Java ClassLoader 1

Posted on | Edited on | In

Class

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public abstract class ClassLoader {
  ...
  protected final Class<?> defineClass(String name, byte[] b, int off, int len)
  ...
  public Class<?> loadClass(String name) {...}
  ...
  public final ClassLoader getParent() {...}
  ...
  public URL getResource(String name) {...}
  ...
  public Enumeration<URL> getResources(String name) {...}
  ...
}

  • Most important method is loadClass(arg) ,
    it accept a class’s full name, and return it’s instance.

  • defineClass(arg) accept a byte array, turn it into instance.
    Usually, it load a file from disk, then pass file byte into JVM.
    Instantiate Class into a instance through JVM’s(native method) define to Class.

  • getResource() and getResources search URL from repository.
    They have agent mechanism as well as loadClass, like:
    defineClass(getResource(name).getBytes())

Code

Because Java late binding and explain character,
Classes’ will be load when it needed like
static method/ construct method or direct allocate in field.

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public class A {
  public void xxx() {
    B b = new B();
    b.xxoo();
  }
}

They are same when B have default constructor.

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B b = new B();

B b = Class.forName("B", false,
    A.class.getClassLoader()).newInstance();