基于Java信号量解决死锁过程解析

package lockTest;
import java.util.Date;
/**
 * 崔素强
 * @author cuisuqiang@163.com
 */
public class LockTest {
	public static String obj1 = "obj1";
	public static String obj2 = "obj2";
	public static void main(String[] args) {
		LockA la = new LockA();
		new Thread(la).start();
		LockB lb = new LockB();
		new Thread(lb).start();
	}
}
class LockA implements Runnable{
	public void run() {
		try {
			System.out.println(new Date().toString() + " LockA 开始执行");
			while(true){
				synchronized (LockTest.obj1) {
					System.out.println(new Date().toString() + " LockA 锁住 obj1");
					Thread.sleep(3000); // 此处等待是给B能锁住机会
					synchronized (LockTest.obj2) {
						System.out.println(new Date().toString() + " LockA 锁住 obj2");
						Thread.sleep(60 * 1000); // 为测试，占用了就不放
					}
				}
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}
class LockB implements Runnable{
	public void run() {
		try {
			System.out.println(new Date().toString() + " LockB 开始执行");
			while(true){
				synchronized (LockTest.obj2) {
					System.out.println(new Date().toString() + " LockB 锁住 obj2");
					Thread.sleep(3000); // 此处等待是给A能锁住机会
					synchronized (LockTest.obj1) {
						System.out.println(new Date().toString() + " LockB 锁住 obj1");
						Thread.sleep(60 * 1000); // 为测试，占用了就不放
					}
				}
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

package lockTest;
import java.util.Date;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
/**
 * 崔素强
 * @author cuisuqiang@163.com
 */
public class UnLockTest {
	public static String obj1 = "obj1";
	public static final Semaphore a1 = new Semaphore(1);
	public static String obj2 = "obj2";
	public static final Semaphore a2 = new Semaphore(1);

	public static void main(String[] args) {
		LockAa la = new LockAa();
		new Thread(la).start();
		LockBb lb = new LockBb();
		new Thread(lb).start();
	}
}
class LockAa implements Runnable {
	public void run() {
		try {
			System.out.println(new Date().toString() + " LockA 开始执行");
			while (true) {
				if (UnLockTest.a1.tryAcquire(1, TimeUnit.SECONDS)) {
					System.out.println(new Date().toString() + " LockA 锁住 obj1");
					if (UnLockTest.a2.tryAcquire(1, TimeUnit.SECONDS)) {
						System.out.println(new Date().toString() + " LockA 锁住 obj2");
						Thread.sleep(60 * 1000); // do something
					}else{
						System.out.println(new Date().toString() + "LockA 锁 obj2 失败");
					}
				}else{
					System.out.println(new Date().toString() + "LockA 锁 obj1 失败");
				}
				UnLockTest.a1.release(); // 释放
				UnLockTest.a2.release();
				Thread.sleep(1000); // 马上进行尝试，现实情况下do something是不确定的
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}
class LockBb implements Runnable {
	public void run() {
		try {
			System.out.println(new Date().toString() + " LockB 开始执行");
			while (true) {
				if (UnLockTest.a2.tryAcquire(1, TimeUnit.SECONDS)) {
					System.out.println(new Date().toString() + " LockB 锁住 obj2");
					if (UnLockTest.a1.tryAcquire(1, TimeUnit.SECONDS)) {
						System.out.println(new Date().toString() + " LockB 锁住 obj1");
						Thread.sleep(60 * 1000); // do something
					}else{
						System.out.println(new Date().toString() + "LockB 锁 obj1 失败");
					}
				}else{
					System.out.println(new Date().toString() + "LockB 锁 obj2 失败");
				}
				UnLockTest.a1.release(); // 释放
				UnLockTest.a2.release();
				Thread.sleep(10 * 1000); // 这里只是为了演示，所以tryAcquire只用1秒，而且B要给A让出能执行的时间，否则两个永远是死锁
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}