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本文主要研究一下AsyncHttpClient的RequestFilter
RequestFilter
org/asynchttpclient/filter/RequestFilter.java
代码语言：javascript复制/**
* 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
代码语言：javascript复制/**
* 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
代码语言：javascript复制/**
* 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
代码语言：javascript复制 /**
* 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
代码语言：javascript复制 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)。
