Java 9 Modules - Multi Module Mode Example

In the last tutorial, we saw different compilation modes in Java 9.

Multi-module mode can be useful if we are developing multiple modules which are closely related. In this mode, we can compile all sub modules at once (by specifying --module-source-path) instead of compiling individual modules.

In this example, we will see a complete example of multi-module mode. We will also learn how to compile, run and package (in jar) the multi-module project.

Example

We are going to create two modules based on multi-module directory structure.

The First Module

core.app/core/util/WorkerUtil.java

package core.util;

public class WorkerUtil {
  public static void doSomething() {
      System.out.println("WorkerUtil working..");
  }
}

core.app/module-info.java

module core.app {
  exports core.util;
}

The Second Module

main.app/example/app/AppMain.java

package example.app;

import core.util.WorkerUtil;

public class AppMain {
    public static void main(String[] args) {
        WorkerUtil.doSomething();
    }
}

main.app/module-info.java

module main.app {
  requires core.app;
}

Here is our project's directory structure.

D:\multi-module-project>"tree /A /F"
\---src
    +---core.app
    |   |   module-info.java
    |   |   
    |   \---core
    |       \---util
    |               WorkerUtil.java
    |               
    \---main.app
        |   module-info.java
        |   
        \---example
            \---app
                    AppMain.java
                    

Compiling

Let's save a list of all java classes in a file, so that we can use @fileName as argument of javac command.

D:\multi-module-project>dir  /B  /S  src\*.java > javaFiles.txt

Confirm our text file has all java source files:

D:\multi-module-project>type javaFiles.txt
D:\multi-module-project\src\core.app\module-info.java
D:\multi-module-project\src\core.app\core\util\WorkerUtil.java
D:\multi-module-project\src\main.app\module-info.java
D:\multi-module-project\src\main.app\example\app\AppMain.java

Compiling:

D:\multi-module-project>javac -d outDir --module-source-path src @javaFiles.txt

The --module-source-path option (used above) is specific to multi-module mode which specifies the main source directory for multiple modules.

After compilation:

D:\multi-module-project>"tree /A /F"
|   javaFiles.txt
|   
+---outDir
|   +---core.app
|   |   |   module-info.class
|   |   |   
|   |   \---core
|   |       \---util
|   |               WorkerUtil.class
|   |               
|   \---main.app
|       |   module-info.class
|       |   
|       \---example
|           \---app
|                   AppMain.class
|                   
\---src
    +---core.app
    |   |   module-info.java
    |   |   
    |   \---core
    |       \---util
    |               WorkerUtil.java
    |               
    \---main.app
        |   module-info.java
        |   
        \---example
            \---app
                    AppMain.java
                    

Running

D:\multi-module-project>java --module-path outDir --module main.app/example.app.AppMain
WorkerUtil working..

Packaging in Jar

We still need to package individual modules in separate jar files.

D:\multi-module-project>mkdir pkg

D:\multi-module-project>jar --create --file pkg\core.app.jar -C outDir\core.app .

D:\multi-module-project>jar --create --file pkg\app.main.jar --main-class=example.app.AppMain  -C outDir\main.app .

D:\multi-module-project\pkg>"tree /A /F"
    app.main.jar
    core.app.jar
    
No subfolders exist 

Checking the content of jars:

D:\multi-module-project>jar --list --file pkg\core.app.jar
META-INF/
META-INF/MANIFEST.MF
module-info.class
core/
core/util/
core/util/WorkerUtil.class

D:\multi-module-project>jar --list --file pkg\app.main.jar
META-INF/
META-INF/MANIFEST.MF
module-info.class
example/
example/app/
example/app/AppMain.class

Running:

D:\multi-module-project>java --module-path pkg --module main.app/example.app.AppMain
WorkerUtil working..

Using --describe-module:

D:\multi-module-project>jar --describe-module --file pkg\core.app.jar
core.app jar:file:///D:/multi-module-project/pkg/core.app.jar/!module-info.class
exports core.util
requires java.base mandated

D:\multi-module-project>jar --describe-module --file pkg\app.main.jar
main.app jar:file:///D:/multi-module-project/pkg/app.main.jar/!module-info.class
requires core.app
requires java.base mandated
contains example.app
main-class example.app.AppMain

Example Project

Dependencies and Technologies Used:

• JDK 9