Java线程同步是保证多线程状态下安全访问竞争资源的一种编程手段，但线程的同步在Java多线程编程中算是一个比较难的难点，很多开发者甚至都不知道什么是竞争资源，也不知道时候需要进行线程同步，当然这是没有明确答案的，一些原则问题还是要考虑。
Java如何实现线程同步？
对于同步，在具体的Java代码中需要完成一下两个操作：
1、把竞争访问的资源标识为private；
2、同步哪些修改变量的代码，使用synchronized关键字同步方法或代码。
当然这不是唯一控制并发安全的途径。
synchronized 关键字使用说明
synchronized 只能标记非抽象的方法，不能标识成员变量。
为了演示同步方法的使用，构建了一个信用卡账户，起初信用额为100w，然后模拟透支、存款等多个操作。显然银行账户User对象是个竞争资源，而多个并发操作的是账户方法oper(int x)，当然应该在此方法上加上同步，并将账户的余额设为私有变量，禁止直接访问。
01/**
02* Java线程：线程的同步
03*
04* @author leizhimin 2009-11-4 11:23:32
05*/
06public class Test {
07public static void main(String[] args) {
08User u = new User("张三", 100);
09MyThread t1 = new MyThread("线程A", u, 20);
10MyThread t2 = new MyThread("线程B", u, -60);
11MyThread t3 = new MyThread("线程C", u, -80);
12MyThread t4 = new MyThread("线程D", u, -30);
13MyThread t5 = new MyThread("线程E", u, 32);
14MyThread t6 = new MyThread("线程F", u, 21);
15t1.start();
16t2.start();
17t3.start();
18t4.start();
19t5.start();
20t6.start();
21}
22}
23class MyThread extends Thread {
24private User u;
25private int y = 0;
26MyThread(String name, User u, int y) {
27super(name);
28this.u = u;
29this.y = y;
30}
31public void run() {
32u.oper(y);
33}
34}
35class User {
36private String code;
37private int cash;
38User(String code, int cash) {
39this.code = code;
40this.cash = cash;
41}
42public String getCode() {
43return code;
44}
45public void setCode(String code) {
46this.code = code;
47}
48/**
49* 业务方法
50* @param x 添加x万元
51*/
52public synchronized void oper(int x) {
53try {
54Thread.sleep(10L);
55this.cash += x;
56System.out.println(Thread.currentThread().getName() + "运行结束，增加“" + x + "”，当前用户账户余额为：" + cash);
57Thread.sleep(10L);
58} catch (InterruptedException e) {
59e.printStackTrace();
60}
61}
62@Override
63public String toString() {
64return "User{" +
65"code='" + code + '\'' +
66", cash=" + cash +
67'}';
68}
69}
复制代码/*** Java线程：线程的同步** @author leizhimin 2009-11-4 11:23:32*/public class Test {public static void main(String[] args) {User u = new User("张三", 100);MyThread t1 = new MyThread("线程A", u, 20);MyThread t2 = new MyThread("线程B", u, -60);MyThread t3 = new MyThread("线程C", u, -80);MyThread t4 = new MyThread("线程D", u, -30);MyThread t5 = new MyThread("线程E", u, 32);MyThread t6 = new MyThread("线程F", u, 21);t1.start();t2.start();t3.start();t4.start();t5.start();t6.start();}}class MyThread extends Thread {private User u;private int y = 0;MyThread(String name, User u, int y) {super(name);this.u = u;this.y = y;}public void run() {u.oper(y);}}class User {private String code;private int cash;User(String code, int cash) {this.code = code;this.cash = cash;}public String getCode() {return code;}public void setCode(String code) {this.code = code;}/*** 业务方法* @param x 添加x万元*/public synchronized void oper(int x) {try {Thread.sleep(10L);this.cash += x;System.out.println(Thread.currentThread().getName() + "运行结束，增加“" + x + "”，当前用户账户余额为：" + cash);Thread.sleep(10L);} catch (InterruptedException e) {e.printStackTrace();}}@Overridepublic String toString() {return "User{" +"code='" + code + '\'' +", cash=" + cash +'}';}}