FastThreadLocal

Netty中的FastThreadLocal

版本：4.1.23
大家都应该接触过Jdk的ThreadLocal，它使用每个Thread中的ThreadLocalMap存储ThreadLocal，ThreadLocalMap内部使用ThreadLocalMap.Entry 数组存储每一个ThreadLocal，存储计算和HashMap类似，要计算key的索引位置=key.threadLocalHashCode&(len-1)，中间还需要计算冲突,使用的是线程探测方法（当前索引在被占用下，使用下一个索引）。达到一定条件后，还需扩充数组长度，rehash，可为效率不是太高。另外，Jdk的ThreadLocal，还需要使用者注意内存泄漏问题。作为高性能框架的Netty为了解决上面的两个问题重构了TheadLocal，产生了FastThreadLocal。下面讲解如何具体解决刚才说的问题的。

1、与TheadLocal内部使用类对比

不同对象	Jdk	Netty	备注
线程	Thead	FastThreadLocalThread:继成JDK的Thread	netty使用自己的DefaultThreadFactory
map	ThreadLocalMap	InternalThreadLocalMap	map
map内部数组	ThreadLocalMap.entry	UnpaddedInternalThreadLocalMap.indexedVariables	存储theadLocal
Runnable	Runnable	FastThreadLocalRunnable	为了防止内存泄漏，netty的Runnable包装了Runable
ThreadLocal	ThreadLocal	FastThreadLocalMap

Thead与FastThreadLocalThread

//继成了Thread，使用InternalThreadLocalMap替代了Thread中的TheadLocal 
public class FastThreadLocalThread extends Thread {
// This will be set to true if we have a chance to wrap the Runnable.
    private final boolean cleanupFastThreadLocals;
    private InternalThreadLocalMap threadLocalMap;
    
    //....省略
}

DefaultThreadFactory

public class DefaultThreadFactory implements ThreadFactory {
    //....省略
@Override
    public Thread newThread(Runnable r) {
        //使用 FastThreadLocalRunnable
        Thread t = newThread(FastThreadLocalRunnable.wrap(r), prefix + nextId.incrementAndGet());
        try {
            if (t.isDaemon() != daemon) {
                t.setDaemon(daemon);
            }

            if (t.getPriority() != priority) {
                t.setPriority(priority);
            }
        } catch (Exception ignored) {
            // Doesn't matter even if failed to set.
        }
        return t;
    }
    //使用FastThreadLocal
    protected Thread newThread(Runnable r, String name) {
        return new FastThreadLocalThread(threadGroup, r, name);
    }
}

FastThreadLocalRunnable

//继成Runnable
final class FastThreadLocalRunnable implements Runnable {
    private final Runnable runnable;

    private FastThreadLocalRunnable(Runnable runnable) {
        this.runnable = ObjectUtil.checkNotNull(runnable, "runnable");
    }

    @Override
    public void run() {
        try {
            runnable.run();
        } finally {
            //线程执行完成。删除theadLocal，防止内存泄漏
            FastThreadLocal.removeAll();
        }
    }

    static Runnable wrap(Runnable runnable) {
        return runnable instanceof FastThreadLocalRunnable ? runnable : new FastThreadLocalRunnable(runnable);
    }
}

UnpaddedInternalThreadLocalMap

class UnpaddedInternalThreadLocalMap {

    static final ThreadLocal slowThreadLocalMap = new ThreadLocal(); //对没有使用netty的FastThreadLocalThread的使用底层统一使用netty的InternalThreadLocalMap封装V，但使用JDk的ThreadLocal来存储
    static final AtomicInteger nextIndex = new AtomicInteger();

    /** Used by {@link FastThreadLocal} */
    Object[] indexedVariables; //底层存储threadLocal的V的数组

    // Core thread-locals
    int futureListenerStackDepth;
    int localChannelReaderStackDepth;
    Map, Boolean> handlerSharableCache;
    IntegerHolder counterHashCode;
    ThreadLocalRandom random;
    Map, TypeParameterMatcher> typeParameterMatcherGetCache;
    Map, Map> typeParameterMatcherFindCache;

    // String-related thread-locals
    StringBuilder stringBuilder;
    Map charsetEncoderCache;
    Map charsetDecoderCache;

    // ArrayList-related thread-locals
    ArrayList