ThreadLocal 的原理及问题，一网打尽！

private ThreadLocal<Boolean> mThreadLocal = new ThreadLocal<>();

private void threadLocal() {
        mThreadLocal.set(true);
        Log.d(TAG, "[Thread#main]threadLocal=" + mThreadLocal.get());
        new Thread() {
            @Override
            public void run() {
                super.run();
                mThreadLocal.set(false);
                Log.d(TAG, "[Thread#1]threadLocal=" + mThreadLocal.get());
            }
        }.start();

        new Thread() {
            @Override
            public void run() {
                super.run();
                Log.d(TAG, "[Thread#2]threadLocal=" + mThreadLocal.get());
            }
        }.start();
    }

    /**
     * Sets the current thread's copy of this thread-local variable
     * to the specified value.  Most subclasses will have no need to
     * override this method, relying solely on the {@link #initialValue}
     * method to set the values of thread-locals.
     *
     * @param value the value to be stored in the current thread's copy of
     *        this thread-local.
     */
    public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

    /**
     * Get the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param  t the current thread
     * @return the map
     */
    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

    /**
     * Create the map associated with a ThreadLocal. Overridden in
     * InheritableThreadLocal.
     *
     * @param t the current thread
     * @param firstValue value for the initial entry of the map
     */
    void createMap(Thread t, T firstValue) {
        t.threadLocals = new ThreadLocalMap(this, firstValue);
    }

public class Thread implements Runnable {
   /* ThreadLocal values pertaining to this thread. This map is maintained
    * by the ThreadLocal class.
    */
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

       static class ThreadLocalMap {

        /**
         * The entries in this hash map extend WeakReference, using
         * its main ref field as the key (which is always a
         * ThreadLocal object).  Note that null keys (i.e. entry.get()
         * == null) mean that the key is no longer referenced, so the
         * entry can be expunged from table.  Such entries are referred to
         * as "stale entries" in the code that follows.
         */
        static class Entry extends WeakReference<ThreadLocal> {
            /** The value associated with this ThreadLocal. */
            Object value;
            // 类似于map的key，value结构，key就是ThreadLocal，value就是需要隔离访问的变量
            Entry(ThreadLocal> k, Object v) {
                super(k);
                value = v;
            }
        }
        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        // 用数组保存了 Entry，因为可能有多个变量需要线程隔离访问
        private Entry[] table;
}

        /**
         * Set the value associated with key.
         *
         * @param key the thread local object
         * @param value the value to be set
         */
        private void set(ThreadLocal key, Object value) {

            // We don't use a fast path as with get() because it is at
            // least as common to use set() to create new entries as
            // it is to replace existing ones, in which case, a fast
            // path would fail more often than not.

            Entry[] tab = table;
            int len = tab.length;
            int i = key.threadLocalHashCode & (len-1);

            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal k = e.get();

                if (k == key) {
                    e.value = value;
                    return;
                }

                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }

            tab[i] = new Entry(key, value);
            int sz = ++size;
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

    /**
     * Returns the value in the current thread's copy of this
     * thread-local variable.  If the variable has no value for the
     * current thread, it is first initialized to the value returned
     * by an invocation of the {@link #initialValue} method.
     *
     * @return the current thread's value of this thread-local
     */
    public T get() {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null) {
                @SuppressWarnings("unchecked")
                T result = (T)e.value;
                return result;
            }
        }
        return setInitialValue();
    }

    /**
     * Variant of set() to establish initialValue. Used instead
     * of set() in case user has overridden the set() method.
     *
     * @return the initial value
     */
    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;
    }

protected T initialValue() {
   return null;
}

/**
 * 类说明：ThreadLocal造成的内存泄漏演示
 */
public class ThreadLocalOOM {
    private static final int TASK_LOOP_SIZE = 500;

    final static ThreadPoolExecutor poolExecutor
            = new ThreadPoolExecutor(5, 5,
            1,
            TimeUnit.MINUTES,
            new LinkedBlockingQueue<>());

    static class LocalVariable {
        private byte[] a = new byte[1024*1024*5];/*5M大小的数组*/
    }

    final static ThreadLocal localVariable
            = new ThreadLocal<>();

    public static void main(String[] args) throws InterruptedException {
        Object o = new Object();
        /*5*5=25*/
        for (int i = 0; i < TASK_LOOP_SIZE; ++i) {
            poolExecutor.execute(new Runnable() {
                public void run() {
                    //localVariable.set(new LocalVariable());
                    new LocalVariable();
                    System.out.println("use local varaible");
                    //localVariable.remove();
                }
            });

            Thread.sleep(100);
        }
        System.out.println("pool execute over");
    }
}

/**
 * 类说明：ThreadLocal的线程不安全演示
 */
public class ThreadLocalUnsafe implements Runnable {

    public static Number number = new Number(0);

    public void run() {
        //每个线程计数加一
        number.setNum(number.getNum() + 1);
        //将其存储到ThreadLocal中
        value.set(number);
        SleepTools.ms(2);
        //输出num值
        System.out.println(Thread.currentThread().getName() + "=" + value.get().getNum());
    }

    public static ThreadLocal value = new ThreadLocal() {
    };

    public static void main(String[] args) {
        for (int i = 0; i < 5; i++) {
            new Thread(new ThreadLocalUnsafe()).start();
        }
    }

    private static class Number {
        public Number(int num) {
            this.num = num;
        }

        private int num;

        public int getNum() {
            return num;
        }

        public void setNum(int num) {
            this.num = num;
        }

        @Override
        public String toString() {
            return "Number [num=" + num + "]";
        }
    }
}