doc: 阅读webclient filter文档

spring/webflux/webclient.md

@@ -394,4 +394,61 @@ WebClient client = webClient.mutate()
 		})
 		.build();
 ```
+### filter处理response
+在使用filter时，需要`确保response content总是被消费`：
+- 当filter在处理response时，需要确保filter总是：
+  - 消费response content
+  - 或将response传递到下游 
+
+示例代码如下所示：
+```java
+public ExchangeFilterFunction renewTokenFilter() {
+	return (request, next) -> next.exchange(request).flatMap(response -> {
+		if (response.statusCode().value() == HttpStatus.UNAUTHORIZED.value()) {
+			return response.releaseBody()
+					.then(renewToken())
+					.flatMap(token -> {
+						ClientRequest newRequest = ClientRequest.from(request).build();
+						return next.exchange(newRequest);
+					});
+		} else {
+			return Mono.just(response);
+		}
+	});
+}
+```
+
+如下示例创建了自定义的filter，用于计算POST和PUT的`multipart/form-data`请求的`Content-Length`
+```java
+public class MultipartExchangeFilterFunction implements ExchangeFilterFunction {
+
+	@Override
+	public Mono<ClientResponse> filter(ClientRequest request, ExchangeFunction next) {
+		if (MediaType.MULTIPART_FORM_DATA.includes(request.headers().getContentType())
+				&& (request.method() == HttpMethod.PUT || request.method() == HttpMethod.POST)) {
+			return next.exchange(ClientRequest.from(request).body((outputMessage, context) ->
+				request.body().insert(new BufferingDecorator(outputMessage), context)).build()
+			);
+		} else {
+			return next.exchange(request);
+		}
+	}
+
+	private static final class BufferingDecorator extends ClientHttpRequestDecorator {
+
+		private BufferingDecorator(ClientHttpRequest delegate) {
+			super(delegate);
+		}
+
+		@Override
+		public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
+			return DataBufferUtils.join(body).flatMap(buffer -> {
+				getHeaders().setContentLength(buffer.readableByteCount());
+				return super.writeWith(Mono.just(buffer));
+			});
+		}
+	}
+}
+```
+