Java修炼——线程同步的俩种方法

当多线程去同时抢占CPU资源时，有多线程的安全问题。这时候就需要将线程同步。线程同步有俩个方法。

1.同步代码块（synchronize），同步代码块需要同步监视器，同步监视器是针对对象进行操作。什么对象时共享的，就可以给他加上同步监视器。

package com.bjsxt.ticker;
public class Ticket implements Runnable {
	private  int ticket=5;
	@Override
	public void run() {
		for (int i = 0; i <100; i++) {//假设100个人再买票
			synchronized (this) {
				if (ticket > 0) {
					System.out.println(Thread.currentThread().getName()+"正在卖第"+(ticket--)+"票");
				}
			}
			try {
				Thread.sleep(30);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
	}
}

package com.bjsxt.ticker;
public class TestTicket01 {
	public static void main(String[] args) {
		Ticket t=new Ticket();
		Thread t1=new Thread(t,"A窗口");
		Thread t2=new Thread(t,"B窗口");
		Thread t3=new Thread(t,"C窗口");
		//启动线程
		t1.start();
		t2.start();
		t3.start();
	}
}

打印输出结果：

2.第二种就是同步方法：

package com.bjsxt.ticker;
public class Ticket1 implements Runnable {
	private  int ticket=5;
	@Override
	public void run() {
		for (int i = 0; i <100; i++) {//假设100个人再买票
				this.saleTicket();
				try {
					Thread.sleep(30);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
		}
	}
	private synchronized void  saleTicket() {//同步方法不需要指定同步监视器，同步监视器只能是当前对象this
		if (ticket > 0) {
			System.out.println(Thread.currentThread().getName()+"正在卖第"+(ticket--)+"票");
		}
	}
}

package com.bjsxt.ticker;
public class TestTicket01 {
	public static void main(String[] args) {
		Ticket1 t=new Ticket1();
		Thread t1=new Thread(t,"A窗口");
		Thread t2=new Thread(t,"B窗口");
		Thread t3=new Thread(t,"C窗口");
		//启动线程
		t1.start();
		t2.start();
		t3.start();
	}
}

运行结果：

进行对比，我们将没有用线程同步的程序拿过来看看：

package com.bjsxt.ticker;
public class Ticket2 implements Runnable {
	private  int ticket=5;
	@Override
	public void run() {
		for (int i = 0; i <100; i++) {//假设100个人再买票
				if (ticket > 0) {
					try {
						Thread.sleep(300);
					} catch (InterruptedException e) {
						// TODO Auto-generated catch block
						e.printStackTrace();
					}
					System.out.println(Thread.currentThread().getName()+"正在卖第"+(ticket--)+"票");
			}
		}
	}
}

package com.bjsxt.ticker;
public class TestTicket01 {
	public static void main(String[] args) {
		Ticket2 t=new Ticket2();
		Thread t1=new Thread(t,"A窗口");
		Thread t2=new Thread(t,"B窗口");
		Thread t3=new Thread(t,"C窗口");
		//启动线程
		t1.start();
		t2.start();
		t3.start();
	}
}

运行结果：

可以看到，没有用同步方法，连0和-1都出来了，甚至还有俩个窗口卖同一张票的。这就是多线程没有用同步方法的线程安全。