（十五）类索引

Posted 2020-11-04 专注改变人生。

一、概念

上一章节讲到了访问标志，如下表可知访问标志之后是this_class（这个类）和super_class（父类）和interfaces_count（继承接口的数量）和interfaces（继承的接口），各占2个字节。

 

类型	名称	数量
u4	magic	1
u2	minor_version	1
u2	major_version	1
u2	constant_pool_count	1
cp_info	constant_pool	constant_pool_count - 1
u2	access_flags	1
u2	this_class	1
u2	super_class	1
u2	interfaces_count	1
u2	interfaces	interfaces_count
u2	fields_count	1
field_info	fields	fields_count
u2	methods_count	1
method_info	methods	methods_count
u2	attribute_count	1
attribute_info	attributes	attributes_count

 

 

二、例子

• 编写一个类，继承一个类，并实现了多个接口，如下

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class Test extends FileInputStream implements Runnable,ActionListener{

    public Test(File file) throws FileNotFoundException {
        super(file);
        // TODO Auto-generated constructor stub
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // TODO Auto-generated method stub
    }

    @Override
    public void run() {
        // TODO Auto-generated method stub
        
    }

}

• 用javac编译后，打开class文件，如下图

 

• 用javap 命令行查看如下代码
• ，thisc_class在常量池中的位置是0x0002 = 2 ,对比下面金色部分#2代表的是Test，所以this_class的值为Test，即本类是Test类。
• super_class在常量池中的位置是0x0002 = 3,对比下面金色部分#3代表的是java/io/FileInputStream
  ，所以super_class的值为java/io/FileInputStream，即父类是FileInputStream类。
• 接口的数量为：0x0002 = 2 ，表示有两个接口
• 两个接口的位置跟this_class和super_class一样，不再解释。

C:\\Users\\Administrator\\Desktop>javap -verbose Test.class
Classfile /C:/Users/Administrator/Desktop/Test.class
  Last modified 2018-5-13; size 471 bytes
  MD5 checksum 150d8c5a88b79752ec7a2200bff2b737
  Compiled from "Test.java"
public class Test extends java.io.FileInputStream implements java.lang.Runnable,java.awt.event.ActionListener
  minor version: 0
  major version: 52
  flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
   #1 = Methodref          #3.#18         // java/io/FileInputStream."<init>":(Ljava/io/File;)V
   #2 = Class              #19            // Test
   #3 = Class              #20            // java/io/FileInputStream
   #4 = Class              #21            // java/lang/Runnable
   #5 = Class              #22            // java/awt/event/ActionListener
   #6 = Utf8               <init>
   #7 = Utf8               (Ljava/io/File;)V
   #8 = Utf8               Code
   #9 = Utf8               LineNumberTable
  #10 = Utf8               Exceptions
  #11 = Class              #23            // java/io/FileNotFoundException
  #12 = Utf8               actionPerformed
  #13 = Utf8               (Ljava/awt/event/ActionEvent;)V
  #14 = Utf8               run
  #15 = Utf8               ()V
  #16 = Utf8               SourceFile
  #17 = Utf8               Test.java
  #18 = NameAndType        #6:#7          // "<init>":(Ljava/io/File;)V
  #19 = Utf8               Test
  #20 = Utf8               java/io/FileInputStream
  #21 = Utf8               java/lang/Runnable
  #22 = Utf8               java/awt/event/ActionListener
  #23 = Utf8               java/io/FileNotFoundException
{
  public Test(java.io.File) throws java.io.FileNotFoundException;
    descriptor: (Ljava/io/File;)V
    flags: ACC_PUBLIC
    Code:
      stack=2, locals=2, args_size=2
         0: aload_0
         1: aload_1
         2: invokespecial #1                  // Method java/io/FileInputStream."<init>":(Ljava/io/File;)V
         5: return
      LineNumberTable:
        line 10: 0
        line 12: 5
    Exceptions:
      throws java.io.FileNotFoundException

  public void actionPerformed(java.awt.event.ActionEvent);
    descriptor: (Ljava/awt/event/ActionEvent;)V
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=2, args_size=2
         0: return
      LineNumberTable:
        line 18: 0

  public void run();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=0, locals=1, args_size=1
         0: return
      LineNumberTable:
        line 24: 0
}
SourceFile: "Test.java"