简单赋值语句的字节码

java代码1：

package com.bytecode;

public class Wizard {
    private int age;
    
    public Wizard() {
        this.age = 100;
    }
}

javap -v Wizard.class 得到字节码：

Classfile /D:/Users/Administrator/workspace/TestJava/bin/com/bytecode/Wizard.class
  Last modified 2018-3-14; size 311 bytes
  MD5 checksum 301ffac27f2fa331e20b98a912914551
  Compiled from "Wizard.java"
public class com.bytecode.Wizard
  SourceFile: "Wizard.java"
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:
   #1 = Class              #2             //  com/bytecode/Wizard
   #2 = Utf8               com/bytecode/Wizard
   #3 = Class              #4             //  java/lang/Object
   #4 = Utf8               java/lang/Object
   #5 = Utf8               age
   #6 = Utf8               I
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Methodref          #3.#11         //  java/lang/Object."<init>":()V
  #11 = NameAndType        #7:#8          //  "<init>":()V
  #12 = Fieldref           #1.#13         //  com/bytecode/Wizard.age:I
  #13 = NameAndType        #5:#6          //  age:I
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               Lcom/bytecode/Wizard;
  #18 = Utf8               SourceFile
  #19 = Utf8               Wizard.java
{
  public com.bytecode.Wizard();
    flags: ACC_PUBLIC

    Code:
      stack=2, locals=1, args_size=1
         0: aload_0       
         1: invokespecial #10                 // Method java/lang/Object."<init>":()V
         4: aload_0       
         5: bipush        100
         7: putfield      #12                 // Field age:I
        10: return        
      LineNumberTable:
        line 6: 0
        line 7: 4
        line 8: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/bytecode/Wizard;
}

在构造函数Wizard()中，局部变量表0处是this引用。

指令	描述	操作数栈
aload_0	this引用入操作数栈	this
invokespecial #10	调用Object类的<init>方法，消耗栈中的this引用
aload_0	this引用再次推入操作数栈	this
bipush 100	把整数100推入操作数栈	this, 100
putfield  #12	调用this -> Wizard.age = 100，消耗栈中的this和100

java代码2：

public class Wizard {
    private int age;
    private void grow() {
        this.age += 1;
    }
}

javap -v -p Wizard.class 得到字节码：

Classfile /D:/Users/Administrator/workspace/TestJava/bin/com/bytecode/Wizard.class
  Last modified 2018-3-15; size 375 bytes
  MD5 checksum 1910939ad3f794132f14da4c1c417e96
  Compiled from "Wizard.java"
public class com.bytecode.Wizard
  SourceFile: "Wizard.java"
  minor version: 0
  major version: 51
  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:
   #1 = Class              #2             //  com/bytecode/Wizard
   #2 = Utf8               com/bytecode/Wizard
   #3 = Class              #4             //  java/lang/Object
   #4 = Utf8               java/lang/Object
   #5 = Utf8               age
   #6 = Utf8               I
   #7 = Utf8               <init>
   #8 = Utf8               ()V
   #9 = Utf8               Code
  #10 = Methodref          #3.#11         //  java/lang/Object."<init>":()V
  #11 = NameAndType        #7:#8          //  "<init>":()V
  #12 = Utf8               LineNumberTable
  #13 = Utf8               LocalVariableTable
  #14 = Utf8               this
  #15 = Utf8               Lcom/bytecode/Wizard;
  #16 = Utf8               grow
  #17 = Fieldref           #1.#18         //  com/bytecode/Wizard.age:I
  #18 = NameAndType        #5:#6          //  age:I
  #19 = Utf8               SourceFile
  #20 = Utf8               Wizard.java
{
  private int age;
    flags: ACC_PRIVATE


  public com.bytecode.Wizard();
    flags: ACC_PUBLIC

    Code:
      stack=1, locals=1, args_size=1
         0: aload_0       
         1: invokespecial #10                 // Method java/lang/Object."<init>":()V
         4: return        
      LineNumberTable:
        line 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lcom/bytecode/Wizard;

  private void grow();
    flags: ACC_PRIVATE

    Code:
      stack=3, locals=1, args_size=1
         0: aload_0       
         1: dup           
         2: getfield      #17                 // Field age:I
         5: iconst_1      
         6: iadd          
         7: putfield      #17                 // Field age:I
        10: return        
      LineNumberTable:
        line 7: 0
        line 8: 10
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      11     0  this   Lcom/bytecode/Wizard;
}

关注grow方法：

指令	描述	操作数栈
aload_0	this入栈	this
dup	栈顶的this复制一份，再入栈	this, this
getfield #17	this出栈，this.age入栈	this, this.age
iconst_1	常数1入栈	this, this.age, 1
iadd	栈顶的2个值相加	this, (this.age + 1)
putfield #17	给成员赋值，消耗两个栈元素