ThreadLocal实例以及底层原理

public class ThreadLocalTest {
    public static ThreadLocal threadLocal = new ThreadLocal<>();
    public static void main(String[] args) {
        Thread thread_1=new Thread(new Runnable() {
            @Override
            public void run() {
                threadLocal.set("-------thread_1-------");
                Print("thread_1");
                System.out.println("thread_1 after remove:"+threadLocal.get());
            }
        });
        Thread thread_2=new Thread(new Runnable() {
            @Override
            public void run() {
                threadLocal.set("-------thread_2-------");
                Print("thread_2");
                System.out.println("thread_2 after remove:"+threadLocal.get());
            }
        });
        thread_1.start();
        thread_2.start();
    }
    public static void Print(String name)
    {
        System.out.println(name+"："+threadLocal.get());
        threadLocal.remove();
    }
}

底层原理:

Thread类中有两个变量，分别为ThreadLocals和inheritableThreadLocals,这两个变量都属于ThreadLocalMap数据类型，而ThreadLocalMap是一种定制的HashMap,每个线程中的这两个变量都是null，只有当调用了ThreadLocal的set()或者get()方法后，这两个变量才会被创建。每个线程的本地变量不是存放在ThreadLocal的实例中的，而是存放在相应的ThreadLocals变量中的，当ThreadLocal调用了set()之后，会把每个线程对应的本地变量存放在ThreadLocals变量中，get()也是从ThreadLocals变量中取出的，remove()同理。

ThreadLocal底层Set()方法实现:

 public void set(T value)
    {
        //获取当前线程
        Thread t = Thread.currentThread();
        //将当前线程当作key，去查找相应的本地变量threadLocals
        ThreadLocalMap map  = getMap(t);
        //如果map不为空，说明不是第一次调用set方法
        if(map!=null)
        {
            map.set(this,value);
        }
        //如果为空，要先创建当前线程的threadLocals变量
        else
            createMap(t,value);
    }
    public ThreadLocalMap getMap(Thread t)
    {
        return t.threadLocals;
    }
    public void createMap(Thread t,T value)
    {
        t.threadLocals=new ThreadLocalMap(this,value);
    }

ThreadLocal底层Get()方法实现:

 public T get()
    {
        //获取当前线程
        Thread t = Thread.currentThread();
        //将当前线程当作key，去查找相应的本地变量threadLocals
        ThreadLocalMap map  = getMap(t);
        //如果map不为空，说明不是第一次调用get方法
        if(map!=null)
        {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if(e!=null)
            {
                T result = (T)e.value;
                return result;
            }
        }
        //如果为空，初始化threadLocals
        else
            return setInitialValue();
    }
    private T setInitialValue() {
        T value = null;
        set(value);
        return null;
    }