The private modifier in Java is invalid
在Java编程中，使用private关键字修饰了某个成员，只有这个成员所在的类和这个类的方法可以使用，其他的类都无法访问到这个private成员。
上面描述了private修饰符的基本职能，今天来研究一下private功能失效的情况。
Java内部类
在Java中相信很多人都用过内部类，Java允许在一个类里面定义另一个类，类里面的类就是内部类，也叫做嵌套类。一个简单的内部类实现可以如下
class OuterClass {
class InnerClass{
}
}今天的问题和Java内部类相关，只涉及到部分和本文研究相关的内部类知识，具体关于Java内部类后续的文章会介绍。
第一次失效?
一个我们在编程中经常用到的场景，就是在一个内部类里面访问外部类的private成员变量或者方法，这是可以的。如下面的代码实现。
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();
}
}这是为什么呢，不是private修饰的成员只能被成员所述的类才能访问么？难道private真的失效了么？
编译器在捣鬼？
我们使用javap命令查看一下生成的两个class文件
OuterClass的反编译结果
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
}
咦？不对，在OuterClass中我们并没有定义这两个方法
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
}从给出来的注释来看，access$0返回outerClass的language属性；access$1返回outerClass的region属性。并且这两个方法都接受OuterClass的实例作为参数。这两个方法为什么生成呢，有什么作用呢？我们看一下内部类的反编译结果就知道了。
OuterClass$InnerClass的反编译结果
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;
注意：在内部类构造的时候，会将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引用。
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_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
小结
这部分private看上去失效可，实际上并没有失效，因为当内部类调用外部类的私有属性时，其真正的执行是调用了编译器生成的属性的静态方法（即acess$0,access$1等）来获取这些属性值。这一切都是编译器的特殊处理。
这次也失效？
如果说上面的写法很常用，那么这样的写法是不是很少接触，但是却可以运行。
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反编译看一下。不过这次我们先看一下InnerClass的结果
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
}又出现了，编译器又自动生成了一个获取私有属性的后门方法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 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
}其中这句调用就是外部类通过内部类的实例获取私有属性x的操作
33: invokestatic #39; //Method AnotherOuterClass$InnerClass.access$0:(LAnotherOuterClass$InnerClass;)I
再来个总结
其中java官方文档 有这样一句话
if the member or constructor is declared private, then access is permitted if and only if it occurs within the body of the top level class (§7.6) that encloses the declaration of the member or constructor.
意思是 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。
如何让内部类私有成员不被外部访问
相信看完上面两部分，你会觉得，内部类的私有成员想不被外部类访问都很困难吧，谁让编译器“爱管闲事”呢，其实也是可以做到的。那就是使用匿名内部类。
由于mRunnable对象的类型为Runnable，而不是匿名内部类的类型（我们无法正常拿到），而Runanble中没有x这个属性，所以mRunnable.x是不被允许的。
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
}
}最后总结
在本文中，private表面上看上去失效了，但实际上是没有的，而是在调用时通过间接的方法来获取私有的属性。
Java的内部类构造时持有对外部类的应用，C++不会，这一点和C++不一样。
更多Java中的private修饰符失效了相关文章请关注PHP中文网！
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
Is the company's security software causing the application to fail to run? How to troubleshoot and solve it?
Apr 19, 2025 pm 04:51 PM
Troubleshooting and solutions to the company's security software that causes some applications to not function properly. Many companies will deploy security software in order to ensure internal network security. ...
How to simplify field mapping issues in system docking using MapStruct?
Apr 19, 2025 pm 06:21 PM
Field mapping processing in system docking often encounters a difficult problem when performing system docking: how to effectively map the interface fields of system A...
How to elegantly obtain entity class variable names to build database query conditions?
Apr 19, 2025 pm 11:42 PM
When using MyBatis-Plus or other ORM frameworks for database operations, it is often necessary to construct query conditions based on the attribute name of the entity class. If you manually every time...
How do I convert names to numbers to implement sorting and maintain consistency in groups?
Apr 19, 2025 pm 11:30 PM
Solutions to convert names to numbers to implement sorting In many application scenarios, users may need to sort in groups, especially in one...
How does IntelliJ IDEA identify the port number of a Spring Boot project without outputting a log?
Apr 19, 2025 pm 11:45 PM
Start Spring using IntelliJIDEAUltimate version...
How to safely convert Java objects to arrays?
Apr 19, 2025 pm 11:33 PM
Conversion of Java Objects and Arrays: In-depth discussion of the risks and correct methods of cast type conversion Many Java beginners will encounter the conversion of an object into an array...
E-commerce platform SKU and SPU database design: How to take into account both user-defined attributes and attributeless products?
Apr 19, 2025 pm 11:27 PM
Detailed explanation of the design of SKU and SPU tables on e-commerce platforms This article will discuss the database design issues of SKU and SPU in e-commerce platforms, especially how to deal with user-defined sales...
How to elegantly get entity class variable name building query conditions when using TKMyBatis for database query?
Apr 19, 2025 pm 09:51 PM
When using TKMyBatis for database queries, how to gracefully get entity class variable names to build query conditions is a common problem. This article will pin...
