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2020
10-24

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 OuterClass
Compiled 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 OuterClassInnerClass
Compiled 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 AnotherOuterClassInnerClass
Compiled 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 AnotherOuterClass
Compiled 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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