ThreadLocal深入理解修订版

本文是传智博客多线程视频的学习笔记。

原版本见

http://blog.csdn.net/dlf123321/article/details/42531979

ThreadLocal是一个和线程安全相关的类。

说的很模糊,咱们看一个图

一个非线程安全的例子

import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class ThreadScopeDataShare {
    static private int data = 0;

    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newCachedThreadPool();
        for (int i = 0; i <2; i++) {
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    int data2= new Random().nextInt(100);
                    System.out.println(Thread.currentThread().getName()+" put "+data2);
                    data=data2;
                    try {
                        Thread.sleep(1000); //为什么要睡1秒 大家懂吗?
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    new A().get();
                    new B().get();

                }
            });
        }
        threadPool.shutdown();
    }

    public static int getData() {
        return data;
    }

}

class A {
    public int get() {
        int data = ThreadScopeDataShare.getData();
        System.out
            .println("a  "+Thread.currentThread().getName() + " getdata " + data);
        return data;
    }
}

class B {
    public int get() {
        int data = ThreadScopeDataShare.getData();
        System.out
            .println("b  "+Thread.currentThread().getName() + " getdata " + data);
        return data;
    }
}

改进版

public class ThreadScopeShareData3 {

    private static Map threadData = new HashMap();
    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newCachedThreadPool();
        for (int i = 0; i <2; i++) {
            threadPool.execute(new Runnable() {
                @Override
                public void run() {
                    int data2= new Random().nextInt(100);
                    System.out.println(Thread.currentThread().getName()+" put "+data2);
                    threadData.put(Thread.currentThread(),data2);
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    new A().get();
                    new B().get();

                }
            });
        }
        threadPool.shutdown();
    }
    
    static class A{
        public void get(){
            int data = threadData.get(Thread.currentThread());
            System.out.println("A from " + Thread.currentThread().getName()
                    + " get data :" + data);
        }
    }
    //省略class B
}

运行结果

pool-1-thread-1 put 2
pool-1-thread-2 put 99
A from pool-1-thread-2 get data :99
A from pool-1-thread-1 get data :2
B from pool-1-thread-1 get data :2
B from pool-1-thread-2 get data :99

ThreadLocal的简单介绍

public class ThreadLocalTest {

    private static ThreadLocal x = new ThreadLocal();
    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(500);
                    System.out.println(Thread.currentThread().getName()
                            + " has put data :" + data);
                    x.set(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);
        }
    }
    
    static class B{
        public void get(){
            int data = x.get();            
            System.out.println("B from " + Thread.currentThread().getName()
                    + " get data :" + data);        
        }        
    }
}

import java.util.Random;

public class ThreadLocalTest3 {

    private static ThreadLocal myThreadScopeData = new ThreadLocal();

    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(500);

                    MyThreadScopeData2 myData = new MyThreadScopeData2();
                    myData.setName("name" + data);
                    myData.setAge(data);
                    myThreadScopeData.set(myData);

                    new A().get();
                    new B().get();
                }
            }).start();
        }
    }

    static class A {
        public void get() {
            MyThreadScopeData2 myData = myThreadScopeData.get();

            System.out
                    .println("A from " + Thread.currentThread().getName()
                            + " getMyData: " + myData.getName() + ","
                            + myData.getAge());
        }
    }

        //省略class B
}

class MyThreadScopeData2 {

    private static ThreadLocal map = new ThreadLocal();

    private String name;
    private int age;

    //省略get set
}

import java.util.Random;

public class ThreadLocalTest2 {


    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(500);

                    MyThreadScopeData.getThreadInstance().setName("name" + data);
                    MyThreadScopeData.getThreadInstance().setAge(data);
                    new A().get();
                    new B().get();
                }
            }).start();
        }
    }
    
    static class A{
        public void get(){

            MyThreadScopeData myData = MyThreadScopeData.getThreadInstance();
            System.out.println("A from " + Thread.currentThread().getName()
                    + " getMyData: " + myData.getName() + "," +
                    myData.getAge());
        }
    }
    //省略Class B
}

class MyThreadScopeData{
    private MyThreadScopeData(){}
    public static  MyThreadScopeData getThreadInstance(){
        MyThreadScopeData instance = map.get();
        if(instance == null){
            instance = new MyThreadScopeData();
            map.set(instance);
        }
        return instance;
    }

    private static ThreadLocal map =
            new ThreadLocal();
    
    private String name;
    private int age;
    //省略getset
}