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(); } }
本文转自博客园Grandyang的博客，原文链接：信号旗[CareerCup] 16.5 Semphore ，如需转载请自行联系原博主。
