官方说明理解一下常见的Java Http请求库
HttpURLConnectionHttpClientOkHttpRestTemplate Feign的原理 官方说明
Feign是一个声明式的Web客户端，一个Http请求调用的轻量级框架，来让Rest服务调用更简单Feign英文表意为“假装，伪装”，所以其提供了Http的请求模板，通过编写简单的接口和注解，就可以定义好Http请求的参数、格式、地址等信息Feign完全代理了Http请求，让我们看起来是调用方法实际是发送服务请求的相关处理同时Feign整合了Ribbon负载均衡和Hystrix熔断，可以不再需要显示的使用这两个组件同时SpringCloud为每个命名的Feign客户端创建了一个默认的FeignClientsConfiguration类的配置集，根据需要，我们可以自定义该类【有空再细讲】 理解一下常见的Java Http请求库
通常，java项目中调用接口会用到以下工具
1、HttpClient: Apache子项目，功能丰富、易用、灵活并高效
2、Okhttp:处理网络请求的开源框架、Okhttp拥有更简洁的API,高效的性能，并支持多种协议
3、Httpurlconnection:java的标准类，继承自URLConnection,发送get/post请求，使用比较复杂
4、RestTemplate:Spring提供的用于访问Rest服务的客户端，便捷高效
依次来看一下吧
HttpURLConnection优点：JDK自带的标准库，不需要额外引入缺点：缺乏连接池管理、域名机制控制等特性支持，在Java9下才支持HTTP/2
具体使用
import java.io.BufferedReader;import java.io.InputStream;import java.io.InputStreamReader;import java.io.OutputStream;import java.net.HttpURLConnection;import java.net.URL; public class HttpUrlConnectionDemo { public static void main(String[] args) throws Exception { String urlString = "https://httpbin.org/post"; String bodyString = "password=e10adc3949ba59abbe56e057f20f883e&username=test3"; URL url = new URL(urlString); HttpURLConnection conn = (HttpURLConnection) url.openConnection(); conn.setRequestMethod("POST"); conn.setDoOutput(true); OutputStream os = conn.getOutputStream(); os.write(bodyString.getBytes("utf-8")); os.flush(); os.close(); if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) { InputStream is = conn.getInputStream(); BufferedReader reader = new BufferedReader(new InputStreamReader(is)); StringBuilder sb = new StringBuilder(); String line; while ((line = reader.readLine()) != null) { sb.append(line); } System.out.println("rsp:" + sb.toString()); } else { System.out.println("rsp code:" + conn.getResponseCode()); } } }
HttpClient依赖
GET请求（Delete请求同Get请求）
//Get请求不带参数public class DoGET { public static void main(String[] args) throws Exception { // 创建Httpclient对象 CloseableHttpClient httpclient = HttpClients.createDefault(); // 创建http GET请求 HttpGet httpGet = new HttpGet("http://www.baidu.com/"); CloseableHttpResponse response = null; try { // 执行请求 response = httpclient.execute(httpGet); // 判断返回状态是否为200 if (response.getStatusLine().getStatusCode() == 200) { String content = EntityUtils.toString(response.getEntity(), "UTF-8"); System.out.println("内容长度：" + content.length());// FileUtils.writeStringToFile(new File("C:\baidu.html"), content); } } finally { if (response != null) { response.close(); } httpclient.close(); } } }
//带参数public class DoGETParam { public static void main(String[] args) throws Exception { // 创建Httpclient对象 CloseableHttpClient httpclient = HttpClients.createDefault(); // 定义请求的参数 URI uri = new URIBuilder("http://www.baidu.com/s").setParameter("wd", "java").build(); System.out.println(uri); // 创建http GET请求 HttpGet httpGet = new HttpGet(uri); CloseableHttpResponse response = null; try { // 执行请求 response = httpclient.execute(httpGet); // 判断返回状态是否为200 if (response.getStatusLine().getStatusCode() == 200) { String content = EntityUtils.toString(response.getEntity(), "UTF-8"); System.out.println(content); } } finally { if (response != null) { response.close(); } httpclient.close(); } } }
Post请求（put请求同Post）
//不带参数public class DoPOST { public static void main(String[] args) throws Exception { // 创建Httpclient对象 CloseableHttpClient httpclient = HttpClients.createDefault(); // 创建http POST请求 HttpPost httpPost = new HttpPost("http://www.oschina.net/"); CloseableHttpResponse response = null; try { // 执行请求 response = httpclient.execute(httpPost); // 判断返回状态是否为200 if (response.getStatusLine().getStatusCode() == 200) { String content = EntityUtils.toString(response.getEntity(), "UTF-8"); System.out.println(content); } } finally { if (response != null) { response.close(); } httpclient.close(); } } }
//带参数public class DoPOSTParam { public static void main(String[] args) throws Exception { // 创建Httpclient对象 CloseableHttpClient httpclient = HttpClients.createDefault(); // 创建http POST请求 HttpPost httpPost = new HttpPost("http://www.oschina.net/search"); // 设置2个post参数，一个是scope、一个是q List
优点：接口设计良好，支持HTTP/2，并且在弱网和无网环境下有自动检测和恢复机制
【当网络出现问题的时候，它会自动恢复一般的连接问题，如果你的服务有多个IP地址，当第一个IP请求失败时，OkHttp会交替尝试匹配其他的IP，OkHttp使用现代TLS技术（SNI,ALPN[我也不懂是啥]）初始化新的连接，当握手失败时会回退到TLS1.0】
依赖
GET请求
public static String get(String url) throws IOException { OkHttpClient client = new OkHttpClient(); Request request = new Request.Builder() .get() .url(url) .build(); Response response = client.newCall(request).execute(); return response.body().string();}// 调用String objStr = OkHttpUtil.get(url);
表单数据提交
public static void postFormBody(String url, FormBody formBody) throws IOException { OkHttpClient client = new OkHttpClient(); final Request request = new Request.Builder() .url(url) .post(formBody) .build(); Response response = client.newCall(request).execute(); return response.body().string();}// 调用FormBody formBody = new FormBody.Builder() .add("id", id.toString()) .build();OkHttpUtil.postFormBody(url, formBody);
Json数据提交
public static String postJsonBody(String url, String jsonBody) throws IOException { OkHttpClient client = new OkHttpClient(); MediaType mediaType = MediaType.parse("application/json"); RequestBody body = RequestBody.create(mediaType, jsonBody); final Request request = new Request.Builder() .url(url) .post(body) .build(); Response response = client.newCall(request).execute(); return response.body().string();}// 调用String config = "{"id": 123}";String response = OkHttpUtil.postJsonBody(url, config);
Delete请求
public static String delete(String url, String jsonBody) throws IOException { OkHttpClient client = new OkHttpClient(); MediaType mediaType = MediaType.parse("application/json"); RequestBody body = RequestBody.create(mediaType, jsonBody); Request request = new Request.Builder() .url(url) .method("DELETE", body) .addHeader("Content-Type", "application/json") .build(); Response response = client.newCall(request).execute(); return response.body().string();}// 调用String response = OkHttpUtil.delete(url, "");
RestTemplateSpring提供的用于访问Rest服务的客户端优点：提供了多种便捷访问远程 Http服务的方法，能够大大提高客户端的编写效率
依赖
RestTemplate需要自己注入
三种方式
//方式一：如果要使用，可以编写配置文件自己注入package com.itunion.config;import org.springframework.context.annotation.Bean;import org.springframework.context.annotation.Configuration;import org.springframework.http.client.ClientHttpRequestFactory;import org.springframework.http.client.SimpleClientHttpRequestFactory;import org.springframework.web.client.RestTemplate;@Configurationpublic class ApiConfig { @Bean public RestTemplate restTemplate(ClientHttpRequestFactory factory) { return new RestTemplate(factory); } @Bean public ClientHttpRequestFactory simpleClientHttpRequestFactory() { //默认的是JDK提供http连接，需要的话可以通过setRequestFactory方法替换为例如Apache HttpComponents、Netty或OkHttp等其它HTTP library。 SimpleClientHttpRequestFactory factory = new SimpleClientHttpRequestFactory(); factory.setReadTimeout(5000);//单位为ms factory.setConnectTimeout(5000);//单位为ms return factory; }}
//方式二：也可以通过自己新建对象直接使用RestTemplate restTemplate = new RestTemplate();
//方式三：RestTemplateBuilder创建@Autowired private RestTemplateBuilder templateBuilder; @Bean public RestTemplate restTemplate() { return templateBuilder.build(); }
GET请求
//方式1：Notice notice = restTemplate.getForObject("http://fantj.top/notice/list/{1}/{2}" , Notice.class,1,5);
//方式2：Map
Post请求
//方式一: ForObjectString url = "http://demo/api/book/";HttpHeaders headers = new HttpHeaders();MediaType type = MediaType.parseMediaType("application/json; charset=UTF-8");headers.setContentType(type);String requestJson = "{...}";HttpEntity
//方式二: ForEntityString url = "http://47.xxx.xxx.96/register/checkEmail";HttpHeaders headers = new HttpHeaders();headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);MultiValueMap
feign的核心就是一个代理，通过一系列的封装和处理，最终转换成Http的请求形式
大致原理如下：
启动时，程序进行包扫描，扫描所有包下添加了@FeignClient注解的类，并将这些类注入到Spring的IOC容器中，当定义中的Feign接口被调用时，通过JDK动态代理来生成RequestTemplateRequestTemplate中包含请求的所有信息，如请求参数，请求URL等RequestTemplate生成一个Request对象，将Request交给Client处理，这个client默认是JDK的HttpUrlConnection，也可以是Okhttp,HttpClient等最后client封装成LoadBalanceClient，结合Ribbon负载均衡发起调用
网络上有一张Feign的处理图可以便于理解