add cache request body filter (#1943)

* add cache request body filter

docs/src/main/asciidoc/spring-cloud-gateway.adoc

@@ -1698,6 +1698,52 @@ NOTE: The default implementation of `ReactiveOAuth2AuthorizedClientService` used
 uses an in-memory data store.  You will need to provide your own implementation `ReactiveOAuth2AuthorizedClientService`
 if you need a more robust solution.
 
+
+=== The `CacheRequestBody` `GatewayFilter` Factory
+There are certain situation need to read body.Since the request body stream can only be read once, we need to cache the request body.
+You can use the `CacheRequestBody` filter to cache request body before it send to the downstream and get body from exchagne attribute.
+
+
+The following listing shows how to cache the request body `GatewayFilter`:
+
+====
+[source,java]
+----
+@Bean
+public RouteLocator routes(RouteLocatorBuilder builder) {
+    return builder.routes()
+        .route("cache_request_body_route", r -> r.path("/downstream/**")
+            .filters(f -> f.prefixPath("/httpbin")
+        		.cacheRequestBody(String.class).uri(uri))
+        .build();
+}
+----
+====
+
+
+.application.yml
+====
+[source,yaml]
+----
+spring:
+  cloud:
+    gateway:
+      routes:
+      - id: cache_request_body_route
+        uri: lb://downstream
+        predicates:
+        - Path=/downstream/**
+        filters:
+        - name: CacheRequestBody
+          args:
+            bodyClass: java.lang.String
+----
+====
+
+`CacheRequestBody` will extract request body and conver it to body class (such as `java.lang.String`, defined in the preceding example). then places it in the `ServerWebExchange.getAttributes()` with a key defined in `ServerWebExchangeUtils.CACHED_REQUEST_BODY_ATTR`.
+
+NOTE: This filter only works with http request (including https).
+
 === Default Filters
 
 To add a filter and apply it to all routes, you can use `spring.cloud.gateway.default-filters`.