Java dynamic loading a class issue

Question

I am trying to write a code that compiles and runs another java class, after it creates it from a String.

My problem is when I run

    Class classToLoad = null;
    ClassLoader classLoader = Server.class.getClassLoader();
    try {
        classToLoad = classLoader.loadClass(className);
    } catch (Exception e) {
        e.printStackTrace();
    }

It throws a ClassNotFoundException. My problem isn't about the package, because if I debug the code and place a breakpoint before the "getClassLoader" and I reload the classes, then my code works fine and it sees the class that was recently created earlier in the app.

How can I reload the classes during runtime so the loadClass will work?

Community · Accepted Answer · 2020-06-20 09:12:55Z

Take a look at this tutorial:

ClassLoader Load / Reload Example

... Let's look at a simple example. Below is an example of a simple ClassLoader subclass. Notice how it delegates class loading to its parent except for the one class it is intended to be able to reload. If the loading of this class is delegated to the parent class loader, it cannot be reloaded later. Remember, a class can only be loaded once by the same ClassLoader instance.

As said earlier, this is just an example that serves to show you the basics of a ClassLoader's behaviour. It is not a production ready template for your own class loaders. Your own class loaders should probably not be limited to a single class, but a collection of classes that you know you will need to reload. In addition, you should probably not hardcode the class paths either.

public class MyClassLoader extends ClassLoader{

    public MyClassLoader(ClassLoader parent) {
        super(parent);
    }

    public Class loadClass(String name) throws ClassNotFoundException {
        if(!"reflection.MyObject".equals(name))
                return super.loadClass(name);

        try {
            String url = "file:C:/data/projects/tutorials/web/WEB-INF/" +
                            "classes/reflection/MyObject.class";
            URL myUrl = new URL(url);
            URLConnection connection = myUrl.openConnection();
            InputStream input = connection.getInputStream();
            ByteArrayOutputStream buffer = new ByteArrayOutputStream();
            int data = input.read();

            while(data != -1){
                buffer.write(data);
                data = input.read();
            }

            input.close();

            byte[] classData = buffer.toByteArray();

            return defineClass("reflection.MyObject",
                    classData, 0, classData.length);

        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }

}

Below is an example use of the MyClassLoader.

public static void main(String[] args) throws
    ClassNotFoundException,
    IllegalAccessException,
    InstantiationException {

    ClassLoader parentClassLoader = MyClassLoader.class.getClassLoader();
    MyClassLoader classLoader = new MyClassLoader(parentClassLoader);
    Class myObjectClass = classLoader.loadClass("reflection.MyObject");

    AnInterface2       object1 =
            (AnInterface2) myObjectClass.newInstance();

    MyObjectSuperClass object2 =
            (MyObjectSuperClass) myObjectClass.newInstance();

    //create new class loader so classes can be reloaded.
    classLoader = new MyClassLoader(parentClassLoader);
    myObjectClass = classLoader.loadClass("reflection.MyObject");

    object1 = (AnInterface2)       myObjectClass.newInstance();
    object2 = (MyObjectSuperClass) myObjectClass.newInstance();

}

Neel · Accepted Answer · 2015-08-30 14:43:44Z

0

Probably asking: "What is the context in which you are loading the class?" will help answer your question better.

Most standard frameworks like Spring handle loading classes internally and exposing only the methods that those classes provide.

Try Class.forName(String name) to attempt to load the class and return the handle to the class object.

If you want to specifically use your own classloader to load the class, use the overloaded: Class.forName(String name, boolean initialize, ClassLoader loader)

But you will need to ensure that your classloader is able to locate the class to load correctly.

For the classloader you are using, try: Thread.currentThread().getContextClassLoader()

answered Aug 30, 2015 at 14:43

Neel

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