线程范围内共享变量的概念与作用演进方式

Posted 2020-09-19 安静

/**
 * 
 * @描述: 线程范围内共享变量的概念与作用 .
 * @作者: Wnj .
 * @创建时间: 2017年5月15日 .
 * @版本: 1.0 .
 */
public class ThreadLocalTest {

    private static ThreadLocal<Integer> x = new ThreadLocal<Integer>();
    private static ThreadLocal<MyThreadScopeData> myThreadScopeData = new ThreadLocal<MyThreadScopeData>();
    public static void main(String[] args) {
        for(int i=0;i<2;i++){
            new Thread(new Runnable(){
                @Override
                public void run() {
                    int data = new Random().nextInt();
                    System.out.println(Thread.currentThread().getName() 
                            + " has put data :" + data);
                    x.set(data);
/*                    MyThreadScopeData myData = new MyThreadScopeData();
                    myData.setName("name" + data);
                    myData.setAge(data);
                    myThreadScopeData.set(myData);*/
                    MyThreadScopeData.getThreadInstance().setName("name" + data);
                    MyThreadScopeData.getThreadInstance().setAge(data);
                    new A().get();
                    new B().get();
                }
            }).start();
        }
    }
    
    static class A{
        public void get(){
            int data = x.get();
            System.out.println("A from " + Thread.currentThread().getName() 
                    + " get data :" + data);
/*            MyThreadScopeData myData = myThreadScopeData.get();;
            System.out.println("A from " + Thread.currentThread().getName() 
                    + " getMyData: " + myData.getName() + "," +
                    myData.getAge());*/
            //获取与本线程有关的实例
            MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
            System.out.println("A from " + Thread.currentThread().getName() 
                    + " getMyData: " + myData.getName() + "," +
                    myData.getAge());
        }
    }
    
    static class B{
        public void get(){
            int data = x.get();            
            System.out.println("B from " + Thread.currentThread().getName() 
                    + " get data :" + data);
            MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
            System.out.println("B from " + Thread.currentThread().getName() 
                    + " getMyData: " + myData.getName() + "," +
                    myData.getAge());            
        }        
    }
}

class MyThreadScopeData{
    private MyThreadScopeData(){}
    public static /*synchronized*/ MyThreadScopeData getThreadInstance(){
        //此时的map.get是获取自身的变量，当多个线程同时进来时，获取的是线程本身的变量，所以此种方式不需要加synchronized
        MyThreadScopeData instance = map.get();
        if(instance == null){ //一个线程进来，判断==null,这时候还没有赋值，刚好这时候被CPU调度到另一个线程，也判断==null
            //那么会被new 两次
            instance = new MyThreadScopeData();
            map.set(instance);
        }
        return instance;
    }
    //private static MyThreadScopeData instance = null;//new MyThreadScopeData();
    private static ThreadLocal<MyThreadScopeData> map = new ThreadLocal<MyThreadScopeData>();
    
    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}