用于解决多线程安全问题的方式：
1. 同步代码块 (隐式锁)
2. 同步方法(隐式锁)
3. 同步锁 Lock( jdk 1.5 后)
 注意：是一个显示锁，需要通过 lock() 方法上锁，必须通过 unlock() 方法进行释放锁

下面举一个售票例子：

1）同步代码块

package com.lxj.juc;public class TestSync {public static void main(String[] args) {Ticket ticket = new Ticket();new Thread(ticket).start();new Thread(ticket).start();new Thread(ticket).start();}}
class Ticket implements Runnable{private int ticket = 100;@Overridepublic void run() {while(true) {synchronized(this) {  //this代表对当前对象上锁if(ticket > 0) {try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName()+" : 购票成功，余票为：" +  --ticket );}else {break;}}}}}

运行结果:
Thread-0 : 购票成功，余票为：99
Thread-0 : 购票成功，余票为：98
Thread-2 : 购票成功，余票为：97
.....
Thread-2 : 购票成功，余票为：7
Thread-0 : 购票成功，余票为：6
Thread-0 : 购票成功，余票为：5
Thread-1 : 购票成功，余票为：4
Thread-1 : 购票成功，余票为：3
Thread-0 : 购票成功，余票为：2
Thread-0 : 购票成功，余票为：1
Thread-2 : 购票成功，余票为：0

2）同步方法

package com.lxj.juc;public class TestSync {public static void main(String[] args) {Ticket ticket = new Ticket();new Thread(ticket).start();new Thread(ticket).start();new Thread(ticket).start();}}class Ticket implements Runnable {private int ticket = 100;@Overridepublic void run() {while (true) {if (ticket > 0) {int i = purchase();if(i == 0) {break;}}else {break;}}}private synchronized int purchase() {if (ticket > 0) {try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " : 购票成功，余票为：" + --ticket);}else {return 0;}return 1;}}

运行结果：
Thread-0 : 购票成功，余票为：99
Thread-0 : 购票成功，余票为：98
Thread-0 : 购票成功，余票为：97
Thread-2 : 购票成功，余票为：96
Thread-1 : 购票成功，余票为：95
....
Thread-1 : 购票成功，余票为：8
Thread-2 : 购票成功，余票为：7
Thread-2 : 购票成功，余票为：6
Thread-0 : 购票成功，余票为：5
Thread-0 : 购票成功，余票为：4
Thread-0 : 购票成功，余票为：3
Thread-2 : 购票成功，余票为：2
Thread-1 : 购票成功，余票为：1
Thread-2 : 购票成功，余票为：0

3）同步锁lock

package com.lxj.juc;import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;public class TestSync {public static void main(String[] args) {Ticket ticket = new Ticket();new Thread(ticket).start();new Thread(ticket).start();new Thread(ticket).start();}}class Ticket implements Runnable {private int ticket = 100;private Lock lock = new ReentrantLock();@Overridepublic void run() {while (true) {lock.lock(); //上锁try {if (ticket > 0) {try {Thread.sleep(200);} catch (InterruptedException e) {e.printStackTrace();}System.out.println(Thread.currentThread().getName() + " : 购票成功，余票为：" + --ticket);}else {break;}} finally {lock.unlock(); //解锁}}}}

运行结果：
Thread-0 : 购票成功，余票为：99
Thread-1 : 购票成功，余票为：98
Thread-1 : 购票成功，余票为：97
Thread-1 : 购票成功，余票为：96
Thread-1 : 购票成功，余票为：95
Thread-2 : 购票成功，余票为：94
......
Thread-0 : 购票成功，余票为：6
Thread-0 : 购票成功，余票为：5
Thread-1 : 购票成功，余票为：4
Thread-2 : 购票成功，余票为：3
Thread-2 : 购票成功，余票为：2
Thread-2 : 购票成功，余票为：1
Thread-2 : 购票成功，余票为：0

注意使用同步锁一定要记得关闭锁，放在try{}finally{}中。