ThreadLocal

作用

• 官方解释
  
  • 这个类提供线程本地变量。这些变量与他们正常对应的变量不同，因为每一个线程都可以通过ThreadLocal的set和get方法访问属于自己的且独立初始化的变量副本。在类中ThreadLocal对象通常是私有的静态字段，用来维护（或关联）与线程相关的状态。
• 网络解释
  
  • ThreadLocal 的作用是提供线程内的局部变量，这种变量在线程的生命周期内起作用，减少同一个线程内多个函数或者组件之间一些公共变量的传递的复杂度

常用方法

• get
  
  • 返回线程本地变量在当前线程中的副本。如果这个变量在当前线程没有值，它将使用initialValue方法的返回值第一次初始化。
• set
  
  • 给线程本地变量在当前线程中的副本设置指定的值。由与线程本地变量副本的初始化单独地依赖于initValue方法，所以大部分的子类不需要重写该方法。
• initialValue
  
  • 为线程本地变量副本返回线程的初始化值。这个方法将在线程内第一次通过get方法访问线程本地变量副本的值时被调用，除非线程之前调用过set方法，在这种情况下initialValue方法将不会被调用。通常，这个方法在每个线程中只会被调用一次，但是在调用过remove方法后再次调用get方法时initialValue会被再次调用。
• remove
  
  • 删除线程本地变量在当前线程中的副本的值。

内部类介绍

• ThreadLocal.ThreadLocalMap
  
  • 官方解释
    
    • ThreadLocalMap是一个适用于维护线程本地值的自定义哈希映射。没有任何操作会导出到ThreadLocal类之外。该类是包私有的，允许在Thread类中声明字段。为了帮助处理非常大的和长期的对象，Entry使用WeakReferences作为key，指向ThreadLocal。但是，由于不使用引用队列，因此，在表空间不足的情况下，保证过期的Entry对象被删除。
  • 补充
    
    • ThreadLocalMap中存储的是Entry对象，Entry的key指向ThreadLocal对象，value是当前线程与该ThreadLocal对象绑定的本地值。
    • ThreadLocalMap是懒加载的，当线程第一次使用ThreadLocal的set方法往里面放入本地变量是才会创建ThreadLocalMap对象。
• ThreadLocal.ThreadLocalMap.Entry
  
  • Entry继承自WeakReference类，使用住引用字段作为key，key指向的总是ThreadLocal对象。注意，key==null的Entry对象以为着key指向的ThreadLocal对象不再被引用，这些Entry对象将会从ThreadLocalMap对象中删除。这些对象又叫做过期的Entry对象。
  -

参考

关键代码注释

package cn.julong.thread;import java.lang.ref.WeakReference;import java.util.Objects;import java.util.concurrent.atomic.AtomicInteger;import java.util.function.Supplier;/** * Created by weicm on 2017/6/23. */public class ThreadLocal<T> {    /**     * 当前ThreadLocal实例的HashCode     */    private final int threadLocalHashCode = nextHashCode();    /**     * 当前累积的HashCode     */    private static AtomicInteger nextHashCode = new AtomicInteger();    /**     * HashCode增量，用来计算下一个ThreadLocal的hashCode：nextHashCode.getAndAdd(HASH_INCREMENT)     */    private static final int HASH_INCREMENT = 0x61c88647;    /**     * 计算下一个ThreadLocal的hashCode     */    private static int nextHashCode() {        return nextHashCode.getAndAdd(HASH_INCREMENT);    }    /**     * Returns the current thread's "initial value" for this     * thread-local variable.  This method will be invoked the first     * time a thread accesses the variable with the {@link #get}     * method, unless the thread previously invoked the {@link #set}     * method, in which case the {@code initialValue} method will not     * be invoked for the thread.  Normally, this method is invoked at     * most once per thread, but it may be invoked again in case of     * subsequent invocations of {@link #remove} followed by {@link #get}.     * <p>     * <p>This implementation simply returns {@code null}; if the     * programmer desires thread-local variables to have an initial     * value other than {@code null}, {@code ThreadLocal} must be     * subclassed, and this method overridden.  Typically, an     * anonymous inner class will be used.     *     * @return the initial value for this thread-local     */    protected T initialValue() {        return null;    }    /**     * Creates a thread local variable. The initial value of the variable is     * determined by invoking the {@code get} method on the {@code Supplier}.     *     * @param <S>      the type of the thread local's value     * @param supplier the supplier to be used to determine the initial value     * @return a new thread local variable     * @throws NullPointerException if the specified supplier is null     * @since 1.8     */    public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {        return new SuppliedThreadLocal<>(supplier);    }    /**     * Creates a thread local variable.     *     * @see #withInitial(java.util.function.Supplier)     */    public ThreadLocal() {    }    /**     * 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;    }    /**     * 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);    }    /**     * Removes the current thread's value for this thread-local     * variable.  If this thread-local variable is subsequently     * {@linkplain #get read} by the current thread, its value will be     * reinitialized by invoking its {@link #initialValue} method,     * unless its value is {@linkplain #set set} by the current thread     * in the interim.  This may result in multiple invocations of the     * {@code initialValue} method in the current thread.     *     * @since 1.5     */    public void remove() {        ThreadLocalMap m = getMap(Thread.currentThread());        if (m != null)            m.remove(this);    }    /**     * 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);    }    /**     * Factory method to create map of inherited thread locals.     * Designed to be called only from Thread constructor.     *     * @param parentMap the map associated with parent thread     * @return a map containing the parent's inheritable bindings     */    static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {        return new ThreadLocalMap(parentMap);    }    /**     * Method childValue is visibly defined in subclass     * InheritableThreadLocal, but is internally defined here for the     * sake of providing createInheritedMap factory method without     * needing to subclass the map class in InheritableThreadLocal.     * This technique is preferable to the alternative of embedding     * instanceof tests in methods.     */    T childValue(T parentValue) {        throw new UnsupportedOperationException();    }    /**     * An extension of ThreadLocal that obtains its initial value from     * the specified {@code Supplier}.     */    static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {        private final Supplier<? extends T> supplier;        SuppliedThreadLocal(Supplier<? extends T> supplier) {            this.supplier = Objects.requireNonNull(supplier);        }        @Override        protected T initialValue() {            return supplier.get();        }    }    /**     * 自定义的hash map，适合管理线程本地变量。操作闲着在ThreadLocal类内。     * Entry 使用弱引用作为key,来帮助处理大量和长生命周期的用例。     * 然而，由与没有使用引用队列，当table超出生命周期时，过期的Entry实例能够保证被删除     */    static class ThreadLocalMap {        /**         * Entry继承了弱引用WeakReference，用它当做key,key永远是ThreadLocal实例。         * 注意：空的key（entry.get（）==null）意味着key不在被引用，因此这个entry可以从table中删除。         * 这样的空key的Entry实例在线面的代码中被称为过期实例         */        static class Entry extends WeakReference<ThreadLocal<?>> {            /**             * The value associated with this ThreadLocal.             */            Object value;            Entry(ThreadLocal<?> k, Object v) {                super(k);                value = v;            }        }        /**         * table的初始容量，必须是2的整数倍         */        private static final int INITIAL_CAPACITY = 16;        /**         * 必要时可调整大小的表         * 长度必须是2的整数倍         */        private Entry[] table;        /**         * 表中Entry实例的数量         */        private int size = 0;        /**         * table调整大小的阀值.         */        private int threshold; // Default to 0        /**         * 设置调整table大小阀值为当前容量的2/3.         */        private void setThreshold(int len) {            threshold = len * 2 / 3;        }        /**         * 计算table的下一个索引，当i>=len-1是，返回0         */        private static int nextIndex(int i, int len) {            return ((i + 1 < len) ? i + 1 : 0);        }        /**         * 计算table的上一个索引，当i<=0时，返回len-1         */        private static int prevIndex(int i, int len) {            return ((i - 1 >= 0) ? i - 1 : len - 1);        }        /**         * 构造一个具有初始容量的map         * map是懒汉式构造，只有在往map中放入Entry实例时才构造出来         * 计算当前Entry实例的索引的算法是：当前ThreadLocal实例的hashCode与上初始容量减1         * 即：firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1)         */        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);        }        /**         * Construct a new map including all Inheritable ThreadLocals         * from given parent map. Called only by createInheritedMap.         *         * @param parentMap the map associated with parent thread.         */        private ThreadLocalMap(ThreadLocalMap parentMap) {            Entry[] parentTable = parentMap.table;            int len = parentTable.length;            setThreshold(len);            table = new Entry[len];            for (int j = 0; j < len; j++) {                Entry e = parentTable[j];                if (e != null) {                    @SuppressWarnings("unchecked")                    ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();                    if (key != null) {                        Object value = key.childValue(e.value);                        Entry c = new Entry(key, value);                        int h = key.threadLocalHashCode & (len - 1);                        while (table[h] != null)                            h = nextIndex(h, len);                        table[h] = c;                        size++;                    }                }            }        }        /**         * 获取与key关联的entry。         * 这个方法本身只处理快速路径：直接命中已存在的key。         * 否则它讲调用getEntryAfterMiss，及没有直接命中时调用getEntryAfterMiss。         * 这样设计的目的是为了最大限度的提高直接命中的性能，部分原因是更容易实现这个方法         * @param key the thread local object         * @return the entry associated with key, or null if no such         */        private Entry getEntry(ThreadLocal<?> key) {            int i = key.threadLocalHashCode & (table.length - 1);            Entry e = table[i];            if (e != null && e.get() == key)                return e;            else                return getEntryAfterMiss(key, i, e);        }        /**         * 当key在直接hash索引位置没有发现是的getEnry方法的版本         * @param key the thread local object         * @param i   the table index for key's hash code         * @param e   the entry at table[i]         * @return the entry associated with key, or null if no such         */        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;        }        /**         * 将key与value绑定         * @param key   the thread local object         * @param value the value to be set         */        private void set(ThreadLocal<?> key, Object value) {            // 我们不像使用get（）一样使用快速路径，因为使用set（）来创建新条目至少是常见的，因为它是替换现有条目，在这种情况下，快速路径将会失败。            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();        }        /**         * Remove the entry for key.         */        private void remove(ThreadLocal<?> key) {            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)]) {                if (e.get() == key) {                    e.clear();                    expungeStaleEntry(i);                    return;                }            }        }        /**         *         * 将设置操作期间遇到的陈旧条目替换为指定键的entry。值参数中传递的值存储在entry中，无论entry是否已存在于指定的键上         *         * 作为负面印象，这个方法会清理掉在run中所有过期的entry，run是两个空槽之前的所有槽位的序列         *         * @param key       the key         * @param value     the value to be associated with key         * @param staleSlot index of the first stale entry encountered while         *                  searching for key.         */        private void replaceStaleEntry(ThreadLocal<?> key, Object value,                                       int staleSlot) {            Entry[] tab = table;            int len = tab.length;            Entry e;            // Back up to check for prior stale entry in current run.            // We clean out whole runs at a time to avoid continual            // incremental rehashing due to garbage collector freeing            // up refs in bunches (i.e., whenever the collector runs).            int slotToExpunge = staleSlot;            for (int i = prevIndex(staleSlot, len);                 (e = tab[i]) != null;                 i = prevIndex(i, len))                if (e.get() == null)                    slotToExpunge = i;            // 找到run中的键或尾部空槽，以先到者为准            for (int i = nextIndex(staleSlot, len);                 (e = tab[i]) != null;                 i = nextIndex(i, len)) {                ThreadLocal<?> k = e.get();                // If we find key, then we need to swap it                // with the stale entry to maintain hash table order.                // The newly stale slot, or any other stale slot                // encountered above it, can then be sent to expungeStaleEntry                // to remove or rehash all of the other entries in run.                if (k == key) {                    e.value = value;                    tab[i] = tab[staleSlot];                    tab[staleSlot] = e;                    // Start expunge at preceding stale entry if it exists                    if (slotToExpunge == staleSlot)                        slotToExpunge = i;                    cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);                    return;                }                // If we didn't find stale entry on backward scan, the                // first stale entry seen while scanning for key is the                // first still present in the run.                if (k == null && slotToExpunge == staleSlot)                    slotToExpunge = i;            }            // If key not found, put new entry in stale slot            tab[staleSlot].value = null;            tab[staleSlot] = new Entry(key, value);            // If there are any other stale entries in run, expunge them            if (slotToExpunge != staleSlot)                cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);        }        /**         * 清理掉staleSlot和下一个空插槽之间的过期entry         * 如果遇到不过期的entry则从新计算其hash索引，该索引肯定落在i之前或i所在的位置         * 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--;            // 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;        }        /**         * 启发性扫描一些细胞寻找陈旧的条目。         * 当添加一个新元素，或者另一个陈旧元素已被清除时，这将被调用。         * 它执行对数数量的扫描，作为在不扫描（快速但保留垃圾）和与元素数量成比例的多个扫描之间的平衡，可以发现所有垃圾，但会导致一些插入取O（n）时间。         * 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;        }        /**         * 重新调整表的大小         * 首先骚表整个表，清理掉过期的entry         * 如果清理没有有效的收缩表的大小，则将表容量加倍         * Re-pack and/or re-size the table. First scan the entire         * table removing stale entries. If this doesn't sufficiently         * shrink the size of the table, double the table size.         */        private void rehash() {            expungeStaleEntries();            // Use lower threshold for doubling to avoid hysteresis            if (size >= threshold - threshold / 4)                resize();        }        /**         * 加倍表容量，并重新做hash索引         */        private void resize() {            Entry[] oldTab = table;            int oldLen = oldTab.length;            int newLen = oldLen * 2;            Entry[] newTab = new Entry[newLen];            int count = 0;            for (int j = 0; j < oldLen; ++j) {                Entry e = oldTab[j];                if (e != null) {                    ThreadLocal<?> k = e.get();                    if (k == null) {                        e.value = null; // Help the GC                    } else {                        int h = k.threadLocalHashCode & (newLen - 1);                        while (newTab[h] != null)                            h = nextIndex(h, newLen);                        newTab[h] = e;                        count++;                    }                }            }            setThreshold(newLen);            size = count;            table = newTab;        }        /**         * Expunge all stale entries in the table.         */        private void expungeStaleEntries() {            Entry[] tab = table;            int len = tab.length;            for (int j = 0; j < len; j++) {                Entry e = tab[j];                if (e != null && e.get() == null)                    expungeStaleEntry(j);            }        }    }}