Java ClassLoader delegation model?

Question

When calling loadClass() on a ClassLoader, does the ClassLoader first check if the class has been loaded, or does it immediately delegate this check to

Answer 1

A proper class loader implementation will:

1. Check if the class has already been loaded.
2. Typically ask the parent class loader to load the class
3. Attempt to find the class in its own class path.

The default implementation of ClassLoader.loadClass is something like:

protected synchronized Class loadClass(String name, boolean resolve) {
  // First, check if this class loader has directly defined the class or if the
  // JVM has initiated the class load with this class loader.
  Class result = findLoadedClass(name);
  if (result == null) {
    try {
      // Next, delegate to the parent.
      result = getParent().loadClass(name);
    } catch (ClassNotFoundException ex) {
      // Finally, search locally if the parent could not find the class.
      result = findClass(ex);
    }
  }
  // As a remnant of J2SE 1.0.2, link the class if a subclass of the class
  // loader class requested it (the JVM never calls the method,
  // loadClass(String) passes false, and the protected access modifier prevents
  // callers from passing true).
  if (resolve) {
    resolveClass(result);
  }
  return result;
}

Some class loader implementations will delegation to other non-parent class loaders (OSGi, for example, delegates to a graph of class loaders depending on the package), and some class loader implementations will look for classes in a local classpath before delegating.