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

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }
        //......
    }

一句话总结就是：Thread维护了ThreadLocalMap，而ThreadLocalMap里维护了Entry，而Entry里存的是以ThreadLocal为key，传入的值为value的键值对。

主要方法

get

public T get() {
        Thread t = Thread.currentThread();
        // 获取当前线程，通过当前线程获取该线程的threadlocalmap
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            // 根据threadlocal实例获取到该map中的指定entry
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                // 取出值
                T result = (T)e.value;
                return result;
            }
        }
        // 如果map为空，则设置默认值
        return setInitialValue();
    }

    private T setInitialValue() {
        T value = initialValue();// null
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            // 默认：this，null
            map.set(this, value);
        else
            // 如果连map都没初始化，则构造map
            createMap(t, value);
        return value;
    }    

    protected T initialValue() {
        return null;
    }

    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

set

public void set(T value) {
        // 获取到当前线程的threadlocalmap
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            // map不为空则设置值
            map.set(this, value);
        else
            // 为空则构造map
            createMap(t, value);
    }

remove，每次使用完threadlocal的value后手动remove也是防止内存泄露的最佳手段。

public void remove() {
         // 根据当前threadlocal实例移除指定entry
         ThreadLocalMap m = getMap(Thread.currentThread());
         if (m != null)
             m.remove(this);
     }

为什么会出现内存泄漏？

内存泄漏原因

原因在于threadlocal的entry的key是弱引用，value是强引用

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

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