Classloaders

Remarks#

A classloader is a class whose primary purpose is to mediate the location and loading of classes used by an application. A class loader can also find and loaded resources.

The standard classloader classes can load classes and resources from directories in the file system and from JAR and ZIP files. They can also download and cache JAR and ZIP files from a remote server.

Classloaders are normally chained, so that the JVM will try to load classes from the standard class libraries in preference to application-provided sources. Custom classloaders allow the programmer to alter this. The also can do such things as decrypting bytecode files and bytecode modification.

Instantiating and using a classloader

This basic example shows how an application can instantiate a classloader and use it to dynamically load a class.

URL[] urls = new URL[] {new URL("file:/home/me/extras.jar")};
Classloader loader = new URLClassLoader(urls);
Class<?> myObjectClass = loader.findClass("com.example.MyObject");

The classloader created in this example will have the default classloader as its parent, and will first try to find any class in the parent classloader before looking in “extra.jar”. If the requested class has already been loaded, the findClass call will return the reference to the previously loaded class.

The findClass call can fail in a variety of ways. The most common are:

• If the named class cannot be found, the call with throw ClassNotFoundException.
• If the named class depends on some other class that cannot be found, the call will throw NoClassDefFoundError.

Implementing a custom classLoader

Every custom loader must directly or indirectly extend the java.lang.ClassLoader class. The main extension points are the following methods:

• findClass(String) - overload this method if your classloader follows the standard delegation model for class loading.
• loadClass(String, boolean) - overload this method to implement an alternative delegation model.
• findResource and findResources - overload these methods to customize resource loading.

The defineClass methods which are responsible for actually loading the class from a byte array are final to prevent overloading. Any custom behavior needs to be performed prior to calling defineClass.

Here is a simple that loads a specific class from a byte array:

public class ByteArrayClassLoader extends ClassLoader {
    private String classname;
    private byte[] classfile;

    public ByteArrayClassLoader(String classname, byte[] classfile) {
        this.classname = classname;
        this.classfile = classfile.clone();
    }

    @Override
    protected Class findClass(String classname) throws ClassNotFoundException {
        if (classname.equals(this.classname)) {
            return defineClass(classname, classfile, 0, classfile.length);
        } else {
            throw new ClassNotFoundException(classname);
        }
    }
}

Since we have only overridden the findClass method, this custom class loader is going to behave as follows when loadClass is called.

1. The classloader’s loadClass method calls findLoadedClass to see if a class with this name has already been loaded by this classloader. If that succeeds, the resulting Class object is returned to the requestor.
2. The loadClass method then delegates to the parent classloader by calling its loadClass call. If the parent can deal with the request, it will return a Class object which is then returned to the requestor.
3. If the parent classloader cannot load the class, findClass then calls our override findClass method, passing the name of the class to be loaded.
4. If the requested name matches this.classname, we call defineClass to load the actual class from the this.classfile byte array. The resulting Class object is then returned.
5. If the name did not match, we throw ClassNotFoundException.

Loading an external .class file

To load a class we first need to define it. The class is defined by the ClassLoader. There’s just one problem, Oracle didn’t write the ClassLoader’s code with this feature available. To define the class we will need to access a method named defineClass() which is a private method of the ClassLoader.

To access it, what we will do is create a new class, ByteClassLoader, and extend it to ClassLoader. Now that we have extended our class to ClassLoader, we can access the ClassLoader’s private methods. To make defineClass() available, we will create a new method that will act like a mirror for the private defineClass() method. To call the private method we will need the class name, name, the class bytes, classBytes, the first byte’s offset, which will be 0 because classBytes’ data starts at classBytes[0], and the last byte’s offset, which will be classBytes.lenght because it represents the size of the data, which will be the last offset.

public class ByteClassLoader extends ClassLoader {

    public Class<?> defineClass(String name, byte[] classBytes) {
        return defineClass(name, classBytes, 0, classBytes.length);
    }

}

Now, we have a public defineClass() method. It can be called by passing the name of the class and the class bytes as arguments.

Let’s say we have class named MyClass in the package stackoverflow…

To call the method we need the class bytes so we create a Path object representing our class’ path by using the Paths.get() method and passing the path of the binary class as an argument. Now, we can get the class bytes with Files.readAllBytes(path). So we create a ByteClassLoader instance and use the method we created, defineClass(). We already have the class bytes but to call our method we also need the class name which is given by the package name (dot) the class canonical name, in this case stackoverflow.MyClass.

Path path = Paths.get("MyClass.class");

ByteClassLoader loader = new ByteClassLoader();
loader.defineClass("stackoverflow.MyClass", Files.readAllBytes(path);

Note: The defineClass() method returns a Class<?> object. You can save it if you want.

To load the class, we just call loadClass() and pass the class name. This method can throw an ClassNotFoundException so we need to use a try cath block

try{
    loader.loadClass("stackoverflow.MyClass");
} catch(ClassNotFoundException e){
    e.printStackTrace();
}