看了这个有趣的例子，你就能秒懂Java中的多线程同步了！

作者：智慧zhuhuix

cnblogs.com/zhuhuix/p/12970326.html

写在前面

把技术概念通过文字的形式写下来，理清逻辑，加深认知；
把知识点通过系列文章的形式分段写下来，让思维进行刻意的训练；
把难懂的东西通过有趣的故事或者例子讲出来，让技术变得生动。

电影票的案例

单线程的例子

我们设定有一个电影院，该电影院开张不久，在入口的旁边只设立了一个售票点A，顾客看电影，需要在售票点排队依次买票，买完票后在入口处检票进入电影院观影。

上面的描述用代码来实现，可以是这样的：

1、首先建立一个电影票的类：主要的属性有票的ID,哪个放映厅，哪一排哪一列，放映的电影名称，放映时间及票价。

/*** 通过卖票程序读懂多线程--电影票的类** @author zhuhuix* @date 2020-05-12*/
public class Ticket {//idprivate int ticketId;//放映厅private String room;//行private Integer row;//列private Integer col;//电影名private String filmName;//价格private BigDecimal price;//放映时间private LocalDateTime datetime;private Ticket(){}public Ticket(int ticketId,String room, Integer row, Integer col, String filmName, BigDecimal price, LocalDateTime datetime) {this.ticketId = ticketId;this.room = room;this.row = row;this.col = col;this.filmName = filmName;this.price = price;this.datetime = datetime;}public int getTicketId() {return ticketId;}public void setTicketId(int ticketId) {this.ticketId = ticketId;}public String getRoom() {return room;}public void setRoom(String room) {this.room = room;}public Integer getRow() {return row;}public void setRow(Integer row) {this.row = row;}public Integer getCol() {return col;}public void setCol(Integer col) {this.col = col;}public String getFilmName() {return filmName;}public void setFilmName(String filmName) {this.filmName = filmName;}public BigDecimal getPrice() {return price;}public void setPrice(BigDecimal price) {this.price = price;}public LocalDateTime getDatetime() {return datetime;}public void setDatetime(LocalDateTime datetime) {this.datetime = datetime;}@Overridepublic String toString() {return "Ticket{" +"ticketId=" + ticketId +", room='" + room + '\'' +", row=" + row +", col=" + col +", filmName='" + filmName + '\'' +", price=" + price +", datetime=" + datetime +'}';}
}

2、其次建立一个顾客的类：主要的属性有票的ID,购买的电影票；成员方法有买票。

/*** 通过卖票程序读懂多线程--顾客类** @author zhuhuix* @date 2020-05-12*/
public class Customer {//顾客idprivate int customerId;//购买的电影票private Ticket ticket;public Customer(int customerId) {this.customerId = customerId;}//顾客买票public void buyTicket(Ticket ticket) {this.ticket = ticket;}public int getCustomerId() {return customerId;}public void setCustomerId(int customerId) {this.customerId = customerId;}public Ticket getTicket() {return ticket;}public void setTicket(Ticket ticket) {this.ticket = ticket;}@Overridepublic String toString() {return "Customer{" +"customerId=" + customerId +", ticket=" + ticket.toString() +'}';}
}

3、最后写一个主程序，生成电影票的列表，设定上门观影的顾客人数，依次买票，输出电影票购买的状态。

/*** 通过卖票程序读懂多线程--单线程程序** @author zhuhuix* @date 2020-05-12*/
public class TicketSingle {private static final String ROOM = "中央放映厅";private static final int ROW = 10;private static final int COL = 20;private static final String FILM_NAME = "战狼3";private static final BigDecimal PRICE = BigDecimal.valueOf(30);private static List<Ticket> tickets = new ArrayList<>();private static final int CUSTOMER_COUNT = 250;private static List<Customer> customers = new ArrayList<>(CUSTOMER_COUNT);public static void main(String[] args) {//中央放映厅总共有250个座位，2020-05-12 18:00 放映战狼3，售票价格为30元int ticketId=1;for (int row = 1; row <= ROW; row++) {for (int col = 1; col <= COL; col++) {Ticket ticket = new Ticket(ticketId++, ROOM, row, col,FILM_NAME, PRICE,LocalDateTime.of(2020, 5, 10, 18, 00));tickets.add(ticket);}}Iterator<Ticket> iterator = tickets.iterator();while (iterator.hasNext()) {System.out.println(iterator.next().toString());}//顾客到售票点进行随机买票Collections.shuffle(tickets);int index = 1;while (tickets.size() > 0 && index <= CUSTOMER_COUNT) {Ticket ticket = tickets.get(tickets.size() - 1);Customer customer = new Customer(index);customer.buyTicket(ticket);customers.add(customer);tickets.remove(ticket);System.out.println(tickets.size() + "," + index);System.out.println(index + "号顾客买到了"+ "第" + customer.getTicket().getRow() + "行，第" + customer.getTicket().getCol() + "列的票");index++;try {TimeUnit.MILLISECONDS.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}}System.out.println("电影票出售情况如下：");//剩余票的状态System.out.println("剩余票数：" + tickets.size());Iterator<Ticket> ticketIterator = tickets.iterator();while (ticketIterator.hasNext()) {System.out.println(ticketIterator.next().toString());}//顾客购买情况System.out.println("买到票的人数：" + customers.size());Iterator<Customer> customerIterator = customers.iterator();while (customerIterator.hasNext()) {System.out.println(customerIterator.next().toString());}System.out.println("未买到票的人数：" +(CUSTOMER_COUNT- customers.size()));}
}

主程序的输出情况是这样的：

从单线程转向多线程

一切井然有序，程序也运行得很好，那我们继续往下看，由于观影顾客人数的增加，电影院对放映厅做了改造：

增加座位；
增设两个卖票窗口。

也就说原来只有一个窗口排队单通道执行的程序变了，要允许原来的售票点与新增的售票点，同时进行卖票了。

有问题的多线程的例子

我们先简单的在单线程的程序上做个多线程的改造：建立一个多线程的类，重写run方法，将顾客买票的过程移至run方法中，在主程中设立”售票点A“，”售票点B“，”售票点C“三个线程让其同时运行，对了，别忘了把ArrayList这个数据结构也改成Vector。改造后的程序是这样的：

/*** 通过卖票程序读懂多线程--多线程** @author zhuhuix* @date 2020-05-12*/
public class TicketThread extends Thread {private static final String ROOM = "中央放映厅";private static final int ROW = 20;private static final int COL = 30;private static final String FILM_NAME = "战狼3";private static final BigDecimal PRICE = BigDecimal.valueOf(30);private static List<Ticket> tickets = new Vector<>();private static final int CUSTOMER_COUNT = 800;private static int customerId = 1;private static List<Customer> customers = new Vector<>(CUSTOMER_COUNT);TicketThread(String name) {super(name);}@Overridepublic void run() {while (tickets.size() > 0 && customerId <= CUSTOMER_COUNT) {Ticket ticket = tickets.get(tickets.size() - 1);ticket.setWindow(Thread.currentThread().getName());Customer customer = new Customer(customerId);customer.buyTicket(ticket);customers.add(customer);tickets.remove(ticket);System.out.println(tickets.size() + "," + customerId);System.out.println(Thread.currentThread().getName() + ":" + customerId + "号顾客买到了"+ "第" + customer.getTicket().getRow() + "行，第" + customer.getTicket().getCol() + "列的票");customerId++;try {TimeUnit.MILLISECONDS.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}}}public static void main(String[] args) throws InterruptedException {//中央放映厅总共有250个座位，2020-05-12 18:00 放映战狼3，售票价格为30元int ticketId = 1;for (int row = 1; row <= ROW; row++) {for (int col = 1; col <= COL; col++) {Ticket ticket = new Ticket(ticketId++, ROOM, row, col,FILM_NAME, PRICE,LocalDateTime.of(2020, 5, 10, 18, 00));tickets.add(ticket);}}Iterator<Ticket> iterator = tickets.iterator();while (iterator.hasNext()) {System.out.println(iterator.next().toString());}//顾客到售票点进行随机买票Collections.shuffle(tickets);TicketThread ticketThreadA = new TicketThread("售票点A");TicketThread ticketThreadB = new TicketThread("售票点B");TicketThread ticketThreadC = new TicketThread("售票点C");ticketThreadA.start();ticketThreadB.start();ticketThreadC.start();ticketThreadA.join();ticketThreadB.join();ticketThreadC.join();System.out.println("电影票出售情况如下：");//剩余票的状态System.out.println("总共票数:" + ROW * COL + ",剩余票数：" + tickets.size());Iterator<Ticket> ticketIterator = tickets.iterator();while (ticketIterator.hasNext()) {System.out.println(ticketIterator.next().toString());}//顾客购买情况System.out.println("买到票的人数：" + customers.size());Iterator<Customer> customerIterator = customers.iterator();while (customerIterator.hasNext()) {System.out.println(customerIterator.next().toString());}System.out.println("未买到票的人数：" + (CUSTOMER_COUNT - customers.size()));}
}

运行一下：总共只有600张票，买到票的人有614人？那进了电影院顾客肯定得投诉。

线程同步问题

我们分析一下：

电影票总共只有600张，三个窗口同时卖这600张票，电影票是个共享的池子，在多线程术语上称为”共享资源“或”临界资源“，每个线程访问这些资源时，要保证”同步“：售票点A要卖第10排第9列的座位时，当且仅当同一时刻只有售票点A才能访问这个座位对应的电影票，也就是所谓的不能一票多卖。

那多线程如何保证同步？通过加锁！！加锁是用来控制多个线程访问共享资源的方式，一般来说，一个锁能够防止多个线程同时访问共享资源。（Java知音公众号中回复“面试题聚合”，送你一份面试题宝典！）

保证线程同步的例子

为了可以简单地说明加锁可以保证多线程同步，在下面的例子中仅实现对电影票共享池进行加锁。

/*** 通过卖票程序读懂多线程--多线程** @author zhuhuix* @date 2020-05-12*/
public class TicketThread extends Thread {private static final String ROOM = "中央放映厅";private static final int ROW = 20;private static final int COL = 30;private static final String FILM_NAME = "战狼3";private static final BigDecimal PRICE = BigDecimal.valueOf(30);private volatile static List<Ticket> tickets = new Vector<>();private static final int CUSTOMER_COUNT = 800;private static int customerId = 1;private volatile static List<Customer> customers = new Vector<>(CUSTOMER_COUNT);TicketThread(String name) {super(name);}@Overridepublic void run() {while (tickets.size() > 0 && customerId <= CUSTOMER_COUNT) {synchronized (TicketThread.class) {//线程内两次判断，防止tickets 数组溢出if (tickets.size()>0) {Ticket ticket = tickets.get(tickets.size() - 1);ticket.setWindow(Thread.currentThread().getName());Customer customer = new Customer(customerId);customer.buyTicket(ticket);customers.add(customer);tickets.remove(ticket);System.out.println(tickets.size() + "," + customerId);System.out.println(Thread.currentThread().getName() + ":" + customerId + "号顾客买到了"+ "第" + customer.getTicket().getRow() + "行，第" + customer.getTicket().getCol() + "列的票");customerId++;try {TimeUnit.MILLISECONDS.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}}} }}public static void main(String[] args) throws InterruptedException {//中央放映厅总共有250个座位，2020-05-12 18:00 放映战狼3，售票价格为30元int ticketId = 1;for (int row = 1; row <= ROW; row++) {for (int col = 1; col <= COL; col++) {Ticket ticket = new Ticket(ticketId++, ROOM, row, col,FILM_NAME, PRICE,LocalDateTime.of(2020, 5, 10, 18, 00));tickets.add(ticket);}}Iterator<Ticket> iterator = tickets.iterator();while (iterator.hasNext()) {System.out.println(iterator.next().toString());}//顾客到售票点进行随机买票Collections.shuffle(tickets);TicketThread ticketThreadA = new TicketThread("售票点A");TicketThread ticketThreadB = new TicketThread("售票点B");TicketThread ticketThreadC = new TicketThread("售票点C");ticketThreadA.start();ticketThreadB.start();ticketThreadC.start();ticketThreadA.join();ticketThreadB.join();ticketThreadC.join();System.out.println("电影票出售情况如下：");//剩余票的状态System.out.println("总共票数:" + ROW * COL + ",剩余票数：" + tickets.size());Iterator<Ticket> ticketIterator = tickets.iterator();while (ticketIterator.hasNext()) {System.out.println(ticketIterator.next().toString());}//顾客购买情况System.out.println("买到票的人数：" + customers.size());Iterator<Customer> customerIterator = customers.iterator();while (customerIterator.hasNext()) {System.out.println(customerIterator.next().toString());}System.out.println("未买到票的人数：" + (CUSTOMER_COUNT - customers.size()));}
}

运行情况如下：

票不超卖了：

每个窗口也实现了同步并发卖票：

同步的代码主要的改变来自于：

1、将卖票的过程用synchronized修饰，实现锁的互斥，具体可以参考java多线程：

https://blog.csdn.net/jpgzhu/article/details/105967947

synchronized (TicketThread.class) {Ticket ticket = tickets.get(tickets.size() - 1);ticket.setWindow(Thread.currentThread().getName());Customer customer = new Customer(customerId);customer.buyTicket(ticket);customers.add(customer);tickets.remove(ticket);System.out.println(tickets.size() + "," + customerId);System.out.println(Thread.currentThread().getName() + ":" + customerId + "号顾客买到了"+ "第" + customer.getTicket().getRow() + "行，第" + customer.getTicket().getCol() + "列的票");customerId++;try {TimeUnit.MILLISECONDS.sleep(10);} catch (InterruptedException e) {e.printStackTrace();}}

2、将共享资源用volatile 修饰，实现线程访问的可视化，具体可以参考上文链接。

private volatile static List<Ticket> tickets = new Vector<>();
private volatile static List<Customer> customers = new Vector<>(CUSTOMER_COUNT);

写在最后

程序所有的表达，归根到底都是逻辑问题。而逻辑的核心是清晰高效地思考问题。对于多线程的理解，大家一定要起手来写一些例程，融汇贯通才能体会到真谛，才能真正应用到工作实践中去。

END

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