如何看破真假美猴王 ？ –java中的Shadowing和Obscuring
- 2019 年 10 月 3 日
- 筆記
故事背景
《西遊記》第五十七回：唐僧因悟空又打死攔路強盜，再次把他攆走。六耳獼猴精趁機變作悟空模樣，搶走行李關文，又把小妖變作唐僧、八戒、沙僧模樣，欲上西天騙取真經。真假二悟空從天上殺到地下，菩薩、玉帝、地藏王等均不能辨認真假，直到雷音寺如來佛處，才被佛祖說出本相，獼猴精被悟空打死。
java之真假美猴王
java中有時候也會出現真假美猴王的事件，請看下面的程式後列印什麼？
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(); } }
我們期望程式列印：
Fido: Mmmmm, beef
Fido: Woof Woof
Fido: Zzzzzzz…
實際上報編譯錯誤。
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！
我不是要調用Thread的sleep方法，而是要調用Pet的sleep方法。為什麼出現這種情況呢？
JSL-6.4定義了這種情況：
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.
簡單的意思是：在作用域內，一個地方的聲明可能被另一個同名的聲明所遮蔽。在這種情況下不能簡單的使用名字來引用他們所聲明的實體。
變數Shadowing舉例：
class Test1 { public static void main(String[] args) { int i; for (int i = 0; i < 10; i++) System.out.println(i); } }
編譯報錯，但編譯檢測也不是萬能的，也有一些trick來逃避：
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，則不會出現Shadowing的問題：
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，那該怎麼解決呢？
問題解決
方式一：執行緒內調用，改成Pet.this.sleep();限定具體的方法：
public void live() { new Thread() { public void run() { while (true) { eat(); play(); Pet.this.sleep(); } } }.start(); }
方式二：
將sleep名稱改為其它不衝突的名稱，如petSleep，然後執行緒內調用該方法
public void petSleep() { System.out.println(name + ": Zzzzzzz..."); }
方式三：也是最好的方式，使用Thread(Runnable)構造器來替代對Thread 的繼承。那個匿名類不會再繼承Thread.sleep 方法，故也不會有衝突了。
public void live(){ new Thread(new Runnable(){ public void run(){ while(true){ eat(); play(); sleep(); } } }).start(); }
參考資料
【1】https://docs.oracle.com/javase/specs/jls/se12/html/jls-6.html#jls-6.4
【2】java解惑
