多线程的并发执行可以提高程序运行的效率，但当多个线程去处理同一个资源时，就容易产生一些安全问题。如模拟抢票程序，如果不加线程安全处理的话，就很容易多个线程抢到同一张票，或者出现余票为负数的情况。
synchronized关键字，就是用来控制线程同步的，保证我们的线程在多线程环境下，synchronized块或synchronized方法中的内容不被多个线程同时执行，确保我们数据的完整性。（在run方法里使用synchronized块，或在实现多线程方法里新建synchronized方法）
案例一，抢票程序
1 package cn.ftf.threadsafe; 2 3 public class SafeTest01 { 4 public static void main(String[] args) { 5 Safe12306 safe=new Safe12306(); 6 Thread th=new Thread(safe); 7 Thread th1=new Thread(safe); 8 Thread th2=new Thread(safe); 9 th1.start();10 th2.start();11 th.start();12 13 }14 }15 class Safe12306 implements Runnable{16 private int i=10;17 private boolean flag=true;18 public void run() {19 while(flag)20 test();21 }22 public synchronized void test() { //synchronized方法，为提高效率，最好使用synchronized块23 if(i<=0) {24 flag=false;25 return; 26 }27 try {28 Thread.sleep(200);29 } catch (InterruptedException e) {30 e.printStackTrace();31 }32 System.out.println(i--);33 } 34 35 //优化,尽可能锁定合理的范围，是指数据的完整性36 public void test2() {37 if(i<=0) { //考虑的是没有票的情况38 flag=false;39 return;40 }41 synchronized (this) {42 if(i<=0) { //多重检测。//考虑的是最后一张票的情况43 flag=false;44 return;45 46 }47 try {48 Thread.sleep(200);49 } catch (InterruptedException e) {50 e.printStackTrace();51 }52 System.out.println(i--);53 } 54 } 55 56 } 案例二，多人操作一个账户取钱
1 package cn.ftf.threadsafe; 2 /** 3 *多人操作同一账户取钱 取钱 4 * @author 房廷飞 5 * 6 */ 7 public class SafeTest02 { 8 public static void main(String[] args) { 9 Account acc=new Account(100, "礼金");10 Drawing you=new Drawing(acc, 80);11 Drawing she=new Drawing(acc, 90);12 new Thread(you,"可悲的你").start();13 new Thread(she,"happy的她").start();14 }15 16 }17 class Account{18 int money;19 String name;20 public Account(int money, String name) {21 super();22 this.money = money;23 this.name = name;24 }25 }26 //模拟取款27 class Drawing implements Runnable{28 Account account;29 int drawMoney; //取地钱数30 int packetTotal; //口袋的钱数31 32 public Drawing(Account account, int drawMoney) {33 super();34 this.account = account;35 this.drawMoney = drawMoney;36 }37 38 @Override39 public void run() {40 synchronized (account) {41 if(account.money-drawMoney>0) {42 account.money-=drawMoney;43 packetTotal+=drawMoney;44 System.out.println(Thread.currentThread().getName()+"账户余额"+account.money);45 System.out.println(Thread.currentThread().getName()+"口袋里的钱"+packetTotal); 46 }else {47 System.out.println(Thread.currentThread().getName()+"取款失败，余额不足！");48 }49 }50 }51 }52 案例三 操作容器,多线程向容器里添加数据会有数据丢失或覆盖的情况
1 package cn.ftf.threadsafe; 2 3 import java.util.ArrayList; 4 import java.util.List; 5 6 /** 7 * 操作容器,多线程向容器里添加数据会有数据丢失或覆盖的情况 8 * @author 房廷飞 9 *10 */11 public class UnsafeTest03 {12 public static void main(String[] args) throws InterruptedException {13 List list=new ArrayList();14 for(int i=0;i<10000;i++) {15 new Thread(()->{16 synchronized (list) {17 list.add(Thread.currentThread().getName());18 }19 }).start();20 }21 Thread.sleep(2000);22 System.out.println(list.size());23 }24 25 } 涉及多线程读写的容器还可以使用CopyOnWriteArrayList类来创建容器，其内部维护了一个安全的用于线程操作的容器，也是用的synchronized代码块实现的容器安全。
1 import java.util.ArrayList; 2 import java.util.concurrent.CopyOnWriteArrayList; 3 import java.util.List; 4 5 public class SynContainer { 6 public static void main(String[] args) throws InterruptedException { 7 CopyOnWriteArrayList list = new CopyOnWriteArrayList (); 8 for(int i=0;i<10000;i++) { 9 new Thread(()->{10 list.add(Thread.currentThread().getName());11 }) .start();12 }13 Thread.sleep(10000);14 System.out.println(list.size());15 }16 }
下面写一个多线程（多用户）买电影票的程序
包内包含三个文件，影院类，用户类和测试类：Cinema.java , Person.java , Test.java
Cinema.java ：共同操作的对象
1 package cn.ftf.cinema; 2 3 import java.util.ArrayList; 4 import java.util.List; 5 6 public class Cinema{ 7 public List getTicket() { 8 return ticket; 9 }10 public void setTicket(List ticket) {11 this.ticket = ticket;12 }13 private List ticket;14 private String name;15 public Cinema(List ticket, String name) {16 super();17 this.ticket = ticket;18 this.name = name;19 System.out.println(name+"欢迎你！现有余座："+ticket.toString());20 }21 public boolean sell(List needTicket){22 List copy=new ArrayList ();23 copy.addAll(ticket);24 25 copy.removeAll(needTicket);26 if(ticket.size()-copy.size()!=needTicket.size()) {27 return false;28 }29 ticket=copy;30 return true;31 }32 } Person.java 实现多线程的对象
1 package cn.ftf.cinema; 2 3 import java.util.List; 4 5 public class Person implements Runnable{ 6 Cinema ci; 7 private String pname; 8 private List needTicket; 9 public Person(Cinema ci, String pname, List needTicket) {10 super();11 this.ci = ci;12 this.pname = pname;13 this.needTicket = needTicket;14 }15 @Override16 public void run() {17 synchronized (ci) {18 boolean flag=ci.sell(needTicket);19 if(flag) {20 System.out.println(pname+"购票成功，座位："+needTicket);21 22 }else {23 System.out.println(pname+"购票失败，余票不足！");24 }25 }26 }27 } Test.java 测试类
1 package cn.ftf.cinema; 2 3 import java.util.List; 4 5 public class Person implements Runnable{ 6 Cinema ci; 7 private String pname; 8 private List needTicket; 9 public Person(Cinema ci, String pname, List needTicket) {10 super();11 this.ci = ci;12 this.pname = pname;13 this.needTicket = needTicket;14 }15 @Override16 public void run() {17 synchronized (ci) {18 boolean flag=ci.sell(needTicket);19 if(flag) {20 System.out.println(pname+"购票成功，座位："+needTicket);21 22 }else {23 System.out.println(pname+"购票失败，余票不足！");24 }25 }26 }27 } 其中还包含了集合类List的部分使用，增删，复制等