一个详细的Java字节码例子

[介绍]

本文给出一个java字节码的例子。 纸上得来终觉浅，绝知此事要躬行。

 

本文将详细解释一个字节码例子。 看过一些相关的文章和书籍，但是给出的例子一般都更加简化，然后给出理论解释。

我自己通过认真分析此字节码，基本上把重要部分都cover了。

 

本文的例子来自于： [KB JVM - How to print readable bytecode from class file -> An example of bytecode - java]

 

 [一些note]

关于字节码的一些note，copy到这里

java字节码与汇编程序(Assembly)有点类似.

区别在于：1）JVM没有寄存器，所有操作均在Stack上执行 2）但是有局部变量表

load : slot# --> stack

store: slot# <-- stack

const: const  --> stack

这里还有个简单的例子: [P161 Ref 1 zzm]

note:每个方法所使用的局部变量表、栈的大小在编译时确定

note:局部变量表 0 - this, 1..m m个方法参数，剩下的是可用的slot

note:调用某个方法之前，会将局部变量表准备好，因为一个线程中的所有方法共享同一个局部变量表

 

[Java Source Code]

 

public class Volatile
{
    private volatile int field = 0;

    public void increase(){
        field++;
    }


    public static void main(String[] args){
        Volatile ex = new Volatile();
        ex.increase();
    }
}

 

[ByteCode]

 

javap -verbose Volatile.class

 

下面的以BY:开头的是我的注释

都是用英文写的，因为我当时的编辑工具中文显示乱码。

D:\baoying_no_index\360CloudDisk_Compiling_Output\BaoyingCleanProject\bin>javap
-verbose Volatile


Compiled from "Volatile.java"
public class Volatile extends java.lang.Object
  SourceFile: "Volatile.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class        #2;     //  Volatile
const #2 = Asciz        Volatile;
const #3 = class        #4;     //  java/lang/Object
const #4 = Asciz        java/lang/Object;
const #5 = Asciz        field;
const #6 = Asciz        I;
const #7 = Asciz        <init>;
const #8 = Asciz        ()V;
const #9 = Asciz        Code;
const #10 = Method      #3.#11; //  java/lang/Object."<init>":()V
const #11 = NameAndType #7:#8;//  "<init>":()V
const #12 = Field       #1.#13; //  Volatile.field:I
const #13 = NameAndType #5:#6;//  field:I
const #14 = Asciz       LineNumberTable;
const #15 = Asciz       LocalVariableTable;
const #16 = Asciz       this;
const #17 = Asciz       LVolatile;;
const #18 = Asciz       increase;
const #19 = Asciz       main;
const #20 = Asciz       ([Ljava/lang/String;)V;
const #21 = Method      #1.#11; //  Volatile."<init>":()V
const #22 = Method      #1.#23; //  Volatile.increase:()V
const #23 = NameAndType #18:#8;//  increase:()V
const #24 = Asciz       args;
const #25 = Asciz       [Ljava/lang/String;;
const #26 = Asciz       ex;
const #27 = Asciz       SourceFile;
const #28 = Asciz       Volatile.java;


BY: https://en.wikipedia.org/wiki/Java_bytecode_instruction_listings


{
public Volatile();
  Code:
  --BY: Stack=2 max Stack size
  --BY: Locals=1, it means the Slot capacity is 1. It contains 'this' only.
  --BY: Args_size=1. only 'this' as default parameter.
   Stack=2, Locals=1, Args_size=1
   0:   aload_0  BY: push slot#0 ('this') to Stack.  after Stack: this
   1:   invokespecial   #10; //Method java/lang/Object."<init>":()V BY: invoke parent constructor. After Stack: empty
   4:   aload_0 BY: push slot#0 ('this') to Stack. Why again? A: it is required by the following(#6) putfield.(20150630)  After Stack: this
   5:   iconst_0 BY: push 0 to Stack. After Stack: (top)0, this (bottom)
   6:   putfield        #12; //Field field:I BY: assign field#12 with 0 (const #12 = Field       #1.#13; //  Volatile.field:I). After Stack: empty.
   9:   return
  LineNumberTable:
   line 9: 0
   line 11: 4
   line 9: 9

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      10      0    this       LVolatile;

//    public void increase(){
//        field++;
//    }
public void increase();
  Code:
   Stack=3, Locals=1, Args_size=1
   0:   aload_0  BY: push 'this' to Stack.  slot #0 is 'this'. why aload_0 is required?. A: It is used by the last ( line #7 put field) (20150630) . after: Stack:(top) this (bottom)
   1:   dup          BY: dup the top of the stack, why dup is required?. A: it is used  by the following(line #2) getfield. Although the getfile/putfied spec does not mention that it is required to put the 'this' side of the field, it is required! another example here http://cs.au.dk/~mis/dOvs/jvmspec/ref--18.html (20150630) after: Stack:(top)this, this (bottom)

   2:   getfield        #12; //Field field:I  BY: push the field #12 reference to Stack. after: Stack:(top)field #12, this(bottom)

   5:   iconst_1 BY: push 1 to Stack. Stack:(top)1, field #12, this, this(bottom)
   6:   iadd        BY: pop 2 values and push adding result back.. after: Stack:(top) value of field #12+1, this(bottom)

   7:   putfield        #12; //Field field:I  BY: pop 2 values, set field #12 value. after: Stack:(top) (bottom)
   10:  return
  LineNumberTable:
   line 14: 0
   line 15: 10

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      11      0    this       LVolatile;

//    public static void main(String[] args){
//        Volatile ex = new Volatile();
//        ex.increase();
//    }
//const #21 = Method      #1.#11; //  Volatile."<init>":()V
//const #22 = Method      #1.#23; //  Volatile.increase:()V
public static void main(java.lang.String[]);
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   new     #1; //class Volatile BY: create a new Volatile object, after: Stack (top)ex(bottom)
   3:   dup   BY:after: Stack (top)ex,ex(bottom)
   4:   invokespecial   #21; //Method "<init>":()V BY:initialize ex, top popped for this instructino after: Stack (top)ex(bottom)
   7:   astore_1 BY: save ex to Slot #1 .after: Stack (top)ex(bottom)
   8:   aload_1 BY: save ex back to stack .after: Stack (top)ex(bottom).  Note, no change for stack, after astore_1 and aload_1, but the Slot #1 saved value 'ex' why it is required? Because the following invokation will take Slot. The Slot should prepare Slot[0] = this, Slot[1..m] as arguments. (20150630)
   9:   invokevirtual   #22; //Method increase:()V
   12:  return
  LineNumberTable:
   line 19: 0
   line 20: 8
   line 21: 12

  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      13      0    args       [Ljava/lang/String;
   8      5      1    ex       LVolatile;


}