Oftentimes we will work on a Java project where certain resources (e.g. images, sounds, movie clips) will be required by our program at runtime. Typically, we can just have a library folder outside of our runnable jar, referenced by the class-path, which will contain our resources. This can be a major problem, however, when we want to have a standalone java application run with just the executable JAR file. There are several complicated solutions out there for single JAR packaging, but for a quick, easy, and pain-free solution we can have our program automatically "unpack" its own library folder! This tutorial is created to help you do just that.

Here is a general overview of what we need to do to achieve our goal:

• Create the desired folder* on the local machine (ex., lib)
• Get a handle on the resource inside our JAR
• Write this resource from the JAR file into an external file
• Start the main application**

First, we need to create the desired library folder in the JAR's environment. This is done using the File class' mkdir() command. For example, lets make a "lib" directory:

  new File("lib").mkdir();

The code above will create the folder "lib" in whatever directory the JAR is being run from.

Second, we need to get a handle on the resource in our JAR file in stream format. This can be done simply with the getClass().getResourceAsStream() method, as illustrated below:

  InputStream input = getClass().getResourceAsStream("/lib/swing-layout-1.0.jar");

This code will get an InputStream from the given resource. In this case, the file "swing-layout-1.0.jar" is located in the "lib" folder of our runnable JAR.

Third, we need to take this InputStream and write it to an actual file. This can be done with a simple method that utilizes a FileOutputStream to write the file to disk. This method, setFile, and an example usage are shown below:

  /**
   *Writes a file given the inputstream and filename.
   *Used for unpacking the JAR.
   *
   *@param io       Source InputStream to be written to a file
   *@param fileName Name of file to be written 
   */
  public void setFile(InputStream io, String fileName)
  throws IOException {
    FileOutputStream fos = new FileOutputStream(fileName);
    byte[] buf = new byte[256];
    int read = 0;
    while ((read = io.read(buf)) > 0) {
      fos.write(buf, 0, read);
    }
  }

  setFile(getClass().getResourceAsStream("/lib/swing-layout-1.0.jar"),"lib/swing-layout-1.0.jar");
  

Lastly, start the main application. Now that the libraries are in place, the program should be up and running!

A piece of sample code that shows how all of this comes together can be seen here: LibUnpacker.java.