近期由于浏览Netty 的源码，想把Netty中一些编程技巧与优化的东西记录下来！来了解Netty中一些编程思想！
FastThreadLocal：对JDK中ThreadLocal进行优化，由于ThreadLocal底层存储数据是一个ThreadLocalMap 结构，是一个数组结构，通过threadLocalHashCode查找在数组中的元素Entry, 当hash冲突时，继续向前检测查找, 所以当Hash冲突时，检索的效率就会降低，具体可查看java.lang.ThreadLocal#get 与 java.lang.ThreadLocal.ThreadLocalMap#getEntry 方法。而FastThreadLocal则正是处理了这个问题，使其时间复杂度一直为O(1)
看一下 FastThredLocal 的系主要代码：
class UnpaddedInternalThreadLocalMap {
//对非FastThreadLocalThread线程做兼容，从中获取InternalThreadLocalMap
static final ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = new ThreadLocal<InternalThreadLocalMap>();
//从此变量中获取线程存储数据的index
static final AtomicInteger nextIndex = new AtomicInteger();
//数据存储的数组
Object[] indexedVariables;
}
public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap {
....
public static InternalThreadLocalMap get() {
Thread thread = Thread.currentThread();
//根据是否是FastThreadLocalThread 两种获取InternalThreadLocalMap 的方式
if (thread instanceof FastThreadLocalThread) {
return fastGet((FastThreadLocalThread) thread);
} else {
return slowGet();
}
}
//对于FastThreadLocalThread 线程，都会有一个InternalThreadLocalMap的引用，直接获取
private static InternalThreadLocalMap fastGet(FastThreadLocalThread thread) {
InternalThreadLocalMap threadLocalMap = thread.threadLocalMap();
if (threadLocalMap == null) {
thread.setThreadLocalMap(threadLocalMap = new InternalThreadLocalMap());
}
return threadLocalMap;
}
//对于非FastThreadLocalThread 线程
private static InternalThreadLocalMap slowGet() {
ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = UnpaddedInternalThreadLocalMap.slowThreadLocalMap;
InternalThreadLocalMap ret = slowThreadLocalMap.get();
if (ret == null) {
ret = new InternalThreadLocalMap();
slowThreadLocalMap.set(ret);
}
return ret;
}
...
}
public class FastThreadLocal<V> {
...
private final int index;
public FastThreadLocal() {
index = InternalThreadLocalMap.nextVariableIndex();
}
//获取ThreadLocal当前线程对应保存的数据， 与ThreadLocal中的get()功能相同
public final V get() {
//InternalThreadLocalMap.get()获取InternalThreadLocalMap实例，从InternalThreadLocalMap中获取存储的数据
return get(InternalThreadLocalMap.get());
}
public final V get(InternalThreadLocalMap threadLocalMap) {
//根据index获取保存在UnpaddedInternalThreadLocalMap 中Object[] indexedVariables的值
Object v = threadLocalMap.indexedVariable(index);
if (v != InternalThreadLocalMap.UNSET) {
return (V) v;
}
//为NULL 则调用initialize(...)方法
return initialize(threadLocalMap);
}
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;
}
...
}
从上面的代码可以看出：
1：每一个FastThreadLocalThread都会有一个对InternalThreadLocalMap 的获取，方法thread.threadLocalMap(), 非FastThreadLocalThread直接从UnpaddedInternalThreadLocalMap.slowThreadLocalMap中获取。得到InternalThreadLocalMap.
2：当FastThreadLocalThread从FastThreadLocal中获取数据时，每一个FastThreadLocal都有一个index属性，此属性InternalThreadLocalMap.nextVariableIndex();赋值，保证了index的唯一性。index表示UnpaddedInternalThreadLocalMap属性indexedVariables数组的下标, 从而获取到保存在indexedVariables中的数据
3：如果获取的数据为null, 则调用FastThreadLocal的initialize()进行一个数据的初始化操作
