Java 训练营第一周习题：01 字节码分析

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发布于: 2021 年 02 月 23 日

题目

自己写一个简单的 Hello.java，里面需要涉及基本类型，四则运行，if 和 for，然后自己分析一下对应的字节码。

程序源码

编译：javac -g ByteCodeAnalyze.java


public class ByteCodeAnalyze {    public static void main(String[] args) {        ByteCodeAnalyze byteCodeAnalyze = new ByteCodeAnalyze();
        for (int i = 0; i < 4; i++) {            if (i == 0) {                System.out.println(byteCodeAnalyze.add(1d, 2d));            } else if (i == 1) {                System.out.println(byteCodeAnalyze.sub(2d, 1d));            } else if (i == 2) {                System.out.println(byteCodeAnalyze.mul(1d, 2d));            } else {                System.out.println(byteCodeAnalyze.div(100d, 3d));            }        }    }
    private double add(double a, double b) {        return a + b;    }
    private double sub(double a, double b) {        return a - b;    }
    private double mul(double a, double b) {        return a * b;    }
    private double div(double a, double b) {        return a / b;    }}

复制代码

程序字节码

用 javap 工具来获取 class 文件中的指令清单。

javap 是标准 JDK 内置的一款工具, 专门用于反编译 class 文件。

查看 class 文件字节码：javap -c -verbose ByteCodeAnalyze

Classfile /Users/zhanghongang/Study/GeekBang/JavaCourse/JAVA-Homework/Week_01/ByteCodeAnalyze.class // class文件的完成路径  Last modified 2021年2月21日; size 1164 bytes // class文件修改时间以及大小  SHA-256 checksum 96f40cd5b0ed04c47715231a8e071c7e3fcf30747cb0fa10e161f18200351f79 // SHA-256 校验和  Compiled from "ByteCodeAnalyze.java" // 从哪个文件编译而来public class ByteCodeAnalyze  minor version: 0  major version: 59 // java主版本  major_version.minor_version 组成我们的版本号59.0  flags: (0x0021) ACC_PUBLIC, ACC_SUPER  this_class: #7                          // ByteCodeAnalyze  super_class: #2                         // java/lang/Object  interfaces: 0, fields: 0, methods: 6, attributes: 1Constant pool: // 常量池   #1 = Methodref          #2.#3          // java/lang/Object."<init>":()V   #2 = Class              #4             // java/lang/Object   #3 = NameAndType        #5:#6          // "<init>":()V   #4 = Utf8               java/lang/Object   #5 = Utf8               <init>   #6 = Utf8               ()V   #7 = Class              #8             // ByteCodeAnalyze   #8 = Utf8               ByteCodeAnalyze   #9 = Methodref          #7.#3            #10 = Fieldref           #11.#12        // java/lang/System.out:Ljava/io/PrintStream;  #11 = Class              #13            // java/lang/System  #12 = NameAndType        #14:#15        // out:Ljava/io/PrintStream;  #13 = Utf8               java/lang/System  #14 = Utf8               out  #15 = Utf8               Ljava/io/PrintStream;  #16 = Double             2.0d  #18 = Methodref          #7.#19         // ByteCodeAnalyze.add:(DD)D  #19 = NameAndType        #20:#21        // add:(DD)D  #20 = Utf8               add  #21 = Utf8               (DD)D  #22 = Methodref          #23.#24        // java/io/PrintStream.println:(D)V  #23 = Class              #25            // java/io/PrintStream  #24 = NameAndType        #26:#27        // println:(D)V  #25 = Utf8               java/io/PrintStream  #26 = Utf8               println  #27 = Utf8               (D)V  #28 = Methodref          #7.#29         // ByteCodeAnalyze.sub:(DD)D  #29 = NameAndType        #30:#21        // sub:(DD)D  #30 = Utf8               sub  #31 = Methodref          #7.#32         // ByteCodeAnalyze.mul:(DD)D  #32 = NameAndType        #33:#21        // mul:(DD)D  #33 = Utf8               mul  #34 = Double             100.0d  #36 = Double             3.0d  #38 = Methodref          #7.#39         // ByteCodeAnalyze.div:(DD)D  #39 = NameAndType        #40:#21        // div:(DD)D  #40 = Utf8               div  #41 = Utf8               Code  #42 = Utf8               LineNumberTable  #43 = Utf8               LocalVariableTable  #44 = Utf8               this  #45 = Utf8               LByteCodeAnalyze;  #46 = Utf8               main  #47 = Utf8               ([Ljava/lang/String;)V  #48 = Utf8               i  #49 = Utf8               I  #50 = Utf8               args  #51 = Utf8               [Ljava/lang/String;  #52 = Utf8               byteCodeAnalyze  #53 = Utf8               StackMapTable  #54 = Utf8               a  #55 = Utf8               D  #56 = Utf8               b  #57 = Utf8               SourceFile  #58 = Utf8               ByteCodeAnalyze.java{  public ByteCodeAnalyze(); // ByteCodeAnalyze类的构造方法    descriptor: ()V    flags: (0x0001) ACC_PUBLIC    Code:      stack=1, locals=1, args_size=1 // 栈深最大1，局部变量1,args_size入参是1(如果是实体方法会把this也算入参)         0: aload_0		// aload_0中的0是局部变量表里的Slot 0,意思是将局部变量表里的Slot 0的东西         							// 压入操作数栈，这个Slot 0里的东西Name是this，也就是ByteCodeAnalyze的实例         1: invokespecial #1 // 调用父类构造方法 // Method java/lang/Object."<init>":()V         4: return      LineNumberTable:        line 2: 0      LocalVariableTable:        Start  Length  Slot  Name   Signature            0       5     0  this   LByteCodeAnalyze;
  public static void main(java.lang.String[]);    descriptor: ([Ljava/lang/String;)V    flags: (0x0009) ACC_PUBLIC, ACC_STATIC    Code:      stack=6, locals=3, args_size=1         0: new           #7 // 创建常量池#2位置的类ByteCodeAnalyze的实例，并将此实例引用入栈，栈深度为1         3: dup              // 复制栈顶的刚刚放入的引用，再次压栈，这时栈里有两个重复的内容，栈深度为2         4: invokespecial #9 // 弹出栈顶ByteCodeAnalyze引用对象并调用<init>方法，即构造方法，栈深度为1         7: astore_1 // 将栈顶引用变量（ByteCodeAnalyze引用对象）存入本地变量表 Slot 1 位置，栈深度为1         8: iconst_0 // 将int类型数值0推送至栈顶，栈深度为2         9: istore_2 // 将栈顶 int 型数值 0 存入本地变量表 Slot 2 位置，即存入代码中i变量的初始值0        10: iload_2  // 将本地变量表 Slot 2 位置的 0 推送至栈顶，栈深度为3        11: iconst_4 // 将int类型数值 4 推送至栈顶，栈深度为4        12: if_icmpge     102 // 取出并比较栈顶两 int 型数值大小，0 不大于等于 4继续执行，否则，跳转到 102 return，栈深度为1        15: iload_2 // 将本地变量表 Slot 2 位置的 0 推送至栈顶，栈深度为2        16: ifne          36 // 当栈顶 int 型数值不等于 0 时跳转到 36，等于0时，则继续执行        // 19:getstatic，获取指定类的静态域，并将其值压入栈顶，栈深度为3        19: getstatic     #10                 // Field java/lang/System.out:Ljava/io/PrintStream;        22: aload_1 // 将本地变量表 Slot 1 位置的引用变量（ByteCodeAnalyze引用对象）入栈，栈深度为4        23: dconst_1 //  将 double 类型数值 1.0d 推送至栈顶，栈深度为5        // 24: ldc2_w，将 long 或 double 型常量值从常量池中推送至栈顶，2.0d入栈，栈深度6        24: ldc2_w        #16                 // double 2.0d        // 27，调用实例方法，即调用常量池#18的 ByteCodeAnalyze.add 方法，栈顶的 2.0 和 1.0 分别出栈，        //     作为 ByteCodeAnalyze.add 方法的入参，获取到的结果3.0(1.0+2.0)入栈，栈深度为4（5-2+1）        27: invokevirtual #18                 // Method add:(DD)D        // 30，调用 PrintStream.println 方法输入栈顶的 3.0        30: invokevirtual #22                 // Method java/io/PrintStream.println:(D)V        33: goto          96 // 无条件跳转到 96，即i++，i由0变为1        36: iload_2        37: iconst_1        38: if_icmpne     58        41: getstatic     #10                 // Field java/lang/System.out:Ljava/io/PrintStream;        44: aload_1        45: ldc2_w        #16                 // double 2.0d        48: dconst_1        49: invokevirtual #28                 // Method sub:(DD)D        52: invokevirtual #22                 // Method java/io/PrintStream.println:(D)V        55: goto          96        58: iload_2        59: iconst_2        60: if_icmpne     80        63: getstatic     #10                 // Field java/lang/System.out:Ljava/io/PrintStream;        66: aload_1        67: dconst_1        68: ldc2_w        #16                 // double 2.0d        71: invokevirtual #31                 // Method mul:(DD)D        74: invokevirtual #22                 // Method java/io/PrintStream.println:(D)V        77: goto          96        80: getstatic     #10                 // Field java/lang/System.out:Ljava/io/PrintStream;        83: aload_1        84: ldc2_w        #34                 // double 100.0d        87: ldc2_w        #36                 // double 3.0d        90: invokevirtual #38                 // Method div:(DD)D        93: invokevirtual #22                 // Method java/io/PrintStream.println:(D)V        96: iinc          2, 1 // 将局部变量表的 Slot 2位置的变量 +1，即 i++        99: goto          10   // 无条件跳转到 10，即 for 循环的条件判断       102: return      LineNumberTable:        line 4: 0        line 6: 8        line 7: 15        line 8: 19        line 9: 36        line 10: 41        line 11: 58        line 12: 63        line 14: 80        line 6: 96        line 17: 102      LocalVariableTable:        Start  Length  Slot  Name   Signature           10      92     2     i   I            0     103     0  args   [Ljava/lang/String;            8      95     1 byteCodeAnalyze   LByteCodeAnalyze;      StackMapTable: number_of_entries = 6        frame_type = 253 /* append */          offset_delta = 10          locals = [ class ByteCodeAnalyze, int ]        frame_type = 25 /* same */        frame_type = 21 /* same */        frame_type = 21 /* same */        frame_type = 15 /* same */        frame_type = 250 /* chop */          offset_delta = 5}SourceFile: "ByteCodeAnalyze.java"