java多线程学习-ThreadLocal

为了凑字，把oracle文档里介绍ThreadLocal抄过来

public class ThreadLocal<T>
extends Object

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable.ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).

For example, the class below generates unique identifiers local to each thread. A thread's id is assigned the first time it invokes ThreadId.get() and remains unchanged on subsequent calls.

 import java.util.concurrent.atomic.AtomicInteger;

 public class ThreadId {
     // Atomic integer containing the next thread ID to be assigned
     private static final AtomicInteger nextId = new AtomicInteger(0);

     // Thread local variable containing each thread's ID
     private static final ThreadLocal<Integer> threadId =
         new ThreadLocal<Integer>() {
             @Override protected Integer initialValue() {
                 return nextId.getAndIncrement();
         }
     };

     // Returns the current thread's unique ID, assigning it if necessary
     public static int get() {
         return threadId.get();
     }
 }
 

Each thread holds an implicit reference to its copy of a thread-local variable as long as the thread is alive and the ThreadLocal instance is accessible; after a thread goes away, all of its copies of thread-local instances are subject to garbage collection (unless other references to these copies exist).

 接下来一个示例

import java.util.Random;

public class ThreadLocalTest {
    
    public static void main(String[] args) {
        for (int i = 0; i < 2; i++) {
            
            new Thread(new Runnable(){
                public void run() {
                    int age = new Random().nextInt();
                    MyData md = MyData.getInstance();
                    md.setAge(age);
                    md.setName(Thread.currentThread().getName());
                    
                    System.out.println(Thread.currentThread().getName()+ " age is "+age);
                    new A().get();
                    new B().get();
                    
                };
            }).start();
        
        }
    }
    
    
    static class A{
        public void get(){
            MyData md = MyData.getInstance();
            System.out.println(Thread.currentThread().getName()+" A age "+md.getAge() + " name = "+md.getName());
        }
    }
    
    static class B{
        public void get(){
            MyData md = MyData.getInstance();
            System.out.println(Thread.currentThread().getName()+" B age "+md.getAge() + " name = "+md.getName());
        }
    }
}



class MyData{
    
    private MyData(){
    }
    
    public static MyData md = null;
    private static ThreadLocal<MyData> tl = new ThreadLocal<MyData>();
    
    public synchronized static MyData getInstance(){
         md  = tl.get();
        if(null==md){
            md = new MyData();
            tl.set(md);
        }
        return md;
    }
    
    
    
    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;
    }
    
}

想了半天不知如何表达，语文太差。

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