近期由于瀏覽Netty 的源碼，想把Netty中一些編程技巧與優化的東西記錄下來！來了解Netty中一些編程思想！
FastThreadLocal：對JDK中ThreadLocal進行優化，由于ThreadLocal底層存儲數據是一個ThreadLocalMap 結構，是一個數組結構，通過threadLocalHashCode查找在數組中的元素Entry, 當hash沖突時，繼續向前檢測查找, 所以當Hash沖突時，檢索的效率就會降低，具體可查看java.lang.ThreadLocal#get 與 java.lang.ThreadLocal.ThreadLocalMap#getEntry 方法。而FastThreadLocal則正是處理了這個問題，使其時間復雜度一直為O(1)
看一下 FastThredLocal 的系主要代碼：
class UnpaddedInternalThreadLocalMap {
//對非FastThreadLocalThread線程做兼容，從中獲取InternalThreadLocalMap
static final ThreadLocal<InternalThreadLocalMap> slowThreadLocalMap = new ThreadLocal<InternalThreadLocalMap>();
//從此變量中獲取線程存儲數據的index
static final AtomicInteger nextIndex = new AtomicInteger();
//數據存儲的數組
Object[] indexedVariables;
}
public final class InternalThreadLocalMap extends UnpaddedInternalThreadLocalMap {
....
public static InternalThreadLocalMap get() {
Thread thread = Thread.currentThread();
//根據是否是FastThreadLocalThread 兩種獲取InternalThreadLocalMap 的方式
if (thread instanceof FastThreadLocalThread) {
return fastGet((FastThreadLocalThread) thread);
} else {
return slowGet();
}
}
//對于FastThreadLocalThread 線程，都會有一個InternalThreadLocalMap的引用，直接獲取
private static InternalThreadLocalMap fastGet(FastThreadLocalThread thread) {
InternalThreadLocalMap threadLocalMap = thread.threadLocalMap();
if (threadLocalMap == null) {
thread.setThreadLocalMap(threadLocalMap = new InternalThreadLocalMap());
}
return threadLocalMap;
}
//對于非FastThreadLocalThread 線程
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();
}
//獲取ThreadLocal當前線程對應保存的數據， 與ThreadLocal中的get()功能相同
public final V get() {
//InternalThreadLocalMap.get()獲取InternalThreadLocalMap實例，從InternalThreadLocalMap中獲取存儲的數據
return get(InternalThreadLocalMap.get());
}
public final V get(InternalThreadLocalMap threadLocalMap) {
//根據index獲取保存在UnpaddedInternalThreadLocalMap 中Object[] indexedVariables的值
Object v = threadLocalMap.indexedVariable(index);
if (v != InternalThreadLocalMap.UNSET) {
return (V) v;
}
//為NULL 則調用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;
}
...
}
從上面的代碼可以看出：
1：每一個FastThreadLocalThread都會有一個對InternalThreadLocalMap 的獲取，方法thread.threadLocalMap(), 非FastThreadLocalThread直接從UnpaddedInternalThreadLocalMap.slowThreadLocalMap中獲取。得到InternalThreadLocalMap.
2：當FastThreadLocalThread從FastThreadLocal中獲取數據時，每一個FastThreadLocal都有一個index屬性，此屬性InternalThreadLocalMap.nextVariableIndex();賦值，保證了index的唯一性。index表示UnpaddedInternalThreadLocalMap屬性indexedVariables數組的下標, 從而獲取到保存在indexedVariables中的數據
3：如果獲取的數據為null, 則調用FastThreadLocal的initialize()進行一個數據的初始化操作
