使用 URLClassLoader 动态加载 JAR？

Question

I have a program which needs to be able to dynamically load JARs at runtime - after looking around I beleive this uses URLClassLoader, but I'm not sure how to get it to work. The JAR "openup.jar" is in the same directory as the program.

Ideally I would like to be able to load this JAR without having to specify each individual class inside it.

Answer 1

What I successfully used:

@SuppressWarnings("unchecked")
public void addURL(URL u) throws IOException {
    URLClassLoader sysLoader = (URLClassLoader) ThisClass.class.getClassLoader();
    URL urls[] = sysLoader.getURLs();
    for (int i = 0; i < urls.length; i++) {
        if (urls[i].toString().equalsIgnoreCase(u.toString())) {
            return;
        }
    }
    Class sysclass = URLClassLoader.class;
    try {
        Method method = sysclass.getDeclaredMethod("addURL", parameters);
        method.setAccessible(true);
        method.invoke(sysLoader, new Object[] { u });
    } catch (Throwable t) {
        throw new IOException("Error, could not add URL to system classloader");
    }
}

An almost identical solution is indeed presented in How should I load Jars dynamically at runtime?

Answer 2

Here I am explaining the full process of creating a jar and then loading a jar dynamically in another project:

Steps to create jar:

1. Create a new Project in any IDE(Eclipse).
2. Add a Sample class in packages(in my case MyClass).
3. Do right-click on the project and then export as jar and give a local path of system location where the jar wants to keep(right-click -> Export -> choose java/jar file -> next ->give path -> finish).
4. Then jar will be available at given location.

package newJar.com.example;

public class MyClass {

public MyClass() {
    // TODO Auto-generated constructor stub
}

public void helloWorld() {
    System.out.println("Hello from the helloWorld");
}

}

Steps to load the jar dynamically:

1. Create a new project and add below code:

   public class CustomClass {

   public CustomClass() { // TODO Auto-generated constructor stub }

   public void getCall(File myJar) {

    try { URLClassLoader loader = new URLClassLoader(new URL[] { myJar.toURI().toURL() }, CustomClass.class.getClass().getClassLoader()); Class classToLoad = Class.forName("newJar.com.example.MyClass", true, loader); System.out.println(classToLoad); Method method = classToLoad.getDeclaredMethod("helloWorld"); Object instance = classToLoad.newInstance(); Object result = method.invoke(instance); } catch (MalformedURLException | ClassNotFoundException | NoSuchMethodException | SecurityException | InstantiationException | IllegalAccessException | IllegalArgumentException | InvocationTargetException e) { // TODO Auto-generated catch block e.printStackTrace(); }

   }

   public static void main(String[] args) { // TODO Auto-generated method stub

    File myJar = new File("Path of your jar ");//"D:\\\\ECLIPSE\\\\myjar.jar" CustomClass cls = new CustomClass(); cls.getCall(myJar);

   }

}

This is how it can make use of the jar and if the jar is on the server then can give the server path instead of the local path.