JAVA多线程学习五：线程范围内共享变量&ThreadLocal

一、概念

可以将每个线程用到的数据与对应的线程号存放到一个map集合中，使用数据时从这个集合中根据线程号获取对应线程的数据，就可以实现线程范围内共享相同的变量。

二、代码

Runnable中的run()方法里面执行Thread.currentThread()都会对应当前Runnable对应的线程，因此A、B中对应的Thread.currentThread()对应所在的Runnable对应的线程

public class ThreadScopeShareData {
 
    private static Map<Thread, Integer> threadData = new HashMap<Thread, Integer>();
    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);
                    threadData.put(Thread.currentThread(), data);
                    new A().get();
                    new B().get();
                }
            }).start();
        }
    }
 
    static class A{
        public void get(){
            int data = threadData.get(Thread.currentThread());
            System.out.println("A from " + Thread.currentThread().getName()
                    + " get data :" + data);
        }
    }
 
    static class B{
        public void get(){
            int data = threadData.get(Thread.currentThread());
            System.out.println("B from " + Thread.currentThread().getName()
                    + " get data :" + data);
        }
    }
}

三、ThreadLocal

JDK1.5提供了ThreadLocal类来方便实现线程范围内的数据共享，它的作用就相当于前面中的Map(内部并不是Map)，也就是让每个线程拥有自己的值
一个ThreadLocal对象只能记录一个线程内部的一个共享变量，需要记录多个共享数据，可以创建多个ThreadLocal对象，或者将这些数据进行封装，将封装后的数据对象存入ThreadLocal对象中。
线程结束后也可以自动释放相关的ThreadLocal变量，也可以调用ThreadLocal.remove()方法用来更快释放内存。

代码：

public class ThreadLocalTest {  
 
    private static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>();
    public static void main(String[] args) {  
 
        //启动两个线程
        for (int i = 0; i < 2; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    //创建每个线程私有的变量
                    int data = new Random().nextInt(100);
                    System.out.println(Thread.currentThread().getName()+" has put data: "+data);
                    //往local里面设置值
                    threadLocal.set(data);
                    new A().get();
                    new B().get();
                }
            }).start();
        }
    }  
 
    static class A{
        public void get(){
            int data =threadLocal.get();
            System.out.println("A from "+Thread.currentThread().getName()+" has get data: "+data);
        }
    }
 
    static class B{
        public void get(){
            int data =threadLocal.get();
            System.out.println("B from "+Thread.currentThread().getName()+" has get data: "+data);
        }
    }
}  

假设需要保存不止一个值，可以把其他属性的值打包成一个类，然后将该类设置成ThreadLocal的值。

下面代码中，在类MyThreadLocalScopeDate里面定义了一个静态变量Map，用来保存所有线程创建的MyThreadLocalScopeDate，并使用单例使得不管多少线程都只创建一个MyThreadLocalScopeDate对象。

public class ThreadLocalTest {  
 
    private static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>();
    public static void main(String[] args) {  
 
        //启动两个线程
        for (int i = 0; i < 2; i++) {
            new Thread(new Runnable() {
                @Override
                public void run() {
                    //创建每个线程私有的变量
                    int data = new Random().nextInt(100);
                    System.out.println(Thread.currentThread().getName()+" has put data: "+data);
                    //往local里面设置值
                    threadLocal.set(data);
                    //获取自己线程的MyThreadLocalScopeDate实例对象
                    MyThreadLocalScopeDate myData = MyThreadLocalScopeDate.getThreadInstance();
                    myData.setName("name"+data);
                    myData.setAge(data);
                    new A().get();
                    new B().get();
                }
            }).start();
        }
    }  
 
    static class A{
        public void get(){
            int data =threadLocal.get();
            System.out.println("A from "+Thread.currentThread().getName()+" has get data: "+data);
            MyThreadLocalScopeDate myData = MyThreadLocalScopeDate.getThreadInstance();
            System.out.println("A from "+Thread.currentThread().getName()+" has get MyThreadLocalScopeDate name: "+myData.getName()+" , age: "+myData.getAge());
        }
    }  
 
    static class B{
        public void get(){
            int data =threadLocal.get();
            System.out.println("B from "+Thread.currentThread().getName()+" has get data: "+data);
            MyThreadLocalScopeDate myData = MyThreadLocalScopeDate.getThreadInstance();
            System.out.println("B from "+Thread.currentThread().getName()+" has get MyThreadLocalScopeDate name: "+myData.getName()+" , age: "+myData.getAge());
        }
    }
}  
 
class MyThreadLocalScopeDate{//单例模式  
 
    private MyThreadLocalScopeDate(){};//构造方法私有化
    private static ThreadLocal<MyThreadLocalScopeDate> map = new ThreadLocal<MyThreadLocalScopeDate>();//封装MyThreadLocalScopeDate是线程实现范围内共享  
 
    //思考AB两个线程过来的情况 自己分析 AB都需要的自己的对象 没有关系 所以不需要同步 如果有关系就需要同步了
    public static /*synchronized*/MyThreadLocalScopeDate getThreadInstance(){
        MyThreadLocalScopeDate instance =map.get();
        if(instance==null){
            instance = new MyThreadLocalScopeDate();
            map.set(instance);
        }
        return instance;
    }  
 
    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;
    }
}

出处：https://www.cnblogs.com/pony1223/p/9251641.html