Compiling a Jar with java

Question

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.

Here .. http://stackoverflow.com/questions/5258159/how-to-make-an-executable-jar-file — vidit, Jul 18 '13 at 22:23
"In Eclipse you can do it simply as follows :", "Compile your classes." - not what I am asking, "how can I create a jar file using java code?" — Lee Fogg, Jul 18 '13 at 23:10
Java includes a fully functional set of zip file utilities (java.util.zip) that can be used to create jar files. And there are some additional "nice to have" facilities in java.util.jar. — Hot Licks, Jul 18 '13 at 23:59
@LeeAllan- Did you look at the accepted answer on that question? It tells you exactly that and with a code example. — vidit, Jul 19 '13 at 01:12
The only java code in that example is the file to be compiled, I need the code in which will compile .java files into a runnable jar. — Lee Fogg, Jul 19 '13 at 01:31
Have you check out the explanation here: http://stackoverflow.com/questions/2946338/how-do-i-programmatically-compile-and-instantiate-a-java-class Then you can take a look at how to build JAR programmatically: http://docs.oracle.com/javase/6/docs/api/java/util/jar/JarOutputStream.html — user2507946, Jul 19 '13 at 03:03

Jon Skeet · Answer 1 · 2013-07-18T22:29:33.363

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.

My IDE is written in Java and needs to compile for the user as explained. Could I get Java to run these Command Prompt commands? Or even if not create the necessary batch file? — Lee Fogg, Jul 18 '13 at 23:18
@LeeAllan: You *could*, but you'd be a lot better off using the JavaCompiler class you linked to. Likewise you can use the classes in `java.util.jar` to build jar files programmatically. — Jon Skeet, Jul 19 '13 at 05:44

Compiling a Jar with java

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