Java多线程设计模式详解学习笔记四——Immutable

public class Main {
	public static void main(String[] args) {
		Person alice = new Person("Alice", "Alaska");
	    new PrintPeronThread(alice).start();
	    new PrintPeronThread(alice).start();
	    new PrintPeronThread(alice).start();
	}
}

public final class Person {
	private final String name;
	private final String address;
	
	public Person(String name,String address) {
		this.name=name;
		this.address=address;
	}

	@Override
	public String toString() {
		return "[Person: name: "+name+" ,address:"+address+"]";
	}
}

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

运行结果：

Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]
Thread-0 prints [Person: name: Alice ,address:Alaska]

何时使用：

1.当实例产生后，状态不再变化时

Immutable参与者是一个字段值无法改变的类，也没有任何用来改变字段的方法，所有字段设置成finall，Person没有setter方法。当Immutable参与者的实例创建以后，状态就完全不再变化。

2.实例需要共享，而且访问很频繁时

Immutable Pattern的优点，在于“不需要用synchronized保护”，这样的优点是在不丧失安全性与生命性的前提下，提高程序的执行性能。

final字段：

1.实例字段：

直接在声明字段的时候就直接赋初值

class Something{
     final int value=123;
}

在构造器中将值赋给字段（blank final）

class Something{
     final int value;
     Something(){
           this.value=123; 
     }
}

2.静态变量

直接在声明字段的时候就直接赋初值

class Something{
     final static int value=123;
}

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在static块（静态初始化）中将值赋给字段（blank fina）

class Something{
     static final int value;
     static{
           value=123; 
     }
}