Java 9 Using exports with to

In this example we will create three module

1. com.moduleone
2. com.moduletwo
3. com.modulethree

We will create an example such that com.moduletwo and com.modulethree is dependent on com.moduleone but com.moduleone will only export its code to com.moduletwo. We will see error that pops up when we try to use the module where it is not exported.

Here’s the directory structure

├── run.sh
└── src
    ├── com.moduleone
    │   ├── com
    │   │   └── moduleone
    │   │       └── ModuleOne.java
    │   └── module-info.java
    ├── com.modulethree
    │   ├── com
    │   │   └── modulethree
    │   │       └── ModuleThree.java
    │   └── module-info.java
    └── com.moduletwo
        ├── com
        │   └── moduletwo
        │       └── ModuleTwo.java
        └── module-info.java

I will be modifying the example from previous post so, if you haven’t read it here’s a link to them

1. Part 1: Java Module Basics/Intro
2. Part 2: Java exports and requires keyword

From previous example I just modified the com.moduleone module-info.java file to use to keyword.

module com.moduleone {
    exports com.moduleone to com.modulethree;
}

package com.moduleone;

public class ModuleOne {
    public static void callMe(){
        System.out.println("Hello from module one");
    }
}

The difference here is com.moduleone is only exporting it’s code to com.modulethree. Others cannot access it.

If we don’t change the com.moduletwo code i.e

module com.moduletwo {
    requires com.moduleone;
}

package com.moduletwo;

import com.moduleone.ModuleOne;

public class ModuleTwo {
    public static void main(String[] args){
        ModuleOne.callMe();
    }
}

If we compile and run: ./run.sh com.moduletwo ModuleTwo our example we get error like below

src/com.moduletwo/com/moduletwo/ModuleTwo.java:3: error: package com.moduleone is not visible
import com.moduleone.ModuleOne;
          ^
  (package com.moduleone is declared in module com.moduleone, which does not export it to module com.moduletwo)
1 error

You can see that we get error saying com.moduleone doesn’t export its code to com.moduletwo.

So let’s comment out the code on com.moduletwo to remove the reference that we are using com.moduleone.

module com.moduletwo {
    // requires com.moduleone;
}

package com.moduletwo;

// import com.moduleone.ModuleOne;

public class ModuleTwo {
    public static void main(String[] args){
        // ModuleOne.callMe();
    }
}

Let’s create a third module com.modulethree which is the module com.moduleone exports its code to.

module com.modulethree {
    requires com.moduleone;
}

package com.modulethree;

import com.moduleone.ModuleOne;

public class ModuleThree {
    public static void main(String[] args){
        ModuleOne.callMe();
    }
}

Compile and run the code: ./run.sh com.modulethree ModuleThree and your code will compile successfully and show the result as well.

NOTE: You can also export your modules to multiple modules by using comma (,) to separate them.

Conclusion #

You can see the power of using exports with to here. If you want your code to be visible to only certain modules you can do that easily.

GitHub Code