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ThreadLocal 是一个线程的工具类,主要用于存储一些线程的共享变量,各个线程之间互不影响,在多线程及高并发环境下,可以实现无状态的存储及调用
好久以前。我一直以为ThreadLocal 可能就是一个Map,以Thead ID为key,然后往里面设置Value即可,但实际上JDK里的ThreadLocal 却没有这样子实现。细想,这么实现其实会带来两个问题
那JDK中ThreadLocal 是怎么实现的呢?
我们从ThreadLocal.set(T value)方法开始进入看看
/** * 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); }
发现是往ThreadLocalMap 这个对象设值,那这个对象哪里来的呢?我们接着跟踪看看
/** * 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; }
原来这个ThreadLocalMap 居然是 Thread的一个局部变量,且该 变量的初始值为空
/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ ThreadLocal.ThreadLocalMap threadLocals = null;
我们再来详细看看ThreadLocalMap 这个类
原来ThreadLocalMap 居然是ThreadLocal下的一个静态内部类。主要的成员是
/** * The table, resized as necessary. * table.length MUST always be a power of two. */ private Entry[] table; //用来保存存进去的Value
而Entry 有点类似于Map Entry 也是一个键值对。我们来看看 Entry的定义
/** * 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> { /** The value associated with this ThreadLocal. */ Object value; Entry(ThreadLocal k, Object v) { super(k); value = v; } }
Entry 继承了弱引用(为什么要弱引用呢,下面会解释,先hold 住),很明显 k 就是 ThreadLocal 对象,而Value 就是我们即将要保存的共享变量。
到了这里,我们基本上对ThreadLocal的存储有一个清晰的认识了,首先ThreadLocal的值不是保存在一个Map中的,他是保存在当前Thread的ThreadLocaoMap 上的Entry数组上了,其中Entry继承弱引用,而Value 就保存在Entry的value 局部变量上。
我们来思考一下,这么实现带来了哪些好处。
接下来我们继续分析一下ThreadLocal.set(T value)方法
当要设值的时候其实调用的是ThreadLocalMap 的set(T value)方法,我们看看它的实现
/** * 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; // 寻找一个存储的下标 int i = key.threadLocalHashCode & (len-1); //下标可能会冲突,如果冲突了采用开放地址法 for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { ThreadLocal k = e.get(); if (k == key) { //已经存在的key 直接更新value e.value = value; return; } if (k == null) { //k 为空有可能是原来存储的被回收了,要清除,防止内存泄露 // 同时将k设置为key value 设置为value replaceStaleEntry(key, value, i); return; } } //获得了存储下标,新生成一个Entry 存进去 tab[i] = new Entry(key, value); int sz = ++size; //扩容,及清除一些key null的entry if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash(); }
我们再来看看两个蛮有意思的方法
/** * 寻找一些过期的key ,将其替换成要设置的key 及value * */ private void replaceStaleEntry(ThreadLocal key, Object value, int staleSlot) { Entry[] tab = table; int len = tab.length; Entry e; // 往前寻找过期的slot int slotToExpunge = staleSlot; for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len)) if (e.get() == null) slotToExpunge = i; // 往后搜索为空的key for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal k = e.get(); // 找到了key 将value 替换 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; } // 用于后面清除过期的slot 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); // 清除一些过期的slot if (slotToExpunge != staleSlot) cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); }
// 清除一些过期的slot 类型与二分法 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); //清除 } } while ( (n >>>= 1) != 0); // n = n / 2 return removed; }
以上就是对ThreadLocalMap.set(T value)的解读。接下来我们看看ThreadLocal.get()方法
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(); }
继续看ThreadLocalMap.getEntry(ThreadLocal<> key )
private Entry getEntry(ThreadLocal key) { // 找下标 int i = key.threadLocalHashCode & (table.length - 1); Entry e = table[i]; if (e != null && e.get() == key) //找到及返回 return e; else //没找到就继续 return getEntryAfterMiss(key, i, e); }
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) // 顺手清除空的 expungeStaleEntry(i); else //继续找下一个 i = nextIndex(i, len); e = tab[i]; } return null; }
我们再来看看ThreadLocal的remove 方法
先找到ThreadLocalMap 对象
public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this); }
/** * 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) { // 找到了key 对ThreadLocal弱引用 进行claar // 这里为什么不顺手把value也置为null呢??? e.clear(); expungeStaleEntry(i); return; } } }
很显然,清除操作只是对ThreadLocalMap的Entry对象中的弱引用clear而已。没有对value置空。这一点在使用中是要引起注意的。
至于为什么不对value置为null,我的理解是这样的。在ThreadLocal.set(T value) 及ThreadLocal.get()两个方法,都会触发清除一些过期的Entry。当中就会对value 置为null..所以这里没有重复操作。可见下列方法
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; }
为什么Entry 要继承WeakReference(当没有强应用时,若发生gc弱引用一定会被回收)?
盗用一张图
从图中可以看出。Key 对ThreadLocal的应用是弱引用。假设该引用是强引用,那么当我们new 的ThreadLocal对象的引用被置为null时,堆中真正的ThreaLocal 依然不会被回收,造成内存泄露,因为我们的key保持了对ThreadLocal的强引用。所以强引用不合理,当为弱应用时,发生GC时ThreadLocal在堆中的东西将会被回收。
为什么不将vlaue 也设置为弱引用
将value 设置为弱引用,当该value没有了强引用,则value会被回收。这时候我们通过ThreadLocal.get()出来的东西是空的,没办法满足存储共享变量的目的。
线程池使用ThreadLocal 有什么需要注意的
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