Dynamically load a JAR using URLClassLoader?

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.

possible duplicate of [How should I load Jars dynamically at runtime?](http://stackoverflow.com/questions/60764/how-should-i-load-jars-dynamically-at-runtime) — dogbane, Jan 11 '11 at 11:13
What exactly do you mean by "load this JAR without having to know what is inside it."? What exactly do you want to load from it, then? — Joachim Sauer, Jan 11 '11 at 11:29
Why would you want to load a jar without knowing what is inside it ? Do you want to handle it as a zip file or are you interested in loading java classes ? — pgras, Jan 11 '11 at 11:30
Apologies, that was poorly phrased - what I mean is, is it possible to load the JAR without having to specify the individual classes to be used. There are a good 100+ classes (all of which I need to use) within each of the JARs I'm using - what I meant was is it possible to load them without using getClass() for each class in each JAR? — Myn, Jan 11 '11 at 11:42
Where *do* you use those classes? In Java classes aren't "loaded" through unknown magic. They are loaded because they are referenced somewhere. What *exactly* references the classes that you want to have loaded from that jar? The "clean" solution is to create a classloader that is able to load the jar in question and have the classes that *depend on that jar* be loaded from a classloader that has set the jar classloader as its parent. — Joachim Sauer, Jan 11 '11 at 11:49
I'm afraid I'm not sure what you mean there but I'm going to take the answer that it is not possible to do what I'm trying to. I beleive I'll have to go through the painstaking task of individually referencing each class I need to use. Thanks for the input everyone. — Myn, Jan 11 '11 at 12:09

score 1 · Accepted Answer · edited May 23 '17 at 12:33

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?

edited May 23 '17 at 12:33

Community

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1

answered Jan 11 '11 at 11:21

morja

8,297
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That looks like a terrible idea, since it makes a lot of assumptions that are in no way guaranteed: 1.) that the classloader of `ThisClass` is an `URLClassLoader`; 2.) that the `URLClassLoader` class has a `addURL` method (that does what you want) and 3.) that you may invoke `setAccessible()` (i.e. no security manager is running and prohibiting that). – Joachim Sauer Jan 11 '11 at 11:29
1

Yes, that is true, but it works well in my case. Do you have a cleaner solution? – morja Jan 11 '11 at 11:31
Thanks Morja, I *think*, if I understand the code correctly, this might do what I was after but if not then it was still an answer, so thank you. – Myn Jan 11 '11 at 12:11

score 0 · Answer 2 · answered Oct 12 '21 at 14:26

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

Steps to create jar:

Create a new Project in any IDE(Eclipse).
Add a Sample class in packages(in my case MyClass).
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).
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:

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.

Dynamically load a JAR using URLClassLoader?

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