Java Tip 127: See JAR run

Enable your unrunnable JARs to run with the java -jar command

You can easily package an application's entire set of classes and resources into a Java Archive (JAR). In fact, that is one goal of having jar files. Another is to let users easily execute the application stored in the archive. Why then are jar files second-class citizens in the Java universe—functioning only as archives—when they can be first class, right alongside native executables?

To execute a jar file, you can use the

java

command's

-jar

option. For example, say you have a runnable jar file called

myjar.jar

. Because the file is runnable, you can execute it like this:

java -jar myjar.jar

.

Alternatively, the Java Runtime Environment (JRE), when installed on an OS like Microsoft Windows, associates jar files with the JVM so you can double-click on them to run the application. These JARs must be runnable.

The question is: How do you make a JAR runnable?

The manifest file and the Main-Class entry

Inside most JARs, a file called

MANIFEST.MF

is stored in a directory called

META-INF

. Inside that file, a special entry called

Main-Class

tells the

java -jar

command which class to execute.

The problem is that you must properly add this special entry to the manifest file yourself—it must go in a certain place and must have a certain format. However, some of us don't like editing configuration files.

Let the API do it for you

Since Java 1.2, a package called java.util.jar has let you work with jar files. (Note: It builds on the java.util.zip package.) Specifically, the jar package lets you easily manipulate that special manifest file via the Manifest class.

Let's write a program that uses this API. First, this program must know about three things:

  1. The JAR we wish to make runnable
  2. The main class we wish to execute (this class must exist inside the JAR)
  3. The name of a new JAR for our output, because we shouldn't simply overwrite files

Write the program

The above list will constitute our program's arguments. At this point, let's choose a suitable name for this application. How does MakeJarRunnable sound?

Check the arguments to main

Assume our main entry point is a standard main(String[]) method. We should first check the program arguments here:

    if (args.length != 3) {
        System.out.println("Usage: MakeJarRunnable "
                           + "<jar file> <Main-Class>
<output>");
        System.exit(0);
    }

Please pay attention to how the argument list is interpreted, as it is important for the following code. The argument order and contents are not set in stone; however, remember to modify the other code appropriately if you change them.

Access the JAR and its manifest file

First, we must create some objects that know about JAR and manifest files:

    //Create the JarInputStream object, and get its manifest
    JarInputStream jarIn = new JarInputStream(new FileInputStream(args[0]));
    Manifest manifest = jarIn.getManifest();
    if (manifest == null) {
        //This will happen if no manifest exists
        manifest = new Manifest();
    }

Set the Main-Class attribute

We put the Main-Class entry in the manifest file's main attributes section. Once we obtain this attribute set from the manifest object, we can set the appropriate main class. However, what if a Main-Class attribute already exists in the original JAR? This program simply prints a warning and exits. Perhaps we could add a command-line argument that tells the program to use the new value instead of the pre-existing one:

    Attributes a = manifest.getMainAttributes();
    String oldMainClass = a.putValue("Main-Class", args[1]);
    //If an old value exists, tell the user and exit
    if (oldMainClass != null) {
        System.out.println("Warning: old Main-Class value is: "
                           + oldMainClass);
        System.exit(1);
    }

Output the new JAR

We need to create a new jar file, so we must use the JarOutputStream class. Note: We must ensure we don't use the same file for output as we do for input. Alternatively, perhaps the program should consider the case where the two jar files are the same and prompt the user if he wishes to overwrite the original. However, I reserve this as an exercise for the reader. On with the code!

    System.out.println("Writing to " + args[2] + "...");
    JarOutputStream jarOut = new JarOutputStream(new FileOutputStream(args[2]),
                                                 manifest);

We must write every entry from the input JAR to the output JAR, so iterate over the entries:

    //Create a read buffer to transfer data from the input
    byte[] buf = new byte[4096];
    //Iterate the entries
    JarEntry entry;
    while ((entry = jarIn.getNextJarEntry()) != null) {
        //Exclude the manifest file from the old JAR
        if ("META-INF/MANIFEST.MF".equals(entry.getName())) continue;
        //Write the entry to the output JAR
        jarOut.putNextEntry(entry);
        int read;
        while ((read = jarIn.read(buf)) != -1) {
            jarOut.write(buf, 0, read);
        }
        jarOut.closeEntry();
    }
    //Flush and close all the streams
    jarOut.flush();
    jarOut.close();
    jarIn.close();

Complete program

Of course, we must place this code inside a main method, inside a class, and with a suitable set of import statements. The Resources section provides the complete program.

Usage example

Let's put this program to use with an example. Suppose you have an application whose main entry point is in a class called HelloRunnableWorld. (This is the full class name.) Also assume that you've created a JAR called myjar.jar, containing the entire application. Run MakeJarRunnable on this jar file like so:

    java MakeJarRunnable myjar.jar HelloRunnableWorld myjar_r.jar

Again, as mentioned earlier, notice how I order the argument list. If you forget the order, just run this program with no arguments and it will respond with a usage message.

Try to run the

java -jar

command on

myjar.jar

and then on

myjar_r.jar

. Note the difference! After you've done that, explore the manifest files (

META-INF/MANIFEST.MF

) in each JAR. (You can find both JARs in the

source code

.)

Here's a suggestion: Try to make the MakeJarRunnable program into a runnable JAR!

Run with it

Running a JAR by double-clicking it or using a simple command is always more convenient than having to include it in your classpath and running a specific main class. To help you do this, the JAR specification provides a Main-Class attribute for the JAR's manifest file. The program I present here lets you utilize Java's JAR API to easily manipulate this attribute and make your JARs runnable.

Shawn Silverman is currently a graduate student in the department of electrical and computer engineering at the University of Manitoba in Canada. He started working with Java in mid-1996, and has been using it almost exclusively ever since. His current interests include the simulation of electric fields and fluids, error-correcting codes, and the implementation of nifty GUI (graphical user interface) tricks. Shawn also teaches a third-year software design course in the computer engineering department at his university.

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