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