Java private修饰符失效的原因

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 OuterClassCompiled from 'OuterClass.java'public class OuterClass extends java.lang.Object{public OuterClass(); Code: 0: aload_0 1: invokespecial #11; //Method java/lang/Object.'<init>':()V 4: aload_0 5: ldc #13; //String en 7: putfield #15; //Field language:Ljava/lang/String; 10: aload_0 11: ldc #17; //String US 13: putfield #19; //Field region:Ljava/lang/String; 16: return public static void main(java.lang.String[]); Code: 0: new #1; //class OuterClass 3: dup 4: invokespecial #27; //Method '<init>':()V 7: astore_1 8: new #28; //class OuterClassInnerClass 11: dup 12: aload_1 13: dup 14: invokevirtual #30; //Method java/lang/Object.getClass:()Ljava/lang/Class; 17: pop 18: invokespecial #34; //Method OuterClassInnerClass.'<init>':(LOuterClass;)V 21: astore_2 22: aload_2 23: invokevirtual #37; //Method OuterClassInnerClass.printOuterClassPrivateFields:()V 26: return static java.lang.String access0(OuterClass); Code: 0: aload_0 1: getfield #15; //Field language:Ljava/lang/String; 4: areturn static java.lang.String access1(OuterClass); Code: 0: aload_0 1: getfield #19; //Field region:Ljava/lang/String; 4: areturn }

在这里有一个OuterClass方法，

static java.lang.String access0(OuterClass); Code: 0: aload_0 1: getfield #15; //Field language:Ljava/lang/String; 4: areturn static java.lang.String access1(OuterClass); Code: 0: aload_0 1: getfield #19; //Field region:Ljava/lang/String; 4: areturn }

根据注释，可以知道access0返回outerClass的language属性，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 OuterClassInnerClassCompiled from 'OuterClass.java'public class OuterClassInnerClass extends java.lang.Object{final OuterClass this0; public OuterClassInnerClass(OuterClass); Code: 0: aload_0 1: aload_1 2: putfield #10; //Field this0:LOuterClass; 5: aload_0 6: invokespecial #12; //Method java/lang/Object.'<init>':()V 9: return public void printOuterClassPrivateFields(); Code: 0: new #20; //class java/lang/StringBuilder 3: dup 4: ldc #22; //String language= 6: invokespecial #24; //Method java/lang/StringBuilder.'<init>':(Ljava/lang/String;)V 9: aload_0 10: getfield #10; //Field this0:LOuterClass; 13: invokestatic #27; //Method OuterClass.access0:(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_0 25: getfield #10; //Field this0:LOuterClass; 28: invokestatic #39; //Method OuterClass.access1:(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_1 38: getstatic #46; //Field java/lang/System.out:Ljava/io/PrintStream; 41: aload_1 42: invokevirtual #52; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 45: 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 this0; public OuterClassInnerClass(OuterClass); Code: 0: aload_0 1: aload_1 2: putfield #10; //Field this$0:LOuterClass; 5: aload_0 6: invokespecial #12; //Method java/lang/Object.'<init>':()V 9: 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 AnotherOuterClassInnerClassCompiled from 'AnotherOuterClass.java'class AnotherOuterClassInnerClass extends java.lang.Object{final AnotherOuterClass this0; AnotherOuterClassInnerClass(AnotherOuterClass); Code: 0: aload_0 1: aload_1 2: putfield #12; //Field this0:LAnotherOuterClass; 5: aload_0 6: invokespecial #14; //Method java/lang/Object.'<init>':()V 9: aload_0 10: bipush 10 12: putfield #17; //Field x:I 15: return static int access0(AnotherOuterClassInnerClass); Code: 0: aload_0 1: getfield #17; //Field x:I 4: 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 AnotherOuterClassCompiled from 'AnotherOuterClass.java'public class AnotherOuterClass extends java.lang.Object{public AnotherOuterClass(); Code: 0: aload_0 1: invokespecial #8; //Method java/lang/Object.'<init>':()V 4: return public static void main(java.lang.String[]); Code: 0: new #16; //class AnotherOuterClassInnerClass 3: dup 4: new #1; //class AnotherOuterClass 7: dup 8: invokespecial #18; //Method '<init>':()V 11: dup 12: invokevirtual #19; //Method java/lang/Object.getClass:()Ljava/lang/Class; 15: pop 16: invokespecial #23; //Method AnotherOuterClassInnerClass.'<init>':(LAnotherOuterClass;)V 19: astore_1 20: getstatic #26; //Field java/lang/System.out:Ljava/io/PrintStream; 23: new #32; //class java/lang/StringBuilder 26: dup 27: ldc #34; //String InnerClass Filed = 29: invokespecial #36; //Method java/lang/StringBuilder.'<init>':(Ljava/lang/String;)V 32: aload_1 33: invokestatic #39; //Method AnotherOuterClassInnerClass.access0:(LAnotherOuterClassInnerClass;)I 36: 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;)V 45: 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。

invokestatic #39; //Method AnotherOuterClassInnerClass.access0:(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; @Override public 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 allowed p.mRunnable.run(); // allowed }}

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