Netty中使用FastThreadLocal替代JDK中的ThreadLocal【JAVA】ThreadLocal源码分析，其用法和ThreadLocal 一样，只不过从名字FastThreadLocal来看，其处理效率要比JDK中的ThreadLocal要高
在类加载的时候，先初始化了一个静态成员：
1 private static final int variablesToRemoveIndex = InternalThreadLocalMap.nextVariableIndex();
实际上FastThreadLocal的操作都是通过对InternalThreadLocalMap的操作来实现的，
而InternalThreadLocalMap是UnpaddedInternalThreadLocalMap的子类，UnpaddedInternalThreadLocalMap的定义比较简单：
1 class UnpaddedInternalThreadLocalMap {
2 static final ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = new ThreadLocal();
3 static final AtomicInteger nextIndex = new AtomicInteger();
4 Object[] indexedVariables;
5 int futureListenerStackDepth;
6 int localChannelReaderStackDepth;
7 Map<Class<?>, Boolean> handlerSharableCache;
8 IntegerHolder counterHashCode;
9 ThreadLocalRandom random;
10 Map<Class<?>, TypeParameterMatcher> typeParameterMatcherGetCache;
11 Map<Class<?>, Map<String, TypeParameterMatcher>> typeParameterMatcherFindCache;
12 StringBuilder stringBuilder;
13 Map<Charset, CharsetEncoder> charsetEncoderCache;
14 Map<Charset, CharsetDecoder> charsetDecoderCache;
15 ArrayList<Object> arrayList;
16
17 UnpaddedInternalThreadLocalMap(Object[] indexedVariables) {
18 this.indexedVariables = indexedVariables;
19 }
20 }
可以看到在类加载时，会初始化一个泛型为InternalThreadLocalMap的JDK的ThreadLocal对象作为其静态成员slowThreadLocalMap ，还有一个原子化的Integer静态成员nextIndex
InternalThreadLocalMap的定义如下：
1 public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap {
2 private static final InternalLogger logger = InternalLoggerFactory.getInstance(InternalThreadLocalMap.class);
3 private static final int DEFAULT_ARRAY_LIST_INITIAL_CAPACITY = 8;
4 private static final int STRING_BUILDER_INITIAL_SIZE = SystemPropertyUtil.getInt("io.netty.threadLocalMap.stringBuilder.initialSize", 1024);
5 private static final int STRING_BUILDER_MAX_SIZE;
6 public static final Object UNSET = new Object();
7 private BitSet cleanerFlags;
InternalThreadLocalMap的nextVariableIndex方法：
1 public static int nextVariableIndex() {
2 int index = nextIndex.getAndIncrement();
3 if (index < 0) {
4 nextIndex.decrementAndGet();
5 throw new IllegalStateException("too many thread-local indexed variables");
6 } else {
7 return index;
8 }
9 }
这是一个CAS滞后自增操作，获取nextIndex自增前的值，那么variablesToRemoveIndex初始化时就是0，且恒为0，nextIndex此时变成了1
FastThreadLocal对象的初始化：
1 private final int index = InternalThreadLocalMap.nextVariableIndex();
2
3 public FastThreadLocal() {
4 }
由上面可知，index成员恒等于nextVariableIndex的返回值，nextIndex 的CAS操作保障了每个FastThreadLocal对象的index是不同的
首先看到set方法：
1 public final void set(V value) {
2 if (value != InternalThreadLocalMap.UNSET) {
3 InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
4 if (this.setKnownNotUnset(threadLocalMap, value)) {
5 this.registerCleaner(threadLocalMap);
6 }
7 } else {
8 this.remove();
9 }
10
11 }
只要set的value不是InternalThreadLocalMap.UNSET，会先调用InternalThreadLocalMap的get方法：
1 public static InternalThreadLocalMap get() {
2 Thread thread = Thread.currentThread();
3 return thread instanceof FastThreadLocalThread ? fastGet((FastThreadLocalThread)thread) : slowGet();
4 }
判断当前线程是否是FastThreadLocalThread，是则调用fastGet，否则调用slowGet
FastThreadLocalThread是经过包装后的Thread：
1 public class FastThreadLocalThread extends Thread {
2 private final boolean cleanupFastThreadLocals;
3 private InternalThreadLocalMap threadLocalMap;
4
5 public FastThreadLocalThread() {
6 this.cleanupFastThreadLocals = false;
7 }
8
9 public FastThreadLocalThread(Runnable target) {
10 super(FastThreadLocalRunnable.wrap(target));
11 this.cleanupFastThreadLocals = true;
12 }
13
14 public FastThreadLocalThread(ThreadGroup group, Runnable target) {
15 super(group, FastThreadLocalRunnable.wrap(target));
16 this.cleanupFastThreadLocals = true;
17 }
18
19 public FastThreadLocalThread(String name) {
20 super(name);
21 this.cleanupFastThreadLocals = false;
22 }
23
24 public FastThreadLocalThread(ThreadGroup group, String name) {
25 super(group, name);
26 this.cleanupFastThreadLocals = false;
27 }
28
29 public FastThreadLocalThread(Runnable target, String name) {
30 super(FastThreadLocalRunnable.wrap(target), name);
31 this.cleanupFastThreadLocals = true;
32 }
33
34 public FastThreadLocalThread(ThreadGroup group, Runnable target, String name) {
35 super(group, FastThreadLocalRunnable.wrap(target), name);
36 this.cleanupFastThreadLocals = true;
37 }
38
39 public FastThreadLocalThread(ThreadGroup group, Runnable target, String name, long stackSize) {
40 super(group, FastThreadLocalRunnable.wrap(target), name, stackSize);
41 this.cleanupFastThreadLocals = true;
42 }
43
44 public final InternalThreadLocalMap threadLocalMap() {
45 return this.threadLocalMap;
46 }
47
48 public final void setThreadLocalMap(InternalThreadLocalMap threadLocalMap) {
49 this.threadLocalMap = threadLocalMap;
50 }
51
52 public boolean willCleanupFastThreadLocals() {
53 return this.cleanupFastThreadLocals;
54 }
55
56 public static boolean willCleanupFastThreadLocals(Thread thread) {
57 return thread instanceof FastThreadLocalThread && ((FastThreadLocalThread)thread).willCleanupFastThreadLocals();
58 }
59 }
如果看过我之前写的ThreadLocal源码分析，看到这就明白，JDK的ThreadLocal中很重要的一点是在Thread类中有一个ThreadLocalMap类型的成员，每个线程都维护这一张ThreadLocalMap，通过ThreadLocalMap来和ThreadLocal对象产生映射关系；而这里和JDK同理绑定的就是InternalThreadLocalMap。
fastGet方法：
1 private static InternalThreadLocalMap fastGet(FastThreadLocalThread thread) {
2 InternalThreadLocalMap threadLocalMap = thread.threadLocalMap();
3 if (threadLocalMap == null) {
4 thread.setThreadLocalMap(threadLocalMap = new InternalThreadLocalMap());
5 }
6
7 return threadLocalMap;
8 }
这里也和JDK的ThreadLocal类似，判断FastThreadLocalThread 线程的threadLocalMap成员是否为null，若是null，则先创建一个InternalThreadLocalMap实例：
1 private InternalThreadLocalMap() {
2 super(newIndexedVariableTable());
3 }
先调用newIndexedVariableTable方法：
1 private static Object[] newIndexedVariableTable() {
2 Object[] array = new Object[32];
3 Arrays.fill(array, UNSET);
4 return array;
5 }
创建了一个大小为32的数组，并且用UNSET这个Object填充了整个数组，然后调用UnpaddedInternalThreadLocalMap的构造，令indexedVariables成员保存该数组
再来看slowGet方法：
1 private static InternalThreadLocalMap slowGet() {
2 ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = UnpaddedInternalThreadLocalMap.slowThreadLocalMap;
3 InternalThreadLocalMap ret = (InternalThreadLocalMap)slowThreadLocalMap.get();
4 if (ret == null) {
5 ret = new InternalThreadLocalMap();
6 slowThreadLocalMap.set(ret);
7 }
8
9 return ret;
10 }
可以看到，其实这里为了提高效率，并没有直接使用JDK的ThreadLocal，而是给当前非FastThreadLocalThread线程绑定了一个ThreadLocal
回到FastThreadLocal的set方法，在取得到了当前线程的InternalThreadLocalMap成员后，调用setKnownNotUnset方法：
1 private boolean setKnownNotUnset(InternalThreadLocalMap threadLocalMap, V value) {
2 if (threadLocalMap.setIndexedVariable(this.index, value)) {
3 addToVariablesToRemove(threadLocalMap, this);
4 return true;
5 } else {
6 return false;
7 }
8 }
首先调用了InternalThreadLocalMap的setIndexedVariable方法：
1 public boolean setIndexedVariable(int index, Object value) {
2 Object[] lookup = this.indexedVariables;
3 if (index < lookup.length) {
4 Object oldValue = lookup[index];
5 lookup[index] = value;
6 return oldValue == UNSET;
7 } else {
8 this.expandIndexedVariableTableAndSet(index, value);
9 return true;
10 }
11 }
因为index是不可更改的常量，所以这里有两种情况：
当indexedVariables这个Object数组的长度大于index时，直接将value放在indexedVariables数组下标为index的位置，返回oldValue是否等于UNSET，若是不等于UNSET，说明已经set过了，直进行替换，若是等于UNSET，还要进行后续的registerCleaner
当indexedVariables这个Object数组的长度小于等于index时，调用expandIndexedVariableTableAndSet方法扩容
expandIndexedVariableTableAndSet方法：
1 private void expandIndexedVariableTableAndSet(int index, Object value) {
2 Object[] oldArray = this.indexedVariables;
3 int oldCapacity = oldArray.length;
4 int newCapacity = index | index >>> 1;
5 newCapacity |= newCapacity >>> 2;
6 newCapacity |= newCapacity >>> 4;
7 newCapacity |= newCapacity >>> 8;
8 newCapacity |= newCapacity >>> 16;
9 ++newCapacity;
10 Object[] newArray = Arrays.copyOf(oldArray, newCapacity);
11 Arrays.fill(newArray, oldCapacity, newArray.length, UNSET);
12 newArray[index] = value;
13 this.indexedVariables = newArray;
14 }
如果读过HashMap源码的话对上述的位运算操作因该不陌生，这个位运算产生的newCapacity的值是大于oldCapacity的最小的二的整数幂（【Java】HashMap中的tableSizeFor方法）
然后申请一个newCapacity大小的数组，将原数组的内容拷贝到新数组，并且用UNSET填充剩余部分，还是将value放在下标为index的位置，用indexedVariables保存新数组。
setIndexedVariable成立后，setKnownNotUnset继续调用addToVariablesToRemove方法：
1 private static void addToVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
2 Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
3 Set variablesToRemove;
4 if (v != InternalThreadLocalMap.UNSET && v != null) {
5 variablesToRemove = (Set)v;
6 } else {
7 variablesToRemove = Collections.newSetFromMap(new IdentityHashMap());
8 threadLocalMap.setIndexedVariable(variablesToRemoveIndex, variablesToRemove);
9 }
10
11 variablesToRemove.add(variable);
12 }
上面说过variablesToRemoveIndex恒为0，调用InternalThreadLocalMap的indexedVariable方法：
1 public Object indexedVariable(int index) {
2 Object[] lookup = this.indexedVariables;
3 return index < lookup.length ? lookup[index] : UNSET;
4 }
由于variablesToRemoveIndex恒等于0，所以这里判断indexedVariables这个Object数组是否为空，若是为空，则返回第0个元素，若不是则返回UNSET
在addToVariablesToRemove中，接着对indexedVariables的返回值进行了判断，
判断不是UNSET，并且不等于null，则说明是set过的，然后将刚才的返回值强转为Set类型
若上述条件不成立，创建一个IdentityHashMap，将其包装成Set赋值给variablesToRemove，然后调用InternalThreadLocalMap的setIndexedVariable方法，这里就和上面不一样了，上面是将value放在下标为index的位置，而这里是将Set放在下标为0的位置。
看到这，再结合上面来看，其实已经有一个大致的想法了，一开始在set时，是将value放在InternalThreadLocalMap的Object数组下标为index的位置，然后在这里获取下标为0的Set，说明value是暂时放在下标为index的位置，然后判断下标为0的位置有没有Set，若是有，取出这个Set ，将当前FastThreadLocal对象放入Set中，则说明这个Set中存放的是FastThreadLocal集合
那么就有如下关系：
回到FastThreadLocal的set方法，在setKnownNotUnset成立后，调用registerCleaner方法：
1 private void registerCleaner(InternalThreadLocalMap threadLocalMap) {
2 Thread current = Thread.currentThread();
3 if (!FastThreadLocalThread.willCleanupFastThreadLocals(current) && !threadLocalMap.isCleanerFlagSet(this.index)) {
4 threadLocalMap.setCleanerFlag(this.index);
5 }
6 }
willCleanupFastThreadLocals的返回值在前面FastThreadLocalThread的初始化时就确定了，看到isCleanerFlagSet方法：
1 public boolean isCleanerFlagSet(int index) {
2 return this.cleanerFlags != null && this.cleanerFlags.get(index);
3 }
cleanerFlags 是一个BitSet对象，在InternalThreadLocalMap初始化时是null，
若不是第一次的set操作，则根据index，获取index在BitSet对应位的值
这里使用BitSet，使其持有的位和indexedVariables这个Object数组形成了一一对应关系，每一位都是0和1代表当前indexedVariables的对应下标位置的使用情况，0表示没有使用对应UNSET，1则代表有value
在上面条件成立的情况下，调用setCleanerFlag方法：
1 public void setCleanerFlag(int index) {
2 if (this.cleanerFlags == null) {
3 this.cleanerFlags = new BitSet();
4 }
5
6 this.cleanerFlags.set(index);
7 }
逻辑比较简单，判断cleanerFlags是否初始化，若没有，则立即初始化，再将cleanerFlags中对应index位的值设为1；
这里通过registerCleaner直接标记了所有set了value的下标可，为以后的removeAll 清除提高效率。
下来看FastThreadLocal的get方法：
1 public final V get() {
2 InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
3 Object v = threadLocalMap.indexedVariable(this.index);
4 if (v != InternalThreadLocalMap.UNSET) {
5 return v;
6 } else {
7 V value = this.initialize(threadLocalMap);
8 this.registerCleaner(threadLocalMap);
9 return value;
10 }
11 }
和上面一样，先取得当前线程持有的InternalThreadLocalMap ，调用indexedVariable方法，根据当前FastThreadLocal的index定位，判断是否是UNSET（set过），若没有set过则和JDK一样调用initialize先set：
1 private V initialize(InternalThreadLocalMap threadLocalMap) {
2 Object v = null;
3
4 try {
5 v = this.initialValue();
6 } catch (Exception var4) {
7 PlatformDependent.throwException(var4);
8 }
9
10 threadLocalMap.setIndexedVariable(this.index, v);
11 addToVariablesToRemove(threadLocalMap, this);
12 return v;
13 }
initialValue（）方法就是对外提供的，需要手动覆盖：
1 protected V initialValue() throws Exception {
2 return null;
3 }
后面的操作就和set的逻辑一样。
remove方法：
1 public final void remove() {
2 this.remove(InternalThreadLocalMap.getIfSet());
3 }
getIfSet方法：
1 public static InternalThreadLocalMap getIfSet() {
2 Thread thread = Thread.currentThread();
3 return thread instanceof FastThreadLocalThread ? ((FastThreadLocalThread)thread).threadLocalMap() : (InternalThreadLocalMap)slowThreadLocalMap.get();
4 }
和上面的get方法思路相似，只不过在这里如果获取不到不会创建
然后调用remove重载：
1 public final void remove(InternalThreadLocalMap threadLocalMap) {
2 if (threadLocalMap != null) {
3 Object v = threadLocalMap.removeIndexedVariable(this.index);
4 removeFromVariablesToRemove(threadLocalMap, this);
5 if (v != InternalThreadLocalMap.UNSET) {
6 try {
7 this.onRemoval(v);
8 } catch (Exception var4) {
9 PlatformDependent.throwException(var4);
10 }
11 }
12
13 }
14 }
先检查threadLocalMap是否存在，若存在才进行后续操作：
调用removeIndexedVariable方法：
1 public Object removeIndexedVariable(int index) {
2 Object[] lookup = this.indexedVariables;
3 if (index < lookup.length) {
4 Object v = lookup[index];
5 lookup[index] = UNSET;
6 return v;
7 } else {
8 return UNSET;
9 }
10 }
和之前的setIndexedVariable逻辑相似，只不过现在是把index位置的元素设置为UNSET
接着调用removeFromVariablesToRemove方法：
1 private static void removeFromVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
2 Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
3 if (v != InternalThreadLocalMap.UNSET && v != null) {
4 Set<FastThreadLocal<?>> variablesToRemove = (Set)v;
5 variablesToRemove.remove(variable);
6 }
7 }
之前说过variablesToRemoveIndex恒为0，在Object数组中下标为0存储的Set<FastThreadLocal<?>>集合（不为UNSET情况下），从集合中，将当前FastThreadLocal移除掉
最后调用了onRemoval方法，该方法需要由用户去覆盖：
1 protected void onRemoval(V value) throws Exception {
2 }
removeAll方法，是一个静态方法：
1 public static void removeAll() {
2 InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.getIfSet();
3 if (threadLocalMap != null) {
4 try {
5 Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
6 if (v != null && v != InternalThreadLocalMap.UNSET) {
7 Set<FastThreadLocal<?>> variablesToRemove = (Set)v;
8 FastThreadLocal<?>[] variablesToRemoveArray = (FastThreadLocal[])variablesToRemove.toArray(new FastThreadLocal[0]);
9 FastThreadLocal[] var4 = variablesToRemoveArray;
10 int var5 = variablesToRemoveArray.length;
11
12 for(int var6 = 0; var6 < var5; ++var6) {
13 FastThreadLocal<?> tlv = var4[var6];
14 tlv.remove(threadLocalMap);
15 }
16 }
17 } finally {
18 InternalThreadLocalMap.remove();
19 }
20
21 }
22 }
首先获取当前线程的InternalThreadLocalMap，若是存在继续后续操作：
通过indexedVariable方法，取出Object数组中下标为0的Set集合（如果不是UNSET情况下），将其转换为FastThreadLocal数组，遍历这个数组调用上面的remove方法。
FastThreadLocal源码分析到此结束。
