Single Threaded Execution

摘要: 对临界区的修改只允许一个线程操作

Single Threaded Execution

这个设计模式是多线程设计时候最基础的一个准则，对于临界区的值只允许一个线程进行修改。定义临界区还是使用synchronized来定义。（虽然jdk 7出了很多并发包api，但还是先只关注最基础的api 关键字）

//演示
public class Main {
public static void main(String args[]){
Gate gate=new Gate();
new UserThread(gate,"chen ","china").start();
new UserThread(gate,"bob","brazil").start();;
}
}

public class Gate {
private int counter=0;
private String name="nobady";
private String address="nowhere";
public synchronized void pass(String name,String address){
this.counter++;
this.name=name;
this.address=address;
check();
}

public String toString(){
return "no."+counter+": "+name+","+address;
}
public void check(){
if(name.charAt(0)!=address.charAt(0)){
System.out.println("******broken******"+toString());
}
}
}

public class UserThread extends Thread {
private final Gate gate;

private final String myname;

private final String myaddress;

public UserThread(Gate gate,String name,String address) {
this.gate=gate;
this.myname=name;
this.myaddress=address;
}

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

因为定义了临界区，所以不会输出check里面的值。去掉关键字就会出错。

但是定义临界区，就会使多线程的性能降低。如何减小临界区的范围是一个比较关键的地方。像多个值需要赋值的时候，volatile 关键字是并没有任何作用的。