单线程相当于一个人做一件事，而多线程程序中，当需要多个线程共享同一个数据时，一个线程对共享的数据进行修改时，在他没有完成相关操作之前不允许其他线程进行相关操作，否则会出现线程安全问题。
package pers.zhb.runnable;
public class MyThread implements Runnable {
private int tickets = 10;
public void run() {
while (true) {
if (tickets > 0)
System.out.println(Thread.currentThread().getName()
+ "售票窗口,售卖了" + tickets-- + "号票");
else
System.exit(0);
}
}
}
package pers.zhb.runnable;
public class RunnableDemo {
public static void main(String[] args) {
MyThread mt=new MyThread();
Thread thread1 = new Thread(mt);
Thread thread2 = new Thread(mt);
Thread thread3 = new Thread(mt);
Thread thread4 = new Thread(mt);
thread1.start();
thread2.start();
thread3.start();
thread4.start();
}
}
可以看出，10号票在被1号窗口卖出后，又被2号窗口卖了一次，显然这是不符合常理的。产生的原因是这样的：2号线程开始执行，tickets为10，进行了 if (tickets > 0)；语句的执行，可以继续往下执行了，但是他没有抢夺到CPU资源，接着是1号线程读取数据10，进行判断可以执行，1号线程执行完后，2号线程依旧没有抢夺到CPU资源，只能继续等待。接着是3号线程、0号线程进行相关操作。当2号线程抢夺到CPU资源时，他的数据还是10，就造成了有重复票的情况。
添加同步语句：
package pers.zhb.runnable;
public class MyThread implements Runnable {
private int tickets = 10;
Object lock = new Object();
public void run() {
synchronized (lock) {
while (true) {
if (tickets > 0)
System.out.println(Thread.currentThread().getName()
+ "售票窗口,售卖了" + tickets-- + "号票");
else
System.exit(0);
}
}
}
}
package pers.zhb.runnable;
public class RunnableDemo {
public static void main(String[] args) {
MyThread mt=new MyThread();
Thread thread1 = new Thread(mt);
Thread thread2 = new Thread(mt);
Thread thread3 = new Thread(mt);
Thread thread4 = new Thread(mt);
thread1.start();
thread2.start();
thread3.start();
thread4.start();
}
}
创建同步方法：
package pers.zhb.runnable;
public class MyThread implements Runnable {
private int tickets = 10;
Object lock = new Object();
public void run() {
while (true)
lock();
}
public synchronized void lock() {
while (true) {
if (tickets > 0)
System.out.println(Thread.currentThread().getName()
+ "售票窗口,售卖了" + tickets-- + "号票");
else
System.exit(0);
}
}
}
package pers.zhb.runnable;
public class RunnableDemo {
public static void main(String[] args) {
MyThread mt=new MyThread();
Thread thread1 = new Thread(mt);
Thread thread2 = new Thread(mt);
Thread thread3 = new Thread(mt);
Thread thread4 = new Thread(mt);
thread1.start();
thread2.start();
thread3.start();
thread4.start();
}
}
