Java Class文件学习

数据类型：

class 文件结构

minor version:副版本

major version:主版本号

文件的版本号 major_version.minor_version

flags:访问标志

ACC_PUBLICH 声明为publich
ACC_FINAL 声明为final
ACC_SUPER 当用到invokespecial指令时，需要对父类方法做特殊处理
ACC_INTERFACE 该class文件定义的是接口而不是类
ACC_ABSTRACT 声明为abstract
ACC_SYNTHETIC 声明为synthetic
ACC_ANNOTATION 标识注解类型
ACC_ENUM 标识枚举类型

constant pool 常量池

方法调用和返回指令

invokevirtual 指令用于调用对象的实例方法，根据对象的实际类型进行分派
invokeinterface 指令用于调用接口方法，它会在运行时搜索由特定对象实现的这个接口方法，并找出适合的方法进行调用
invokespecial 指令用于调用一些需要特殊的实例方法，包括实例的初始化，私有方法和父类方法
invokestatic 指令用于调用命名类中的静态方法
invokedynamic 指令用于调用可以绑定invokedynamic指令的调用点对象作为目标方法。

aaload_<n>:从局部变量表中加载索引值为n的引用

aconst_null:将一个null值压入到操作数栈顶

putfield:为制定类的字段赋值

通过执行javac xxx.java javap -verbose xxx.class 查看class文件

public class AconstNullDemo {
    Integer i=null;

    void test(){
        i=1;
        if(i.equals(Integer.valueOf(1))){
            System.out.println("eq");
        }
    }
}


public class common.jvm.AconstNullDemo
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #9.#20         // java/lang/Object."<init>":()V
   #2 = Fieldref           #8.#21         // common/jvm/AconstNullDemo.i:Ljava/lang/Integer;
   #3 = Methodref          #22.#23        // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
   #4 = Methodref          #22.#24        // java/lang/Integer.equals:(Ljava/lang/Object;)Z
   #5 = Fieldref           #25.#26        // java/lang/System.out:Ljava/io/PrintStream;
   #6 = String             #27            // eq
   #7 = Methodref          #28.#29        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #8 = Class              #30            // common/jvm/AconstNullDemo
   #9 = Class              #31            // java/lang/Object
  #10 = Utf8               i
  #11 = Utf8               Ljava/lang/Integer;
  #12 = Utf8               <init>
  #13 = Utf8               ()V
  #14 = Utf8               Code
  #15 = Utf8               LineNumberTable
  #16 = Utf8               test
  #17 = Utf8               StackMapTable
  #18 = Utf8               SourceFile
  #19 = Utf8               AconstNullDemo.java
  #20 = NameAndType        #12:#13        // "<init>":()V
  #21 = NameAndType        #10:#11        // i:Ljava/lang/Integer;
  #22 = Class              #32            // java/lang/Integer
  #23 = NameAndType        #33:#34        // valueOf:(I)Ljava/lang/Integer;
  #24 = NameAndType        #35:#36        // equals:(Ljava/lang/Object;)Z
  #25 = Class              #37            // java/lang/System
  #26 = NameAndType        #38:#39        // out:Ljava/io/PrintStream;
  #27 = Utf8               eq
  #28 = Class              #40            // java/io/PrintStream
  #29 = NameAndType        #41:#42        // println:(Ljava/lang/String;)V
  #30 = Utf8               common/jvm/AconstNullDemo
  #31 = Utf8               java/lang/Object
  #32 = Utf8               java/lang/Integer
  #33 = Utf8               valueOf
  #34 = Utf8               (I)Ljava/lang/Integer;
  #35 = Utf8               equals
  #36 = Utf8               (Ljava/lang/Object;)Z
  #37 = Utf8               java/lang/System
  #38 = Utf8               out
  #39 = Utf8               Ljava/io/PrintStream;
  #40 = Utf8               java/io/PrintStream
  #41 = Utf8               println
  #42 = Utf8               (Ljava/lang/String;)V
{
  java.lang.Integer i;
    descriptor: Ljava/lang/Integer;
    flags:

  public common.jvm.AconstNullDemo();
    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
         //将null压入栈顶
         5: aconst_null
         //将null赋值给属性i
         6: putfield      #2                  // Field i:Ljava/lang/Integer;
         9: return
      LineNumberTable:
        line 6: 0
        line 7: 4

  void test();
    descriptor: ()V
    flags:
    Code:
      stack=2, locals=1, args_size=1
        //加载this到操作栈
         0: aload_0  
         //加载常量1到操作栈
         1: iconst_1
         //调用静态方法
         2: invokestatic  #3                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
         //将计算结果赋值给属性，为什么putfield的索引不是连续的？这是因为invokestatic操作和操作数长度为3个slot
         5: putfield      #2                  // Field i:Ljava/lang/Integer;
         //加载this到操作栈
         8: aload_0
         //将属性压入栈顶
         9: getfield      #2                  // Field i:Ljava/lang/Integer;
        //加载常量1到操作栈顶
        12: iconst_1
        //调用静态方法Integer.valueOf()
        13: invokestatic  #3                  // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
        //调用虚方法equals
        16: invokevirtual #4                  // Method java/lang/Integer.equals:(Ljava/lang/Object;)Z
        //如果ifeq的结果为0则挑战到30行
        19: ifeq          30
        22: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
        25: ldc           #6                  // String eq
        27: invokevirtual #7                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        30: return
      LineNumberTable:
        line 10: 0
        line 11: 8
        line 12: 22
        line 14: 30
      StackMapTable: number_of_entries = 1
        frame_type = 30 /* same */
}

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