Java: Dynamically loading external classes

Question

I am writing an application that will load Java scripts. I currently have a GUI which utilizes a JFileChooser to allow the user to select a script from their machine. The script file can be anywhere. It is not on the classpath. Having only a File object to represent that script file, how can I obtain a Class representation of it?

I know that to load a class you need its binary name, so in.this.format . However, the problem with that is I don't know how the script writer may have packaged it. For example, he/she may have, while developing it, put the script file in the package foo.bar . After I download this script and place it in my documents (ie, in foo/bar ), I can't load the script without knowing that it was packaged in foo.bar . foo/bar ），在不知道脚本已打包在foo.bar情况下无法加载该脚本。 If the class name is Test and I try to create a URLClassLoader pointing to the script file by doing new URLClassLoader(new URL[] { new URL(scriptFile.toURI().toURL()) }) and I do classLoader.loadClass("Test") I will get an exception saying that the class had the wrong name, and the correct name is foo.bar.Test . But how am I supposed to know that ahead of time?

This is what I have right now:

public class ScriptClassLoader extends URLClassLoader {

    private final File script;

    public ScriptClassLoader(File script) throws MalformedURLException {
        super(new URL[] { script.toURI().toURL() });
        this.script = script;
    }

    public Class<?> load() throws ClassNotFoundException {
        String fileName = script.getName();
        String className = fileName.substring(0, fileName.indexOf(".class"));
        return loadClass(className);
    }
}

How do people load scripts at runtime that are not part of the program's classpath, and the binary name of the class is not known?

Answer 1

If you just need to load a class from a given .class file, no matter how this classes is named, you can load the data yourself and then call ClassLoader's defineClass() method:

RandomAccessFile raf = new RandomAccessFile(script, "r");
try {
    byte[] classData = new byte[(int) raf.length()];
    raf.readFully(classData);
    return super.defineClass(null, classData, 0, classData.length);
} finally {
    raf.close();
}