ThreadLocal 源码分析

最新推荐文章于 2022-08-21 13:16:18 发布

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ThreadLocal用于多线程环境下为每个线程提供独立的变量副本，确保线程间数据隔离。本文详细解析了ThreadLocal的工作原理，包括其内部实现机制、源码分析及应用场景。

Threadlocal 主要用于在多线程的情况下，每个线程可以保存一份本地的变量，该变量在线程之间相互独立，互不影响，并伴随着该线程的运行过程可以随时取出。

应用场景：比如，在web系统中，每个用户进行访问都有各自的不同的用户信息，可以在访问接收的时候做一个拦截器，把用户的信息放入到threadlocal中，在该线程的整个流程中，可以取出当前用户的相关信息，并且和其他线程不相互影响。

原理：每个线程都有一个threadlocalMap对象，在向threadlocal放入变量的时候，实际上是向每个线程的threadlocalMap中放入变量，放入的变量是key value形式的，其中key是当前的threadLocal对象，value是放入的变量。threadlocalMap中有个Entity的kev value 数组，该entity其中的key是弱引用，也就是存放threadLocal对象的地方。

源码分析

首先从ThreadLocal的 get方法看起

/**
 * 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();
}

（1）先获取当前的线程

（2）然后获取当前线程的ThreadLocalMap ThreadLocalMap map = getMap(t);

如果是第一次调用该方法，之前没有set过值，那么map应该为null。会调用 setInitialValue()方法;

/**
 * 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;
}

（1）初始化的时候value 是为null ，map根据上面第一次调用的话为null

（2）所有会调用createMap(t, value)方法

/**
 * 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);
}

（1）获取当前线程threadLocalMap 然后new 了一个

/**
 * 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);
}

（1）该构造方法为threadLocalMap内部的构造方法，主要的值存放在table数组中

（2）int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); 获取数组的下标，& (INITIAL_CAPACITY - 1)相当于对数组的初始容量取模，下标值一定是在数组容量范围内的。

（3）因为是第一次初始化，数组的大小为1.

（4） setThreshold(INITIAL_CAPACITY); 设置重新分配数组的空间的阀值，为数组大小的2/3，超过这个数组，从新分配空间。

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();
}

接下来看最开始的那段代码，假设不是第一次调用，当map不为空的时候，会走到

ThreadLocalMap.Entry e = map.getEntry(this); 这段代码

/**
 * 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) {
    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);
}

（1）if (e != null && e.get() == key) 如果当前位置值存在，并且key也相等，直接返回该值。

（2）存在一种情况，在放入值得时候，key的位置i已经存在了，则用线性探测法，去找下一个位置，直到找到不存在的位置，

所以上面的判断if不成立的时候，会调用getEntryAfterMiss(key, i, e)，该方法用线性探测的方式查找值。

/**
 * Version of getEntry method for use when key is not found in
 * its direct hash slot.
 *
 * @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;
}

（1）从 i的位置向下遍历整张表，因为table不会一定会有为null的位置，所以找不到则返回null，找到key相等的值则直接返回。

（2）因为key 为弱引用，所有当key为空的时候，调用方法expungeStaleEntry(i) 情况value的值，并且遍历整张表，清空所有key为null的值。

/**
 * 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--;

    // 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;
}

接下来我们看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);
}

（1）第一次set的时候同理上面已经说过createMap相关

（2）我们只看不是第一次的情况，这时候map不为空，该方法会调用map.set(this, value)

/**
 * 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();
}

（1）查找到有相同的key 则value进行覆盖，没有相同的key ，则在 i的位置创建一个新的entry

接下来看remove方法

/**
 * 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);
 }

直接可以看m.remove(this)方法

/**
 * 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;
        }
    }
}