JVM-04

复杂点的例子：

Java代码如下：

1561102354042.png

编译

$ javac -g HelloWorld.java

汇编指令如下：

$ javap -v HelloWorld.class
Classfile /D:/ideaproject/jvm/HelloWorld.class
  Last modified 2019-6-21; size 824 bytes
  MD5 checksum 18f78c76b5e4f36814f54b846739fc02
  Compiled from "HelloWorld.java"
public class HelloWorld
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #8.#33         // java/lang/Object."<init>":()V
   #2 = Class              #34            // HelloWorld
   #3 = Methodref          #2.#33         // HelloWorld."<init>":()V
   #4 = Methodref          #2.#35         // HelloWorld.record:(D)V
   #5 = Methodref          #2.#36         // HelloWorld.getAverage:()D
   #6 = Fieldref           #37.#38        // java/lang/System.out:Ljava/io/PrintStream;
   #7 = Methodref          #39.#40        // java/io/PrintStream.println:(D)V
   #8 = Class              #41            // java/lang/Object
   #9 = Utf8               <init>
  #10 = Utf8               ()V
  #11 = Utf8               Code
  #12 = Utf8               LineNumberTable
  #13 = Utf8               LocalVariableTable
  #14 = Utf8               this
  #15 = Utf8               LHelloWorld;
  #16 = Utf8               record
  #17 = Utf8               (D)V
  #18 = Utf8               score
  #19 = Utf8               D
  #20 = Utf8               getAverage
  #21 = Utf8               ()D
  #22 = Utf8               main
  #23 = Utf8               ([Ljava/lang/String;)V
  #24 = Utf8               args
  #25 = Utf8               [Ljava/lang/String;
  #26 = Utf8               calculator
  #27 = Utf8               score1
  #28 = Utf8               I
  #29 = Utf8               score2
  #30 = Utf8               avg
  #31 = Utf8               SourceFile
  #32 = Utf8               HelloWorld.java
  #33 = NameAndType        #9:#10         // "<init>":()V
  #34 = Utf8               HelloWorld
  #35 = NameAndType        #16:#17        // record:(D)V
  #36 = NameAndType        #20:#21        // getAverage:()D
  #37 = Class              #42            // java/lang/System
  #38 = NameAndType        #43:#44        // out:Ljava/io/PrintStream;
  #39 = Class              #45            // java/io/PrintStream
  #40 = NameAndType        #46:#17        // println:(D)V
  #41 = Utf8               java/lang/Object
  #42 = Utf8               java/lang/System
  #43 = Utf8               out
  #44 = Utf8               Ljava/io/PrintStream;
  #45 = Utf8               java/io/PrintStream
  #46 = Utf8               println
{
  public HelloWorld(); //构造方法，默认添加
    descriptor: ()V  //构造方法的描述信息
    flags: ACC_PUBLIC //构造方法的修饰符
    Code: //Code属性
      stack=1, locals=1, args_size=1 //最大栈深1，最大局部变量表容量：1，参数个数：1
         0: aload_0 //加载该方法局部变量表索引0的变量：这里是this
         1: invokespecial #1                  // Method java/lang/Object."<init>":()V
         4: return //返回
      LineNumberTable:
        line 1: 0
      LocalVariableTable: //局部变量表
        Start  Length  Slot  Name   Signature
            0       5     0  this   LHelloWorld; //索引0，this

  public void record(double);//public void record(double) 方法
    descriptor: (D)V //方法参数和返回值信息
    flags: ACC_PUBLIC //方法的修饰符
    Code:
      stack=0, locals=3, args_size=2 //最大栈深0，最大局部变量表容量：3，参数个数：2
         0: return
      LineNumberTable:
        line 4: 0
      LocalVariableTable://局部变量表
        Start  Length  Slot  Name   Signature
            0       1     0  this   LHelloWorld;//索引0，this，LHelloWorld表示该类的引用
            0       1     1 score   D           //索引1，score，D表示double

  public double getAverage();//public double getAverage();
    descriptor: ()D
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=1, args_size=1
         0: dconst_0
         1: dreturn
      LineNumberTable:
        line 7: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       2     0  this   LHelloWorld;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: ACC_PUBLIC, ACC_STATIC //方法的修饰符 public static
    Code:
      stack=3, locals=6, args_size=1
         0: new           #2                  // class HelloWorld
         3: dup  //这个后面解释
         4: invokespecial #3                  // Method "<init>":()V
         7: astore_1 //存储局部变量表索引为1的变量
         8: iconst_1 //将 int类型 1 压入操作数栈顶
         9: istore_2 //将1 存储到 局部变量表索引为2的变量，这里是score1
        10: iconst_2
        11: istore_3
        12: aload_1 //将 局部变量表索引为1 的变量 加载到操作数栈
        13: iload_2 //
        14: i2d  // int类型转换为double类型
        15: invokevirtual #4                  // Method record:(D)V
        18: aload_1
        19: iload_3
        20: i2d
        21: invokevirtual #4                  // Method record:(D)V
        24: aload_1
        25: invokevirtual #5                  // Method getAverage:()D
        28: dstore        4
        30: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
        33: dload         4
        35: invokevirtual #7                  // Method java/io/PrintStream.println:(D)V
        38: return
      LineNumberTable:
        line 12: 0
        line 14: 8
        line 15: 10
        line 17: 12
        line 18: 18
        line 20: 24
        line 21: 30
        line 22: 38
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      39     0  args   [Ljava/lang/String;
            8      31     1 calculator   LHelloWorld;
           10      29     2 score1   I
           12      27     3 score2   I
           30       9     4   avg   D
}
SourceFile: "HelloWorld.java"

思考题：

用 Java 写一个简单的 class 文件解析工具支持输出函数列表