本篇將會說到失效的private修飾符
前言
在Java編程裏，使用private關鍵字修飾了一個成員，只有成員內部可以訪問，其餘成員都不可訪問，今天說明一下private功能失效的問題。
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失效之Java內部類
在一個內部類裏訪問外部類的private成員變量或者方法。
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 OuterClass$InnerClass
11: dup
12: aload_1
13: dup
14: invokevirtual #30; //Method java/lang/Object.getClass:()Ljava/lang/Class;
17: pop
18: invokespecial #34; //Method OuterClass$InnerClass."<init>":(LOuterClass;)V
21: astore_2
22: aload_2
23: invokevirtual #37; //Method OuterClass$InnerClass.printOuterClassPrivateFields:()V
26: return
static java.lang.String access$0(OuterClass);
Code:
0: aload_0
1: getfield #15; //Field language:Ljava/lang/String;
4: areturn
static java.lang.String access$1(OuterClass);
Code:
0: aload_0
1: getfield #19; //Field region:Ljava/lang/String;
4: areturn
}
在這裏有一個OuterClass方法，
static java.lang.String access$0(OuterClass);
Code:
0: aload_0
1: getfield #15; //Field language:Ljava/lang/String;
4: areturn
static java.lang.String access$1(OuterClass);
Code:
0: aload_0
1: getfield #19; //Field region:Ljava/lang/String;
4: areturn
}
根據註釋，可以知道access1返回outerClass的region屬性，並且這兩個方法都接受OuterClass的實例作爲參數， 對這兩個方法進行反編譯。
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_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
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 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_0
25: 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_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
}
下面代碼調用access$0的代碼,其目的是得到OuterClass的language 私有屬性。
13: invokestatic #27; //Method OuterClass.access$0:(LOuterClass;)Ljava/lang/String;
下面代碼調用了access$1的代碼，其目的是得到OutherClass的region 私有屬性。
28: invokestatic #39; //Method OuterClass.access$1:(LOuterClass;)Ljava/lang/String;
即，在內部類構造的時候，會有外部類的引用傳遞進來，並且作爲內部類的一個屬性，所以內部類會持有一個其外部類的應用。this$0就是內部類持有的外部類引用，通過構造方法傳遞引用並賦值。
final OuterClass this$0;
public OuterClass$InnerClass(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
繼續失效
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;
}
}
和上面一樣，使用Javap反編譯一下
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_0
1: aload_1
2: putfield #12; //Field this$0: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 access$0(AnotherOuterClass$InnerClass);
Code:
0: aload_0
1: getfield #17; //Field x:I
4: ireturn
}
編譯器自動生成了一個access$0一次來獲取x的值 AnotherOuterClass.class的反編譯結果
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 AnotherOuterClass$InnerClass
3: dup
4: new #1; //class AnotherOuterClass
7: dup
8: invokespecial #18; //Method "<init>":()V
11: dup
12: invokevirtual #19; //Method java/lang/Object.getClass:()Ljava/lang/Class;
15: pop
16: invokespecial #23; //Method AnotherOuterClass$InnerClass."<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 AnotherOuterClass$InnerClass.access$0:(LAnotherOuterClass$InnerClass;)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
}
其中這句話，直接說明通過內部類的實例，獲取到私有屬性x的操作。
33: invokestatic #39; //Method AnotherOuterClass$InnerClass.access$0:(LAnotherOuterClass$InnerClass;)I
在官網文檔中是這樣說道的，如果（內部類的）成員和構造方法設定成了私有修飾符，當且僅當其外部類訪問時是允許的。
如何保證不被訪問
使用的方法相當簡單，使用匿名內部類的方法實現
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
}
}