CODEWITHSUNDEEP
javajava-compiler-api
Lee Fogg
Lee Fogg

Reputation: 795

Compiling a Jar with java

For years I have been using an IDE (Eclipse) to compile my jar files for me, through the years I have learned about how they work however I still don't fully understand how the jar knows where the main method is, I am also curious about how simple (or not) it is to compile one manually.

I have a (sort of) IDE I'm working on that will need to be able to compile and run a jar that includes both the file from the user and either a jar or a bunch of other classes (the API), I have seen some questions here mentioning Java JavaCompiler class but never giving demo code and there seems to be a next to no one that knows how to compile manually so I would like to contribute. So, how can I create a jar file using java code? Please provide demo code.

Upvotes: 1

Views: 290

Answers (1)

Jon Skeet
Jon Skeet

Reputation: 1503280

I still don't fully understand how the jar knows where the main method is

That's the job of the manifest file.

I am also curious about how simple (or not) it is to compile one manually.

It's pretty straightforward - you use the jar tool after you've built the class files.

Let's do a full walk through.

Create a directory called src and a directory called bin. Under src, create a directory demo and a file called Test.java in that directory:

package demo;

public class Test {
    public static void main(String[] args) {
        System.out.println("Working!");
    }
}

Now compile the code:

javac -d bin src/demo/Test.java

(That will work on both Unix and Windows.)

Then create a manifest file called manifest.txt - it doesn't matter where it goes really, but I'll just keep it in src for the moment:

Main-Class: demo.Test

Now build a jar file:

jar cfm test.jar src\manifest.txt -C bin demo/Test.class

And run it:

java -jar test.jar

These days you can specify the entry point on the command line instead of building a manifest file yourself:

jar cfe test.jar demo.Test -C bin demo/Test.class

See the linked docs for more details on how to use the jar tool, and the potential contents of the manifest.

Upvotes: 4

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