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。
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。