Immutable

Immutable是“不能改变“的意思。如果多线程中共享的数据是不可变的（例如String、Integer），那么无论多少线程来访问这个数据都不会对使这个数据发生改变。这是也不需要synchronized来设定临界区。

public final class Person {
	private String name;
	private String address;
	public Person(String name,String address){
		this.name=name;
		this.address=address;
	}
	public String getName(){return name;}//只有访问器没有修改器
	public String address(){return address;}
	@Override
	public String toString(){
		return "["+name+","+address+"]";
	}
}

 

public class VisitThread extends Thread{
	private Person person;
	public VisitThread(Person person){this.person=person;}
	@Override
	public void run(){
		while(true)
			visit();
	}
	private void visit(){
		System.out.println(Thread.currentThread()+" visit "+person);
	}
}

 

public class ImmutableTest {
	public static void main(String[] args){
		Person p=new Person("LiLei","Beijing");
		new VisitThread(p).start();
		new VisitThread(p).start();
		new VisitThread(p).start();
	}
}

 运行结果：

Thread[Thread-2,5,main] visit [LiLei,Beijing]
Thread[Thread-2,5,main] visit [LiLei,Beijing]
Thread[Thread-1,5,main] visit [LiLei,Beijing]
Thread[Thread-1,5,main] visit [LiLei,Beijing]
Thread[Thread-1,5,main] visit [LiLei,Beijing]
Thread[Thread-0,5,main] visit [LiLei,Beijing]
Thread[Thread-2,5,main] visit [LiLei,Beijing]
Thread[Thread-2,5,main] visit [LiLei,Beijing]
Thread[Thread-2,5,main] visit [LiLei,Beijing]
Thread[Thread-2,5,main] visit [LiLei,Beijing]
Thread[Thread-1,5,main] visit [LiLei,Beijing]
Thread[Thread-1,5,main] visit [LiLei,Beijing]
Thread[Thread-1,5,main] visit [LiLei,Beijing]

 

        这种模式的参与者是不可变的对象，尽管它是共享数据，但是它不会得到修改，它不需要用synchronized，能够自由被访问。这时可以将对象的字段设置为final。

        如果一个类声明为final，那么它无法被继承；如果的实例方法声明为final，那么它无法被覆盖（override），如果将类方法声明为final，那么子类无法看到这个方法，如果字段声明为final，那么final字段的值只能被指定一次。通常，final字段初始化在定义时、构造器里面静态初始化块里面实现。