threadlocal翻译成中文比较准确的叫法应该是：线程局部变量。
这个玩意有什么用处，或者说为什么要有这么一个东东？先解释一下，在并发编程的时候，成员变量如果不做任何处理其实是线程不安全的，各个线程都在操作同一个变量，显然是不行的，并且我们也知道volatile这个关键字也是不能保证线程安全的。那么在有一种情况之下，我们需要满足这样一个条件：变量是同一个，但是每个线程都使用同一个初始值，也就是使用同一个变量的一个新的副本。这种情况之下ThreadLocal就非常使用，比如说DAO的数据库连接，我们知道DAO是单例的，那么他的属性Connection就不是一个线程安全的变量。而我们每个线程都需要使用他，并且各自使用各自的。这种情况，ThreadLocal就比较好的解决了这个问题。
我们从源码的角度来分析这个问题。
首先定义一个ThreadLocal：
ThreadLocaltl = ThreadLocal Connection initConn = = DriverManager.getConnection("url, name and password"=( ==
package java.lang;import java.lang.ref.*;import java.util.concurrent.atomic.AtomicInteger;public class ThreadLocal{ private final int threadLocalHashCode = nextHashCode(); private static AtomicInteger nextHashCode =new AtomicInteger();private static final int HASH_INCREMENT = 0x61c88647;private static int nextHashCode() {return nextHashCode.getAndAdd(HASH_INCREMENT); }protected T initialValue() {return null; }public ThreadLocal() { }public T get() { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t);if (map != null) { ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value; }return setInitialValue(); }private T setInitialValue() { T value = initialValue(); Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t);if (map != null) map.set(this, value);elsecreateMap(t, value);return value; }public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t);if (map != null) map.set(this, value);elsecreateMap(t, value); } public void remove() { ThreadLocalMap m = getMap(Thread.currentThread()); if (m != null) m.remove(this); }ThreadLocalMap getMap(Thread t) {return t.threadLocals; }void createMap(Thread t, T firstValue) { t.threadLocals = new ThreadLocalMap(this, firstValue); }static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {return new ThreadLocalMap(parentMap); }T childValue(T parentValue) {throw new UnsupportedOperationException(); }static class ThreadLocalMap {static class Entry extends WeakReference {/** The value associated with this ThreadLocal. */Object value; Entry(ThreadLocal k, Object v) {super(k); value = v; } }private static final int INITIAL_CAPACITY = 16;private Entry[] table;private int size = 0;private int threshold; // Default to 0private void setThreshold(int len) { threshold = len * 2 / 3; }private static int nextIndex(int i, int len) {return ((i + 1 < len) ? i + 1 : 0); }private static int prevIndex(int i, int len) {return ((i - 1 >= 0) ? i - 1 : len - 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); }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) { ThreadLocal key = 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++; } } } }private Entry getEntry(ThreadLocal key) {int i = key.threadLocalHashCode & (table.length - 1); Entry e = table[i];if (e != null && e.get() == key)return e;elsereturn 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) expungeStaleEntry(i);elsei = nextIndex(i, len); e = tab[i]; }return null; }private void set(ThreadLocal key, Object value) {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(); }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; } } }private void replaceStaleEntry(ThreadLocal key, Object value, int staleSlot) { Entry[] tab = table;int len = tab.length; Entry e;int slotToExpunge = staleSlot;for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len))if (e.get() == null) slotToExpunge = i;for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal k = e.get();if (k == key) { e.value = value; tab[i] = tab[staleSlot]; tab[staleSlot] = e;if (slotToExpunge == staleSlot) slotToExpunge = i; cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);return; }if (k == null && slotToExpunge == staleSlot) slotToExpunge = i; }tab[staleSlot].value = null; tab[staleSlot] = new Entry(key, value);if (slotToExpunge != staleSlot) cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); }private int expungeStaleEntry(int staleSlot) { Entry[] tab = table;int len = tab.length;tab[staleSlot].value = null; tab[staleSlot] = null; size--; 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;while (tab[h] != null) h = nextIndex(h, len); tab[h] = e; } } }return i; }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; }private void rehash() { expungeStaleEntries();// Use lower threshold for doubling to avoid hysteresisif (size >= threshold - threshold / 4) resize(); }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; }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); } } } }
这样子，都是用同一个连接，但是每个连接都是新的，是同一个连接的副本。
那么实现机制是如何的呢？
1、每个Thread对象内部都维护了一个ThreadLocalMap这样一个ThreadLocal的Map，可以存放若干个ThreadLocal
/* ThreadLocal values pertaining to this thread. This map is maintained * by the ThreadLocal class. */ThreadLocal.ThreadLocalMap threadLocals = null;
2、当我们在调用get()方法的时候，先获取当前线程，然后获取到当前线程的ThreadLocalMap对象，如果非空，那么取出ThreadLocal的value，否则进行初始化，初始化就是将initialValue的值set到ThreadLocal中。
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);if (e != null)return (T)e.value;
}return setInitialValue();
}
3、当我们调用set()方法的时候，很常规，就是将值设置进ThreadLocal中。
4、总结：当我们调用get方法的时候，其实每个当前线程中都有一个ThreadLocal。每次获取或者设置都是对该ThreadLocal进行的操作，是与其他线程分开的。
5、应用场景：当很多线程需要多次使用同一个对象，并且需要该对象具有相同初始化值的时候最适合使用ThreadLocal。
6、其实说再多也不如看一下源码来得清晰。如果要看源码，其中涉及到一个WeakReference和一个Map，这两个地方需要了解下，这两个东西分别是a.Java的弱引用，也就是GC的时候会销毁该引用所包裹(引用)的对象，这个threadLocal作为key可能被销毁，但是只要我们定义成他的类不卸载，tl这个强引用就始终引用着这个ThreadLocal的，永远不会被gc掉。b.和HashMap差不多。
