深入解析J.U.C并发包（五）——Single Threaded Execution模式

 

一、定义

Single Threaded Execution 是指“以一个线程执行”的意思，有时也称为Critical Section（临界区）。

二、模式案例

案例：

假设三个人，频繁地通过一扇门，规定每次只能通过一个人，当通过一个人时，程序会将通过的总人次加1，同时记录该次通过人的姓名和出生地。

门.java

package Entity;

public class Gate {

    private int counter = 0;
    private String name = "Nobody";
    private String address = "Nowhere";

    public void pass(String name, String address) {
        this.counter ++;
        this.name = name;
        this.address = address;

        check();
    }

    private void check() {
        if (name.charAt(0) != address.charAt(0)) {
            System.out.println("*******BROKEN*******" + toString());
        }
    }

    @Override
    public String toString() {
        return "Gate{" +
                "counter=" + counter +
                ", name='" + name + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

人.java

package Entity;

public class UserThread extends Thread{
    private final Gate gate;
    private final String myName;
    private final String myAddress;

    public UserThread(Gate gate, String myName, String myAddress) {
        this.gate = gate;
        this.myName = myName;
        this.myAddress = myAddress;
    }

    @Override
    public void run() {
        System.out.println(myName + " BEGIN");
        while (true) {
            gate.pass(myName,myAddress);
        }
    }
}

 main.java

import Entity.Gate;
import Entity.UserThread;

public class Demo1 {
    public static void main(String[] args) {
        System.out.println("Testing Gate, hit CTRL+C to exit.");
        Gate gate = new Gate();
        new UserThread(gate,"Alice","Alaska").start();
        new UserThread(gate,"Bobby","Brazil").start();
        new UserThread(gate,"Chris","Canada").start();
    }
}

Testing Gate, hit CTRL+C to exit.
Alice BEGIN
Chris BEGIN
Bobby BEGIN
*******BROKEN*******Gate{counter=959, name='Alice', address='Alaska'}
*******BROKEN*******Gate{counter=1232, name='Alice', address='Alaska'}
*******BROKEN*******Gate{counter=1450, name='Alice', address='Alaska'}
*******BROKEN*******Gate{counter=1664, name='Alice', address='Alaska'}
*******BROKEN*******Gate{counter=2048, name='Alice', address='Alaska'}
*******BROKEN*******Gate{counter=2467, name='Alice', address='Alaska'}
*******BROKEN*******Gate{counter=2820, name='Bobby', address='Brazil'}
*******BROKEN*******Gate{counter=2904, name='Bobby', address='Brazil'}
*******BROKEN*******Gate{counter=3025, name='Alice', address='Alaska'}

分析：

可以看到，上述Gate类并非线程安全的，因为pass方法会被多个线程同时调用，且该方法中会修改Gate类字段的值。

优化：

package Entity;

public class Gate {

    private int counter = 0;
    private String name = "Nobody";
    private String address = "Nowhere";

    public synchronized void pass(String name, String address) {
        this.counter++;
        this.name = name;
        this.address = address;

        check();
    }

    private void check() {
        if (name.charAt(0) != address.charAt(0)) {
            System.out.println("*******BROKEN*******" + toString() );
        }
    }

    @Override
    public String toString() {
        return "Gate{" +
                "counter=" + counter +
                ", name='" + name + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

三、模式讲解

角色：

Single Threaded Execution 模式的角色如下：

•  SharedResource（共享资源）参与者

SharedResource就是多线程会同时访问的资源类，该类通常具有两类方法：

1、SafeMethod -- 被多个线程同时调用也不会发生问题的方法

2、UnsafeMethod -- 被多个线程同时调用会发生问题，这类方法需要加以防护，指定只能由单线程访问区域，即临界区（critical section）