Netty|Netty 源码之 FastThreadLocal Netty源码之FastThreadLocal

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 slowThreadLocalMap = new ThreadLocal(); //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 slowThreadLocalMap = UnpaddedInternalThreadLocalMap.slowThreadLocalMap; InternalThreadLocalMap ret = slowThreadLocalMap.get(); if (ret == null) { ret = new InternalThreadLocalMap(); slowThreadLocalMap.set(ret); } return ret; } ... }

public class FastThreadLocal { ... 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; } ... }

【Netty|Netty 源码之 FastThreadLocal】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