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 tickets = new ArrayList<>();    private static final int CUSTOMER_COUNT = 250;    private static List 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 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 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 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 tickets = new Vector<>();    private static final int CUSTOMER_COUNT = 800;    private static int customerId = 1;    private static List 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 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 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 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 tickets = new Vector<>();    private static final int CUSTOMER_COUNT = 800;    private static int customerId = 1;    private volatile static List 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 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 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 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 tickets = new Vector<>();private volatile static List 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。