digdeep

凡是过去,皆是序幕。Read the fucking manual and source code.

  博客园 :: 首页 :: 博问 :: 闪存 :: 新随笔 :: 联系 :: 订阅 订阅 :: 管理 ::
  339 随笔 :: 0 文章 :: 207 评论 :: 0 引用

ThreadLocal是Java语言提供的用于支持线程局部变量的类。所谓的线程局部变量,就是仅仅只能被本线程访问,不能在线程之间进行共享访问的变量(每个线程一个拷贝)。在各个Java web的各种框架中ThreadLocal几乎已经被用烂了,spring中有使用,mybatis中也有使用,hibernate中也有使用,甚至我们写个分页也用ThreadLocal来传递参数......这也从侧面说明了ThreadLocal十分的给力。

从使用者的角度而言,一般我们可以将ThreadLocal看做是一个:ConcurrentHashMap<Thread, Object>,以Thread引用为key, 来保存本线程的局部变量。但是从实现的角度而言,ThreadLocal的实现根本就不是这样的。下面从源码分析ThreadLocal的实现。

1. 既然是线程局部变量,那么理所当然就应该存储在自己的线程对象中,我们可以从 Thread 的源码中找到线程局部变量存储的地方:

public class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }
    // ... ...
    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;
    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

 我们可以看到线程局部变量是存储在Thread对象的 threadLocals 属性中,而 threadLocals 属性是一个 ThreadLocal.ThreadLocalMap 对象。

2. 我们接着看 ThreadLocal.ThreadLocalMap 是何方神圣

    /**
     * ThreadLocalMap is a customized hash map suitable only for
     * maintaining thread local values. No operations are exported
     * outside of the ThreadLocal class. The class is package private to
     * allow declaration of fields in class Thread.  To help deal with
     * very large and long-lived usages, the hash table entries use
     * WeakReferences for keys. However, since reference queues are not
     * used, stale entries are guaranteed to be removed only when
     * the table starts running out of space.
     */
    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;
        // ... ...
        /**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        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);
        }

可以看到ThreadLocal.ThreadLocalMap 是 ThreadLocal 的一个静态内部类。ThreadLocalMap从字面上就可以看出这是一个保存ThreadLocal对象的map(其实是以它为Key),没错,不过是经过了两层包装的ThreadLocal对象。第一层包装是使用 WeakReference<ThreadLocal<?>> 将ThreadLocal对象变成一个弱引用的对象;第二层包装是 定义了一个专门的类 Entry 来扩展 WeakReference<ThreadLocal<?>>:

        static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;
            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

类 Entry 很显然是一个保存map键值对的实体,ThreadLocal<?>为key, 要保存的线程局部变量的值为value。super(k)调用的WeakReference的构造函数,表示将ThreadLocal<?>对象转换成弱引用对象,用做key。

从 ThreadLocalMap 的构造函数:

        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);
        }

可以看出,ThreadLocalMap这个map的实现是使用一个数组 private Entry[] table 来保存键值对的实体,初始大小为16,ThreadLocalMap自己实现了如何从 key  到 value 的映射: firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1)

    /**
     * ThreadLocals rely on per-thread linear-probe hash maps attached
     * to each thread (Thread.threadLocals and
     * inheritableThreadLocals).  The ThreadLocal objects act as keys,
     * searched via threadLocalHashCode.  This is a custom hash code
     * (useful only within ThreadLocalMaps) that eliminates collisions
     * in the common case where consecutively constructed ThreadLocals
     * are used by the same threads, while remaining well-behaved in
     * less common cases.
     */
    private final int threadLocalHashCode = nextHashCode();
    /**
     * The next hash code to be given out. Updated atomically. Starts at
     * zero.
     */
    private static AtomicInteger nextHashCode = new AtomicInteger();
    /**
     * The difference between successively generated hash codes - turns
     * implicit sequential thread-local IDs into near-optimally spread
     * multiplicative hash values for power-of-two-sized tables.
     */
    private static final int HASH_INCREMENT = 0x61c88647;
    /**
     * Returns the next hash code.
     */
    private static int nextHashCode() {
        return nextHashCode.getAndAdd(HASH_INCREMENT);
    }

 使用一个 static 的原子属性 AtomicInteger nextHashCode,通过每次增加 HASH_INCREMENT = 0x61c88647 ,然后 & (INITIAL_CAPACITY - 1) 取得在数组  private Entry[] table 中的索引。

3. 我们先看一下 Thread 对象中的 ThreadLocal.ThreadLocalMap threadLocals = null; 如何初始化:

    /**
     * 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);
    }

ThreadLocal在调用set方法时,如果 getMap(注意是以Thread引用为key) 返回的 t.threadLocals 为null,那么表示该线程的 ThreadLocalMap 还没有初始化,所以调用createMap进行初始化:t.threadLocals = new ThreadLocalMap(this, firstValue);

注意这里使用到了延迟初始化的技术:

        /**
         * Construct a new map initially containing (firstKey, firstValue).
         * ThreadLocalMaps are constructed lazily, so we only create
         * one when we have at least one entry to put in it.
         */
        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);
        }

这里仅仅是初始化了16个元素的引用数组,并没有初始化16个 Entry 对象。而是一个线程中有多少个线程局部对象要保存,那么就初始化多少个 Entry 对象来保存它们

到了这里,我们可以思考一下,为什么要这样实现了。为什么要用 ThreadLocalMap 来保存线程局部对象呢?原因是一个线程拥有的的局部对象可能有很多,这样实现的话,那么不管你一个线程拥有多少个局部变量,都是使用同一个 ThreadLocalMap 来保存的ThreadLocalMap 中 private Entry[] table 的初始大小是16。超过容量的2/3时,会扩容。

4. 我们在看一下 ThreadLocal.set 方法:

    /**
     * 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);
    }

 我们看到是以当前 thread 的引用为 key, 获得 ThreadLocalMap ,然后调用 map.set(this, value); 保存进 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();
        }

5. ThreadLocal 涉及到的两个层面的内存自动回收

1)在 ThreadLocal 层面的内存回收:

/*
 * Each thread holds an implicit reference to its copy of a thread-local
 * variable as long as the thread is alive and the {@code ThreadLocal}
 * instance is accessible; after a thread goes away, all of its copies of
 * thread-local instances are subject to garbage collection (unless other
 * references to these copies exist).

当线程死亡时,那么所有的保存在的线程局部变量就会被回收,其实这里是指线程Thread对象中的 ThreadLocal.ThreadLocalMap threadLocals 会被回收,这是显然的。

2)ThreadLocalMap 层面的内存回收:

    /**
     * ThreadLocalMap is a customized hash map suitable only for
     * maintaining thread local values. No operations are exported
     * outside of the ThreadLocal class. The class is package private to
     * allow declaration of fields in class Thread.  To help deal with
     * very large and long-lived usages, the hash table entries use
     * WeakReferences for keys. However, since reference queues are not
     * used, stale entries are guaranteed to be removed only when
     * the table starts running out of space.
     */

如果线程可以活很长的时间,并且该线程保存的线程局部变量有很多(也就是 Entry 对象很多),那么就涉及到在线程的生命期内如何回收 ThreadLocalMap 的内存了,不然的话,Entry对象越多,那么ThreadLocalMap 就会越来越大,占用的内存就会越来越多,所以对于已经不需要了的线程局部变量,就应该清理掉其对应的Entry对象。使用的方式是,Entry对象的key是WeakReference 的包装,当ThreadLocalMap private Entry[] table,已经被占用达到了三分之二时 threshold = 2/3(也就是线程拥有的局部变量超过了10个) ,就会尝试回收 Entry 对象,我们可以看到 ThreadLocalMap.set方法中有下面的代码:

            if (!cleanSomeSlots(i, sz) && sz >= threshold)
                rehash();

  cleanSomeSlots 就是进行回收内存:

        /**
         * Heuristically scan some cells looking for stale entries.
         * This is invoked when either a new element is added, or
         * another stale one has been expunged. It performs a
         * logarithmic number of scans, as a balance between no
         * scanning (fast but retains garbage) and a number of scans
         * proportional to number of elements, that would find all
         * garbage but would cause some insertions to take O(n) time.
         *
         * @param i a position known NOT to hold a stale entry. The
         * scan starts at the element after i.
         *
         * @param n scan control: {@code log2(n)} cells are scanned,
         * unless a stale entry is found, in which case
         * {@code log2(table.length)-1} additional cells are scanned.
         * When called from insertions, this parameter is the number
         * of elements, but when from replaceStaleEntry, it is the
         * table length. (Note: all this could be changed to be either
         * more or less aggressive by weighting n instead of just
         * using straight log n. But this version is simple, fast, and
         * seems to work well.)
         *
         * @return true if any stale entries have been removed.
         */
        private boolean cleanSomeSlots(int i, int n) {
            boolean removed = false;
            Entry[] tab = table;
            int len = tab.length;
            do {
                i = nextIndex(i, len);
                Entry e = tab[i];
                if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }
e.get() == null 调用的是 Entry 的父类 WeakReference<ThreadLocal<?>> 的方法:
    /**
     * Returns this reference object's referent.  If this reference object has
     * been cleared, either by the program or by the garbage collector, then
     * this method returns <code>null</code>.
     *
     * @return   The object to which this reference refers, or
     *           <code>null</code> if this reference object has been cleared
     */
    public T get() {
        return this.referent;
    }

返回 null ,表示 Entry 的 key 已经被回收了,所以可以回收该 Entry 对象了expungeStaleEntry(i)

        /**
         * Expunge a stale entry by rehashing any possibly colliding entries
         * lying between staleSlot and the next null slot.  This also expunges
         * any other stale entries encountered before the trailing null.  See
         * Knuth, Section 6.4
         *
         * @param staleSlot index of slot known to have null key
         * @return the index of the next null slot after staleSlot
         * (all between staleSlot and this slot will have been checked
         * for expunging).
         */
        private int expungeStaleEntry(int staleSlot) {
            Entry[] tab = table;
            int len = tab.length;

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

 6. ThreadLocal常用的接口:

1)需要制定初始值时,可以覆盖protected T initialValue()方法;

2)public T get();

3)public void set(T value);

4)public void remove();

7. 总结

1)一个线程中的所有的局部变量其实存储在该线程自己的同一个map属性中;

2)线程死亡时,线程局部变量会自动回收内存;

3)线程局部变量时通过一个 Entry 保存在map中,该Entry 的key是一个 WeakReference包装的ThreadLocal, value为线程局部变量; 

     key 到 value 的映射是通过:ThreadLocal.threadLocalHashCode & (INITIAL_CAPACITY - 1) 来完成的;

4)当线程拥有的局部变量超过了容量的2/3(没有扩大容量时是10个),会涉及到ThreadLocalMap中Entry的回收;

 

posted on 2015-05-17 23:39  digdeep  阅读(4825)  评论(2编辑  收藏
不懂数据库和Web安全的架构师不是一个好的程序员。