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