Java字节码例子解析

举个简单的例子：

public class Hello {
    public static void main(String[] args) {
        String string1 = "ab";
        String string2 = "c";
        String string3 = string1 + "c";
        System.out.println(string1 == string3);
    }
}

过程大致分析如图：

第一步 将线程栈中的string1、string2引用分别指向了常量池ab、c的地址。

第二步 轮到了string3 = string1 + "c"，首先会初始化StringBuilder到堆中，然后调append将string1字符串拼接、然后调append再拼接"c"。

第三步 StringBuilder指向常量池的"abc"地址，然后通过toString返回值.

然后执行如下指令：

$ javac Hello.java   //编译

$ javap -verbose Hello   //反编译，结果如下：

Classfile /C:/Users/lisam/Desktop/新建文件夹/Hello.class

  Last modified 2018-9-17; size 706 bytes

  MD5 checksum 4d2164bd48f0690cad84271e27b237a5

  Compiled from "Hello.java"

public class Hello

  SourceFile: "Hello.java"

  minor version: 0

  major version: 51

  flags: ACC_PUBLIC, ACC_SUPER

//方法常量池

Constant pool:

   #1 = Methodref          #11.#24        //  java/lang/Object."<init>":()V

   #2 = String             #25            //  ab

   #3 = String             #26            //  c

   #4 = Class              #27            //  java/lang/StringBuilder

   #5 = Methodref          #4.#24         //  java/lang/StringBuilder."<init>":()V

   #6 = Methodref          #4.#28         //  java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

   #7 = Methodref          #4.#29         //  java/lang/StringBuilder.toString:()Ljava/lang/String;

   #8 = Fieldref           #30.#31        //  java/lang/System.out:Ljava/io/PrintStream;

   #9 = Methodref          #32.#33        //  java/io/PrintStream.println:(Z)V

  #10 = Class              #34            //  Hello

  #11 = Class              #35            //  java/lang/Object

  #12 = Utf8               <init>

  #13 = Utf8               ()V

  #14 = Utf8               Code

  #15 = Utf8               LineNumberTable

  #16 = Utf8               main

  #17 = Utf8               ([Ljava/lang/String;)V

  #18 = Utf8               StackMapTable

  #19 = Class              #36            //  "[Ljava/lang/String;"

  #20 = Class              #37            //  java/lang/String

  #21 = Class              #38            //  java/io/PrintStream

  #22 = Utf8               SourceFile

  #23 = Utf8               Hello.java

  #24 = NameAndType        #12:#13        //  "<init>":()V

  #25 = Utf8               ab

  #26 = Utf8               c

  #27 = Utf8               java/lang/StringBuilder

  #28 = NameAndType        #39:#40        //  append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

  #29 = NameAndType        #41:#42        //  toString:()Ljava/lang/String;

  #30 = Class              #43            //  java/lang/System

  #31 = NameAndType        #44:#45        //  out:Ljava/io/PrintStream;

  #32 = Class              #38            //  java/io/PrintStream

  #33 = NameAndType        #46:#47        //  println:(Z)V

  #34 = Utf8               Hello

  #35 = Utf8               java/lang/Object

  #36 = Utf8               [Ljava/lang/String;

  #37 = Utf8               java/lang/String

  #38 = Utf8               java/io/PrintStream

  #39 = Utf8               append

  #40 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;

  #41 = Utf8               toString

  #42 = Utf8               ()Ljava/lang/String;

  #43 = Utf8               java/lang/System

  #44 = Utf8               out

  #45 = Utf8               Ljava/io/PrintStream;

  #46 = Utf8               println

  #47 = Utf8               (Z)V

{

  public Hello();

    flags: ACC_PUBLIC

    Code:

      stack=1, locals=1, args_size=1

         0: aload_0       

         1: invokespecial #1                  // Method java/lang/Object."<init>":()V

         4: return        

      LineNumberTable:

        line 1: 0

  public static void main(java.lang.String[]);

    flags: ACC_PUBLIC, ACC_STATIC

    Code:

      stack=3, locals=4, args_size=1

         //将字符串"ab"压入常量池

         0: ldc           #2                  // String ab

         //存储到局部变量表

         2: astore_1      

         //将字符串"c"压入常量池

         3: ldc           #3                  // String c

         //存储到局部变量表

         5: astore_2      

         //String3的拼接过程中，会先new

         6: new           #4                  // class java/lang/StringBuilder

         9: dup           

         //然后invokespecial调用初始化StringBuilder构造器

        10: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V

        //把局部变量表中n=1的引用(即步骤2: astore_1)的值"ab"装在到操作数栈中。

        13: aload_1       

        //然后invokevirtual来调用方法append  string1

        14: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        //将字符串"c"压入常量池

        17: ldc           #3                  // String c

        //然后append "c"

        19: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

        //调用toString返回值

        22: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

        //然后将string3的值"abc"存储到局部变量表

        25: astore_3      

        //获取类的静态域，这里拿到了静态System.out，并将值压入栈顶

        26: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;

        //把局部变量表中n=1的引用(即步骤2: astore_1)的值"ab"装在到操作数栈中。

        29: aload_1      

        //把局部变量表中n=3的引用(即步骤25: astore_3)的值"abc"装在到操作数栈中。

        30: aload_3       

        //比较上面栈顶两引用型数值，当结果不相等时跳转

        31: if_acmpne     38

        //将int为1压入栈顶

        34: iconst_1      

        35: goto          39

        38: iconst_0      

        //调用静态类中System.out.println

        39: invokevirtual #9                  // Method java/io/PrintStream.println:(Z)V

        //返回空

        42: return        

      LineNumberTable:

        line 3: 0

        line 4: 3

        line 5: 6

        line 6: 26

        line 7: 42

      StackMapTable: number_of_entries = 2

           frame_type = 255 /* full_frame */

          offset_delta = 38

          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String ]

          stack = [ class java/io/PrintStream ]

           frame_type = 255 /* full_frame */

          offset_delta = 0

          locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String ]

          stack = [ class java/io/PrintStream, int ]

}
 

原文链接：Java字节码例子解析 - Lisam Blog - CSDN博客  https://blog.csdn.net/qq_28666081/article/details/82749655

参考链接：一文让你明白Java字节码 - 简书  https://www.jianshu.com/p/252f381a6bc4