如何看破真假美猴王 ？ –java中的Shadowing和Obscuring
- 2019 年 10 月 3 日
- 笔记
故事背景
《西游记》第五十七回：唐僧因悟空又打死拦路强盗，再次把他撵走。六耳猕猴精趁机变作悟空模样，抢走行李关文，又把小妖变作唐僧、八戒、沙僧模样，欲上西天骗取真经。真假二悟空从天上杀到地下，菩萨、玉帝、地藏王等均不能辨认真假，直到雷音寺如来佛处，才被佛祖说出本相，猕猴精被悟空打死。
java之真假美猴王
java中有时候也会出现真假美猴王的事件，请看下面的程序后打印什么？
public class Pet { public final String name; public final String food; public final String sound; public Pet(String name, String food, String sound) { this.name = name; this.food = food; this.sound = sound; } public void eat() { System.out.println(name + ": Mmmmm, " + food); } public void play() { System.out.println(name + ": " + sound + " " + sound); } public void sleep() { System.out.println(name + ": Zzzzzzz..."); } public void live() { new Thread() { public void run() { while (true) { eat(); play(); sleep(); } } }.start(); } public static void main(String[] args) { new Pet("Fido", "beef", "Woof").live(); } }
我们期望程序打印：
Fido: Mmmmm, beef
Fido: Woof Woof
Fido: Zzzzzzz…
实际上报编译错误。
The method sleep(long) in the type Thread is not applicable for the arguments ()
查看Thread的sleep方法：
/** * Causes the currently executing thread to sleep (temporarily cease * execution) for the specified number of milliseconds, subject to * the precision and accuracy of system timers and schedulers. The thread * does not lose ownership of any monitors. * * @param millis * the length of time to sleep in milliseconds * * @throws IllegalArgumentException * if the value of {@code millis} is negative * * @throws InterruptedException * if any thread has interrupted the current thread. The * <i>interrupted status</i> of the current thread is * cleared when this exception is thrown. */ public static native void sleep(long millis) throws InterruptedException; /** * Causes the currently executing thread to sleep (temporarily cease * execution) for the specified number of milliseconds plus the specified * number of nanoseconds, subject to the precision and accuracy of system * timers and schedulers. The thread does not lose ownership of any * monitors. * * @param millis * the length of time to sleep in milliseconds * * @param nanos * {@code 0-999999} additional nanoseconds to sleep * * @throws IllegalArgumentException * if the value of {@code millis} is negative, or the value of * {@code nanos} is not in the range {@code 0-999999} * * @throws InterruptedException * if any thread has interrupted the current thread. The * <i>interrupted status</i> of the current thread is * cleared when this exception is thrown. */ public static void sleep(long millis, int nanos) throws InterruptedException { if (millis < 0) { throw new IllegalArgumentException("timeout value is negative"); } if (nanos < 0 || nanos > 999999) { throw new IllegalArgumentException( "nanosecond timeout value out of range"); } if (nanos >= 500000 || (nanos != 0 && millis == 0)) { millis++; } sleep(millis); }
等等！
我不是要调用Thread的sleep方法，而是要调用Pet的sleep方法。为什么出现这种情况呢？
JSL-6.4定义了这种情况：
It is a compile-time error if the name of a formal parameter is used to declare a new variable within the body of the method, constructor, or lambda expression, unless the new variable is declared within a class declaration contained by the method, constructor, or lambda expression. It is a compile-time error if the name of a local variable v is used to declare a new variable within the scope of v, unless the new variable is declared within a class whose declaration is within the scope of v. It is a compile-time error if the name of an exception parameter is used to declare a new variable within the Block of the catch clause, unless the new variable is declared within a class declaration contained by the Block of the catch clause. It is a compile-time error if the name of a local class C is used to declare a new local class within the scope of C, unless the new local class is declared within another class whose declaration is within the scope of C.
java中有Shadowing(遮蔽)的描述，其中：
Shadowing：Some declarations may be shadowed in part of their scope by another declaration of the same name, in which case a simple name cannot be used to refer to the declared entity.
简单的意思是：在作用域内，一个地方的声明可能被另一个同名的声明所遮蔽。在这种情况下不能简单的使用名字来引用他们所声明的实体。
变量Shadowing举例：
class Test1 { public static void main(String[] args) { int i; for (int i = 0; i < 10; i++) System.out.println(i); } }
编译报错，但编译检测也不是万能的，也有一些trick来逃避：
class Test2 { public static void main(String[] args) { int i; class Local { { for (int i = 0; i < 10; i++) System.out.println(i); } } new Local(); } }
如果在不同block，则不会出现Shadowing的问题：
class Test3 { public static void main(String[] args) { for (int i = 0; i < 10; i++) System.out.print(i + " "); for (int i = 10; i > 0; i--) System.out.print(i + " "); System.out.println(); } }
原因找到了，那该怎么解决呢？
问题解决
方式一：线程内调用，改成Pet.this.sleep();限定具体的方法：
public void live() { new Thread() { public void run() { while (true) { eat(); play(); Pet.this.sleep(); } } }.start(); }
方式二：
将sleep名称改为其它不冲突的名称，如petSleep，然后线程内调用该方法
public void petSleep() { System.out.println(name + ": Zzzzzzz..."); }
方式三：也是最好的方式，使用Thread(Runnable)构造器来替代对Thread 的继承。那个匿名类不会再继承Thread.sleep 方法，故也不会有冲突了。
public void live(){ new Thread(new Runnable(){ public void run(){ while(true){ eat(); play(); sleep(); } } }).start(); }
参考资料
【1】https://docs.oracle.com/javase/specs/jls/se12/html/jls-6.html#jls-6.4
【2】java解惑
