《Cracking the Coding Interview》——第16章：线程与锁——题目5

2014-04-27 20:16

题目：假设一个类Foo有三个公有的成员方法first()、second()、third()。请用锁的方法来控制调用行为，使得他们的执行循序总是遵从first、second、third的顺序。

解法：你应该想到了用lock的方法类阻塞，不过这里面有个概念问题使得直接用ReentrantLock不能通过编译（对于一个锁对象，不同在A线程中锁定，又在B线程中解锁，不允许这样的归属关系），可以用Semaphore来达到相同的目的。请看下面的代码。

代码：

// 16.5 There're three methods in a class FooBar, how would you make sure that they're executed in a fixed order, in whichever order they're called?
public class FirstRun implements Runnable {
private FooBar fooBar;

public FirstRun(FooBar fooBar) {
// TODO Auto-generated constructor stub
this.fooBar = fooBar;
}

@Override
public void run() {
// TODO Auto-generated method stub
fooBar.first();
}
}

// -----------------------------------------------------------------------------
public class SecondRun implements Runnable {
private FooBar fooBar;

public SecondRun(FooBar fooBar) {
// TODO Auto-generated constructor stub
this.fooBar = fooBar;
}

@Override
public void run() {
// TODO Auto-generated method stub
fooBar.second();
}
}

// -----------------------------------------------------------------------------
public class ThirdRun implements Runnable {
private FooBar fooBar;

public ThirdRun(FooBar fooBar) {
// TODO Auto-generated constructor stub
this.fooBar = fooBar;
}

@Override
public void run() {
// TODO Auto-generated method stub
fooBar.third();
}
}

// -----------------------------------------------------------------------------
import java.util.concurrent.Semaphore;

public class FooBar {
private Semaphore sem1;
private Semaphore sem2;
private Semaphore sem3;

public FooBar() {
// TODO Auto-generated constructor stub
sem1 = new Semaphore(1);
sem2 = new Semaphore(1);
sem3 = new Semaphore(1);

try {
sem1.acquire();
sem2.acquire();
sem3.acquire();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

public void first() {
System.out.println("first");

sem1.release();
}

public void second() {
try {
sem1.acquire();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
sem1.release();
System.out.println("second");
sem2.release();
}

public void third() {
try {
sem2.acquire();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
sem2.release();
System.out.println("third");
sem3.release();
}

public static void main(String[] args) {
FooBar fooBar = new FooBar();
Thread t1 = new Thread(new FirstRun(fooBar));
Thread t2 = new Thread(new SecondRun(fooBar));
Thread t3 = new Thread(new ThirdRun(fooBar));

t3.start();
t1.start();
t2.start();
}
}