解析 Javap 的结果文件

﹏ヽ暗。殇╰゛Y 2022-09-03 10:20 110阅读 0赞

# 一 代码 #

package chapter01.java1;
    
    public class JavapTest {
        private int num;
        boolean flag;
        protected char gender;
        public String info;
    
        public static final int COUNTS = 1;
    
        static {
            String url = "www.atguigu.com";
        }
    
        {
            info = "java";
        }
    
        public JavapTest() {
    
        }
    
        private JavapTest(boolean flag) {
            this.flag = flag;
        }
    
        private void methodPrivate() {
    
        }
    
        int getNum(int i) {
            return num + i;
        }
    
        protected char showGender() {
            return gender;
        }
    
        public void showInfo() {
            int i = 10;
            System.out.println(info + i);
        }
    }

# 二 运行命令 #

> C:\\Users\\chengqiuming\\Desktop\\2>javap -v -p JavapTest.class>javaptest.txt

# 三 解析 #

Classfile /C:/Users/chengqiuming/Desktop/2/JavapTest.class     // 字节码文件所属的路径
      Last modified 2021-8-5; size 1354 bytes					   // 最后修改时间、字节码文件的大小
      MD5 checksum 1e39e428fd4b2abbbf17f01ea9bd2655				   // MD5 散列值
      Compiled from "JavapTest.java"							   // 源文件的名称
    public class chapter01.java1.JavapTest
      minor version: 0											   // 副版本
      major version: 52											   // 主版本
      flags: ACC_PUBLIC, ACC_SUPER								   // 访问标识
    Constant pool:												   // 常量池信息
       #1 = Methodref          #16.#46        // java/lang/Object."<init>":()V
       #2 = String             #47            // java
       #3 = Fieldref           #15.#48        // chapter01/java1/JavapTest.info:Ljava/lang/String;
       #4 = Fieldref           #15.#49        // chapter01/java1/JavapTest.flag:Z
       #5 = Fieldref           #15.#50        // chapter01/java1/JavapTest.num:I
       #6 = Fieldref           #15.#51        // chapter01/java1/JavapTest.gender:C
       #7 = Fieldref           #52.#53        // java/lang/System.out:Ljava/io/PrintStream;
       #8 = Class              #54            // java/lang/StringBuilder
       #9 = Methodref          #8.#46         // java/lang/StringBuilder."<init>":()V
      #10 = Methodref          #8.#55         // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      #11 = Methodref          #8.#56         // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
      #12 = Methodref          #8.#57         // java/lang/StringBuilder.toString:()Ljava/lang/String;
      #13 = Methodref          #58.#59        // java/io/PrintStream.println:(Ljava/lang/String;)V
      #14 = String             #60            // www.atguigu.com
      #15 = Class              #61            // chapter01/java1/JavapTest
      #16 = Class              #62            // java/lang/Object
      #17 = Utf8               num
      #18 = Utf8               I
      #19 = Utf8               flag
      #20 = Utf8               Z
      #21 = Utf8               gender
      #22 = Utf8               C
      #23 = Utf8               info
      #24 = Utf8               Ljava/lang/String;
      #25 = Utf8               COUNTS
      #26 = Utf8               ConstantValue
      #27 = Integer            1
      #28 = Utf8               <init>
      #29 = Utf8               ()V
      #30 = Utf8               Code
      #31 = Utf8               LineNumberTable
      #32 = Utf8               LocalVariableTable
      #33 = Utf8               this
      #34 = Utf8               Lchapter01/java1/JavapTest;
      #35 = Utf8               (Z)V
      #36 = Utf8               methodPrivate
      #37 = Utf8               getNum
      #38 = Utf8               (I)I
      #39 = Utf8               i
      #40 = Utf8               showGender
      #41 = Utf8               ()C
      #42 = Utf8               showInfo
      #43 = Utf8               <clinit>
      #44 = Utf8               SourceFile
      #45 = Utf8               JavapTest.java
      #46 = NameAndType        #28:#29        // "<init>":()V
      #47 = Utf8               java
      #48 = NameAndType        #23:#24        // info:Ljava/lang/String;
      #49 = NameAndType        #19:#20        // flag:Z
      #50 = NameAndType        #17:#18        // num:I
      #51 = NameAndType        #21:#22        // gender:C
      #52 = Class              #63            // java/lang/System
      #53 = NameAndType        #64:#65        // out:Ljava/io/PrintStream;
      #54 = Utf8               java/lang/StringBuilder
      #55 = NameAndType        #66:#67        // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      #56 = NameAndType        #66:#68        // append:(I)Ljava/lang/StringBuilder;
      #57 = NameAndType        #69:#70        // toString:()Ljava/lang/String;
      #58 = Class              #71            // java/io/PrintStream
      #59 = NameAndType        #72:#73        // println:(Ljava/lang/String;)V
      #60 = Utf8               www.atguigu.com
      #61 = Utf8               chapter01/java1/JavapTest
      #62 = Utf8               java/lang/Object
      #63 = Utf8               java/lang/System
      #64 = Utf8               out
      #65 = Utf8               Ljava/io/PrintStream;
      #66 = Utf8               append
      #67 = Utf8               (Ljava/lang/String;)Ljava/lang/StringBuilder;
      #68 = Utf8               (I)Ljava/lang/StringBuilder;
      #69 = Utf8               toString
      #70 = Utf8               ()Ljava/lang/String;
      #71 = Utf8               java/io/PrintStream
      #72 = Utf8               println
      #73 = Utf8               (Ljava/lang/String;)V
    ############################################################################# 字段表信息 #########################
    {									// 字段表集合的信息
      private int num;					// 字段名
        descriptor: I					// 字段描述符：字段的类型
        flags: ACC_PRIVATE				// 字段的访问标识
    
      boolean flag;
        descriptor: Z
        flags:
    
      protected char gender;
        descriptor: C
        flags: ACC_PROTECTED
    
      public java.lang.String info;
        descriptor: Ljava/lang/String;
        flags: ACC_PUBLIC
    
      public static final int COUNTS;
        descriptor: I
        flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
        ConstantValue: int 1			// 常量字段的属性：ConstantValue
    
    ############################################################################ 方法表信息 ################
      public chapter01.java1.JavapTest();			// 构造器1的信息
        descriptor: ()V
        flags: ACC_PUBLIC
        Code:
          stack=2, locals=1, args_size=1
             0: aload_0
             1: invokespecial #1                  // Method java/lang/Object."<init>":()V
             4: aload_0
             5: ldc           #2                  // String java
             7: putfield      #3                  // Field info:Ljava/lang/String;
            10: return
          LineNumberTable:
            line 19: 0
            line 16: 4
            line 21: 10
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      11     0  this   Lchapter01/java1/JavapTest;
    
      private chapter01.java1.JavapTest(boolean);	// 构造器2的信息
        descriptor: (Z)V
        flags: ACC_PRIVATE
        Code:
          stack=2, locals=2, args_size=2
             0: aload_0
             1: invokespecial #1                  // Method java/lang/Object."<init>":()V
             4: aload_0
             5: ldc           #2                  // String java
             7: putfield      #3                  // Field info:Ljava/lang/String;
            10: aload_0
            11: iload_1
            12: putfield      #4                  // Field flag:Z
            15: return
          LineNumberTable:
            line 23: 0
            line 16: 4
            line 24: 10
            line 25: 15
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      16     0  this   Lchapter01/java1/JavapTest;
                0      16     1  flag   Z
    
      private void methodPrivate();
        descriptor: ()V
        flags: ACC_PRIVATE
        Code:
          stack=0, locals=1, args_size=1
             0: return
          LineNumberTable:
            line 29: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       1     0  this   Lchapter01/java1/JavapTest;
    
      int getNum(int);
        descriptor: (I)I
        flags:
        Code:
          stack=2, locals=2, args_size=2
             0: aload_0
             1: getfield      #5                  // Field num:I
             4: iload_1
             5: iadd
             6: ireturn
          LineNumberTable:
            line 32: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       7     0  this   Lchapter01/java1/JavapTest;
                0       7     1     i   I
    
      protected char showGender();
        descriptor: ()C
        flags: ACC_PROTECTED
        Code:
          stack=1, locals=1, args_size=1
             0: aload_0
             1: getfield      #6                  // Field gender:C
             4: ireturn
          LineNumberTable:
            line 36: 0
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0       5     0  this   Lchapter01/java1/JavapTest;
    
      public void showInfo();					 // 
        descriptor: ()V		                     // 方法描述符：方法的形参列表、返回值类型
        flags: ACC_PUBLIC						 // 方法的访问标识
        Code:									 // 方法的 Code 属性
          stack=3, locals=2, args_size=1		 // stack:操作数栈的最大深度 locals：局部变量表长度  args_size：方法接收参数的个数，没形参的是1，反之依次加1
      // 偏移量 操作码        操作数
             0: bipush        10
             2: istore_1
             3: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
             6: new           #8                  // class java/lang/StringBuilder
             9: dup
            10: invokespecial #9                  // Method java/lang/StringBuilder."<init>":()V
            13: aload_0
            14: getfield      #3                  // Field info:Ljava/lang/String;
            17: invokevirtual #10                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
            20: iload_1
            21: invokevirtual #11                 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
            24: invokevirtual #12                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
            27: invokevirtual #13                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
            30: return
    	  // 行号表:字节码指令的偏移量与Java 源代码中代码行号的一一对应关系
          LineNumberTable:
            line 40: 0
            line 41: 3
            line 42: 30
    	  // 局部变量表：描述内部局部变量的相关信息
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
                0      31     0  this   Lchapter01/java1/JavapTest;
                3      28     1     i   I
      
      static {};
        descriptor: ()V
        flags: ACC_STATIC
        Code:
          stack=1, locals=1, args_size=0
             0: ldc           #14                 // String www.atguigu.com
             2: astore_0
             3: return
          LineNumberTable:
            line 12: 0
            line 13: 3
          LocalVariableTable:
            Start  Length  Slot  Name   Signature
    }
    SourceFile: "JavapTest.java"     // 附件属性：字节码文件对应的源文件名

# 四 小结 #

1 通过 javap 命令可以查看一个 java 类的反汇编得到的 Class 文件版本号、常量池、访问标识、局部变量表、指令代码行号表等信息。不显示类索引、父类索引、接口索引、<clint>()、<init>()等结构。

2 通过对前面例子反汇编文件的简单分析，可以发现，一个方法的执行通常会涉及下面几块内存操作：

a java 栈中：局部变量表、操作数栈。

b java堆：通过对象的地址引用去操作。

c 常量池

d 其他如帧数据区、方法区的剩余部分等情况，测试中没有显示出来，这里说明一下。

3 平常，我们比较关注的 Java 类中每个方法的反汇编的指令操作过程，这些指令都是按顺序执行的，可以参考官网。

[https://docs.oracle.com/javase/specs/jvms/se8/html/index.html][https_docs.oracle.com_javase_specs_jvms_se8_html_index.html]

[https_docs.oracle.com_javase_specs_jvms_se8_html_index.html]: https://docs.oracle.com/javase/specs/jvms/se8/html/index.html