How to disassemble Java code - java

How to disassemble Java code - java

八月 05, 2019

Introduction

In order to do some researchs about the internal structure of the Java code, we need to disassemble the Java code.

How

Create a Java file(Singleton.java)

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/**
 * Author: steven
 * Email: steven.wang@deepnetsecurity.com
 * Datetime: 12/06/19 17:58
 **/
public class Singleton {
    private volatile static Singleton uniqueInstance;

    private Singleton() {
    }

    public static Singleton getUniqueInstance() {
        if (uniqueInstance == null) {
            synchronized (Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }
        return uniqueInstance;
    }
}

Java file to class file, we will get Singleton.class which will be similar with Java file.

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javac Singleton.java
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//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

public class Singleton {
    private static volatile Singleton uniqueInstance;

    private Singleton() {
    }

    public static Singleton getUniqueInstance() {
        if (uniqueInstance == null) {
            Class var0 = Singleton.class;
            synchronized(Singleton.class) {
                if (uniqueInstance == null) {
                    uniqueInstance = new Singleton();
                }
            }
        }

        return uniqueInstance;
    }
}

Class file to assembly code

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javap -c Singleton.class
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Compiled from "Singleton.java"
public class Singleton {
  public static Singleton getUniqueInstance();
    Code:
       0: getstatic     #2                  // Field uniqueInstance:LSingleton;
       3: ifnonnull     37
       6: ldc           #3                  // class Singleton
       8: dup
       9: astore_0
      10: monitorenter
      11: getstatic     #2                  // Field uniqueInstance:LSingleton;
      14: ifnonnull     27
      17: new           #3                  // class Singleton
      20: dup
      21: invokespecial #4                  // Method "<init>":()V
      24: putstatic     #2                  // Field uniqueInstance:LSingleton;
      27: aload_0
      28: monitorexit
      29: goto          37
      32: astore_1
      33: aload_0
      34: monitorexit
      35: aload_1
      36: athrow
      37: getstatic     #2                  // Field uniqueInstance:LSingleton;
      40: areturn
    Exception table:
       from    to  target type
          11    29    32   any
          32    35    32   any
}