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本文主要研究一下AsyncHttpClient的RequestFilter

RequestFilter

org/asynchttpclient/filter/RequestFilter.java

/**
 * A Filter interface that gets invoked before making an actual request.
 */
public interface RequestFilter {

  /**
   * An {@link org.asynchttpclient.AsyncHttpClient} will invoke {@link RequestFilter#filter} and will use the
   * returned {@link FilterContext#getRequest()} and {@link FilterContext#getAsyncHandler()} to continue the request
   * processing.
   *
   * @param ctx a {@link FilterContext}
   * @param <T> the handler result type
   * @return {@link FilterContext}. The {@link FilterContext} instance may not the same as the original one.
   * @throws FilterException to interrupt the filter processing.
   */
  <T> FilterContext<T> filter(FilterContext<T> ctx) throws FilterException;
}

RequestFilter定义了filter方法

ThrottleRequestFilter

org/asynchttpclient/filter/ThrottleRequestFilter.java

/**
 * A {@link org.asynchttpclient.filter.RequestFilter} throttles requests and block when the number of permits is reached,
 * waiting for the response to arrives before executing the next request.
 */
public class ThrottleRequestFilter implements RequestFilter {
  private static final Logger logger = LoggerFactory.getLogger(ThrottleRequestFilter.class);
  private final Semaphore available;
  private final int maxWait;

  public ThrottleRequestFilter(int maxConnections) {
    this(maxConnections, Integer.MAX_VALUE);
  }

  public ThrottleRequestFilter(int maxConnections, int maxWait) {
    this(maxConnections, maxWait, false);
  }

  public ThrottleRequestFilter(int maxConnections, int maxWait, boolean fair) {
    this.maxWait = maxWait;
    available = new Semaphore(maxConnections, fair);
  }

  /**
   * {@inheritDoc}
   */
  @Override
  public <T> FilterContext<T> filter(FilterContext<T> ctx) throws FilterException {
    try {
      if (logger.isDebugEnabled()) {
        logger.debug("Current Throttling Status {}", available.availablePermits());
      }
      if (!available.tryAcquire(maxWait, TimeUnit.MILLISECONDS)) {
        throw new FilterException(String.format("No slot available for processing Request %s with AsyncHandler %s",
                ctx.getRequest(), ctx.getAsyncHandler()));
      }
    } catch (InterruptedException e) {
      throw new FilterException(String.format("Interrupted Request %s with AsyncHandler %s",
              ctx.getRequest(), ctx.getAsyncHandler()));
    }

    return new FilterContext.FilterContextBuilder<>(ctx)
            .asyncHandler(ReleasePermitOnComplete.wrap(ctx.getAsyncHandler(), available))
            .build();
  }
}

ThrottleRequestFilter实现了RequestFilter接口，它使用Semaphore来对request进行限流，限流不通过抛出FilterException，若通过则通过ReleasePermitOnComplete.wrap(ctx.getAsyncHandler(), available)包装一下asyncHandler以释放信号量ReleasePermitOnComplete

ReleasePermitOnComplete

org/asynchttpclient/filter/ReleasePermitOnComplete.java

/**
 * Wrapper for {@link AsyncHandler}s to release a permit on {@link AsyncHandler#onCompleted()}. This is done via a dynamic proxy to preserve all interfaces of the wrapped handler.
 */
public class ReleasePermitOnComplete {

  /**
   * Wrap handler to release the permit of the semaphore on {@link AsyncHandler#onCompleted()}.
   *
   * @param handler   the handler to be wrapped
   * @param available the Semaphore to be released when the wrapped handler is completed
   * @param <T>       the handler result type
   * @return the wrapped handler
   */
  @SuppressWarnings("unchecked")
  public static <T> AsyncHandler<T> wrap(final AsyncHandler<T> handler, final Semaphore available) {
    Class<?> handlerClass = handler.getClass();
    ClassLoader classLoader = handlerClass.getClassLoader();
    Class<?>[] interfaces = allInterfaces(handlerClass);

    return (AsyncHandler<T>) Proxy.newProxyInstance(classLoader, interfaces, (proxy, method, args) -> {
        try {
          return method.invoke(handler, args);
        } finally {
          switch (method.getName()) {
            case "onCompleted":
            case "onThrowable":
              available.release();
            default:
          }
        }
    });
  }

  //......
}  

ReleasePermitOnComplete的wrap对原来的handler进行代理，在finally里头执行available.release()

preProcessRequest

org/asynchttpclient/DefaultAsyncHttpClient.java

  /**
   * Configure and execute the associated {@link RequestFilter}. This class
   * may decorate the {@link Request} and {@link AsyncHandler}
   *
   * @param fc {@link FilterContext}
   * @return {@link FilterContext}
   */
  private <T> FilterContext<T> preProcessRequest(FilterContext<T> fc) throws FilterException {
    for (RequestFilter asyncFilter : config.getRequestFilters()) {
      fc = asyncFilter.filter(fc);
      assertNotNull(fc, "filterContext");
    }

    Request request = fc.getRequest();
    if (fc.getAsyncHandler() instanceof ResumableAsyncHandler) {
      request = ResumableAsyncHandler.class.cast(fc.getAsyncHandler()).adjustRequestRange(request);
    }

    if (request.getRangeOffset() != 0) {
      RequestBuilder builder = new RequestBuilder(request);
      builder.setHeader("Range", "bytes=" + request.getRangeOffset() + "-");
      request = builder.build();
    }
    fc = new FilterContext.FilterContextBuilder<>(fc).request(request).build();
    return fc;
  }

DefaultAsyncHttpClient的preProcessRequest方法遍历config.getRequestFilters()，挨个执行asyncFilter.filter(fc)

executeRequest

org/asynchttpclient/DefaultAsyncHttpClient.java

  public <T> ListenableFuture<T> executeRequest(Request request, AsyncHandler<T> handler) {
    if (config.getCookieStore() != null) {
      try {
        List<Cookie> cookies = config.getCookieStore().get(request.getUri());
        if (!cookies.isEmpty()) {
          RequestBuilder requestBuilder = new RequestBuilder(request);
          for (Cookie cookie : cookies) {
            requestBuilder.addOrReplaceCookie(cookie);
          }
          request = requestBuilder.build();
        }
      } catch (Exception e) {
        handler.onThrowable(e);
        return new ListenableFuture.CompletedFailure<>("Failed to set cookies of request", e);
      }
    }

    if (noRequestFilters) {
      return execute(request, handler);
    } else {
      FilterContext<T> fc = new FilterContext.FilterContextBuilder<T>().asyncHandler(handler).request(request).build();
      try {
        fc = preProcessRequest(fc);
      } catch (Exception e) {
        handler.onThrowable(e);
        return new ListenableFuture.CompletedFailure<>("preProcessRequest failed", e);
      }

      return execute(fc.getRequest(), fc.getAsyncHandler());
    }
  }

executeRequest方法对于noRequestFilters为false会执行preProcessRequest

小结

AsyncHttpClient的RequestFilter定义了filter方法，它有一个实现类为ThrottleRequestFilter，使用信号量用于对请求进行限流；DefaultAsyncHttpClient的executeRequest方法对于noRequestFilters为false会执行preProcessRequest，而preProcessRequest方法遍历config.getRequestFilters()，挨个执行asyncFilter.filter(fc)。