Define a compile-time constant from an external value

Question

I would like to define a version number in a main class in each jar file that is assigned at compile time, like what can be easily done in C with an #include statement with a value from an external file. I would like to only set a value in that external location once, so any jar files that get compiled/built until I change it gets that same value.

My first thought was to define it in a common class then simply reference it like this:

I create a Base.java file:

class Base
{
    public final static String version = "1.2.3";
}

Then I compile Base.java and jar it up.

And then I create a Module1.java file:

class Module1
{
    public final static String version = Base.version;

    public static void main( String[] args )
    {
        Module1();
    }

    Module1()
    {
        System.out.println( "Module1: "+this.version );
    }
}

But of course, this won't compile without importing Base class, so I insert this just before the Module1 class:

import Base;

And I compile Module1.java and jar it up, and execute it; and as expected it returns:

Module1: 1.2.3

So far so good. But then I edit the Base.java file and change the version value to something different, like, say, "1.3.0", then compile Base.java and jar it up.

And now I want to create a Module2.java file:

import Base;
class Module2
{
    public final static String version = Base.version;

    public static void main( String[] args )
    {
        Module2();
    }

    Module2()
    {
        System.out.println( "Module2: "+this.version );
    }
}

And I compile and jar up Module2, and execute it it correctly returns:

Module2: 1.3.0

Also good. But as a sanity check I expect (want/hope) Module1 to return the same results as before, so I rerun Module1, but Bogus! It returns:

Module1: 1.3.0

Any advice on how to pull this off? So the version in a module remains as it was at compile-time, not set during each session at run-time?

Answer 1

In Java, the standard place for storing the version of a .jar file is the manifest file (META-INF/MANIFEST.MF), not a class file. Specifically, put this line there:

Implementation-Version: 1.2.3

See here for more details.

To access this information from your code, use the java.util.jar.Manifest class, and specifically the getMainAttributes() method.