How to disassemble Java class files

Use javap.

Explain that you want to read test / Hello.class in the following state.

$ cat test/Hello.java
package test;

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

$ javac test/Hello.java

javap test.Hello

When no options are used, then the javap command prints the package, protected and public fields, and methods of the classes passed to it.

$ javap test.Hello
Compiled from "Hello.java"
class test.Hello {
  test.Hello();
  public static void main(java.lang.String[]);
}

javap -c test.Hello

-c Prints disassembled code, for example, the instructions that comprise the Java bytecodes, for each of the methods in the class.

$ javap -c test.Hello
Compiled from "Hello.java"
class test.Hello {
  test.Hello();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String Hello world
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

javap -v test.Hello

-verbose Prints stack size, number of locals and arguments for methods.

Classfile /home/k0kubun/src/github.com/k0kubun/jjvm/test/Hello.class
  Last modified May 11, 2019; size 420 bytes
  MD5 checksum c614506092a303bb2b0c4133ed455e3e
  Compiled from "Hello.java"
class test.Hello
  minor version: 0
  major version: 52
  flags: ACC_SUPER
Constant pool:
   #1 = Methodref          #6.#15         // java/lang/Object."<init>":()V
   #2 = Fieldref           #16.#17        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = String             #18            // Hello world
   #4 = Methodref          #19.#20        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #5 = Class              #21            // test/Hello
   #6 = Class              #22            // java/lang/Object
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Utf8               LineNumberTable
  #11 = Utf8               main
  #12 = Utf8               ([Ljava/lang/String;)V
  #13 = Utf8               SourceFile
  #14 = Utf8               Hello.java
  #15 = NameAndType        #7:#8          // "<init>":()V
  #16 = Class              #23            // java/lang/System
  #17 = NameAndType        #24:#25        // out:Ljava/io/PrintStream;
  #18 = Utf8               Hello world
  #19 = Class              #26            // java/io/PrintStream
  #20 = NameAndType        #27:#28        // println:(Ljava/lang/String;)V
  #21 = Utf8               test/Hello
  #22 = Utf8               java/lang/Object
  #23 = Utf8               java/lang/System
  #24 = Utf8               out
  #25 = Utf8               Ljava/io/PrintStream;
  #26 = Utf8               java/io/PrintStream
  #27 = Utf8               println
  #28 = Utf8               (Ljava/lang/String;)V
{
  test.Hello();
    descriptor: ()V
    flags:
    Code:
      stack=1, locals=1, args_size=1
         0: aload_0
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return
      LineNumberTable:
        line 3: 0

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #3                  // String Hello world
         5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 5: 0
        line 6: 8
}
SourceFile: "Hello.java"