1 restTemplate — spring 提供

特点：

1、RestOperations 提供了各种封装方法，非常方便直接将返回转成实体类。

2、默认使用JDK 的HttpURLConnection进行通信，但是可以通过RestTemplate.setRequestFactory 切换到不同的HTTP源：如Apache HttpComponents、Netty、OkHttp。

3、支持同步、异步请求；

4、支持更多的定制，比如拦截器等。

ps：支持 get 请求，参数是 body 的形式。

参考：https://www.huaweicloud.com/articles/7f32ca1acff12162ce8d3bace872ae04.html

国外知名博客Baeldung的博客 The Guide to RestTemplate: https://www.baeldung.com/rest-template

1.1 底层是JAVA的HttpURLConnection(默认使用，可以定制)

所有的请求都需要执行 doExecute() 方法

@Nullable
protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback, @Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {
    Assert.notNull(url, "URI is required");
    Assert.notNull(method, "HttpMethod is required");
    ClientHttpResponse response = null;

    Object var14;
    try {
        // 创建请求
        ClientHttpRequest request = this.createRequest(url, method);
        if (requestCallback != null) {
            requestCallback.doWithRequest(request);
        }

        response = request.execute();
        this.handleResponse(url, method, response);
        var14 = responseExtractor != null ? responseExtractor.extractData(response) : null;
    } catch (IOException var12) {
        String resource = url.toString();
        String query = url.getRawQuery();
        resource = query != null ? resource.substring(0, resource.indexOf(63)) : resource;
        throw new ResourceAccessException("I/O error on " + method.name() + " request for "" + resource + "": " + var12.getMessage(), var12);
    } finally {
        if (response != null) {
            response.close();
        }

    }

    return var14;
}

HttpAccessor 创建请求

public abstract class HttpAccessor {
    ... // 省略代码无数
    protected ClientHttpRequest createRequest(URI url, HttpMethod method) throws IOException {
        // 使用 ClientHttpRequestFactory 创建请求
        ClientHttpRequest request = this.getRequestFactory().createRequest(url, method);
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("HTTP " + method.name() + " " + url);
        }

        return request;
    }
}

ClientHttpRequestFactory接口的具体实现，如：SimpleClientHttpRequestFactory 创建请求

public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
    // 使用 HttpURLConnection 创建请求
    HttpURLConnection connection = this.openConnection(uri.toURL(), this.proxy);
    this.prepareConnection(connection, httpMethod.name());
    return (ClientHttpRequest)(this.bufferRequestBody ? new SimpleBufferingClientHttpRequest(connection, this.outputStreaming) : new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming));
}

1.2 post 请求，返回直接封装为实体

RestTemplate restTemplate = new RestTemplate();
HttpEntity<Foo> request = new HttpEntity<>(new Foo("bar"));
Foo foo = restTemplate.postForObject(fooResourceUrl, request, Foo.class);
assertThat(foo, notNullValue());
assertThat(foo.getName(), is("bar"));

1.3 get请求，但是参数是body形式

一般 get 请求，不支持 body 传参。

参考：https://stackoverflow.com/questions/43421126/how-to-use-httpclient-to-send-content-in-body-of-get-request/68812976#68812976

HTTP GET with a body is a somewhat unconventional construct that falls in a gray area of the HTTP specification - the end result is that many older pieces of software either cannot handle such a request at all, or will explicitly reject it because they believe it to be malformed.

带有body参数的HTTP GET是一种非传统的构造，属于HTTP规范的灰色区域。最终的结果是，许多旧的软件要么根本不能处理这样的请求，要么会明确拒绝，因为他们认为它是格式错误的请求。

/**
 * 注意：get请求，但是参数是body形式
 *
 * @param url
 * @param paramBody
 * @return
 */
private String getWithBody(String url, Map<String, Object> paramBody) {
    HttpHeaders httpHeaders = new HttpHeaders();
    httpHeaders.setContentType(MediaType.AppLICATION_JSON);
    httpHeaders.setAccept(Arrays.asList(MediaType.APPLICATION_JSON));
    HttpEntity requestEntity = new HttpEntity(JsonUtil.of(paramBody), httpHeaders);
    RestTemplate template = getTemplate();
    ResponseEntity response = template.exchange(url, HttpMethod.GET, requestEntity, String.class);
    Object result = response.getBody();
    logger.info("/invokeThirdPartyRequest/getWithBody/result/[{}]", result.toString());
    return result.toString();
}

/**
 * 获取 RestTemplate
 *
 * @return
 */
private RestTemplate getTemplate() {
    RestTemplate restTemplate = new RestTemplate();
    //修改restTemplate的RequestFactory使其支持Get携带body参数
    restTemplate.setRequestFactory(new HttpComponentsClientRestfulHttpRequestFactory());
    return restTemplate;
}

import org.apache.http.client.methods.HttpEntityEnclosingRequestBase;
import org.apache.http.client.methods.HttpUriRequest;
import org.springframework.http.HttpMethod;
import org.springframework.http.client.HttpComponentsClientHttpRequestFactory;

import java.net.URI;

public class HttpComponentsClientRestfulHttpRequestFactory extends HttpComponentsClientHttpRequestFactory {

    @Override
    protected HttpUriRequest createHttpUriRequest(HttpMethod httpMethod, URI uri) {
        if (httpMethod == HttpMethod.GET) {
            return new HttpGetRequestWithEntity(uri);
        }
        return super.createHttpUriRequest(httpMethod, uri);
    }

    /**
     * 定义HttpGetRequestWithEntity实现HttpEntityEnclosingRequestBase抽象类，以支持GET请求携带body数据
     */
    private static final class HttpGetRequestWithEntity extends HttpEntityEnclosingRequestBase {
        public HttpGetRequestWithEntity(final URI uri) {
            super.setURI(uri);
        }
        @Override
        public String getMethod() {
            return HttpMethod.GET.name();
        }
    }
}

2 HttpUtil — hutool 提供

HttpUtil 其实是 HttpRequest 的封装。

它支持各种封装好的get、post、put请求。

2.1 get 请求

public static String get(String urlString, Charset customCharset) {
    return ((HttpRequest)HttpRequest.get(urlString).charset(customCharset)).execute().body();
}

public static String get(String urlString) {
    return get(urlString, HttpGlobalConfig.timeout);
}

public static String get(String urlString, int timeout) {
    return HttpRequest.get(urlString).timeout(timeout).execute().body();
}

// form 表单格式的入参
public static String get(String urlString, Map<String, Object> paramMap) {
    return HttpRequest.get(urlString).form(paramMap).execute().body();
}

// form 表单格式的入参，并设置超时时间
public static String get(String urlString, Map<String, Object> paramMap, int timeout) {
    return HttpRequest.get(urlString).form(paramMap).timeout(timeout).execute().body();
}

2.2 post 请求

这些请求最终调用的都是 HttpRequest 的 execute() 方法。

// form 表单格式的入参
public static String post(String urlString, Map<String, Object> paramMap) {
    return post(urlString, paramMap, HttpGlobalConfig.timeout);
}

// form 表单格式的入参，并设置超时时间
public static String post(String urlString, Map<String, Object> paramMap, int timeout) {
    return HttpRequest.post(urlString).form(paramMap).timeout(timeout).execute().body();
}

// body 格式入参
public static String post(String urlString, String body) {
    return post(urlString, body, HttpGlobalConfig.timeout);
}

// body 格式入参，并设置超时时间
public static String post(String urlString, String body, int timeout) {
    return HttpRequest.post(urlString).timeout(timeout).body(body).execute().body();
}

2.3 一个例子

Map<String, Object> param = new HashMap<>();
param.put("userId", userId);
String res = HttpUtil.post(url, JsonUtil.of(param));

3 HttpRequest — hutool 提供

HttpRequest 提供了非常方便构造请求的构造函数。当参数比较多、header比较多的时候，可以使用这种方式。（这里使用了构造模式）

3.1 底层是Java的HttpURLConnection

HttpRequest 底层又是使用了 java 提供的 HttpURLConnection

上源码：

最终都需要执行这个execute方法，这个方法调用了hutool封装的HttpConnection，这个HttpConnection又使用了java提供的HttpURLConnection。

// hutool 执行方法
public HttpResponse execute(boolean isAsync) {
    this.urlWithParamIfGet();
    this.initConnection();
    this.send();
    HttpResponse httpResponse = this.sendRedirectIfPossible();
    if (null == httpResponse) {
        httpResponse = new HttpResponse(this.httpConnection, this.charset, isAsync, this.isIgnoreResponseBody());
    }

    return httpResponse;
}

public class HttpConnection {
    private final URL url;
    private final Proxy proxy;
    // 这个连接 HttpURLConnection ，是java提供的
    private HttpURLConnection conn;
    ...// 省略无数代码
}

3.2 一个例子

private String invoke(String url, String isMock, Map<String, Object> map) {
    String result = HttpRequest.post(url).body(JSONUtil.toJsonStr(map)).execute().body();
    return result;
}