« back

Java 9 exports and requires

Tags:  java

When we are dealing with Java 9 modules we can create an application such that one modules depends on other. We use exports keyword to define what visible to outside world and requires keyword to define module dependency.

For this example we will create two modules: com.moduleone and com.moduletwo

Scenario #

com.moduleone will have some code that will be used my com.moduletwo

Code for com.moduleone #

For now just create a simple method that will print some message

package com.moduleone;

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

Code for com.moduletwo #

Create a class that will uses code from com.moduleone

package com.moduletwo;

import com.moduleone.ModuleOne;

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

Compile and run the code #

Compile: javac -d mods --module-source-path src $(find src -name "*.java")

Run: java --module-path mods -m com.moduleone/com.moduleone.ModuleOne

OR

You can use the script I created to compile and run the program.

We run the code using ./run.sh com.moduletwo ModuleTwo

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, but module com.moduletwo does not read it)
1 error

We get this error because we have not specified that com.moduletwo depends on com.moduleone. To do this we will use requires keyword

Declare dependencies / requires module.name #

So we tell com.moduletwo require com.moduleone using requires statement. Simply add requires com.moduletwo; on module-info.java of com.moduletwo.

module com.moduletwo {
    requires com.moduleone;
}

Compile and run again #

But this time you will get another error

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)
1 error

If you look closely it says, com.moduleone does not exports it. So let’s do that.

Make code visible outside module / exports module.name #

We need to add exports statement on module declartion file. On com.moduleone module-info.java add exports com.moduleone; where com.modulenone is package you want to export.

module com.moduleone {
    exports com.moduleone;
}

Compile and run again #

Now you will get console output: ./run.sh com.moduletwo ModuleTwo

Output #

Hello from module one

Conclusion #

Hope this simple application helped you understand how exports and requires keyword works.