(三)Single Threaded Execution模式

一、定义

一个线程执行。有时也称为Critical Section(临界区)。

二、模式案例

案例：

假设有三个人，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。

门的定义：

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());
        }
    }

    public String toString(){
        return "No."+counter+": "+name+", "+address;
    }
}

人的定义

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;
    }

    public void run(){
        System.out.println(myname+" BEGIN");
        while(true){
            gate.pass(myname,myaddress);
        }
    }
}

测试类

public class Main {
    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();
    }
}

多次执行后发现可能出现以下结果：

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：

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());
        }
    }

    public synchronized String toString(){
        return "No."+counter+": "+name+", "+address;
    }
}