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：
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：
class UnpaddedInternalThreadLocalMap {
static final ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = new ThreadLocal();
static final AtomicInteger nextIndex = new AtomicInteger();
Object[] indexedVariables;
int futureListenerStackDepth;
int localChannelReaderStackDepth;
Map<Class<?>, Boolean> handlerSharableCache;
IntegerHolder counterHashCode;
ThreadLocalRandom random;
Map<Class<?>, TypeParameterMatcher> typeParameterMatcherGetCache;
Map<Class<?>, Map<String, TypeParameterMatcher>> typeParameterMatcherFindCache;
StringBuilder stringBuilder;
Map<Charset, CharsetEncoder> charsetEncoderCache;
Map<Charset, CharsetDecoder> charsetDecoderCache;
ArrayList<Object> arrayList;
UnpaddedInternalThreadLocalMap(Object[] indexedVariables) {
this.indexedVariables = indexedVariables;
}
}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：
public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap {
private static final InternalLogger logger = InternalLoggerFactory.getInstance(InternalThreadLocalMap.class);
private static final int DEFAULT_ARRAY_LIST_INITIAL_CAPACITY = 8;
private static final int STRING_BUILDER_INITIAL_SIZE = SystemPropertyUtil.getInt("io.netty.threadLocalMap.stringBuilder.initialSize", 1024);
private static final int STRING_BUILDER_MAX_SIZE;
public static final Object UNSET = new Object();
private BitSet cleanerFlags;InternalThreadLocalMap的nextVariableIndex方法：
public static int nextVariableIndex() {
int index = nextIndex.getAndIncrement();
if (index < 0) {
nextIndex.decrementAndGet();
throw new IllegalStateException("too many thread-local indexed variables");
} else {
return index;
}
}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：
private final int index = InternalThreadLocalMap.nextVariableIndex();
public FastThreadLocal() {
}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：
public final void set(V value) {
if (value != InternalThreadLocalMap.UNSET) {
InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
if (this.setKnownNotUnset(threadLocalMap, value)) {
this.registerCleaner(threadLocalMap);
}
} else {
this.remove();
}
}只要set的value不是InternalThreadLocalMap.UNSET，会先调用InternalThreadLocalMap的get方法：
public static InternalThreadLocalMap get() {
Thread thread = Thread.currentThread();
return thread instanceof FastThreadLocalThread ? fastGet((FastThreadLocalThread)thread) : slowGet();
}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：
public class FastThreadLocalThread extends Thread {
private final boolean cleanupFastThreadLocals;
private InternalThreadLocalMap threadLocalMap;
public FastThreadLocalThread() {
this.cleanupFastThreadLocals = false;
}
public FastThreadLocalThread(Runnable target) {
super(FastThreadLocalRunnable.wrap(target));
this.cleanupFastThreadLocals = true;
}
public FastThreadLocalThread(ThreadGroup group, Runnable target) {
super(group, FastThreadLocalRunnable.wrap(target));
this.cleanupFastThreadLocals = true;
}
public FastThreadLocalThread(String name) {
super(name);
this.cleanupFastThreadLocals = false;
}
public FastThreadLocalThread(ThreadGroup group, String name) {
super(group, name);
this.cleanupFastThreadLocals = false;
}
public FastThreadLocalThread(Runnable target, String name) {
super(FastThreadLocalRunnable.wrap(target), name);
this.cleanupFastThreadLocals = true;
}
public FastThreadLocalThread(ThreadGroup group, Runnable target, String name) {
super(group, FastThreadLocalRunnable.wrap(target), name);
this.cleanupFastThreadLocals = true;
}
public FastThreadLocalThread(ThreadGroup group, Runnable target, String name, long stackSize) {
super(group, FastThreadLocalRunnable.wrap(target), name, stackSize);
this.cleanupFastThreadLocals = true;
}
public final InternalThreadLocalMap threadLocalMap() {
return this.threadLocalMap;
}
public final void setThreadLocalMap(InternalThreadLocalMap threadLocalMap) {
this.threadLocalMap = threadLocalMap;
}
public boolean willCleanupFastThreadLocals() {
return this.cleanupFastThreadLocals;
}
public static boolean willCleanupFastThreadLocals(Thread thread) {
return thread instanceof FastThreadLocalThread && ((FastThreadLocalThread)thread).willCleanupFastThreadLocals();
}
}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：
private static InternalThreadLocalMap fastGet(FastThreadLocalThread thread) {
InternalThreadLocalMap threadLocalMap = thread.threadLocalMap();
if (threadLocalMap == null) {
thread.setThreadLocalMap(threadLocalMap = new InternalThreadLocalMap());
}
return threadLocalMap;
}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：
private InternalThreadLocalMap() {
super(newIndexedVariableTable());
}先调用newIndexedVariableTable方法：
private static Object[] newIndexedVariableTable() {
Object[] array = new Object[32];
Arrays.fill(array, UNSET);
return array;
}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：
private static InternalThreadLocalMap slowGet() {
ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = UnpaddedInternalThreadLocalMap.slowThreadLocalMap;
InternalThreadLocalMap ret = (InternalThreadLocalMap)slowThreadLocalMap.get();
if (ret == null) {
ret = new InternalThreadLocalMap();
slowThreadLocalMap.set(ret);
}
return ret;
}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<InternalThreadLocalMap>对象，避免直接使用JDK的ThreadLocal效率低。
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：
private boolean setKnownNotUnset(InternalThreadLocalMap threadLocalMap, V value) {
if (threadLocalMap.setIndexedVariable(this.index, value)) {
addToVariablesToRemove(threadLocalMap, this);
return true;
} else {
return false;
}
}首先调用了InternalThreadLocalMap的setIndexedVariable方法：
public boolean setIndexedVariable(int index, Object value) {
Object[] lookup = this.indexedVariables;
if (index < lookup.length) {
Object oldValue = lookup[index];
lookup[index] = value;
return oldValue == UNSET;
} else {
this.expandIndexedVariableTableAndSet(index, value);
return true;
}
}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：
private void expandIndexedVariableTableAndSet(int index, Object value) {
Object[] oldArray = this.indexedVariables;
int oldCapacity = oldArray.length;
int newCapacity = index | index >>> 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;
this.indexedVariables = newArray;
}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：
private static void addToVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
Set variablesToRemove;
if (v != InternalThreadLocalMap.UNSET && v != null) {
variablesToRemove = (Set)v;
} else {
variablesToRemove = Collections.newSetFromMap(new IdentityHashMap());
threadLocalMap.setIndexedVariable(variablesToRemoveIndex, variablesToRemove);
}
variablesToRemove.add(variable);
}上面说过variablesToRemoveIndex恒为0，调用InternalThreadLocalMap的indexedVariable方法：
public Object indexedVariable(int index) {
Object[] lookup = this.indexedVariables;
return index < lookup.length ? lookup[index] : UNSET;
}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：
private void registerCleaner(InternalThreadLocalMap threadLocalMap) {
Thread current = Thread.currentThread();
if (!FastThreadLocalThread.willCleanupFastThreadLocals(current) && !threadLocalMap.isCleanerFlagSet(this.index)) {
threadLocalMap.setCleanerFlag(this.index);
}
}willCleanupFastThreadLocals的返回值在前面FastThreadLocalThread的初始化时就确定了，看到isCleanerFlagSet方法：
public boolean isCleanerFlagSet(int index) {
return this.cleanerFlags != null && this.cleanerFlags.get(index);
}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：
public void setCleanerFlag(int index) {
if (this.cleanerFlags == null) {
this.cleanerFlags = new BitSet();
}
this.cleanerFlags.set(index);
}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：
public final V get() {
InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.get();
Object v = threadLocalMap.indexedVariable(this.index);
if (v != InternalThreadLocalMap.UNSET) {
return v;
} else {
V value = this.initialize(threadLocalMap);
this.registerCleaner(threadLocalMap);
return value;
}
}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：
private V initialize(InternalThreadLocalMap threadLocalMap) {
Object v = null;
try {
v = this.initialValue();
} catch (Exception var4) {
PlatformDependent.throwException(var4);
}
threadLocalMap.setIndexedVariable(this.index, v);
addToVariablesToRemove(threadLocalMap, this);
return v;
}initialValue（）方法就是对外提供的，需要手动覆盖：
protected V initialValue() throws Exception {
return null;
}后面的操作就和set的逻辑一样。
remove方法：
public final void remove() {
this.remove(InternalThreadLocalMap.getIfSet());
}getIfSet方法：
public static InternalThreadLocalMap getIfSet() {
Thread thread = Thread.currentThread();
return thread instanceof FastThreadLocalThread ? ((FastThreadLocalThread)thread).threadLocalMap() : (InternalThreadLocalMap)slowThreadLocalMap.get();
}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：
public final void remove(InternalThreadLocalMap threadLocalMap) {
if (threadLocalMap != null) {
Object v = threadLocalMap.removeIndexedVariable(this.index);
removeFromVariablesToRemove(threadLocalMap, this);
if (v != InternalThreadLocalMap.UNSET) {
try {
this.onRemoval(v);
} catch (Exception var4) {
PlatformDependent.throwException(var4);
}
}
}
}先检查threadLocalMap是否存在，若存在才进行后续操作：
调用removeIndexedVariable方法：
public Object removeIndexedVariable(int index) {
Object[] lookup = this.indexedVariables;
if (index < lookup.length) {
Object v = lookup[index];
lookup[index] = UNSET;
return v;
} else {
return UNSET;
}
}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：
private static void removeFromVariablesToRemove(InternalThreadLocalMap threadLocalMap, FastThreadLocal<?> variable) {
Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
if (v != InternalThreadLocalMap.UNSET && v != null) {
Set<FastThreadLocal<?>> variablesToRemove = (Set)v;
variablesToRemove.remove(variable);
}
}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：
protected void onRemoval(V value) throws Exception {
}removeAll方法，是一个静态方法：
public static void removeAll() {
InternalThreadLocalMap threadLocalMap = InternalThreadLocalMap.getIfSet();
if (threadLocalMap != null) {
try {
Object v = threadLocalMap.indexedVariable(variablesToRemoveIndex);
if (v != null && v != InternalThreadLocalMap.UNSET) {
Set<FastThreadLocal<?>> variablesToRemove = (Set)v;
FastThreadLocal<?>[] variablesToRemoveArray = (FastThreadLocal[])variablesToRemove.toArray(new FastThreadLocal[0]);
FastThreadLocal[] var4 = variablesToRemoveArray;
int var5 = variablesToRemoveArray.length;
for(int var6 = 0; var6 < var5; ++var6) {
FastThreadLocal<?> tlv = var4[var6];
tlv.remove(threadLocalMap);
}
}
} finally {
InternalThreadLocalMap.remove();
}
}
}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。