[CareerCup] 16.5 Semphore 信号旗

 

16.5 Suppose we have the following code:
public class Foo {
　　public Foo() { . . . }
　　public void first() { ... }
　　public void second() { ... }
　　public void thirdQ { ... }
}
The same instance of Foo will be passed to three different threads. ThreadA will call first, threads will call second, and threadC will call third. Design a mechanism to ensure that first is called before second and second is called before third.

 

按照题目中要求的顺序即需满足在执行second()前检测first()是否完成了，在执行third()前检测second()是否完成了，由于我们需要考虑线程安全，所以布尔型的标记不行，那么使用锁呢，看如下代码：

 

import java.util.concurrent.locks.ReentrantLock;public class FooBad {public int pauseTime = 10000;public ReentrantLock lock1;public ReentrantLock lock2;public FooBad() {try {lock1 = new ReentrantLock();lock2 = new ReentrantLock();lock1.lock();lock2.lock();} catch (Exception ex) {ex.printStackTrace();}}public void first() {try {System.out.println("Started Executing 1");Thread.sleep(pauseTime);System.out.println("Finished Executing 1");lock1.unlock();} catch (Exception ex) {ex.printStackTrace();}}public void second () {try {lock1.lock();lock1.unlock();System.out.println("Started Executing 2");Thread.sleep(pauseTime);System.out.println("Finished Executing 2");lock2.unlock();} catch (Exception ex) {ex.printStackTrace();}}public void third() {try {lock2.lock();lock2.unlock();System.out.println("Started Executing 3");Thread.sleep(pauseTime);System.out.println("Finished Executing 3");} catch (Exception ex) {ex.printStackTrace();}} }public class MyThread extends Thread {private String method;private FooBad foo;public MyThread(FooBad foo, String method) {this.method = method;this.foo = foo;}public void run() {if (method == "first") {foo.first();} else if (method == "second") {foo.second();} else if (method == "third") {foo.third();}} }public class j {public static void main(String[] args) {FooBad foo = new FooBad();MyThread thread1 = new MyThread(foo, "first");MyThread thread2 = new MyThread(foo, "second");MyThread thread3 = new MyThread(foo, "third");thread3.start();thread2.start();thread1.start();} }

 

上述代码并不能很好的完成题目中要求的顺序，因为锁的所有权的问题。一个线程操作一个锁，当别的线程是无法解这个线程的锁的，所以用锁是不行的。我们可以使用信号旗Semaphores，参见代码如下：

 

import java.util.concurrent.Semaphore; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReentrantLock;public class Foo {public int pauseTime = 1000;public Semaphore sem1;public Semaphore sem2;public Foo() {try {sem1 = new Semaphore(1);sem2 = new Semaphore(1);sem1.acquire();sem2.acquire();} catch (InterruptedException e) {e.printStackTrace();}}public void first() {try {System.out.println("Started Executing 1");Thread.sleep(pauseTime);System.out.println("Finished Executing");sem1.release();} catch (Exception ex) {ex.printStackTrace();}}public void second() {try {sem1.acquire();sem1.release();System.out.println("Started Executing 2");Thread.sleep(pauseTime);System.out.println("Finished Executing 2");sem2.release();} catch (Exception ex) {ex.printStackTrace();}}public void third() {try {sem2.acquire();sem2.release();System.out.println("Started Executing 3");Thread.sleep(pauseTime);System.out.println("Finished Executing 3");} catch (Exception ex) {ex.printStackTrace();}} }public class MyThread extends Thread {private String method;private Foo foo;public MyThread(Foo foo, String method) {this.method = method;this.foo = foo;}public void run() {if (method == "first") {foo.first();} else if (method == "second") {foo.second();} else if (method == "third") {foo.third();}} }public class j {public static void main(String[] args) {Foo foo = new Foo();MyThread thread1 = new MyThread(foo, "first");MyThread thread2 = new MyThread(foo, "second");MyThread thread3 = new MyThread(foo, "third");thread3.start();thread2.start();thread1.start();} }

 

CareerCup All in One 题目汇总

 

转载于:https://www.cnblogs.com/grandyang/p/5400632.html

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