轻松读懂字节码文件-白红宇

轻松读懂字节码文件

阅读量：4181 次

发布时间：2019-05-26

本文共 9490 字，大约阅读时间需要 31 分钟。

invokeinterface: 调用接口方法

invokespecial: 调用初始化方法、私有方法、或父类中定义的方法

invokestatic: 调用静态方法

invokevirtual: 调用实例方法

package com.xiaobu.test.StringDemo;/** * @author xiaobu * @version JDK1.8.0_171 * @date on  2019/4/30 16:04 * @description V1.0 走StringBuilder */public class StringDemo3 {
       public static void main(String[] args) {
       test1();    }    //结果为false    private static void test1() {
           String str = "123";        String a = "123";        String str2 = a + "";        System.out.println(str==str2);    }}

使用idea插件反编译字节码文件

Classfile /E:/Practise/SpringBoot/demo/target/classes/com/xiaobu/test/StringDemo/StringDemo3.class  Last modified 2019-10-22; size 1020 bytes  MD5 checksum fd43b6950ccbd87c2bdd41457f4e50b1  Compiled from "StringDemo3.java"public class com.xiaobu.test.StringDemo.StringDemo3  minor version: 0  major version: 52   //表明是JDK1.8  flags: ACC_PUBLIC, ACC_SUPER  //类的访问标志Constant pool:        //常量池   #1 = Methodref          #12.#35        // java/lang/Object."
   
    ":()V   #+数字表示第几个常量 这个指向父类的构造方法   #2 = Methodref          #11.#36        // com/xiaobu/test/StringDemo/StringDemo3.test1:()V  指向test1()方法 返回值void   #3 = String             #37            // 123   #4 = Class              #38            // java/lang/StringBuilder   #5 = Methodref          #4.#35         // java/lang/StringBuilder."
    
     ":()V   #6 = Methodref          #4.#39         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;   #7 = String             #40            //   #8 = Methodref          #4.#41         // java/lang/StringBuilder.toString:()Ljava/lang/String;   #9 = Fieldref           #42.#43        // java/lang/System.out:Ljava/io/PrintStream;  #10 = Methodref          #44.#45        // java/io/PrintStream.println:(Z)V  #11 = Class              #46            // com/xiaobu/test/StringDemo/StringDemo3  #12 = Class              #47            // java/lang/Object  #13 = Utf8               
     
        #14 = Utf8               ()V  #15 = Utf8               Code  #16 = Utf8               LineNumberTable  #17 = Utf8               LocalVariableTable  #18 = Utf8               this  #19 = Utf8               Lcom/xiaobu/test/StringDemo/StringDemo3;  #20 = Utf8               main  #21 = Utf8               ([Ljava/lang/String;)V  #22 = Utf8               args  #23 = Utf8               [Ljava/lang/String;  #24 = Utf8               MethodParameters  #25 = Utf8               test1  #26 = Utf8               str  #27 = Utf8               Ljava/lang/String;  #28 = Utf8               a  #29 = Utf8               str2  #30 = Utf8               StackMapTable  #31 = Class              #48            // java/lang/String  #32 = Class              #49            // java/io/PrintStream  #33 = Utf8               SourceFile  #34 = Utf8               StringDemo3.java  #35 = NameAndType        #13:#14        // "
      
       ":()V  #36 = NameAndType        #25:#14        // test1:()V  #37 = Utf8               123  #38 = Utf8               java/lang/StringBuilder  #39 = NameAndType        #50:#51        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;  #40 = Utf8  #41 = NameAndType        #52:#53        // toString:()Ljava/lang/String;  #42 = Class              #54            // java/lang/System  #43 = NameAndType        #55:#56        // out:Ljava/io/PrintStream;  #44 = Class              #49            // java/io/PrintStream  #45 = NameAndType        #57:#58        // println:(Z)V  #46 = Utf8               com/xiaobu/test/StringDemo/StringDemo3  #47 = Utf8               java/lang/Object  #48 = Utf8               java/lang/String  #49 = Utf8               java/io/PrintStream  #50 = Utf8               append  #51 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;  #52 = Utf8               toString  #53 = Utf8               ()Ljava/lang/String;  #54 = Utf8               java/lang/System  #55 = Utf8               out  #56 = Utf8               Ljava/io/PrintStream;  #57 = Utf8               println  #58 = Utf8               (Z)V{
     public com.xiaobu.test.StringDemo.StringDemo3(); //默认构造方法    descriptor: ()V    flags: ACC_PUBLIC    Code:                                    //code表示代码 stack:最大操作数栈 locals:局部变量锁存储的空间:4个字节大小      stack=1, locals=1, args_size=1        // args_size方法参数个数         0: aload_0                         //将第一个引用类型本地变量推送至栈顶         1: invokespecial #1                  // Method java/lang/Object."
       
        ":()V         4: return                            //方法返回      LineNumberTable:        line 9: 0                             //第九行源码对应 字节码行号0: aload_0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       5     0  this   Lcom/xiaobu/test/StringDemo/StringDemo3;  public static void main(java.lang.String[]);    descriptor: ([Ljava/lang/String;)V    flags: ACC_PUBLIC, ACC_STATIC    Code:      stack=0, locals=1, args_size=1         0: invokestatic  #2                  // Method test1:()V         3: return                            //方法返回      LineNumberTable:        line 11: 0                            //第十一行源码对应 字节码行号0: invokestatic  #2        line 12: 3                            //第十二行源码对应 字节码行号3: return      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       4     0  args   [Ljava/lang/String;    MethodParameters:      Name                           Flags      args}SourceFile: "StringDemo3.java"

可以看出上面代码底层走了StringBuilder，进行字符串拼接，然后调用了StringBuilder的toString方法。

package com.xiaobu.test.StringDemo;/** * @author xiaobu * @version JDK1.8.0_171 * @date on  2019/4/30 16:04 * @description V1.0 走String */public class StringDemo4 {
       public static void main(String[] args) {
       test1();    }    //结果为true    private static void test1() {
           String str = "11";        String a = "11";        String str2 = "11" + "";        System.out.println(str==str2);    }}

反编译字节码文件

Classfile /E:/Practise/SpringBoot/demo/target/classes/com/xiaobu/test/StringDemo/StringDemo4.class  Last modified 2019-10-22; size 851 bytes  MD5 checksum 045bcada94c8d6c922754d92f6242259  Compiled from "StringDemo4.java"public class com.xiaobu.test.StringDemo.StringDemo4  minor version: 0  major version: 52  flags: ACC_PUBLIC, ACC_SUPERConstant pool:   #1 = Methodref          #7.#30         // java/lang/Object."
   
    ":()V   #2 = Methodref          #6.#31         // com/xiaobu/test/StringDemo/StringDemo4.test1:()V   #3 = String             #32            // 11   #4 = Fieldref           #33.#34        // java/lang/System.out:Ljava/io/PrintStream;   #5 = Methodref          #35.#36        // java/io/PrintStream.println:(Z)V   #6 = Class              #37            // com/xiaobu/test/StringDemo/StringDemo4   #7 = Class              #38            // java/lang/Object   #8 = Utf8               
    
        #9 = Utf8               ()V  #10 = Utf8               Code  #11 = Utf8               LineNumberTable  #12 = Utf8               LocalVariableTable  #13 = Utf8               this  #14 = Utf8               Lcom/xiaobu/test/StringDemo/StringDemo4;  #15 = Utf8               main  #16 = Utf8               ([Ljava/lang/String;)V  #17 = Utf8               args  #18 = Utf8               [Ljava/lang/String;  #19 = Utf8               MethodParameters  #20 = Utf8               test1  #21 = Utf8               str  #22 = Utf8               Ljava/lang/String;  #23 = Utf8               a  #24 = Utf8               str2  #25 = Utf8               StackMapTable  #26 = Class              #39            // java/lang/String  #27 = Class              #40            // java/io/PrintStream  #28 = Utf8               SourceFile  #29 = Utf8               StringDemo4.java  #30 = NameAndType        #8:#9          // "
     
      ":()V  #31 = NameAndType        #20:#9         // test1:()V  #32 = Utf8               11  #33 = Class              #41            // java/lang/System  #34 = NameAndType        #42:#43        // out:Ljava/io/PrintStream;  #35 = Class              #40            // java/io/PrintStream  #36 = NameAndType        #44:#45        // println:(Z)V  #37 = Utf8               com/xiaobu/test/StringDemo/StringDemo4  #38 = Utf8               java/lang/Object  #39 = Utf8               java/lang/String  #40 = Utf8               java/io/PrintStream  #41 = Utf8               java/lang/System  #42 = Utf8               out  #43 = Utf8               Ljava/io/PrintStream;  #44 = Utf8               println  #45 = Utf8               (Z)V{
     public com.xiaobu.test.StringDemo.StringDemo4();    descriptor: ()V    flags: ACC_PUBLIC    Code:      stack=1, locals=1, args_size=1         0: aload_0         1: invokespecial #1                  // Method java/lang/Object."
      
       ":()V         4: return      LineNumberTable:        line 9: 0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       5     0  this   Lcom/xiaobu/test/StringDemo/StringDemo4;  public static void main(java.lang.String[]);    descriptor: ([Ljava/lang/String;)V    flags: ACC_PUBLIC, ACC_STATIC    Code:      stack=0, locals=1, args_size=1         0: invokestatic  #2                  // Method test1:()V         3: return      LineNumberTable:        line 11: 0        line 12: 3      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       4     0  args   [Ljava/lang/String;    MethodParameters:      Name                           Flags      args}SourceFile: "StringDemo4.java"

上面直接走的字符串拼接。

参考:

转载地址：http://zwgai.baihongyu.com/

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