一、定义
Single Threaded Execution 是指“以1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);
}
}
}
执行：
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();
}
}
结果：
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：
//将Gate类变为线程安全的类：
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就是多线线程会同时访问的资源类，该类通常具有2类方法：
①SafeMethod——从多个线程同时调用也不会发生问题的方法
②UnsafeMethod——从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）。
