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 { //id private 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; } @Override public 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id private 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; } @Override public 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：1、增加座位；2、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); } @Override public 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。
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); } @Override public 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：synchronized的深度理解
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java多线程：volatile的深度理解
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。