JAVA-000

演示 for 循环和静态方法调用的字节码

注：本篇未完成，只弄明白了 for 循环，尚未掌握静态方法的调用，尤其是未能明白dup指令
请先阅读：

Source Code

package org.example.loop;

public class ForLoopTest {
    private static int[] numbers = {1, 6, 8};

    public static void main(String[] args) {
        MovingAverage ma = new MovingAverage();
        for (int number : numbers) {
            ma.submit(number);
        }
        double avg = ma.getAvg();
    }
}

bytecode 字节码

执行如下命令：

javap -c -v -p ForLoopTest.class

得到字节码文件：

Classfile /C:/Data/Code/GitHub/GeekbangJavaTrainingCamp/Week_01/Course_01/bytecode/target/classes/org/example/loop/ForLoopTest.class
  Last modified Oct 20, 2020; size 817 bytes
  MD5 checksum 97d55e36844eaf624fd166f6346bd2e9
  Compiled from "ForLoopTest.java"
public class org.example.loop.ForLoopTest
  minor version: 0
  major version: 51
  flags: (0x0021) ACC_PUBLIC, ACC_SUPER
  this_class: #7                          // org/example/loop/ForLoopTest
  super_class: #8                         // java/lang/Object
  interfaces: 0, fields: 1, methods: 3, attributes: 1
Constant pool:
   #1 = Methodref          #8.#34         // java/lang/Object."<init>":()V
   #2 = Class              #35            // org/example/loop/MovingAverage
   #3 = Methodref          #2.#34         // org/example/loop/MovingAverage."<init>":()V
   #4 = Fieldref           #7.#36         // org/example/loop/ForLoopTest.numbers:[I
   #5 = Methodref          #2.#37         // org/example/loop/MovingAverage.submit:(D)V
   #6 = Methodref          #2.#38         // org/example/loop/MovingAverage.getAvg:()D
   #7 = Class              #39            // org/example/loop/ForLoopTest
   #8 = Class              #40            // java/lang/Object
   #9 = Utf8               numbers
  #10 = Utf8               [I
  #11 = Utf8               <init>
  #12 = Utf8               ()V
  #13 = Utf8               Code
  #14 = Utf8               LineNumberTable
  #15 = Utf8               LocalVariableTable
  #16 = Utf8               this
  #17 = Utf8               Lorg/example/loop/ForLoopTest;
  #18 = Utf8               main
  #19 = Utf8               ([Ljava/lang/String;)V
  #20 = Utf8               number
  #21 = Utf8               I
  #22 = Utf8               args
  #23 = Utf8               [Ljava/lang/String;
  #24 = Utf8               ma
  #25 = Utf8               Lorg/example/loop/MovingAverage;
  #26 = Utf8               avg
  #27 = Utf8               D
  #28 = Utf8               StackMapTable
  #29 = Class              #23            // "[Ljava/lang/String;"
  #30 = Class              #10            // "[I"
  #31 = Utf8               <clinit>
  #32 = Utf8               SourceFile
  #33 = Utf8               ForLoopTest.java
  #34 = NameAndType        #11:#12        // "<init>":()V
  #35 = Utf8               org/example/loop/MovingAverage
  #36 = NameAndType        #9:#10         // numbers:[I
  #37 = NameAndType        #41:#42        // submit:(D)V
  #38 = NameAndType        #43:#44        // getAvg:()D
  #39 = Utf8               org/example/loop/ForLoopTest
  #40 = Utf8               java/lang/Object
  #41 = Utf8               submit
  #42 = Utf8               (D)V
  #43 = Utf8               getAvg
  #44 = Utf8               ()D
{
  private static int[] numbers;
    descriptor: [I
    flags: (0x000a) ACC_PRIVATE, ACC_STATIC

  public org.example.loop.ForLoopTest();
    descriptor: ()V
    flags: (0x0001) 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 3: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       5     0  this   Lorg/example/loop/ForLoopTest;

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=6, args_size=1
         0: new           #2                  // class org/example/loop/MovingAverage
         3: dup
         4: invokespecial #3                  // Method org/example/loop/MovingAverage."<init>":()V
         7: astore_1
         8: getstatic     #4                  // Field numbers:[I
        11: astore_2
        12: aload_2
        13: arraylength
        14: istore_3
        15: iconst_0
        16: istore        4
        18: iload         4
        20: iload_3
        21: if_icmpge     43
        24: aload_2
        25: iload         4
        27: iaload
        28: istore        5
        30: aload_1
        31: iload         5
        33: i2d
        34: invokevirtual #5                  // Method org/example/loop/MovingAverage.submit:(D)V
        37: iinc          4, 1
        40: goto          18
        43: aload_1
        44: invokevirtual #6                  // Method org/example/loop/MovingAverage.getAvg:()D
        47: dstore_2
        48: return
      LineNumberTable:
        line 7: 0
        line 8: 8
        line 9: 30
        line 8: 37
        line 11: 43
        line 12: 48
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           30       7     5 number   I
            0      49     0  args   [Ljava/lang/String;
            8      41     1    ma   Lorg/example/loop/MovingAverage;
           48       1     2   avg   D
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 18
          locals = [ class "[Ljava/lang/String;", class org/example/loop/MovingAverage, class "[I", int, int ]
          stack = []
        frame_type = 248 /* chop */
          offset_delta = 24

  static {};
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=4, locals=0, args_size=0
         0: iconst_3
         1: newarray       int
         3: dup
         4: iconst_0
         5: iconst_1
         6: iastore
         7: dup
         8: iconst_1
         9: bipush        6
        11: iastore
        12: dup
        13: iconst_2
        14: bipush        8
        16: iastore
        17: putstatic     #4                  // Field numbers:[I
        20: return
      LineNumberTable:
        line 4: 0
}
SourceFile: "ForLoopTest.java"

解析

array 初始化

  static {};
    descriptor: ()V
    flags: (0x0008) ACC_STATIC
    Code:
      stack=4, locals=0, args_size=0
         0: iconst_3
         1: newarray       int
         3: dup
         4: iconst_0
         5: iconst_1
         6: iastore
         7: dup
         8: iconst_1
         9: bipush        6
        11: iastore
        12: dup
        13: iconst_2
        14: bipush        8
        16: iastore
        17: putstatic     #4                  // Field numbers:[I
        20: return
      LineNumberTable:
        line 4: 0

0: iconst_3 把常量3放入operand stack，此时operand stack中只有一个元素3。
1: newarray int 初始化一个int类型的数据，长度为operand stack的第一个元素，此处为3。 3从operand stack中弹出，同时新数组起始地址放入至operand stack。此时operand stack中只有一个元素：即数组的起始地址。
3: dup 复制operand stack的第一个元素，同时放入operand stack中。此时operand stack中有两个元素：3和3。
4: iconst_0 把常量0放入operand stack中。
5: iconst_1 把常量1放入operand stack中。此是operand stack有四个元素：0、1、3、3。
6: iastore 数组的第0个位置放入int 1。此时operand stack中有两个元素：3和3。
7: dup
8: iconst_1 把常量1放入operand stack中。
9: bipush 6 把常量6放入operand stack中。此是operand stack有两个元素：1和6。
11: iastore 数组的第1个位置放入int 6。
12: dup
13: iconst_2 把常量2放入operand stack中。
14: bipush 8 把常量8放入operand stack中。
16: iastore 数组的第2个位置放入int 8。
17: putstatic #4 把结果保存为static field #4 = Fieldref #7.#36，最终解析为org/example/loop/ForLoopTest.numbers:[I
20: return 执行结束。

##　main method

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=3, locals=6, args_size=1
         0: new           #2                  // class org/example/loop/MovingAverage
         3: dup
         4: invokespecial #3                  // Method org/example/loop/MovingAverage."<init>":()V
         7: astore_1
         8: getstatic     #4                  // Field numbers:[I
        11: astore_2
        12: aload_2
        13: arraylength
        14: istore_3
        15: iconst_0
        16: istore        4
        18: iload         4
        20: iload_3
        21: if_icmpge     43
        24: aload_2
        25: iload         4
        27: iaload
        28: istore        5
        30: aload_1
        31: iload         5
        33: i2d
        34: invokevirtual #5                  // Method org/example/loop/MovingAverage.submit:(D)V
        37: iinc          4, 1
        40: goto          18
        43: aload_1
        44: invokevirtual #6                  // Method org/example/loop/MovingAverage.getAvg:()D
        47: dstore_2
        48: return
      LineNumberTable:
        line 7: 0
        line 8: 8
        line 9: 30
        line 8: 37
        line 11: 43
        line 12: 48
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           30       7     5 number   I
            0      49     0  args   [Ljava/lang/String;
            8      41     1    ma   Lorg/example/loop/MovingAverage;
           48       1     2   avg   D
      StackMapTable: number_of_entries = 2
        frame_type = 255 /* full_frame */
          offset_delta = 18
          locals = [ class "[Ljava/lang/String;", class org/example/loop/MovingAverage, class "[I", int, int ]
          stack = []
        frame_type = 248 /* chop */
          offset_delta = 24

0: new #2 新建Constant pool #2 = Class #35，最终解析为org/example/loop/MovingAverage的一个实例对象。
3: dup
4: invokespecial #3 调用Constant pool #3方法，最终解析为Class org/example/loop/MovingAverage 的构造函数。
7: astore_1 把operand stack中的第一个元素存入LocalVariableTable的Slot为1变量，即ma中。
8: getstatic #4　把static 变量加载进operand stack，最终解析为把 org/example/loop/ForLoopTest.numbers 加载进operand stack。
11: astore_2 把operand stack中的第一个元素存入LocalVariableTable的Slot为2变量，即avg中。
12: aload_2 把LocalVariableTable的Slot为2变量，即avg加载进operand stack。
13: arraylength 获取数据的长度
14: istore_3
15: iconst_0 把常量0放入operand stack中。
16: istore 4 把operand stack的第一个元素
18: iload 4 把LocalVariableTable的Slot为4变量加载进operand stack。
20: iload_3 把LocalVariableTable的Slot为3变量加载进operand stack。
21: if_icmpge 43 如果 >= ，则跳转至43: aload_1，即结束循环。
24: aload_2
25: iload 4
27: iaload
28: istore 5
30: aload_1
31: iload 5
33: i2d
34: invokevirtual #5
37: iinc 4, 1
40: goto 18 跳转至18: iload 4，即开始下一次循环。
43: aload_1 把LocalVariableTable的Slot为1变量加载进operand stack。
44: invokevirtual #6 调用方法org/example/loop/MovingAverage.getAvg:()D
47: dstore_2 把operand stack的第一个元素保存至LocalVariableTable的Slot为2变量中。
48: return 退出方法，不返回任何值。

JAVA-000

返回

演示 for 循环和静态方法调用的字节码

Source Code

bytecode 字节码

解析

array 初始化