ThreadLocal 源碼剖析

時間  2019-11-05
標籤 threadlocal 源碼 剖析 欄目 Java 简体版
原文   原文鏈接

ThreadLocal是Java語言提供的用於支持線程局部變量的類。所謂的線程局部變量，就是僅僅只能被本線程訪問，不能在線程之間進行共享訪問的變量(每一個線程一個拷貝)。在各個Java web的各類框架中ThreadLocal幾乎已經被用爛了，spring中有使用，mybatis中也有使用，hibernate中也有使用，甚至咱們寫個分頁也用ThreadLocal來傳遞參數......這也從側面說明了ThreadLocal十分的給力。java

從使用者的角度而言，通常咱們能夠將ThreadLocal看作是一個：ConcurrentHashMap<Thread, Object>，以Thread引用爲key, 來保存本線程的局部變量。可是從實現的角度而言，ThreadLocal的實現根本就不是這樣的。下面從源碼分析ThreadLocal的實現。web

1. 既然是線程局部變量，那麼理所固然就應該存儲在本身的線程對象中，咱們能夠從 Thread 的源碼中找到線程局部變量存儲的地方：spring

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;

 咱們能夠看到線程局部變量是存儲在Thread對象的 threadLocals 屬性中，而 threadLocals 屬性是一個 ThreadLocal.ThreadLocalMap 對象。數組

2. 咱們接着看 ThreadLocal.ThreadLocalMap 是何方神聖mybatis

    /**
     * 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);
        }

能夠看到ThreadLocal.ThreadLocalMap 是 ThreadLocal 的一個靜態內部類。ThreadLocalMap從字面上就能夠看出這是一個保存ThreadLocal對象的map(實際上是以它爲Key)，沒錯，不過是通過了兩層包裝的ThreadLocal對象。第一層包裝是使用 WeakReference<ThreadLocal<?>> 將ThreadLocal對象變成一個弱引用的對象；第二層包裝是 定義了一個專門的類 Entry 來擴展 WeakReference<ThreadLocal<?>>：框架

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

類 Entry 很顯然是一個保存map鍵值對的實體，ThreadLocal<?>爲key, 要保存的線程局部變量的值爲value。super(k)調用的WeakReference的構造函數，表示將ThreadLocal<?>對象轉換成弱引用對象，用作key。less

從 ThreadLocalMap 的構造函數：ide

        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);
        }

能夠看出，ThreadLocalMap這個map的實現是使用一個數組 private Entry[] table 來保存鍵值對的實體，初始大小爲16，ThreadLocalMap本身實現瞭如何從 key  到 value 的映射： firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1)函數

    /**
     * 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);
    }

 使用一個 static 的原子屬性 AtomicInteger nextHashCode，經過每次增長 HASH_INCREMENT = 0x61c88647 ，而後 & (INITIAL_CAPACITY - 1) 取得在數組  private Entry[] table 中的索引。源碼分析

3. 咱們先看一下 Thread 對象中的 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在調用set方法時，若是 getMap(注意是以Thread引用爲key) 返回的 t.threadLocals 爲null，那麼表示該線程的 ThreadLocalMap 尚未初始化，因此調用createMap進行初始化：t.threadLocals = new ThreadLocalMap(this, firstValue);

注意這裏使用到了延遲初始化的技術：

        /**
         * 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);
        }

這裏僅僅是初始化了16個元素的引用數組，並無初始化16個 Entry 對象。而是一個線程中有多少個線程局部對象要保存，那麼就初始化多少個 Entry 對象來保存它們。

到了這裏，咱們能夠思考一下，爲何要這樣實現了。爲何要用 ThreadLocalMap 來保存線程局部對象呢？緣由是一個線程擁有的的局部對象可能有不少，這樣實現的話，那麼無論你一個線程擁有多少個局部變量，都是使用同一個 ThreadLocalMap 來保存的，ThreadLocalMap 中 private Entry[] table 的初始大小是16。超過容量的2/3時，會擴容。

4. 咱們在看一下 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);
    }

 咱們看到是以當前 thread 的引用爲 key, 得到 ThreadLocalMap ，而後調用 map.set(this, value); 保存進 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 涉及到的兩個層面的內存自動回收

1）在 ThreadLocal 層面的內存回收：

/*
 * 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).

當線程死亡時，那麼全部的保存在的線程局部變量就會被回收，其實這裏是指線程Thread對象中的 ThreadLocal.ThreadLocalMap threadLocals 會被回收，這是顯然的。

2）ThreadLocalMap 層面的內存回收：

    /**
     * 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.
     */

若是線程能夠活很長的時間，而且該線程保存的線程局部變量有不少(也就是 Entry 對象不少)，那麼就涉及到在線程的生命期內如何回收 ThreadLocalMap 的內存了，否則的話，Entry對象越多，那麼ThreadLocalMap 就會愈來愈大，佔用的內存就會愈來愈多，因此對於已經不須要了的線程局部變量，就應該清理掉其對應的Entry對象。使用的方式是，Entry對象的key是WeakReference 的包裝，當ThreadLocalMap 的 private Entry[] table，已經被佔用達到了三分之二時 threshold = 2/3(也就是線程擁有的局部變量超過了10個) ，就會嘗試回收 Entry 對象，咱們能夠看到 ThreadLocalMap.set方法中有下面的代碼：

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

  cleanSomeSlots 就是進行回收內存：

        /**
         * 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 調用的是 Entry 的父類 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 已經被回收了，因此能夠回收該 Entry 對象了：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經常使用的接口：

1）須要制定初始值時，能夠覆蓋protected T initialValue()方法；

2）public T get()；

3）public void set(T value)；

4）public void remove()；

7. 總結

1）一個線程中的全部的局部變量其實存儲在該線程本身的同一個map屬性中；

2）線程死亡時，線程局部變量會自動回收內存；

3）線程局部變量時經過一個 Entry 保存在map中，該Entry 的key是一個 WeakReference包裝的ThreadLocal, value爲線程局部變量; 

     key 到 value 的映射是經過：ThreadLocal.threadLocalHashCode & (INITIAL_CAPACITY - 1) 來完成的；

4）當線程擁有的局部變量超過了容量的2/3(沒有擴大容量時是10個)，會涉及到ThreadLocalMap中Entry的回收；

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