这是小小本周的第五篇，本篇将会说到失效的private修饰符

前言

在Java编程里，使用private关键字修饰了一个成员，只有成员内部可以访问，其余成员都不可访问，今天说明一下private功能失效的问题。

失效之Java内部类

在一个内部类里访问外部类的private成员变量或者方法。

public class OuterClass {private String language = "en";private String region = "US";public class InnerClass {public void printOuterClassPrivateFields() {String fields = "language=" + language + ";region=" + region;System.out.println(fields);}}public static void main(String[] args) {OuterClass outer = new OuterClass();OuterClass.InnerClass inner = outer.new InnerClass();inner.printOuterClassPrivateFields();}
}

查看原因

使用javap命令查看一下生成的class文件

15:30 $ javap -c  OuterClass
Compiled from "OuterClass.java"
public class OuterClass extends java.lang.Object{
public OuterClass();Code:0:  aload_01:  invokespecial    #11; //Method java/lang/Object."<init>":()V4:  aload_05:  ldc  #13; //String en7:  putfield #15; //Field language:Ljava/lang/String;10: aload_011: ldc  #17; //String US13: putfield #19; //Field region:Ljava/lang/String;16: returnpublic static void main(java.lang.String[]);Code:0:  new  #1; //class OuterClass3:  dup4:  invokespecial    #27; //Method "<init>":()V7:  astore_18:  new  #28; //class OuterClass$InnerClass11: dup12: aload_113: dup14: invokevirtual    #30; //Method java/lang/Object.getClass:()Ljava/lang/Class;17: pop18: invokespecial    #34; //Method OuterClass$InnerClass."<init>":(LOuterClass;)V21: astore_222: aload_223: invokevirtual    #37; //Method OuterClass$InnerClass.printOuterClassPrivateFields:()V26: returnstatic java.lang.String access$0(OuterClass);Code:0:  aload_01:  getfield #15; //Field language:Ljava/lang/String;4:  areturnstatic java.lang.String access$1(OuterClass);Code:0:  aload_01:  getfield #19; //Field region:Ljava/lang/String;4:  areturn}

在这里有一个OuterClass方法，

static java.lang.String access$0(OuterClass);Code:0:  aload_01:  getfield #15; //Field language:Ljava/lang/String;4:  areturnstatic java.lang.String access$1(OuterClass);Code:0:  aload_01:  getfield #19; //Field region:Ljava/lang/String;4:  areturn}

根据注释，可以知道access1返回outerClass的region属性，并且这两个方法都接受OuterClass的实例作为参数， 对这两个方法进行反编译。

15:37 $ javap -c OuterClass$InnerClass
Compiled from "OuterClass.java"
public class OuterClass$InnerClass extends java.lang.Object{
final OuterClass this$0;public OuterClass$InnerClass(OuterClass);Code:0:  aload_01:  aload_12:  putfield #10; //Field this$0:LOuterClass;5:  aload_06:  invokespecial    #12; //Method java/lang/Object."<init>":()V9:  returnpublic void printOuterClassPrivateFields();Code:0:  new  #20; //class java/lang/StringBuilder3:  dup4:  ldc  #22; //String language=6:  invokespecial    #24; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V9:  aload_010: getfield #10; //Field this$0:LOuterClass;13: invokestatic #27; //Method OuterClass.access$0:(LOuterClass;)Ljava/lang/String;16: invokevirtual    #33; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;19: ldc  #37; //String ;region=21: invokevirtual    #33; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;24: aload_025: getfield #10; //Field this$0:LOuterClass;28: invokestatic #39; //Method OuterClass.access$1:(LOuterClass;)Ljava/lang/String;31: invokevirtual    #33; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;34: invokevirtual    #42; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;37: astore_138: getstatic    #46; //Field java/lang/System.out:Ljava/io/PrintStream;41: aload_142: invokevirtual    #52; //Method java/io/PrintStream.println:(Ljava/lang/String;)V45: return
}

下面代码调用access$0的代码,其目的是得到OuterClass的language 私有属性。

13:   invokestatic #27; //Method OuterClass.access$0:(LOuterClass;)Ljava/lang/String;

下面代码调用了access$1的代码，其目的是得到OutherClass的region 私有属性。

28:   invokestatic #39; //Method OuterClass.access$1:(LOuterClass;)Ljava/lang/String;

即，在内部类构造的时候，会有外部类的引用传递进来，并且作为内部类的一个属性，所以内部类会持有一个其外部类的应用。this$0就是内部类持有的外部类引用，通过构造方法传递引用并赋值。

final OuterClass this$0;public OuterClass$InnerClass(OuterClass);Code:0:  aload_01:  aload_12:  putfield #10; //Field this$0:LOuterClass;5:  aload_06:  invokespecial    #12; //Method java/lang/Object."<init>":()V9:  return

继续失效

public class AnotherOuterClass {public static void main(String[] args) {InnerClass inner = new AnotherOuterClass().new InnerClass();System.out.println("InnerClass Filed = " + inner.x);}class InnerClass {private int x = 10;}}

和上面一样，使用Javap反编译一下

16:03 $ javap -c AnotherOuterClass$InnerClass
Compiled from "AnotherOuterClass.java"
class AnotherOuterClass$InnerClass extends java.lang.Object{
final AnotherOuterClass this$0;AnotherOuterClass$InnerClass(AnotherOuterClass);Code:0:  aload_01:  aload_12:  putfield #12; //Field this$0:LAnotherOuterClass;5:  aload_06:  invokespecial    #14; //Method java/lang/Object."<init>":()V9:  aload_010: bipush   1012: putfield #17; //Field x:I15: returnstatic int access$0(AnotherOuterClass$InnerClass);Code:0:  aload_01:  getfield #17; //Field x:I4:  ireturn}

编译器自动生成了一个access$0一次来获取x的值 AnotherOuterClass.class的反编译结果

16:08 $ javap -c AnotherOuterClass
Compiled from "AnotherOuterClass.java"
public class AnotherOuterClass extends java.lang.Object{
public AnotherOuterClass();Code:0:  aload_01:  invokespecial    #8; //Method java/lang/Object."<init>":()V4:  returnpublic static void main(java.lang.String[]);Code:0:  new  #16; //class AnotherOuterClass$InnerClass3:  dup4:  new  #1; //class AnotherOuterClass7:  dup8:  invokespecial    #18; //Method "<init>":()V11: dup12: invokevirtual    #19; //Method java/lang/Object.getClass:()Ljava/lang/Class;15: pop16: invokespecial    #23; //Method AnotherOuterClass$InnerClass."<init>":(LAnotherOuterClass;)V19: astore_120: getstatic    #26; //Field java/lang/System.out:Ljava/io/PrintStream;23: new  #32; //class java/lang/StringBuilder26: dup27: ldc  #34; //String InnerClass Filed =29: invokespecial    #36; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V32: aload_133: invokestatic #39; //Method AnotherOuterClass$InnerClass.access$0:(LAnotherOuterClass$InnerClass;)I36: invokevirtual    #43; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;39: invokevirtual    #47; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;42: invokevirtual    #51; //Method java/io/PrintStream.println:(Ljava/lang/String;)V45: return}

其中这句话，直接说明通过内部类的实例，获取到私有属性x的操作。

33:   invokestatic #39; //Method AnotherOuterClass$InnerClass.access$0:(LAnotherOuterClass$InnerClass;)I

在官网文档中是这样说道的，如果（内部类的）成员和构造方法设定成了私有修饰符，当且仅当其外部类访问时是允许的。

如何保证不被访问

使用的方法相当简单，使用匿名内部类的方法实现

public class PrivateToOuter {Runnable mRunnable = new Runnable(){private int x=10;@Overridepublic void run() {System.out.println(x);}};public static void main(String[] args){PrivateToOuter p = new PrivateToOuter();//System.out.println("anonymous class private filed= "+ p.mRunnable.x); //not allowedp.mRunnable.run(); // allowed}
}

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