doc: 阅读webclient文档

spring/webflux/webclient.md

@@ -163,3 +163,69 @@ WebClient.create().get()
 #### connectTimeout
 以ms为单位指定一个超时时长，当与服务端建立连接时，如果超过该时长后连接仍未建立，那么将会抛出`java.net.SocketTimeoutException`异常。
 
+### JDK HttpClient
+如果要自定义jdk httpclient配置，欸可以使用如下方式：
+```java
+HttpClient httpClient = HttpClient.newBuilder()
+	.followRedirects(Redirect.NORMAL)
+	.connectTimeout(Duration.ofSeconds(20))
+	.build();
+
+ClientHttpConnector connector =
+		new JdkClientHttpConnector(httpClient, new DefaultDataBufferFactory());
+
+WebClient webClient = WebClient.builder().clientConnector(connector).build();
+```
+
+### Jetty
+自定义jetty httpclient配置的示例如下所示：
+```java
+HttpClient httpClient = new HttpClient();
+httpClient.setCookieStore(...);
+
+WebClient webClient = WebClient.builder()
+		.clientConnector(new JettyClientHttpConnector(httpClient))
+		.build();
+```
+
+### HttpComponents
+如下示例展示了如何定义Apache HttpComponents：
+```java
+HttpAsyncClientBuilder clientBuilder = HttpAsyncClients.custom();
+clientBuilder.setDefaultRequestConfig(...);
+CloseableHttpAsyncClient client = clientBuilder.build();
+
+ClientHttpConnector connector = new HttpComponentsClientHttpConnector(client);
+
+WebClient webClient = WebClient.builder().clientConnector(connector).build();
+```
+
+## retrieve()
+`retrieve()`方法用于定义`如何对相应进行提取`，示例如下：
+```java
+WebClient client = WebClient.create("https://example.org");
+
+Mono<ResponseEntity<Person>> result = client.get()
+		.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
+		.retrieve()
+		.toEntity(Person.class);
+```
+上述示例获取的是`ResponseEntity<Person>`，如果想要直接获取`Persion`类型的response body，可以通过如下方式进行获取:
+```java
+WebClient client = WebClient.create("https://example.org");
+
+Mono<Person> result = client.get()
+		.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
+		.retrieve()
+		.bodyToMono(Person.class);
+```
+默认情况下，`4xx`或`5xx`的http响应将会导致`WebClientResponseException`，如果需要自定义error handling逻辑，需要使用`onStatus`，示例如下：
+```java
+Mono<Person> result = client.get()
+		.uri("/persons/{id}", id).accept(MediaType.APPLICATION_JSON)
+		.retrieve()
+		.onStatus(HttpStatusCode::is4xxClientError, response -> ...)
+		.onStatus(HttpStatusCode::is5xxServerError, response -> ...)
+		.bodyToMono(Person.class);
+```
+