前言
上一篇文章分析了netty将对象进行回收利用，而内部没用使用JAVA的ThreadLoacl，而是使用了FastThreadLocal来保存变量，FastThreadLocal其Fast在哪里，一看源码便知。
正文
ThreadLocal原理
首先看一下传统ThreadLocal实现细节
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();
}
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
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);
}
这里会获取当线程作为参数，通过getMap方法获取一个ThreadLocalMap，ThreadLocalMap其实就是线程的一个变量，而通过getEntry方法可以看出来是使用的HashCode的方法来获取Entry；使用hashMap的方法有两个开销：计算hash和hash冲突
这里初步怀疑FastThreadLocal是在Hash上面做文章，只有这样能让ThreadLocal变快
FastThreadLocal原理
private static final int variablesToRemoveIndex = InternalThreadLocalMap.nextVariableIndex();
private final int index;
public FastThreadLocal() {
index = InternalThreadLocalMap.nextVariableIndex();
}
public static int nextVariableIndex() {
int index = nextIndex.getAndIncrement();
if (index < 0) {
nextIndex.decrementAndGet();
throw new IllegalStateException("too many thread-local indexed variables");
}
return index;
}
首先内部有一个静态方法和普通变量都调用了nextVariableIndex()方法，这是一个自增的索引，variablesToRemoveIndex全局唯一值为0，index根据FastThreadLocal实例化的个数而自增
看一下get方法
public final V get() {
InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
Object v = threadLocalMap.indexedVariable(index);
if (v != InternalThreadLocalMap.UNSET) {
return (V) v;
}
return initialize(threadLocalMap);
}
首先是获取InternalThreadLocalMap，调用了静态的get方法
public static InternalThreadLocalMap get() {
Thread thread = Thread.currentThread();
if (thread instanceof FastThreadLocalThread) {
return fastGet((FastThreadLocalThread) thread);
} else {
return slowGet();
}
}
使用FastThreadLocalThread线程创建FastThreadLocal才会获得加速效果，如果是普通线程则和ThreadLoacl没什么区别
private static InternalThreadLocalMap fastGet(FastThreadLocalThread thread) {
InternalThreadLocalMap threadLocalMap = thread.threadLocalMap();
if (threadLocalMap == null) {
thread.setThreadLocalMap(threadLocalMap = new InternalThreadLocalMap());
}
return threadLocalMap;
}
private static final ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap =
new ThreadLocal<InternalThreadLocalMap>();
private static InternalThreadLocalMap slowGet() {
InternalThreadLocalMap ret = slowThreadLocalMap.get();
if (ret == null) {
ret = new InternalThreadLocalMap();
slowThreadLocalMap.set(ret);
}
return ret;
}
可以看到，如果当前线程是FastThreadLocalThread，线程重写了threadLocalMap方法，直接就获取到了InternalThreadLocalMap，如果没有就new一个出来；如果是普通线程，还是调用了ThreadLocal
ThreadLocalMap是个什么？看这一段代码
Object v = threadLocalMap.indexedVariable(index);
public Object indexedVariable(int index) {
Object[] lookup = indexedVariables;
return index < lookup.length? lookup[index] : UNSET;
}
indexedVariable竟然是通过数组索引获取元素，数组的速度要比Map快很多；而这个index前面讲过，每new一个FastThreadLocal，就会自增1，同一个线程使用多个FastThreadLocal，每一个实例都对应了数组的一个下标，所以支持快速访问
private V initialize(InternalThreadLocalMap threadLocalMap) {
V v = null;
try {
v = initialValue();
} catch (Exception e) {
PlatformDependent.throwException(e);
}
threadLocalMap.setIndexedVariable(index, v);
addToVariablesToRemove(threadLocalMap, this);
return v;
}
public boolean setIndexedVariable(int index, Object value) {
Object[] lookup = indexedVariables;
if (index < lookup.length) {
Object oldValue = lookup[index];
lookup[index] = value;
return oldValue == UNSET;
} else {
expandIndexedVariableTableAndSet(index, value);
return true;
}
}
//扩容
private void expandIndexedVariableTableAndSet(int index, Object value) {
Object[] oldArray = indexedVariables;
final int oldCapacity = oldArray.length;
int newCapacity = index;
newCapacity |= newCapacity >>> 1;
newCapacity |= newCapacity >>> 2;
newCapacity |= newCapacity >>> 4;
newCapacity |= newCapacity >>> 8;
newCapacity |= newCapacity >>> 16;
newCapacity ++;
Object[] newArray = Arrays.copyOf(oldArray, newCapacity);
Arrays.fill(newArray, oldCapacity, newArray.length, UNSET);
newArray[index] = value;
indexedVariables = newArray;
}
initialValue是交给用户重写的方法，当get出来一个null值的时候会初始化，然后设置数组的下标，如果空间不够会进行扩容
private static void addToVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
Set<FastThreadLocal<?>> variablesToRemove;
if (v == InternalThreadLocalMap.UNSET || v == null) {
variablesToRemove = Collections.newSetFromMap(new IdentityHashMap<FastThreadLocal<?>, Boolean>());
threadLocalMap.setIndexedVariable(variablesToRemoveIndex, variablesToRemove);
} else {
variablesToRemove = (Set<FastThreadLocal<?>>) v;
}
variablesToRemove.add(variable);
}
由于FastThreadLocal有一个静态变量variablesToRemoveIndex，其值为0；作用是在Map的数组的起始位置新建一个Set来保存所有FastThreadLocal实例
总结
想要实现加速效果，创建的线程必须是FastThreadLocalThread，这个线程内部保存了InternalThreadLocalMap实例，而这个Map内部使用了数组作为底层存储，当一个线程有多个FastThreadLocal的时候，每个FTL使用index去访问数组，相比于ThreadLocal使用的HashMap方法减少了Hash碰撞；就算多个线程修改同一个FastThreadLocal实例，内部保存的元素也是线程唯一的，并不会因为多线程导致一致性问题。