Optional dependency using modular programming in Java 9 not working with provided module path

Question

I have written source codes of two modules in the directory ~/Desktop/src

code of the module myModuleA

M1.java

package myPack1;
import myPack2.M2;
public class M1
{   
   public static void main(String[] args)   
   {    
     System.out.println("I am M1"); 
     M2.print(); 
     System.out.println("I am M1 Again");
   }
}

module-info.java

module myModuleA
{
      requires myModuleB;
}

code of the module myModuleB

M2.java

package myPack2;
public class M2
{   
   public static void print()   
   {    
     System.out.println("I am M2"); 
   }
}

module-info.java

module myModuleB
{
      exports myPack2;
}

I compiled both modules as:

gyan@#ns:~/Desktop$ javac --module-source-path src -d out -m myModuleA,myModuleB

The directory out is created on the Desktop, which is my current directory.

Thereafter I created another directory on the Desktop with name OtherModule. Cut and paste the compile module directory of myModuleB from the directory out to the directory OtherModule. Deleted the source directory of the module myModuleB from the directory src. Deleted the directory out.

Again I compiled the module myModuleA with the compiled code of the module myModuleB as:

gyan@#ns:~/Desktop$ javac --module-source-path src --module-path OtherModule -d out -m myModuleA

And executed the code successfully as:

gyan@#ns:~/Desktop$ java --module-path out/:OtherModule/ -m myModuleA/myPack1.M1
I am M1
I am M2
I am M1 Again
gyan@#ns:~/Desktop$

But then I changed the module-info.java file of the module myModuleA as:

module myModuleA
{
    requires static myModuleB;
}

Again I deleted the directory out and compiled the code as:

gyan@#ns:~/Desktop$ javac --module-source-path src --module-path OtherModule -d out -m myModuleA

The code compiles successfully. Then I tried to execute the code as:

gyan@#ns:~/Desktop$ java --module-path out/:OtherModule/ -m myModuleA/myPack1.M1
I am M1
Exception in thread "main" java.lang.NoClassDefFoundError: myPack2/M2
    at myModuleA/myPack1.M1.main(M1.java:8)
Caused by: java.lang.ClassNotFoundException: myPack2.M2
    at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:582)
    at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:185)
    at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:496)
    ... 1 more
gyan@#ns:~/Desktop$

What is the reason behind this error, even if I am providing the path to the both compiled modules.

What for is the --upgrade-module-path in use for your example? What happens if you simply let it be the --module-path? — Naman
– Naman, Commented Jan 26, 2018 at 11:34
myModuleB is observable but it will not be resolved unless some only modules requires it. If myModuleB is resolved then myModuleA will read it). So add --add-module myModuleB to the command line and I assume it will work. BTW: What is the reason for using --upgrade-module-path in these examples? Which JDK modules are you overriding. — Alan Bateman
– Alan Bateman, Commented Jan 26, 2018 at 11:35
@nullpointer How is this a duplicate of that question? That one asks "Does X exist?", this one "When doing X, why doesn't Y work?". Even though their answers might overlap partially, these are totally different angles and no one having the OP's problem will assume the answer is hidden in "Does X exist?". — Nicolai Parlog
– Nicolai Parlog, Commented Jan 26, 2018 at 11:52
@AlanBateman Yes It works. And no need to use --upgrade-module-path. We can use --module-path instead. This was stored command in my terminal, so I was reusing it. I am going to change my question and use --module-path instead of --upgrade-module-path. Thanks. — my name is GYAN
– my name is GYAN, Commented Jan 26, 2018 at 11:53
@nullpointer As Nicolai said this is totally different question. I already have seen that question before writing this one. — my name is GYAN
– my name is GYAN, Commented Jan 26, 2018 at 11:59

Nicolai Parlog · Accepted Answer · 2018-01-27 15:52:28Z

2

Optional dependencies are not resolved on their own - even if they are present on the module path. If they are not resolved, they don't make it into the readability graph and so they aren't available at run time - hence the error message.

For the optional dependency to be available at run time, it must either be non-optionally required by third module, resolved during service binding, or added manually with --add-modules.

edited Jan 27, 2018 at 15:52

answered Jan 26, 2018 at 13:32

Nicolai Parlog

51.7k25 gold badges138 silver badges262 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Stephan Herrmann Over a year ago

really --add-exports? The anchor of your link hints at --add-module, which makes more sense (though when I follow the link the anchor doesn't appear to exist).

Nicolai Parlog Over a year ago

@StephanHerrmann, thanks for pointing those mistakes out, I fixed them.

Collectives™ on Stack Overflow

Optional dependency using modular programming in Java 9 not working with provided module path

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related