一、前言
对于内部类平时编码时使用的场景不多，比较常用的地方应该就是绑定事件处理程序的时候了（从C#、JS转向Java阵营的孩子总不不习惯用匿名内部类来做事件订阅:(）。本文将结合Bytecode对四种内部类作介绍，当作一次梳理以便日后查阅。
首先要明确的是内部类是编译器提供的特性，编译器会将含内部类的java文件编译成外部类和内部类的N个文件（N>=2） ，然后JVM就按普通类的方式运行。就如下面的源码会被编译为Outer.class和和Outer$Inner.class文件。
class Outer{ class Inner{} }
三、成员内部类
定义在一个类的内部。相对外部类仅有默认和public两种访问修饰符而言，成员内部类可有默认、private、proteced和public四种访问修饰符，效果与成员字段和方法的一样。
示例：
import java.io.*; // Main.java文件 class Main{ public static void main(String[] args) throws IOException{ MemberCls outer = new MemberCls(); Inner inner1 = outer.new Inner(); Inner inner2 = outer.getInner(); System.out.println(inner1.getVal()); System.out.println(inner2.getVal()); inner1.setVal(2); System.out.println(inner1.getVal()); System.out.println(inner2.getVal()); inner2.setVal(3); System.out.println(inner1.getVal()); System.out.println(inner2.getVal()); System.in.read(); } } // MemberCls.java文件 class MemberCls{ private int val = 1; class Inner{ void setVal(int val){ MemberCls.this.val = val; } int getVal(){ return val; } } Inner getInner(){ return new Inner(); } // 运行结果 // 1 // 1 // 2 // 2 // 3 // 3
并生成MemberCls.class和MemberCls$Inner.class两个类文件。
Classfile /F:/skyDrive/repos/self/jottings/java/sample/01/MemberCls.class Last modified 2015-2-3; size 1117 bytes MD5 checksum aea71084f78ab319a339717e4d0e1e79 Compiled from "MemberCls.java" class MemberCls SourceFile: "MemberCls.java" InnerClasses: #16= #3 of #5; //Inner=class MemberCls$Inner of class MemberCls minor version: 0 major version: 51 flags: ACC_SUPER Constant pool: #1 = Fieldref #5.#36 // MemberCls.val:I #2 = Methodref #15.#37 // java/lang/Object."<init>":()V #3 = Class #38 // MemberCls$Inner #4 = Methodref #3.#39 // MemberCls$Inner."<init>":(LMemberCls;)V #5 = Class #40 // MemberCls #6 = Methodref #5.#37 // MemberCls."<init>":()V #7 = Methodref #15.#41 // java/lang/Object.getClass:()Ljava/lang/Class; #8 = Methodref #5.#42 // MemberCls.getInner:()LMemberCls$Inner; #9 = Fieldref #43.#44 // java/lang/System.out:Ljava/io/PrintStream; #10 = Methodref #3.#45 // MemberCls$Inner.getVal:()I #11 = Methodref #46.#47 // java/io/PrintStream.println:(I)V #12 = Methodref #3.#48 // MemberCls$Inner.setVal:(I)V #13 = Fieldref #43.#49 // java/lang/System.in:Ljava/io/InputStream; #14 = Methodref #50.#51 // java/io/InputStream.read:()I #15 = Class #52 // java/lang/Object #16 = Utf8 Inner #17 = Utf8 InnerClasses #18 = Utf8 val #19 = Utf8 I #20 = Utf8 <init> #21 = Utf8 ()V #22 = Utf8 Code #23 = Utf8 LineNumberTable #24 = Utf8 getInner #25 = Utf8 ()LMemberCls$Inner; #26 = Utf8 main #27 = Utf8 ([Ljava/lang/String;)V #28 = Utf8 Exceptions #29 = Class #53 // java/io/IOException #30 = Utf8 access$002 #31 = Utf8 (LMemberCls;I)I #32 = Utf8 access$000 #33 = Utf8 (LMemberCls;)I #34 = Utf8 SourceFile #35 = Utf8 MemberCls.java #36 = NameAndType #18:#19 // val:I #37 = NameAndType #20:#21 // "<init>":()V #38 = Utf8 MemberCls$Inner #39 = NameAndType #20:#54 // "<init>":(LMemberCls;)V #40 = Utf8 MemberCls #41 = NameAndType #55:#56 // getClass:()Ljava/lang/Class; #42 = NameAndType #24:#25 // getInner:()LMemberCls$Inner; #43 = Class #57 // java/lang/System #44 = NameAndType #58:#59 // out:Ljava/io/PrintStream; #45 = NameAndType #60:#61 // getVal:()I #46 = Class #62 // java/io/PrintStream #47 = NameAndType #63:#64 // println:(I)V #48 = NameAndType #65:#64 // setVal:(I)V #49 = NameAndType #66:#67 // in:Ljava/io/InputStream; #50 = Class #68 // java/io/InputStream #51 = NameAndType #69:#61 // read:()I #52 = Utf8 java/lang/Object #53 = Utf8 java/io/IOException #54 = Utf8 (LMemberCls;)V #55 = Utf8 getClass #56 = Utf8 ()Ljava/lang/Class; #57 = Utf8 java/lang/System #58 = Utf8 out #59 = Utf8 Ljava/io/PrintStream; #60 = Utf8 getVal #61 = Utf8 ()I #62 = Utf8 java/io/PrintStream #63 = Utf8 println #64 = Utf8 (I)V #65 = Utf8 setVal #66 = Utf8 in #67 = Utf8 Ljava/io/InputStream; #68 = Utf8 java/io/InputStream #69 = Utf8 read { MemberCls(); flags: Code: stack=2, locals=1, args_size=1 0: aload_0 1: invokespecial #2 // Method java/lang/Object."<init>":()V 4: aload_0 5: iconst_1 6: putfield #1 // Field val:I 9: return LineNumberTable: line 2: 0 line 3: 4 line 5: 9 MemberCls$Inner getInner(); flags: Code: stack=3, locals=1, args_size=1 0: new #3 // class MemberCls$Inner 3: dup 4: aload_0 5: invokespecial #4 // Method MemberCls$Inner."<init>":(LMemberCls;)V 8: areturn LineNumberTable: line 15: 0 public static void main(java.lang.String[]) throws java.io.IOException; flags: ACC_PUBLIC, ACC_STATIC Code: stack=4, locals=4, args_size=1 0: new #5 // class MemberCls 3: dup 4: invokespecial #6 // Method "<init>":()V 7: astore_1 8: new #3 // class MemberCls$Inner 11: dup 12: aload_1 13: dup 14: invokevirtual #7 // Method java/lang/Object.getClass:()Ljava/lang/Class; 17: pop 18: invokespecial #4 // Method MemberCls$Inner."<init>":(LMemberCls;)V 21: astore_2 22: aload_1 23: invokevirtual #8 // Method getInner:()LMemberCls$Inner; 26: astore_3 27: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 30: aload_2 31: invokevirtual #10 // Method MemberCls$Inner.getVal:()I 34: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 37: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 40: aload_3 41: invokevirtual #10 // Method MemberCls$Inner.getVal:()I 44: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 47: aload_2 48: iconst_2 49: invokevirtual #12 // Method MemberCls$Inner.setVal:(I)V 52: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 55: aload_2 56: invokevirtual #10 // Method MemberCls$Inner.getVal:()I 59: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 62: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 65: aload_3 66: invokevirtual #10 // Method MemberCls$Inner.getVal:()I 69: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 72: aload_3 73: iconst_3 74: invokevirtual #12 // Method MemberCls$Inner.setVal:(I)V 77: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 80: aload_2 81: invokevirtual #10 // Method MemberCls$Inner.getVal:()I 84: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 87: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream; 90: aload_3 91: invokevirtual #10 // Method MemberCls$Inner.getVal:()I 94: invokevirtual #11 // Method java/io/PrintStream.println:(I)V 97: getstatic #13 // Field java/lang/System.in:Ljava/io/InputStream; 100: invokevirtual #14 // Method java/io/InputStream.read:()I 103: pop 104: return LineNumberTable: line 19: 0 line 20: 8 line 21: 22 line 23: 27 line 24: 37 line 25: 47 line 26: 52 line 27: 62 line 28: 72 line 29: 77 line 30: 87 line 32: 97 line 33: 104 Exceptions: throws java.io.IOException static int access$002(MemberCls, int); flags: ACC_STATIC, ACC_SYNTHETIC Code: stack=3, locals=2, args_size=2 0: aload_0 1: iload_1 2: dup_x1 3: putfield #1 // Field val:I 6: ireturn LineNumberTable: line 2: 0 static int access$000(MemberCls); flags: ACC_STATIC, ACC_SYNTHETIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #1 // Field val:I 4: ireturn LineNumberTable: line 2: 0 }