用于解决多线程安全问题的三种方式

用于解决多线程安全问题的方式：
1. 同步代码块 (隐式锁)
2. 同步方法(隐式锁)
3. 同步锁 Lock( jdk 1.5 后)
 注意：是一个显示锁，需要通过 lock() 方法上锁，必须通过 unlock() 方法进行释放锁

下面举一个售票例子：

1）同步代码块

package com.lxj.juc;

public class TestSync {

	public static void main(String[] args) {
		Ticket ticket = new Ticket();
		new Thread(ticket).start();
		new Thread(ticket).start();
		new Thread(ticket).start();
	}
	
}
class Ticket implements Runnable{

	private int ticket = 100;
	
	@Override
	public void run() {
		while(true) {
			synchronized(this) {  //this代表对当前对象上锁
				if(ticket > 0) {
					try {
						Thread.sleep(200);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println(Thread.currentThread().getName()+" : 购票成功，余票为：" +  --ticket );
				}else {
					break;
				}
			}
		}
	}
	
}

运行结果:
Thread-0 : 购票成功，余票为：99
Thread-0 : 购票成功，余票为：98
Thread-2 : 购票成功，余票为：97
.....
Thread-2 : 购票成功，余票为：7
Thread-0 : 购票成功，余票为：6
Thread-0 : 购票成功，余票为：5
Thread-1 : 购票成功，余票为：4
Thread-1 : 购票成功，余票为：3
Thread-0 : 购票成功，余票为：2
Thread-0 : 购票成功，余票为：1
Thread-2 : 购票成功，余票为：0

2）同步方法

package com.lxj.juc;

public class TestSync {

	public static void main(String[] args) {
		Ticket ticket = new Ticket();
		new Thread(ticket).start();
		new Thread(ticket).start();
		new Thread(ticket).start();
	}

}

class Ticket implements Runnable {

	private int ticket = 100;

	@Override
	public void run() {
		while (true) {
			if (ticket > 0) {
				int i = purchase();
				if(i == 0) {
					break;
				}
			}else {
				break;
			}
		}
	}

	private synchronized int purchase() {
		if (ticket > 0) {
			try {
				Thread.sleep(200);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println(Thread.currentThread().getName() + " : 购票成功，余票为：" + --ticket);
		}else {
			return 0;
		}
		return 1;
	}

}

运行结果：
Thread-0 : 购票成功，余票为：99
Thread-0 : 购票成功，余票为：98
Thread-0 : 购票成功，余票为：97
Thread-2 : 购票成功，余票为：96
Thread-1 : 购票成功，余票为：95
....
Thread-1 : 购票成功，余票为：8
Thread-2 : 购票成功，余票为：7
Thread-2 : 购票成功，余票为：6
Thread-0 : 购票成功，余票为：5
Thread-0 : 购票成功，余票为：4
Thread-0 : 购票成功，余票为：3
Thread-2 : 购票成功，余票为：2
Thread-1 : 购票成功，余票为：1
Thread-2 : 购票成功，余票为：0

3）同步锁lock

package com.lxj.juc;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class TestSync {

	public static void main(String[] args) {
		Ticket ticket = new Ticket();
		new Thread(ticket).start();
		new Thread(ticket).start();
		new Thread(ticket).start();
	}

}

class Ticket implements Runnable {

	private int ticket = 100;

    private Lock lock = new ReentrantLock();
	
	@Override
	public void run() {
		while (true) {
			lock.lock(); //上锁
			try {
				if (ticket > 0) {
					try {
						Thread.sleep(200);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println(Thread.currentThread().getName() + " : 购票成功，余票为：" + --ticket);
				}else {
					break;
				}
			} finally {
				lock.unlock(); //解锁
			}
		}
	}


}

运行结果：
Thread-0 : 购票成功，余票为：99
Thread-1 : 购票成功，余票为：98
Thread-1 : 购票成功，余票为：97
Thread-1 : 购票成功，余票为：96
Thread-1 : 购票成功，余票为：95
Thread-2 : 购票成功，余票为：94
......
Thread-0 : 购票成功，余票为：6
Thread-0 : 购票成功，余票为：5
Thread-1 : 购票成功，余票为：4
Thread-2 : 购票成功，余票为：3
Thread-2 : 购票成功，余票为：2
Thread-2 : 购票成功，余票为：1
Thread-2 : 购票成功，余票为：0

注意使用同步锁一定要记得关闭锁，放在try{}finally{}中。