ThreadLocal解析

简介

This class provides thread-local variables. These variables differ from their normal counterparts in that each thread that accesses one (via its get or set method) has its own, independently initialized copy of the variable. ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread (e.g., a user ID or Transaction ID).

简而言之，ThreadLocal可以创建一个只能被当前线程访问或修改的变量。

分析

Q：如何实现线程隔离？

使用Thread对象中的ThreadLocalMap进行数据存储。也就是ThreadLocal将数据存储到当前的线程对象中，通过Thread.currentThread()来获取线程，再通过getMap(t)来获取ThreadLocalMap。具体内容通过阅读源码来逐步分析。

get

返回当前线程存储的ThreadLocal值，如果不存在，会进行初始化并返回。通过map.getEntry(this)。

/**
 * 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() {
    //获取当前对象
    Thread t = Thread.currentThread();
    //通过getMap获取ThreadLocalMap
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        //获取entry
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //不存在则进行初始化
    return setInitialValue();
}

getMap

返回指定线程的ThreadLocalMap。

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

Thread中关于ThreadLocalMap的部分：

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

}

setInitialValue

get方法获取不到值时，通过该方法设置初始值。

/**
 * 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;
}

createMap

ThreadLocalMap不存在时，通过该方法创建，这里有一个疑惑：

Q：为什么在get和setInitialValue进行两次为空检查才进行createMap？

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

set

通过调用map.set(this,value)实现。map中的key是当前ThreadLocal对象。

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

ThreadLocalMap

内部类和成员变量代码如下：

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;

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

        /**
         * The initial capacity -- MUST be a power of two.
         */
        private static final int INITIAL_CAPACITY = 16;

        /**
         * The table, resized as necessary.
         * table.length MUST always be a power of two.
         */
        private Entry[] table;

        /**
         * The number of entries in the table.
         */
        private int size = 0;

        /**
         * The next size value at which to resize.
         */
        private int threshold; // Default to 0
}

set

流程：

• 计算下标，得到下标i
• 进行遍历，如果i对应的key和传入相等，直接返回。如果对象已被回收，调用replaceStaleEntry()并返回。
• 如果i对应的不相等，则从i开始从table数组完后找。
• 能找到则返回，找不到新建entry进行存储。
• 检查是否需要rehash()

/**
         * 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;
            //获取hash后的下标
            int i = key.threadLocalHashCode & (len-1);
			//遍历
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                ThreadLocal<?> k = e.get();
				//如果对应下标的ThreadLocal与当前的相等，直接更新
                if (k == key) {
                    e.value = value;
                    return;
                }
				//如果ThreadLocal对象已被回收，调用replaceStaleEntry
                if (k == null) {
                    replaceStaleEntry(key, value, i);
                    return;
                }
            }
			//如果遍历一圈还是找不到，新建entry进行存储
            tab[i] = new Entry(key, value);
            int sz = ++size;
            //检查是否需要rehash
            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();
        }

下标计算方式：

通过AtomicInteger + 0x61c88647 & (len-1)来获取下标。

int i = key.threadLocalHashCode & (len-1);

private final int threadLocalHashCode = nextHashCode();

private static AtomicInteger nextHashCode = new AtomicInteger();

private static final int HASH_INCREMENT = 0x61c88647;

private static int nextHashCode() {
    return nextHashCode.getAndAdd(HASH_INCREMENT);
}

getEntry

流程：

• 计算下标，得到下标i
• 判断对应下标的entry是否存在，key是否相等，符合要求则直接返回。
• 不符合则调用getEntryAfterMiss()

/**
 * Get the entry associated with key.  This method
 * itself handles only the fast path: a direct hit of existing
 * key. It otherwise relays to getEntryAfterMiss.  This is
 * designed to maximize performance for direct hits, in part
 * by making this method readily inlinable.
 *
 * @param  key the thread local object
 * @return the entry associated with key, or null if no such
 */
private Entry getEntry(ThreadLocal<?> key) {
    //计算下标
    int i = key.threadLocalHashCode & (table.length - 1);
    Entry e = table[i];
    if (e != null && e.get() == key)
        return e;
    else
        //entry不存在或key不相等
        return getEntryAfterMiss(key, i, e);
}

getEntryAfterMiss

也就是对table数组进行遍历查找，找一圈还没有则返回null。

private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) {
    Entry[] tab = table;
    int len = tab.length;

    while (e != null) {
        ThreadLocal<?> k = e.get();
        if (k == key)
            return e;
        if (k == null)
            expungeStaleEntry(i);
        else
            i = nextIndex(i, len);
        e = tab[i];
    }
    return null;
}

remove

流程：

• 计算下标
• 判断key是否相等，符合要求则调用e.clear()来清除key，并调用expungeStaleEntry清除value。

/**
         * Remove the entry for key.
         */
        private void remove(ThreadLocal<?> key) {
            Entry[] tab = table;
            int len = tab.length;
            //计算下标
            int i = key.threadLocalHashCode & (len-1);
            for (Entry e = tab[i];
                 e != null;
                 e = tab[i = nextIndex(i, len)]) {
                //如果key相等
                if (e.get() == key) {
                    e.clear();
                    expungeStaleEntry(i);
                    return;
                }
            }
        }

内存泄露问题

问题分析

从上文可以看到，Entry继承自WeakReference，弱引用指向的对象会在下一次GC时被回收。

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

​ 在初始化时，将弱引用指向ThreadLocal实例。如果外部没有强引用指向ThreadLocal的话，那么Thread Local实例就没有一条引用链路可达，则会被回收。此时entry的key未null，但是有value，但是不能通过key找到value，这样就会存在内存泄漏问题。但是如果线程结束，以上内存都会被回收，也就不存在上述问题。

​ 如果使用线程池去维护线程，线程池并不会主动销毁内部的线程，总会存在着强引用，那么还是会存在问题。

总结问题出现的原因：

1. 线程一直运行，不终止（线程池）
2. null-value：某个弱引用key被回收

如何解决

上述的getEntryAfterMiss()如果判断到了key == null则会调用expungeStaleEntry()将value删除。

private int expungeStaleEntry(int staleSlot) {
    Entry[] tab = table;
    int len = tab.length;

    // expunge entry at staleSlot
    tab[staleSlot].value = null;
    tab[staleSlot] = null;
    size--;

    // Rehash until we encounter null
    Entry e;
    int i;
    for (i = nextIndex(staleSlot, len);
         (e = tab[i]) != null;
         i = nextIndex(i, len)) {
        ThreadLocal<?> k = e.get();
        if (k == null) {
            e.value = null;
            tab[i] = null;
            size--;
        } else {
            int h = k.threadLocalHashCode & (len - 1);
            if (h != i) {
                tab[i] = null;

                // Unlike Knuth 6.4 Algorithm R, we must scan until
                // null because multiple entries could have been stale.
                while (tab[h] != null)
                    h = nextIndex(h, len);
                tab[h] = e;
            }
        }
    }
    return i;
}

该方法在set、get、remove中都会调用。因此还有一个问题：如果创建了ThreadLocal但不调用以上方法，还是会存在问题。

所以最能解决办法的就是用完ThreadLocal之后显式执行ThreadLocal.remove()。

为什么key不设置为强引用

既然弱引用会存在内存泄露问题，为什么不使用强引用呢？

如果将key设置为强引用，当ThreadLocal实例释放后，但是ThreadLocal强引用指向threadLocalMap，threadLocalMap.Entry又强引用指向ThreadLocal，这样会导致ThreadLocal无法回收。也就会出现更为严重的问题。

参考

Java SE 8 Docs API

https://www.jianshu.com/p/dde92ec37bd1