat src/com.socket/com/socket/spi/NetworkSocketProvider.java
    package com.socket.spi;

    import com.socket.NetworkSocket;

    public abstract class NetworkSocketProvider {
        protected NetworkSocketProvider() { }

        public abstract NetworkSocket openNetworkSocket();
    }
The following is the source code for module org.fastsocket .
    $ cat src/org.fastsocket/module-info.java
    module org.fastsocket {
        requires com.socket;
        provides com.socket.spi.NetworkSocketProvider
            with org.fastsocket.FastNetworkSocketProvider;
    }

    $ cat src/org.fastsocket/org/fastsocket/FastNetworkSocketProvider.java
    package org.fastsocket;

    import com.socket.NetworkSocket;
    import com.socket.spi.NetworkSocketProvider;

    public class FastNetworkSocketProvider extends NetworkSocketProvider {
        public FastNetworkSocketProvider() { }

        @Override
        public NetworkSocket openNetworkSocket() {
            return new FastNetworkSocket();
        }
    }

    $ cat src/org.fastsocket/org/fastsocket/FastNetworkSocket.java
    package org.fastsocket;

    import com.socket.NetworkSocket;

    class FastNetworkSocket extends NetworkSocket {
        FastNetworkSocket() { }
        public void close() { }
    }
For simplicity, we compile both modules together. In practice then the service consumer module and service provider modules will nearly always be compiled separately.
    $ mkdir mods
    $ javac -d mods -modulesourcepath src $(find src -name "*.java")

Finally we modify our module com.greetings to use the API.

    $ cat src/com.greetings/module-info.java
    module com.greetings {
        requires com.socket;
    }

    $ cat src/com.greetings/com/greetings/Main.java
    package com.greetings;

    import com.socket.NetworkSocket;

    public class Main {
        public static void main(String[] args) {
            NetworkSocket s = NetworkSocket.open();
            System.out.println(s.getClass());
        }
    }


    $ javac -d mods/com.greetings/ -mp mods $(find src/com.greetings/ -name "*.java")
Finally we run it:
    $ java -mp mods -m com.greetings/com.greetings.Main
    class org.fastsocket.FastNetworkSocket
The output confirms that the service provider has been located and that it was used as the factory for the NetworkSocket.

The linker

jlink is the linker tool and can be used to link a set of modules, along with their transitive dependences, to create a custom modular run-time image (see JEP 220).

The tool currently requires that modules on the module path be packaged in modular JAR or JMOD format. The JDK build packages the standard and JDK-specific modules in JMOD format.

The following example creates a run-time image that contains the module com.greetings and its transitive dependences:

    jlink --modulepath $JAVA_HOME/jmods:mlib --addmods com.greetings --output greetingsapp
The value to --modulepath is a PATH of directories containing the packaged modules. Replace the path separator ':' with ';' on Microsoft Windows.

$JAVA_HOME/jmods is the directory containing java.base.jmod and the other standard and JDK modules. If you using your own build of OpenJDK then the jmod files are in $BUILDOUTPUT/images/jmods, where $BUILDOUTPUT is the build output directory.

The directory mlib on the module path contains the artifact for module com.greetings.

The jlink tool supports many advanced options to customize the generated image, see jlink --help for more options.

javac -Xmodule and java -Xpatch

Developers that checkout java.util.concurrent classes from Doug Lea's CVS will be used to compiling the source files and deploying those classes with -Xbootclasspath/p.

-Xbootclasspath/p has been removed, its module replacement is the option -Xpatch to override classes in a module. Furthermore, javac will emit a warning when compiling classes in a package of an existing module. To compile classes for an existing module requires using the javac option -Xmodule.

Here's an example that compiles a new version of java.util.concurrent.ConcurrentHashMap and uses it at run-time:

    javac -Xmodule:java.base -d mypatches/java.base \
        src/java.base/java/util/concurrent/ConcurrentHashMap.java

    java -Xpatch:java.base=mypatches/java.base ...

More information

Feedback

Please send usage questions and experience reports to the jigsaw-dev list. Specific suggestions about the design of the module system should be sent to the JSR 376 Expert Group's comments list.