Java基础-内部类-为什么成员内部类可以无条件访问外部类

  在此之前,我们已经讨论过了成员内部类可以无条件访问外部类的成员,那具体究竟是如何实现的呢?下面通过反编译字节码文件看看究竟。事实上,编译器在进行编译的时候,会将成员内部类单独编译成一个字节码文件,下面是Outter.java的代码:

public class Outter {
    private Inner inner = null;
    public Outter() {
         
    }
     
    public Inner getInnerInstance() {
        if(inner == null)
            inner = new Inner();
        return inner;
    }
      
    protected class Inner {
        public Inner() {
             
        }
    }
}

  编译之后,出现了两个字节码文件:

  反编译Outter$Inner.class文件得到下面信息:

E:WorkspaceTestincomcxh	est2>javap -v Outter$Inner
Compiled from "Outter.java"
public class com.cxh.test2.Outter$Inner extends java.lang.Object
  SourceFile: "Outter.java"
  InnerClass:
   #24= #1 of #22; //Inner=class com/cxh/test2/Outter$Inner of class com/cxh/tes
t2/Outter
  minor version: 0
  major version: 50
  Constant pool:
const #1 = class        #2;     //  com/cxh/test2/Outter$Inner
const #2 = Asciz        com/cxh/test2/Outter$Inner;
const #3 = class        #4;     //  java/lang/Object
const #4 = Asciz        java/lang/Object;
const #5 = Asciz        this$0;
const #6 = Asciz        Lcom/cxh/test2/Outter;;
const #7 = Asciz        <init>;
const #8 = Asciz        (Lcom/cxh/test2/Outter;)V;
const #9 = Asciz        Code;
const #10 = Field       #1.#11; //  com/cxh/test2/Outter$Inner.this$0:Lcom/cxh/t
est2/Outter;
const #11 = NameAndType #5:#6;//  this$0:Lcom/cxh/test2/Outter;
const #12 = Method      #3.#13; //  java/lang/Object."<init>":()V
const #13 = NameAndType #7:#14;//  "<init>":()V
const #14 = Asciz       ()V;
const #15 = Asciz       LineNumberTable;
const #16 = Asciz       LocalVariableTable;
const #17 = Asciz       this;
const #18 = Asciz       Lcom/cxh/test2/Outter$Inner;;
const #19 = Asciz       SourceFile;
const #20 = Asciz       Outter.java;
const #21 = Asciz       InnerClasses;
const #22 = class       #23;    //  com/cxh/test2/Outter
const #23 = Asciz       com/cxh/test2/Outter;
const #24 = Asciz       Inner;
 
{
final com.cxh.test2.Outter this$0;
 
public com.cxh.test2.Outter$Inner(com.cxh.test2.Outter);
  Code:
   Stack=2, Locals=2, Args_size=2
   0:   aload_0
   1:   aload_1
   2:   putfield        #10; //Field this$0:Lcom/cxh/test2/Outter;
   5:   aload_0
   6:   invokespecial   #12; //Method java/lang/Object."<init>":()V
   9:   return
  LineNumberTable:
   line 16: 0
   line 18: 9
 
  LocalVariableTable:
   Start  Length  Slot  Name   Signature
   0      10      0    this       Lcom/cxh/test2/Outter$Inner;
 
 
}

  第11行到35行是常量池的内容,下面先看看第38行的内容:

final com.cxh.test2.Outter this$0;

  这行是一个指向外部类对象的指针,看到这里想必大家豁然开朗了。也就是说编译器会默认为成员内部类添加了一个指向外部类对象的引用,那么这个引用是如何赋初值的呢?下面接着看内部类的构造器:

public com.cxh.test2.Outter$Inner(com.cxh.test2.Outter);

  从这里可以看出,虽然我们在定义的内部类的构造器是无参构造器,编译器还是会默认添加一个参数,该参数的类型为指向外部类对象的一个引用,所以成员内部类中的Outter this&0 指针便指向了外部类对象,因此可以在成员内部类中随意访问外部类的成员。从这里也间接说明了成员内部类是依赖于外部类的,如果没有创建外部类的对象,则无法对Outter this&0引用进行初始化赋值,也就无法创建成员内部类的对象了。

原文地址:https://www.cnblogs.com/hwaggLee/p/4492415.html