同步与死锁的问题

比如一个卖票程序，如果多线程同时操作时，就有可能出现卖出票为负数的问题。

例子：用Runnable接口实现多线程，并产生3个线程对象。同时卖五张票

package test2;

class MyThread11 implements Runnable {
	private int ticket = 5;

	public void run() {
		for (int i = 0; i < 100; i++) {
			if (ticket > 0) {
				try {
					Thread.sleep(300);
				} catch (InterruptedException e) {
					// TODO: handle exception
					e.printStackTrace();
				}
				System.out.println("卖票：ticket=" + ticket--);
			}
		}
	}
}

public class SyncDemo01 {
	public static void main(String[] args) {
		MyThread11 mt = new MyThread11();
		Thread t1 = new Thread(mt);
		Thread t2 = new Thread(mt);
		Thread t3 = new Thread(mt);
		t1.start();
		t2.start();
		t3.start();
	}
}

　　结果：

卖票：ticket=5
卖票：ticket=4
卖票：ticket=3
卖票：ticket=2
卖票：ticket=1
卖票：ticket=0
卖票：ticket=-1

 

出现了-1，所以就引出同步代码块

【同步代码块】

synchronized（同步对象）{需要同步的代码}

范例：

package test2;

class MyThread11 implements Runnable {
	private int ticket = 5;

	public void run() {
		for (int i = 0; i < 100; i++) {
			synchronized (this) {
				if (ticket > 0) {
					try {
						Thread.sleep(3000);
					} catch (InterruptedException e) {
						// TODO: handle exception
						e.printStackTrace();
					}
					System.out.println("卖票：ticket=" + ticket--);
				}
			}

		}
	}
}

public class SyncDemo01 {
	public static void main(String[] args) {
		MyThread11 mt = new MyThread11();
		Thread t1 = new Thread(mt);
		Thread t2 = new Thread(mt);
		Thread t3 = new Thread(mt);
		t1.start();
		t2.start();
		t3.start();
	}
}

　　结果：

卖票：ticket=5
卖票：ticket=4
卖票：ticket=3
卖票：ticket=2
卖票：ticket=1

这样就可以避免买票出现负数的问题！！！

还有一个方法是同步方法：

【同步方法】

synchronized  方法返回值  方法名称(参数列表)｛｝

package test2;

class MyThread11 implements Runnable {
	private int ticket = 5;

	public void run() {
		for (int i = 0; i < 100; i++) {

			this.sale1();

		}
	}

	public synchronized void sale1() {
		if (ticket > 0) {
			try {
				Thread.sleep(3000);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			System.out.println("卖票：ticket=" + ticket--);
		}
	}

}

public class SyncDemo01 {
	public static void main(String[] args) {
		MyThread11 mt = new MyThread11();
		Thread t1 = new Thread(mt);
		Thread t2 = new Thread(mt);
		Thread t3 = new Thread(mt);
		t1.start();
		t2.start();
		t3.start();
	}
}

　　

　　结果：

卖票：ticket=5
卖票：ticket=4
卖票：ticket=3
卖票：ticket=2
卖票：ticket=1