ThreadLocal

1.线程隔离

//常用操作
static ThreadLocal<threadLocaltlTest> myThreadLocal= new ThreadLocal<>();
myThreadLocal.set(new threadLocaltlTest());
myThreadLocal.get();

    /**
     * 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);
    }

其中 ThreadLocalMap map = getMap(t);获取的是t线程的成员变量threadLocals：

    ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

每次执行

myThreadLocal.set(new threadLocaltlTest());

时都是向每个线程各自独立的成员变量ThreadLocal.ThreadLocalMap threadLocals中填充数据，从而实现线程隔离。

2.内存泄露

情况1：

map.set(this.value)方法的核心是

        /**
         * 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;

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

map中每个entry对象的key都是通过弱引用指向传入的实例化的ThreadLocal对象

使用弱引用的原因：

如果是使用强引用，则当myThreadLocal = null时，map中还有强引用指向原来myThreadLocal 指向的那个ThreadLocal对象，那么ThreadLocal对象则无法被gc回收，造成内存泄露。

如果是使用弱引用，则当myThreadLocal = null时，map中指向原来myThreadLocal 指向的哪个ThreadLocal对象的引用是弱引用，那么gc执行时ThreadLocal对象就会被清理。

情况2：

当myThreadLocal = null时，通过gc，使得map中的key也是null，那么就无法通过key得到value，需要对该entry进行remove操作，如果不能及时的对该entry进行remove就会造成内存泄露。