深入理解ThreadLocal的原理

 final ThreadLocal threadLocal = new ThreadLocal<>();
        Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                //threadLocal.set("11");
                System.out.println(Thread.currentThread().getName()+"@"+threadLocal.get());
            }
        },"thread1");
        Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                threadLocal.set("22");
                System.out.println(Thread.currentThread().getName()+"@"+threadLocal.get());
            }
        },"thread2");

        thread2.start();
        thread1.start();

运行结果---------------------
thread1@null
thread2@22

 从源码分析原因：

public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

 从set方法中，可以明确看出，每个thread 一定维护一个ThreadLocalMap map ；因为getMap方法是根据当前线程对象返回的；接下来看creatMap方法；

void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }
ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
            table = new Entry[INITIAL_CAPACITY];
            int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
            table[i] = new Entry(firstKey, firstValue);
            size = 1;
            setThreshold(INITIAL_CAPACITY);
        }

    在createMap方法中，每个thread维护一个threadLocals属性；其中的this，指向ThreadLocal 对象引用； 而ThreadLocalMap是在ThreadLocal 内部维护的内部类；因此当   指向threadLocal.set（"22"） 时，先判断当前线程对象的threadLocals存不存在，不存在直接调用createMap 方法，将threadLocal当做ThreadLocalMap 的key,value 当做ThreadLocalMap 的value值，封装一个对象数组；

static class Entry extends WeakReference {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal k, Object v) {
                super(k);
                value = v;
            }
        }

由于threadLocal 作为弱引用key，因此在get方法调用完成后，及时调用remove方法，GC掉ThreadLocalMap 对象，否则容易造成内存泄漏；

接着从get方法理解为什么thread1 线程对象调用threadLocal.get（）方法获取到值为null；因为在代码执行到ThreadLocalMap map = getMap(t);这一行ThreadLocalMap map 对象为空；直接走到setInitialValue()；方法；而该方法会创建一个ThreadLocalMap 对象，而默认的value值为null;从initialValue方法返回值得知；

 public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }
 private Entry getEntry(ThreadLocal key) {
            int i = key.threadLocalHashCode & (table.length - 1);
            Entry e = table[i];
            if (e != null && e.get() == key)
                return e;
            else
                return getEntryAfterMiss(key, i, e);
        }

protected T initialValue() {
        return null;
    }

 private T setInitialValue() {
        T value = initialValue();
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
        return value;
    }