汇编与java字节码

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问题

搞懂两个数相加的基本原理

汇编实现

参考汇编教程
macOS上的汇编入门

java实现

jvm规范-操作码列表

jvm内存模型

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JVM内存区域
    |_线程共享
        |_方法区(常量、静态变量、类信息)
        |_堆（存放对象的实例）
    |_线程私有
        |_程序计数器(代码执行到第几行)
        |_虚拟机栈(方法的出入栈)
        |_本地方法栈(native方法)

两个整数相加

$vim Test.java

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public class Test {

    public static void main(String[] args) {
        int a = 3;
        int b = 4;
        int i = a + b;
        System.out.println(i);
    }

}

$ javac Test.java
$ javap -c -l Test

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Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 1: 0

  public static void main(java.lang.String[]);
    Code:
       0: iconst_3  从常量池获取3(int)
       1: istore_1  压入操作数栈位置是1
       2: iconst_4  从常量池获取4(int)
       3: istore_2  压入操作数栈位置是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:
      line 4: 0
      line 5: 2
      line 6: 4
      line 7: 8
      line 8: 15
}

方法调用

$vim Foo.java

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public class Foo {
    public static void main(String[] args) {
        /*boolean flag = true;
        if (flag) System.out.println("Hello, Java!");
        if (flag == true) System.out.println("Hello, JVM!");*/
        A a = new A();
        a.setI(10);
        int c = a.hashCode();
        System.out.println(c);
        System.out.println(a.getI());
    }

}

class A {
    private int i = 0;

    public int getI() {
        return i;
    }

    public void setI(int i) {
        this.i = i;
    }

    @Override
    public int hashCode() {
        return super.hashCode();
    }
}

$javac Foo.java

$javap -c -l Foo

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Compiled from "Foo.java"
public class Foo {
  public Foo();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 1: 0

  public static void main(java.lang.String[]);
    Code:
       0: new           #2                  // class A 创建对象
       3: dup
       4: invokespecial #3                  // Method A."<init>":()V 调用构造方法
       7: astore_1
       8: aload_1
       9: bipush        10
      11: invokevirtual #4                  // Method A.setI:(I)V
      14: aload_1
      15: invokevirtual #5                  // Method A.hashCode:()I
      18: istore_2
      19: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      22: iload_2
      23: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
      26: getstatic     #6                  // Field java/lang/System.out:Ljava/io/PrintStream;
      29: aload_1
      30: invokevirtual #8                  // Method A.getI:()I
      33: invokevirtual #7                  // Method java/io/PrintStream.println:(I)V
      36: return
    LineNumberTable:
      line 6: 0
      line 7: 8
      line 8: 14
      line 9: 19
      line 10: 26
      line 11: 36
}