The story behind Javac--empty class

The story behind Javac--empty class

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Http://www.blogbus.com/dreamhead-logs/5999838.html

Programmers are most familiar with the source code, but in order to make the program really effective, without the help of the compiler. Javac's role is to compile Java code into JVM directives. Because the Java language and the JVM are out of the same door, so, a little familiarity, it is not difficult to find that they are almost directly corresponding. Of course, in order to simplify the writing of code, JAVAC in addition to direct translation, but also secretly help us do a lot of work, we look at the simplest situation.

public class Test {}

We compile this code with JAVAC, and JAVAP can help us decompile the generated class file.

Javap-c Test

Here is the result of the anti-compilation.

public class Test extends Java.lang.object{public test (); code:0: Aload_0 1:invokespecial #1; Method java/lang/object. " <init> ":() V 4:return}

Aside from the specific contents of the instruction, the results of the above anti-compilation clearly tell us that the empty class we have written is not empty at all, because there is also a constructor. This is the work that Javac did for us. Yes, it's javac, but it's not necessarily the JVM's requirement. In fact, classes running on the JVM can have no constructors at all. However, the previous example has clearly told us that, because the role of Javac, directly in the Java language is unable to construct a real empty class. So let's start with the bytecode and construct the real "empty" class with ObjectWeb asm.

Public class noctorgenerator {    public static void main ( String[] args)  throws exception {        string  className =  "Noctor";        classwriter cw  = new classwriter (CLASSWRITER.COMPUTE_MAXS);         cw.visit (opcodes.v1_2, opcodes.acc_public, classname, null,  "Java/lang/object",  null);         cw.visitend ();                 try {             os = new fileoutputstream (classname +  ". Class");             os.write (Cw.tobytearray ());         } finally {            if  (os !=  null)  {                 os.close ();            }         }    }}

With JAVAP, we can see the result of the build, and indeed there is no constructor.

public class Noctor extends java.lang.object{}

However, because no constructors exist, we cannot create objects with this class, but the following code proves that the classes generated by this class are actually available.

public class Noctormain {public static void main (string[] args) {System.out.println (noctor.class); }}

Running this code, we can get the following output:

Class Noctor

The story behind Javac--empty class