Dynamic Class Loading

 This article was originally posted on JRoller on June 30, 2005.

The other day, I wanted to write an Eclipse plugin (maybe more about that in a different post), in which I need to read a selected class file from the project I am working on and execute a method in it. Since I can not have my project in the classpath, I found out that the only solution is to have the class loaded dynamically. If there is a better solution in Eclipse, please somebody tell me.

Before starting to write my plugin, I decided to write a small test application, because I never used class loading before. So here is the class I want to load:

package hello;

public class HelloWorld
{
  public void run()
  {
    System.out.println ("Hello World!");
  }
}

To load it and execute the run method, you can then use the following lines of code:

        ClassLoader loader = new ClassLoader(getClass().getClassLoader())
        {
            public Class findClass(String name) {
                try
                {
                    String path = "C:\\mypath\\hello";
                    File file = new File(path, name + ".class");
                    RandomAccessFile raf = new RandomAccessFile(file, "r");
                    byte[] content = new byte[(int)file.length()];
                    raf.readFully(content);

                    return defineClass("hello." + name, content, 0, content.length);
                }
                catch (Exception e)
                {
                    e.printStackTrace();
                }
                
                return null;
            }
        };
        
        try
        {
            Class helloClass = loader.loadClass("HelloWorld");
            Object hello = helloClass.newInstance();
            Method m = helloClass.getMethod("run", new Class[0]);
            m.invoke(hello);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }

 I did not try the code on more recent java, but since the whole Class Loader API was in the process of being removed, I guess there are other ways to perform this nowadays. I tried asking ChatGPT to produce this code, and the result is quite similar, except it was using the URLClassLoader object which handles reading the file content for us.