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Java魔法堂：内部类详解
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$Inner of class MemberCls minor version: 0 major version: 51 flags: ACC_SUPERConstant pool: #1 = Fieldref #5.#36 // MemberCls.val:I #2 = Methodref #15.#37 // java/lang/Object."<init>":()V #3 = Class #38 // MemberCls$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$000(MemberCls); flags: ACC_STATIC, ACC_SYNTHETIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #1 // Field val:I 4: ireturn LineNumberTable: line 2: 0}
Classfile /F:/skyDrive/repos/self/jottings/java/sample/01/MemberCls$Inner.class Last modified 2015-2-3; size 525 bytes MD5 checksum b092ffe3c5b358c786d99d98c104dc40 Compiled from "MemberCls.java"class MemberCls$Inner SourceFile: "MemberCls.java" InnerClasses: #25= #5 of #21; //Inner=class MemberCls$Inner of class MemberCls minor version: 0 major version: 51 flags: ACC_SUPERConstant pool: #1 = Fieldref #5.#19 // MemberCls$Inner.this$0:LMemberCls; #2 = Methodref #6.#20 // java/lang/Object."<init>":()V #3 = Methodref #21.#22 // MemberCls.access$002:(LMemberCls;I)I #4 = Methodref #21.#23 // MemberCls.access$000:(LMemberCls;)I #5 = Class #24 // MemberCls$Inner #6 = Class #27 // java/lang/Object #7 = Utf8 this$0 #8 = Utf8 LMemberCls; #9 = Utf8 <init> #10 = Utf8 (LMemberCls;)V #11 = Utf8 Code #12 = Utf8 LineNumberTable #13 = Utf8 setVal #14 = Utf8 (I)V #15 = Utf8 getVal #16 = Utf8 ()I #17 = Utf8 SourceFile #18 = Utf8 MemberCls.java #19 = NameAndType #7:#8 // this$0:LMemberCls; #20 = NameAndType #9:#28 // "<init>":()V #21 = Class #29 // MemberCls #22 = NameAndType #30:#31 // access$002:(LMemberCls;I)I #23 = NameAndType #32:#33 // access$000:(LMemberCls;)I #24 = Utf8 MemberCls$Inner #25 = Utf8 Inner #26 = Utf8 InnerClasses #27 = Utf8 java/lang/Object #28 = Utf8 ()V #29 = Utf8 MemberCls #30 = Utf8 access$002 #31 = Utf8 (LMemberCls;I)I #32 = Utf8 access$000 #33 = Utf8 (LMemberCls;)I{ final MemberCls this$0; flags: ACC_FINAL, ACC_SYNTHETIC MemberCls$Inner(MemberCls); flags: Code: stack=2, locals=2, args_size=2 0: aload_0 1: aload_1 2: putfield #1 // Field this$0:LMemberCls; 5: aload_0 6: invokespecial #2 // Method java/lang/Object."<init>":()V 9: return LineNumberTable: line 5: 0 void setVal(int); flags: Code: stack=2, locals=2, args_size=2 0: aload_0 1: getfield #1 // Field this$0:LMemberCls; 4: iload_1 5: invokestatic #3 // Method MemberCls.access$002:(LMemberCls;I)I 8: pop 9: return LineNumberTable: line 7: 0 line 8: 9 int getVal(); flags: Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #1 // Field this$0:LMemberCls; 4: invokestatic #4 // Method MemberCls.access$000:(LMemberCls;)I 7: ireturn LineNumberTable: line 10: 0}
由于成员内部类依赖于外部类实例，因此创建内部类实例时要先创建外部类实例，然后通过下列两种形式来创建内部类实例：
// 方式一内部类 内部类实例 = 外部类实例.new 内部类();// 方式二外部类{ 内部类{} 内部类 get内部类(){ return new 内部类(); }}
内部类 内部类实例 = 外部类实例.get内部类();
注意：
1. 当成员内部类拥有与外部类同名的成员变量或方法时，默认是使用成员内部类的成员。若要访问外部类的同名成员，则需要进行如下操作：
外部类.this.成员变量;外部类.this.成员方法;
2. 对于同一个外部类实例创建的内部类实例，这些内部类实例均操作同一个外部实例。像上述例子那样，均操作同一个val字段。
看Bytecodes可知，编译器自动为MemberCls创建创建两个静态方法access$002和access$000，而MemberCls$Inner实例则通过这两个静态方法访问私有私有字段val的。
// MemberCls.class文件/** 等价于 * static int setVal(MemberCls outer, int val){ * outer.val = val; * return val; * } */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/** 等价于 * static int getVal(MemberCls outer){ * return outer.val; * } */ static int access$000(MemberCls); flags: ACC_STATIC, ACC_SYNTHETIC Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #1 // Field val:I 4: ireturn LineNumberTable: line 2:
// MemberCls$Inner.class文件/** 等价于 * void setVal(int val){ * MemberCls实例.setVal(val); * } */void setVal(int); flags: Code: stack=2, locals=2, args_size=2 0: aload_0 1: getfield #1 // Field this$0:LMemberCls; 4: iload_1 5: invokestatic #3 // Method MemberCls.access$002:(LMemberCls;I)I 8: pop 9: return LineNumberTable: line 7: 0 line 8: 9/** 等价于 * int getVal(int val){ * MemberCls实例.getVal(); * } */ int getVal(); flags: Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #1 // Field this$0:LMemberCls; 4: invokestatic #4 // Method MemberCls.access$000:(LMemberCls;)I 7: ireturn LineNumberTable: line 10: 0
因此内部类可以访问外部类的所有类型的字段和方法（包括private）。
四、局部内部类
局部内部类定义在方法或某个作用域里面，并且仅限于方法和该作用域内访问。
示例：
import java.io.*;class Main{ public static void main(String[] args) throws IOException{ LocalCls outer = new LocalCls(); outer.print(); System.in.read(); }}class LocalCls{ private int val = 1; void print(){ final String name = "fsjohnhuang"; class Inner{ int getVal(){ return val; } void setVal(int val){ LocalCls.this.val = val; } String getName(){ return name; } } Inner inner = new Inner(); System.out.println(inner.getVal()); inner.setVal(2); System.out.println(inner.getVal()); System.out.println(inner.getName()); }}// 结果：// 1// 2// fsjohnhuang
生成LocalCls.class和LocalCls$1Inner.class两个类文件。
Classfile /F:/skyDrive/repos/self/jottings/java/sample/02/LocalCls.class Last modified 2015-2-3; size 1052 bytes MD5 checksum a636f470da37d8c1cb9370dde083d6d8 Compiled from "LocalCls.java"class LocalCls SourceFile: "LocalCls.java" InnerClasses: #17= #8; //Inner=class LocalCls$1Inner minor version: 0 major version: 51 flags: ACC_SUPERConstant pool: #1 = Fieldref #3.#36 // LocalCls.val:I #2 = Methodref #16.#37 // java/lang/Object."<init>":()V #3 = Class #38 // LocalCls #4 = Methodref #3.#37 // LocalCls."<init>":()V #5 = Methodref #3.#39 // LocalCls.print:()V #6 = Fieldref #40.#41 // java/lang/System.in:Ljava/io/InputStream; #7 = Methodref #42.#43 // java/io/InputStream.read:()I #8 = Class #44 // LocalCls$1Inner #9 = Methodref #8.#45 // LocalCls$1Inner."<init>":(LLocalCls;)V #10 = Fieldref #40.#46 // java/lang/System.out:Ljava/io/PrintStream; #11 = Methodref #8.#47 // LocalCls$1Inner.getVal:()I #12 = Methodref #48.#49 // java/io/PrintStream.println:(I)V #13 = Methodref #8.#50 // LocalCls$1Inner.setVal:(I)V #14 = Methodref #8.#51 // LocalCls$1Inner.getName:()Ljava/lang/String; #15 = Methodref #48.#52 // java/io/PrintStream.println:(Ljava/lang/String;)V #16 = Class #53 // java/lang/Object #17 = Utf8 Inner #18 = Utf8 InnerClasses #19 = Utf8 val #20 = Utf8 I #21 = Utf8 <init> #22 = Utf8 ()V #23 = Utf8 Code #24 = Utf8 LineNumberTable #25 = Utf8 main #26 = Utf8 ([Ljava/lang/String;)V #27 = Utf8 Exceptions #28 = Class #54 // java/io/IOException #29 = Utf8 print #30 = Utf8 access$000 #31 = Utf8 (LLocalCls;)I #32 = Utf8 access$002 #33 = Utf8 (LLocalCls;I)I #34 = Utf8 SourceFile #35 = Utf8 LocalCls.java #36 = NameAndType #19:#20 // val:I #37 = NameAndType #21:#22 // "<init>":()V #38 = Utf8 LocalCls #39 = NameAndType #29:#22 // print:()V #40 = Class #55 // java/lang/System #41 = NameAndType #56:#57 // in:Ljava/io/InputStream; #42 = Class #58 // java/io/InputStream #43 = NameAndType #59:#60 // read:()I #44 = Utf8 LocalCls$1Inner #45 = NameAndType #21:#61 // "<init>":(LLocalCls;)V #46 = NameAndType #62:#63 // out:Ljava/io/PrintStream; #47 = NameAndType #64:#60 // getVal:()I #48 = Class #65 // java/io/PrintStream #49 = NameAndType #66:#67 // println:(I)V #50 = NameAndType #68:#67 // setVal:(I)V #51 = NameAndType #69:#70 // getName:()Ljava/lang/String; #52 = NameAndType #66:#71 // println:(Ljava/lang/String;)V #53 = Utf8 java/lang/Object #54 = Utf8 java/io/IOException #55 = Utf8 java/lang/System #56 = Utf8 in #57 = Utf8 Ljava/io/InputStream; #58 = Utf8 java/io/InputStream #59 = Utf8 read #60 = Utf8 ()I #61 = Utf8 (LLocalCls;)V #62 = Utf8 out #63 = Utf8 Ljava/io/PrintStream; #64 = Utf8 getVal #65 = Utf8 java/io/PrintStream #66 = Utf8 println #67 = Utf8 (I)V #68 = Utf8 setVal #69 = Utf8 getName #70 = Utf8 ()Ljava/lang/String; #71 = Utf8 (Ljava/lang/String;)V{ LocalCls(); 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 3: 0 line 11: 4 public static void main(java.lang.String[]) throws java.io.IOException; flags: ACC_PUBLIC, ACC_STATIC Code: stack=2, locals=2, args_size=1 0: new #3 // class LocalCls 3: dup 4: invokespecial #4 // Method "<init>":()V 7: astore_1 8: aload_1 9: invokevirtual #5 // Method print:()V 12: getstatic #6 // Field java/lang/System.in:Ljava/io/InputStream; 15: invokevirtual #7 // Method java/io/InputStream.read:()I 18: pop 19: return LineNumberTable: line 5: 0 line 6: 8 line 8: 12 line 9: 19 Exceptions: throws java.io.IOException void print(); flags: Code: stack=3, locals=3, args_size=1 0: new #8 // class LocalCls$1Inner 3: dup 4: aload_0 5: invokespecial #9 // Method LocalCls$1Inner."<init>":(LLocalCls;)V 8: astore_2 9: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream; 12: aload_2 13: invokevirtual #11 // Method LocalCls$1Inner.getVal:()I 16: invokevirtual #12 // Method java/io/PrintStream.println:(I)V 19: aload_2 20: iconst_2 21: invokevirtual #13 // Method LocalCls$1Inner.setVal:(I)V 24: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream; 27: aload_2 28: invokevirtual #11 // Method LocalCls$1Inner.getVal:()I 31: invokevirtual #12 // Method java/io/PrintStream.println:(I)V 34: getstatic #10 // Field java/lang/System.out:Ljava/io/PrintStream; 37: aload_2 38: invokevirtual #14 // Method LocalCls$1Inner.getName:()Ljava/lang/String; 41: invokevirtual #15 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 44: return LineNumberTable: line 26: 0 line 27: 9 line 28: 19 line 29: 24 line 30: 34 line 31: 44 static int access$000(LocalCls); 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 3: 0 static int access$002(LocalCls, 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 3: 0}
Classfile /F:/skyDrive/repos/self/jottings/java/sample/02/LocalCls$1Inner.class Last modified 2015-2-3; size 651 bytes MD5 checksum a4bf7c12f15f22b2ebb3f79438a555ab Compiled from "LocalCls.java"class LocalCls$1Inner SourceFile: "LocalCls.java" EnclosingMethod: #23.#24 // LocalCls.print InnerClasses: #31= #6; //Inner=class LocalCls$1Inner minor version: 0 major version: 51 flags: ACC_SUPERConstant pool: #1 = Fieldref #6.#25 // LocalCls$1Inner.this$0:LLocalCls; #2 = Methodref #7.#26 // java/lang/Object."<init>":()V #3 = Methodref #23.#27 // LocalCls.access$000:(LLocalCls;)I #4 = Methodref #23.#28 // LocalCls.access$002:(LLocalCls;I)I #5 = String #29 // fsjohnhuang #6 = Class #30 // LocalCls$1Inner #7 = Class #33 // java/lang/Object #8 = Utf8 this$0 #9 = Utf8 LLocalCls; #10 = Utf8 <init> #11 = Utf8 (LLocalCls;)V #12 = Utf8 Code #13 = Utf8 LineNumberTable #14 = Utf8 getVal #15 = Utf8 ()I #16 = Utf8 setVal #17 = Utf8 (I)V #18 = Utf8 getName #19 = Utf8 ()Ljava/lang/String; #20 = Utf8 SourceFile #21 = Utf8 LocalCls.java #22 = Utf8 EnclosingMethod #23 = Class #34 // LocalCls #24 = NameAndType #35:#36 // print:()V #25 = NameAndType #8:#9 // this$0:LLocalCls; #26 = NameAndType #10:#36 // "<init>":()V #27 = NameAndType #37:#38 // access$000:(LLocalCls;)I #28 = NameAndType #39:#40 // access$002:(LLocalCls;I)I #29 = Utf8 fsjohnhuang #30 = Utf8 LocalCls$1Inner #31 = Utf8 Inner #32 = Utf8 InnerClasses #33 = Utf8 java/lang/Object #34 = Utf8 LocalCls #35 = Utf8 print #36 = Utf8 ()V #37 = Utf8 access$000 #38 = Utf8 (LLocalCls;)I #39 = Utf8 access$002 #40 = Utf8 (LLocalCls;I)I{ final LocalCls this$0; flags: ACC_FINAL, ACC_SYNTHETIC LocalCls$1Inner(LocalCls); flags: Code: stack=2, locals=2, args_size=2 0: aload_0 1: aload_1 2: putfield #1 // Field this$0:LLocalCls; 5: aload_0 6: invokespecial #2 // Method java/lang/Object."<init>":()V 9: return LineNumberTable: line 15: 0 int getVal(); flags: Code: stack=1, locals=1, args_size=1 0: aload_0 1: getfield #1 // Field this$0:LLocalCls; 4: invokestatic #3 // Method LocalCls.access$000:(LLocalCls;)I 7: ireturn LineNumberTable: line 17: 0 void setVal(int); flags: Code: stack=2, locals=2, args_size=2 0: aload_0 1: getfield #1 // Field this$0:LLocalCls; 4: iload_1 5: invokestatic #4 // Method LocalCls.access$002:(LLocalCls;I)I 8: pop 9: return LineNumberTable: line 20: 0 line 21: 9 java.lang.String getName(); flags: Code: stack=1, locals=1, args_size=1 0: ldc #5 // String fsjohnhuang 2: areturn LineNumberTable: line 23: 0}
上述两个类文件与成员内部类的几乎一模一样，那么就是说内部类作用范围的限制其实是编译器的限制，而不是JVM的限制了。
注意：
1. 不能有public、protected、private和static作修饰；
2. 局部内部类中仅能访问方法或作用域内的常量，若访问的是变量则编译时会出错。
Q：为什么不能访问局部变量呢？
A：假设可以访问局部变量，那么要考虑的是如何引用到局部变量。
首先局部变量是存放在JVM栈帧中的局部变量表中，并且当方法执行完栈帧也随之弹出，也就是说局部变量所占的内存空间是短暂的（不稳定）。
假如局部变量A是基本类型的话，那么数据直接就存放在局部变量表中相应的Slots中，方法执行完就没了。那局部内部类中所访问的局部变量A到底是什么就无从得知了！ 假如局部变量A是String类型或其他类类型，那么局部内部类中访问的局部变量A时就有两种方式了，第一种是访问String常量池中该字符串的地址，第二种是指向局部变量A的地址，然后通过变量A去访问String常量池中该字符串。
但上述这些均是在运行时才能决定，而编译时是无法正确地被描述出来。并且由于内部类将被编译成独立的类文件，访问其他类方法的局部变量的操作无法在类文件中描述。而常量则可以在内部类文件的常量池部分中被正确地描述，而JVM中处理时也十分简单高效。类文件的常量池条目将合并到运行时常量池中，因此外部和内部量访问的是同一个常量。
下面的Bytecodes表示内部类中直接将常量池中的常量压栈后作为返回值返回。
java.lang.String getName(); flags: Code: stack=1, locals=1, args_size=1 0: ldc #5 // String fsjohnhuang 2: areturn LineNumberTable: line 23:
五、匿名内部类
匿名内部类其实是局部内部类的特殊形式。一般用来绑定事件监听处理程序上。Android示例：
class Outer{ public void subs(){ scan_bt.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { // TODO Auto-generated method stub } }); }}
上述代码生成了一个继承OnClickListener类的匿名内部类，然后实例化匿名类的一个实例，然后以该实例作为参数调用setOnClickListener方法。
并生成一个Outer.class和Outer$1.class类文件。
注意事项与局部内部一样。
六、静态内部类
静态内部类定义在类下，只不过多了个关键字static。静态内部类只能访问外部类的静态字段和静态方法。
而实例化静态内部类时只需 new 外部类.静态内部类() 。
七、总结
尊重原创，转载请注明来自：http://www.cnblogs.com/fsjohnhuang/p/4270044.html ^_^肥仔John
八、参考
http://www.cnblogs.com/dolphin0520/p/3811445.html
Java魔法堂：内部类详解
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