微服务中使用阿里开源的TTL，优雅的实现身份信息的线程间复用三

所以，重点的核心逻辑应该是在TtlCallable#call()或者TtlRunnable#run()中。以下以TtlCallable为例，TtlRunnable同理类似。在分析call()方法之前，先看一个类Transmitter

public static class Transmitter {
  /**
    * 捕获当前线程中的是所有TransimittableThreadLocal和注册ThreadLocal的值。
    */
  @NonNull
  public static Object capture() {
    return new Snapshot(captureTtlValues(), captureThreadLocalValues());
  }
 
    /**
    * 捕获TransimittableThreadLocal的值,将holder中的所有值都添加到HashMap后返回。
    */
  private static HashMap<TransmittableThreadLocal<Object>, Object> captureTtlValues() {
    HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value = 
      new HashMap<TransmittableThreadLocal<Object>, Object>();
    for (TransmittableThreadLocal<Object> threadLocal : holder.get().keySet()) {
      ttl2Value.put(threadLocal, threadLocal.copyValue());
    }
    return ttl2Value;
  }

  /**
    * 捕获注册的ThreadLocal的值,也就是原本线程中的ThreadLocal,可以注册到TTL中，在
    * 进行线程池本地变量传递时也会被传递。
    */
  private static HashMap<ThreadLocal<Object>, Object> captureThreadLocalValues() {
    final HashMap<ThreadLocal<Object>, Object> threadLocal2Value = 
      new HashMap<ThreadLocal<Object>, Object>();
    for(Map.Entry<ThreadLocal<Object>,TtlCopier<Object>>entry:threadLocalHolder.entrySet()){
      final ThreadLocal<Object> threadLocal = entry.getKey();
      final TtlCopier<Object> copier = entry.getValue();
      threadLocal2Value.put(threadLocal, copier.copy(threadLocal.get()));
    }
    return threadLocal2Value;
  }

  /**
    * 将捕获到的本地变量进行替换子线程的本地变量，并且返回子线程现有的本地变量副本backup。
    * 用于在执行run/call方法之后，将本地变量副本恢复。
    */
  @NonNull
  public static Object replay(@NonNull Object captured) {
    final Snapshot capturedSnapshot = (Snapshot) captured;
    return new Snapshot(replayTtlValues(capturedSnapshot.ttl2Value), 
                        replayThreadLocalValues(capturedSnapshot.threadLocal2Value));
  }
 
  /**
    * 替换TransmittableThreadLocal
    */
  @NonNull
  private static HashMap<TransmittableThreadLocal<Object>, Object> replayTtlValues(@NonNull HashMap<TransmittableThreadLocal<Object>, Object> captured) {
    // 创建副本backup
    HashMap<TransmittableThreadLocal<Object>, Object> backup = 
      new HashMap<TransmittableThreadLocal<Object>, Object>();

    for (final Iterator<TransmittableThreadLocal<Object>> iterator = holder.get().keySet().iterator(); iterator.hasNext(); ) {
      TransmittableThreadLocal<Object> threadLocal = iterator.next();
      // 对当前线程的本地变量进行副本拷贝
      backup.put(threadLocal, threadLocal.get());

      // 若出现调用线程中不存在某个线程变量，而线程池中线程有，则删除线程池中对应的本地变量
      if (!captured.containsKey(threadLocal)) {
        iterator.remove();
        threadLocal.superRemove();
      }
    }
    // 将捕获的TTL值打入线程池获取到的线程TTL中。
    setTtlValuesTo(captured);
    // 是一个扩展点，调用TTL的beforeExecute方法。默认实现为空
    doExecuteCallback(true);
    return backup;
  }

  private static HashMap<ThreadLocal<Object>, Object> replayThreadLocalValues(@NonNull HashMap<ThreadLocal<Object>, Object> captured) {
    final HashMap<ThreadLocal<Object>, Object> backup = 
      new HashMap<ThreadLocal<Object>, Object>();
    for (Map.Entry<ThreadLocal<Object>, Object> entry : captured.entrySet()) {
      final ThreadLocal<Object> threadLocal = entry.getKey();
      backup.put(threadLocal, threadLocal.get());
      final Object value = entry.getValue();
      if (value == threadLocalClearMark) threadLocal.remove();
      else threadLocal.set(value);
    }
    return backup;
  }

  /**
    * 清除单线线程的所有TTL和TL，并返回清除之气的backup
    */
  @NonNull
  public static Object clear() {
    final HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value = 
      new HashMap<TransmittableThreadLocal<Object>, Object>();

    final HashMap<ThreadLocal<Object>, Object> threadLocal2Value = 
      new HashMap<ThreadLocal<Object>, Object>();
    for(Map.Entry<ThreadLocal<Object>,TtlCopier<Object>>entry:threadLocalHolder.entrySet()){
      final ThreadLocal<Object> threadLocal = entry.getKey();
      threadLocal2Value.put(threadLocal, threadLocalClearMark);
    }
    return replay(new Snapshot(ttl2Value, threadLocal2Value));
  }

  /**
    * 还原
    */
  public static void restore(@NonNull Object backup) {
    final Snapshot backupSnapshot = (Snapshot) backup;
    restoreTtlValues(backupSnapshot.ttl2Value);
    restoreThreadLocalValues(backupSnapshot.threadLocal2Value);
  }

  private static void restoreTtlValues(@NonNull HashMap<TransmittableThreadLocal<Object>, Object> backup) {
    // 扩展点，调用TTL的afterExecute
    doExecuteCallback(false);

    for (final Iterator<TransmittableThreadLocal<Object>> iterator = holder.get().keySet().iterator(); iterator.hasNext(); ) {
      TransmittableThreadLocal<Object> threadLocal = iterator.next();

      if (!backup.containsKey(threadLocal)) {
        iterator.remove();
        threadLocal.superRemove();
      }
    }

    // 将本地变量恢复成备份版本
    setTtlValuesTo(backup);
  }

  private static void setTtlValuesTo(@NonNull HashMap<TransmittableThreadLocal<Object>, Object> ttlValues) {
    for (Map.Entry<TransmittableThreadLocal<Object>, Object> entry : ttlValues.entrySet()) {
      TransmittableThreadLocal<Object> threadLocal = entry.getKey();
      threadLocal.set(entry.getValue());
    }
  }

  private static void restoreThreadLocalValues(@NonNull HashMap<ThreadLocal<Object>, Object> backup) {
    for (Map.Entry<ThreadLocal<Object>, Object> entry : backup.entrySet()) {
      final ThreadLocal<Object> threadLocal = entry.getKey();
      threadLocal.set(entry.getValue());
    }
  }

  /**
   * 快照类，保存TTL和TL
   */
  private static class Snapshot {
    final HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value;
    final HashMap<ThreadLocal<Object>, Object> threadLocal2Value;

    private Snapshot(HashMap<TransmittableThreadLocal<Object>, Object> ttl2Value,
                     HashMap<ThreadLocal<Object>, Object> threadLocal2Value) {
      this.ttl2Value = ttl2Value;
      this.threadLocal2Value = threadLocal2Value;
    }
  }

进入TtlCallable#call()方法。

@Override
public V call() throws Exception {
  Object captured = capturedRef.get();
  if (captured == null || releaseTtlValueReferenceAfterCall && 
      !capturedRef.compareAndSet(captured, null)) {
    throw new IllegalStateException("TTL value reference is released after call!");
  }
  // 调用replay方法将捕获到的当前线程的本地变量，传递给线程池线程的本地变量，
  // 并且获取到线程池线程覆盖之前的本地变量副本。
  Object backup = replay(captured);
  try {
    // 线程方法调用
    return callable.call();
  } finally {
    // 使用副本进行恢复。
    restore(backup);
  }
}

到这基本上线程池方式传递本地变量的核心代码已经大概看完了。总的来说在创建TtlCallable对象是，调用capture()方法捕获调用方的本地线程变量，在call()执行时，将捕获到的线程变量，替换到线程池所对应获取到的线程的本地变量中，并且在执行完成之后，将其本地变量恢复到调用之前。

总结
本文介绍了使用阿里开源的TransmittableThreadLocal 优雅的实现父子线程的数据传递，应用场景很多，企业中应用也比较广泛。

文章转自公众号：码猿技术专栏