Why does Class.forName appear in the bytecode for this class?

Question

I've been doing some personal research in Java bytecode and I came across a bit of an oddity. If I decompile this class, I find a reference to Class.forName() hanging out in the constant pool. However, there is no reference in the source code to this method.

I assume that something about this code is causing javac to emit a bit of code that dynamically loads a class, but I'm not sure why this happens. It strikes me as inefficient, but mainly I'm just curious as to why this happens.

Answer 1

After disassembling the code with javap, I noticed that there is a method that doesn't exist in the source code:

static java.lang.Class class$(java.lang.String);
  Code:
     0: aload_0
     1: invokestatic  #1                  // Method java/lang/Class.forName:(Ljava/lang/String;)Ljava/lang/Class;
     4: areturn
     5: astore_1
     6: new           #3                  // class java/lang/NoClassDefFoundError
     9: dup
    10: invokespecial #4                  // Method java/lang/NoClassDefFoundError."<init>":()V
    13: aload_1
    14: invokevirtual #5                  // Method java/lang/NoClassDefFoundError.initCause:(Ljava/lang/Throwable;)Ljava/lang/Throwable;
    17: athrow
  Exception table:
     from    to  target type
         0     4     5   Class java/lang/ClassNotFoundException

It looks like this is generated in bytecode compiled for version < JDK1.5 whenever there is a class literal referenced in the code [1]. Basically, this:

if (getClass() == Level.class) {}

turns into this:

if (getClass() == class$("org.apache.log4j.Level")) {}

and class$() looks like this:

static Class class$(java.lang.String className) {
    try {
       return Class.forName(className);
    } catch (ClassNotFoundException e) {
       throw new NoClassDefFoundError();
    }
}

Apparently in JDK1.5, the ldc_w instruction was given the ability to load class constants and the class$() method was no longer necessary.