javap in a programmable way

Question

Can we use javap in our own java code in a programmable way?

for example, the following code:

public class TestClass {
    public static void main(String[] args) {
        System.out.println("hello world");
    }
}

using javap in command line, we got :

// Header + consts 1..22 snipped
const #22 = String      #23;    //  hello world
const #23 = Asciz       hello world;

public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   getstatic       #16; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #22; //String hello world
   5:   invokevirtual   #24; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return
  // Debug info snipped
}

can I print only the Constant Pool using javap's API?

Answer 1

Apache BCEL provides encapsulations of .class file parsing, which provides a set of API. Almost for every element in .class file, there's a corresponding Class in BECL API to represent it. So in some way, it is not that straightforward if you just want to print out certain sections of the class file. Here is a simple example you can refer, pay attention to the org.apache.bcel.classfile.ClassParser:

    ClassParser cp = new ClassParser("TestClass.class");
    JavaClass jc = cp.parse();
    ConstantPool constantPool = jc.getConstantPool(); // Get the constant pool here.
    for (Constant c : constantPool.getConstantPool()) {
        System.out.println(c); // Do what you need to do with all the constants.
    }

Answer 2

There is no API for javap internals, but you can look for the source code of javap, which is in the package com.sun.tools.javap. The entry class is com.sun.tools.javap.Main. So another way to run javap is java -cp $JAVA_HOME/lib/tools.jar com.sun.tools.javap.Main YourTestClass