一、定義

Single Threaded Execution 是指“以1個線程執(zhí)行”的意思，有時也稱為Critical Section（臨界區(qū)）。

二、模式案例

案例：
假設(shè)有三個人，頻繁地通過一扇門，規(guī)定每次只能通過一個人，當(dāng)通過一個人時，程序會將通過的總?cè)舜渭?，同時記錄該次通過人的姓名和出生地。

門的定義：

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;
    }
}

人的定義：

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);
        }
    }
}

執(zhí)行：

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();
    }
}

結(jié)果：

Testing Gate, hit CTRL+C to exit.
Alice BEGIN
Bobby BEGIN
Chris BEGIN
***** BROKEN ***** No.3: Chris, Alaska
***** BROKEN ***** No.3: Chris, Alaska
***** BROKEN ***** No.3: Chris, Alaska
***** BROKEN ***** No.6: Alice, Canada
***** BROKEN ***** No.6: Alice, Canada
***** BROKEN ***** No.6: Alice, Canada
***** BROKEN ***** No.9: Bobby, Brazil
***** BROKEN ***** No.10: Bobby, Alaska

分析：
可以看到，上述Gate類并非線程安全的，因?yàn)閜ass方法會被多個線程同時調(diào)用，且該方法中會修改Gate類字段的值。

優(yōu)化：

//將Gate類變?yōu)榫€程安全的類：
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();
    }
    public synchronized String toString() {
        return "No." + counter + ": " + name + ", " + address;
    }
    private void check() {
        if (name.charAt(0) != address.charAt(0)) {
            System.out.println("***** BROKEN ***** " + toString());
        }
    }
}

三、模式講解

角色：
Single Threaded Execution 模式的角色如下：

• SharedResource(共享資源)參與者
  SharedResource就是多線線程會同時訪問的資源類，該類通常具有2類方法：
  ①SafeMethod——從多個線程同時調(diào)用也不會發(fā)生問題的方法
  ②UnsafeMethod——從多個線程同時調(diào)用會發(fā)生問題，這類方法需要加以防護(hù)，指定只能由單線程訪問區(qū)域，即臨界區(qū)（critical section）。