Javap命令查看反汇编字节码

矫情吗；* 2022-03-31 09:35 245阅读 0赞

# Javap命令查看反汇编字节码 #

*  **当有人问你代码执行效率高低怎么判断时你应该怎么回答呢？**
 *  **是靠经常分析还是有一定的理论依据呢？**
 *  **现在可以通过Javap进行反编译，查看底层代码分析效率高低**  
    我们编写的类文件是.java类型的文件，可以使用 `javac -类文件名加扩展名` 命令进行编译，也可以用我们的ideal或eclipse进行打包。我们会得到.class类型的文件
 *  **首先定位到需要把编译（.class类型）文件的目录里**
 *  **右击打开命令窗口输入`javap -verbose 文件名.class > test.txt`**
 *  **>号后面是输入文件为test的类型为txt的文件**
 *  **txt文件会生成在和class文件同目录下打开**

这个编译之前的代码

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2gyNzM5Nzk1ODY_size_16_color_FFFFFF_t_70]  
这是把编译生成的文件，进行了简单解释，感兴趣的小伙伴可以更深入了解

Classfile /G:test/target/test-classes/test.class
      Last modified 2019-1-2; size 674 bytes
      MD5 checksum abc3194b42db340c1b4a66568c418c6e
      Compiled from "test.java"     //类名
    public class test               //包名加类名，我这个没有包名所以只有类名
      minor version: 0      		//jdk版本号
      major version: 52             //jdk版本号
      flags: ACC_PUBLIC, ACC_SUPER
    Constant pool:					//常量池
       #1 = Methodref          #5.#24         // java/lang/Object."<init>":()V
       #2 = Fieldref           #25.#26        // java/lang/System.out:Ljava/io/PrintStream;
       #3 = Methodref          #27.#28        // java/io/PrintStream.println:(I)V
       #4 = Class              #29            // test
       #5 = Class              #30            // java/lang/Object
       #6 = Utf8               <init>
       #7 = Utf8               ()V
       #8 = Utf8               Code
       #9 = Utf8               LineNumberTable
      #10 = Utf8               LocalVariableTable
      #11 = Utf8               this
      #12 = Utf8               Ltest;
      #13 = Utf8               main
      #14 = Utf8               ([Ljava/lang/String;)V
      #15 = Utf8               args
      #16 = Utf8               [Ljava/lang/String;
      #17 = Utf8               a
      #18 = Utf8               I
      #19 = Utf8               b
      #20 = Utf8               c
      #21 = Utf8               aaa
      #22 = Utf8               SourceFile
      #23 = Utf8               test.java
      #24 = NameAndType        #6:#7          // "<init>":()V
      #25 = Class              #31            // java/lang/System
      #26 = NameAndType        #32:#33        // out:Ljava/io/PrintStream;
      #27 = Class              #34            // java/io/PrintStream
      #28 = NameAndType        #35:#36        // println:(I)V
      #29 = Utf8               test
      #30 = Utf8               java/lang/Object
      #31 = Utf8               java/lang/System
      #32 = Utf8               out
      #33 = Utf8               Ljava/io/PrintStream;
      #34 = Utf8               java/io/PrintStream
      #35 = Utf8               println
      #36 = Utf8               (I)V
    {
      public test();
        descriptor: ()V        //无参函数返回值是void
        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 6: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Ltest;
    
      public static void main(java.lang.String[]);
        descriptor: ([Ljava/lang/String;)V                        //返回值是void参数类型是string
        flags: ACC_PUBLIC, ACC_STATIC
        Code:
    	  // stack操作数栈的深度2
          // locals本地变量表最大长度（slot为单位），64位的是2，其他是1，索引从0开始，如果是非static方法索引0代表this，后面是入参，后面是本地变量
          // args_size1个参数，实例方法多一个this参数
          stack=2, locals=4, args_size=1
          //下面是代码所对应的字节码
             0: iconst_2
             1: istore_1
             2: iconst_3
             3: istore_2
             4: iload_1
             5: iload_2
             6: iadd
             7: istore_3
             8: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
            11: iload_3
            12: invokevirtual #3                  // Method java/io/PrintStream.println:(I)V
            15: return
          LineNumberTable:                 //行号表，代表咱们代码行号和编译好的行号
            line 9: 0
            line 10: 2
            line 11: 4
            line 12: 8
            line 13: 15
          LocalVariableTable:            //本地变量表
            Start  Length  Slot  Name   Signature
                0      16     0  args   [Ljava/lang/String;
                2      14     1     a   I
                4      12     2     b   I
                8       8     3     c   I
     }
    SourceFile: "test.java"

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