Java how to load a Class from another jar to my jar and how to use it

Question

//UPDATE//

Now I can load a Jar, but how can I run methods from it?

Answer 1

From the main JAR file where you need to plug games.jar:

• First get the URL for your games.jar file (use File.toURI().toURL()) and put it in an array (let's call the array urlArray).

• Next get a URLClassLoader instance to load games.jar:

  URLClassLoader urlClassLoader = URLClassLoader.newInstance(urlArray);
  

• Now you can instantiate any class from the games.jar with the Class.forName if you pass the new class loader you just created as third parameter:

  Class<? extends Main> mainClass = (Class<? extends Main>)
  Class.forName("Main", true, urlClassLoader);
  

• On mainClass you can now call your method: mainClass.loaded();

Answer 2

There are two parts to this. First, you need to import the jar. This can be done at runtime using a slight hack with reflection:

import java.io.File;
import java.lang.reflect.Method;
import java.net.URL;
import java.net.URLClassLoader;

public void importJar(File file) {
    Method addURL = URLClassLoader.class.getDeclaredMethod("addURL", URL.class);
    addURL.setAccessible(True);
    addURL.invoke(URLClassLoader.getSystemClassLoader(), new Object[] {file.toURI().toURL()});
}    

Now you can import a jar at runtime. If you simply have the path to the jar, you can easily convert it to a File:

importJar(new File("path/to/jar"));

Second, you need to find the classes defined in your jar. There does not appear to be a built-in way to do this, so you'll have to manually examine the jar itself for classes.

import java.lang.Class;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.jar.JarFile;
import java.util.jar.JarEntry;

public List<Class<?>> findJarClasses(File file) {
    ArrayList<Class<?>> classes = new ArrayList<Class<?>>();
    JarFile jar = new JarFile(file);
    Enumeration<JarEntry> entries = jar.entries();
    while (entries.hasMoreElements()) {
        JarEntry entry = entries.nextElement();
        String name = entry.getName();
        if (name.endsWith(".class")) {
            name = name.substring(0, name.lastIndexOf('.'));
            Class<?> cls = Class.forName(name);
            classes.add(cls);
        }
    }
    return classes;
}

Finally, you can iterate over the classes to determine if any of them implement your desired interface.

File jarFile = new File("path/to/Games.jar");
importJar(jarFile);
List<Class<?>> classes = findJarClasses(jarFile);
for (Class<?> cls : classes) {
    if (cls.isInstance(YourInterface.class)) {
        // Instantiate class.
        YearInterface obj = (YourInterface)cls.newInstance();
        // Call `loaded()` method.
        obj.loaded();
    }
}

Answer 3

1. Create a classloader with the jar's file url.
2. Ask the classloader to load the game's main class by name
3. Create a new instance of the game's main class
4. Cast it to some 'Game' interface your plugin loader knows
5. Invoke the methods on the interface to start the game.