It's not my fault, I don't think you have used this awesome network tool library in Spring.

1. Introduction

In today's IT projects, the scene where the server initiates network requests to the outside world is basically everywhere!

Traditionally, when accessing http services in server-side code, we generally use JDK's HttpURLConnection or Apache's HttpClient, but this method is too cumbersome to use, and the api is very complicated to use, and we have to worry about resource recycling.

Take downloading files as an example, download files through Apache's HttpClient method, the following is the code logic I encapsulated before, see how complicated it is!

In fact, Spring has provided us with a simple and convenient template class to operate, which is RestTemplate.

RestTemplate is a synchronous blocking tool class that executes HTTP requests. It just encapsulates a simpler and easier-to-use template method API based on HTTP client libraries (such as JDK HttpURLConnection, Apache HttpComponents, okHttp, etc.), which is convenient for programmers to use The provided template method initiates network requests and processing, which can greatly improve our development efficiency.

Okay, no more BBs, let's start the code!

2. Environment configuration

2.1. Using RestTemplate in a non-Spring environment

If the current project is not a Spring project, add the spring-web package to introduce the RestTemplate class

<dependency>
  <groupId>org.springframework</groupId>
  <artifactId>spring-web</artifactId>
  <version>5.2.6.RELEASE</version>
</dependency>

Write a unit test class that uses RestTemplate to send a GET request to see if the program runs properly

@Test
public void simpleTest() {
    RestTemplate restTemplate = new RestTemplate();
    String url = "http://jsonplaceholder.typicode.com/posts/1";
    String str = restTemplate.getForObject(url, String.class);
    System.out.println(str);
}

2.2. Using RestTemplate in Spring environment

If the current project is SpringBoot, add the following dependency interface!

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

At the same time, initialize the RestTemplate configuration as a Bean.

@Configuration
public class RestTemplateConfig {

    /**
     * 没有实例化RestTemplate时，初始化RestTemplate
     * @return
     */
    @ConditionalOnMissingBean(RestTemplate.class)
    @Bean
    public RestTemplate restTemplate(){
        RestTemplate restTemplate = new RestTemplate();
        return restTemplate;
    }
}

Note that this initialization method uses JDK's own HttpURLConnection as the underlying HTTP client implementation.

Of course, we can also modify the default client of RestTemplate, such as changing it to an HttpClient client, as follows:

@Configuration
public class RestTemplateConfig {


    /**
     * 没有实例化RestTemplate时，初始化RestTemplate
     * @return
     */
    @ConditionalOnMissingBean(RestTemplate.class)
    @Bean
    public RestTemplate restTemplate(){
        RestTemplate restTemplate = new RestTemplate(getClientHttpRequestFactory());
        return restTemplate;
    }

    /**
     * 使用HttpClient作为底层客户端
     * @return
     */
    private ClientHttpRequestFactory getClientHttpRequestFactory() {
        int timeout = 5000;
        RequestConfig config = RequestConfig.custom()
                .setConnectTimeout(timeout)
                .setConnectionRequestTimeout(timeout)
                .setSocketTimeout(timeout)
                .build();
        CloseableHttpClient client = HttpClientBuilder
                .create()
                .setDefaultRequestConfig(config)
                .build();
        return new HttpComponentsClientHttpRequestFactory(client);
    }

}

Where you need to use RestTemplate, inject and use it!

@Autowired
private RestTemplate restTemplate;

From the feedback of developers, and the evaluation of various HTTP client performance and ease of use on the Internet, OkHttp is better than Apache's HttpClient, and Apache's HttpClient is better than HttpURLConnection.

Therefore, we can also replace the underlying http client with OkHttp in the following ways!

/**
 * 使用OkHttpClient作为底层客户端
 * @return
 */
private ClientHttpRequestFactory getClientHttpRequestFactory(){
    OkHttpClient okHttpClient = new OkHttpClient.Builder()
            .connectTimeout(5, TimeUnit.SECONDS)
            .writeTimeout(5, TimeUnit.SECONDS)
            .readTimeout(5, TimeUnit.SECONDS)
            .build();
    return new OkHttp3ClientHttpRequestFactory(okHttpClient);
}

3. API Practice

The biggest feature of RestTemplate is that it wraps various network request methods, which can greatly simplify the workload of developers. Let's take GET, POST, PUT, DELETE, file upload and download as examples to introduce the use of each API. Way!

3.1. GET request

There are two commonly used methods for sending HTTP GET protocol requests through RestTemplate:

• getForObject()
• getForEntity()

The main difference between the two is that the return value of getForObject() is the response body of the HTTP protocol.

getForEntity() returns ResponseEntity, which encapsulates the HTTP response. In addition to the response body, it also includes HTTP status code, contentType, contentLength, Header and other information.

To write a unit test case in the Spring Boot environment, first create an Api interface, and then write a unit test for service testing.

• get request without parameters

@RestController
public class TestController {

    /**
     * 不带参的get请求
     * @return
     */
    @RequestMapping(value = "testGet", method = RequestMethod.GET)
    public ResponseBean testGet(){
        ResponseBean result = new ResponseBean();
        result.setCode("200");
        result.setMsg("请求成功，方法：testGet");
        return result;
    }
}

public class ResponseBean {

    private String code;

    private String msg;

    public String getCode() {
        return code;
    }

    public void setCode(String code) {
        this.code = code;
    }

    public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }

    @Override
    public String toString() {
        return "ResponseBean{" +
                "code='" + code + '\'' +
                ", msg='" + msg + '\'' +
                '}';
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 单元测试（不带参的get请求）
 */
@Test
public void testGet(){
    //请求地址
    String url = "http://localhost:8080/testGet";

    //发起请求,直接返回对象
    ResponseBean responseBean = restTemplate.getForObject(url, ResponseBean.class);
    System.out.println(responseBean.toString());
}

• get request with parameters (restful style)

@RestController
public class TestController {

    /**
     * 带参的get请求(restful风格)
     * @return
     */
    @RequestMapping(value = "testGetByRestFul/{id}/{name}", method = RequestMethod.GET)
    public ResponseBean testGetByRestFul(@PathVariable(value = "id") String id, @PathVariable(value = "name") String name){
        ResponseBean result = new ResponseBean();
        result.setCode("200");
        result.setMsg("请求成功，方法：testGetByRestFul，请求参数id：" +  id + "请求参数name：" + name);
        return result;
    }
}

@Autowired
private RestTemplate restTemplate;


 /**
 * 单元测试（带参的get请求）
 */
@Test
public void testGetByRestFul(){
    //请求地址
    String url = "http://localhost:8080/testGetByRestFul/{1}/{2}";

    //发起请求,直接返回对象（restful风格）
    ResponseBean responseBean = restTemplate.getForObject(url, ResponseBean.class, "001", "张三");
    System.out.println(responseBean.toString());
}

• get request with parameters (using placeholders to pass parameters)

@RestController
public class TestController {

    /**
     * 带参的get请求(使用占位符号传参)
     * @return
     */
    @RequestMapping(value = "testGetByParam", method = RequestMethod.GET)
    public ResponseBean testGetByParam(@RequestParam("userName") String userName,
                                             @RequestParam("userPwd") String userPwd){
        ResponseBean result = new ResponseBean();
        result.setCode("200");
        result.setMsg("请求成功，方法：testGetByParam，请求参数userName：" +  userName + ",userPwd：" + userPwd);
        return result;
    }
}

@Autowired
private RestTemplate restTemplate;

 /**
 * 单元测试（带参的get请求）
 */
@Test
public void testGetByParam(){
    //请求地址
    String url = "http://localhost:8080/testGetByParam?userName={userName}&userPwd={userPwd}";

    //请求参数
    Map<String, String> uriVariables = new HashMap<>();
    uriVariables.put("userName", "唐三藏");
    uriVariables.put("userPwd", "123456");

    //发起请求,直接返回对象（带参数请求）
    ResponseBean responseBean = restTemplate.getForObject(url, ResponseBean.class, uriVariables);
    System.out.println(responseBean.toString());
}

All of the above getForObject request parameter transfer methods, getForEntity can be used, and the usage methods are almost the same, but there are slight differences in the return results received.

Use ResponseEntity<T> responseEntity to receive the response result. Get the response body with responseEntity.getBody().

 /**
 * 单元测试
 */
@Test
public void testAllGet(){
    //请求地址
    String url = "http://localhost:8080/testGet";

    //发起请求，返回全部信息
    ResponseEntity<ResponseBean> response = restTemplate.getForEntity(url, ResponseBean.class);

    // 获取响应体
    System.out.println("HTTP 响应body：" + response.getBody().toString());

    // 以下是getForEntity比getForObject多出来的内容
    HttpStatus statusCode = response.getStatusCode();
    int statusCodeValue = response.getStatusCodeValue();
    HttpHeaders headers = response.getHeaders();

    System.out.println("HTTP 响应状态：" + statusCode);
    System.out.println("HTTP 响应状态码：" + statusCodeValue);
    System.out.println("HTTP Headers信息：" + headers);
}

3.2, POST request

In fact, the POST request method is similar to the GET request method. RestTemplate's POST request also contains two main methods:

• postForObject ()
• postForEntity ()

postForEntity() returns all the information, and the postForObject() method returns the body object. The specific usage is as follows!

• Simulate form request, post method test

@RestController
public class TestController {

    /**
     * 模拟表单请求，post方法测试
     * @return
     */
    @RequestMapping(value = "testPostByForm", method = RequestMethod.POST)
    public ResponseBean testPostByForm(@RequestParam("userName") String userName,
                                        @RequestParam("userPwd") String userPwd){
        ResponseBean result = new ResponseBean();
        result.setCode("200");
        result.setMsg("请求成功，方法：testPostByForm，请求参数userName：" + userName + ",userPwd:" + userPwd);
        return result;
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 模拟表单提交，post请求
 */
@Test
public void testPostByForm(){
    //请求地址
    String url = "http://localhost:8080/testPostByForm";

    // 请求头设置,x-www-form-urlencoded格式的数据
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);

    //提交参数设置
    MultiValueMap<String, String> map = new LinkedMultiValueMap<>();
    map.add("userName", "唐三藏");
    map.add("userPwd", "123456");

    // 组装请求体
    HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(map, headers);

    //发起请求
    ResponseBean responseBean = restTemplate.postForObject(url, request, ResponseBean.class);
    System.out.println(responseBean.toString());
}

• Simulate form request, post method test (object accepted)

@RestController
public class TestController {

    /**
     * 模拟表单请求，post方法测试
     * @param request
     * @return
     */
    @RequestMapping(value = "testPostByFormAndObj", method = RequestMethod.POST)
    public ResponseBean testPostByForm(RequestBean request){
        ResponseBean result = new ResponseBean();
        result.setCode("200");
        result.setMsg("请求成功，方法：testPostByFormAndObj，请求参数：" + JSON.toJSONString(request));
        return result;
    }
}

public class RequestBean {


    private String userName;


    private String userPwd;


    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getUserPwd() {
        return userPwd;
    }

    public void setUserPwd(String userPwd) {
        this.userPwd = userPwd;
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 模拟表单提交，post请求
 */
@Test
public void testPostByForm(){
    //请求地址
    String url = "http://localhost:8080/testPostByFormAndObj";

    // 请求头设置,x-www-form-urlencoded格式的数据
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);

    //提交参数设置
    MultiValueMap<String, String> map = new LinkedMultiValueMap<>();
    map.add("userName", "唐三藏");
    map.add("userPwd", "123456");

    // 组装请求体
    HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(map, headers);

    //发起请求
    ResponseBean responseBean = restTemplate.postForObject(url, request, ResponseBean.class);
    System.out.println(responseBean.toString());
}

• Mock JSON request, post method test

@RestController
public class TestController {

    /**
     * 模拟JSON请求，post方法测试
     * @param request
     * @return
     */
    @RequestMapping(value = "testPostByJson", method = RequestMethod.POST)
    public ResponseBean testPostByJson(@RequestBody RequestBean request){
        ResponseBean result = new ResponseBean();
        result.setCode("200");
        result.setMsg("请求成功，方法：testPostByJson，请求参数：" + JSON.toJSONString(request));
        return result;
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 模拟JSON提交，post请求
 */
@Test
public void testPostByJson(){
    //请求地址
    String url = "http://localhost:8080/testPostByJson";

    //入参
    RequestBean request = new RequestBean();
    request.setUserName("唐三藏");
    request.setUserPwd("123456789");

    //发送post请求，并打印结果，以String类型接收响应结果JSON字符串
    ResponseBean responseBean = restTemplate.postForObject(url, request, ResponseBean.class);
    System.out.println(responseBean.toString());
}

• Simulate page redirection, post request

@Controller
public class LoginController {

    /**
     * 重定向
     * @param request
     * @return
     */
    @RequestMapping(value = "testPostByLocation", method = RequestMethod.POST)
    public String testPostByLocation(@RequestBody RequestBean request){
        return "redirect:index.html";
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 重定向，post请求
 */
@Test
public void testPostByLocation(){
    //请求地址
    String url = "http://localhost:8080/testPostByLocation";

    //入参
    RequestBean request = new RequestBean();
    request.setUserName("唐三藏");
    request.setUserPwd("123456789");

    //用于提交完成数据之后的页面跳转，返回跳转url
    URI uri = restTemplate.postForLocation(url, request);
    System.out.println(uri.toString());
}

The output is as follows:

http://localhost:8080/index.html

3.3. PUT request

The put request method may not be used by many people. It refers to modifying an existing resource or inserting a resource. This method will send an HTTP PUT method request to the resource represented by the URL. The example is as follows!

@RestController
public class TestController {

    /**
     * 模拟JSON请求，put方法测试
     * @param request
     * @return
     */
    @RequestMapping(value = "testPutByJson", method = RequestMethod.PUT)
    public void testPutByJson(@RequestBody RequestBean request){
        System.out.println("请求成功，方法：testPutByJson，请求参数：" + JSON.toJSONString(request));
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 模拟JSON提交，put请求
 */
@Test
public void testPutByJson(){
    //请求地址
    String url = "http://localhost:8080/testPutByJson";

    //入参
    RequestBean request = new RequestBean();
    request.setUserName("唐三藏");
    request.setUserPwd("123456789");

    //模拟JSON提交，put请求
    restTemplate.put(url, request);
}

3.4. DELETE request

Corresponding to it is the delete method protocol, which means to delete an existing resource. This method will send an HTTP DELETE method request to the resource represented by the URL.

@RestController
public class TestController {

    /**
     * 模拟JSON请求，delete方法测试
     * @return
     */
    @RequestMapping(value = "testDeleteByJson", method = RequestMethod.DELETE)
    public void testDeleteByJson(){
        System.out.println("请求成功，方法：testDeleteByJson");
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 模拟JSON提交，delete请求
 */
@Test
public void testDeleteByJson(){
    //请求地址
    String url = "http://localhost:8080/testDeleteByJson";

    //模拟JSON提交，delete请求
    restTemplate.delete(url);
}

3.5, general request method exchange method

If the above methods do not meet your requirements. In the RestTemplate tool class, there is also an exchange general protocol request method, which can send HTTP method requests such as GET, POST, DELETE, PUT, OPTIONS, PATCH, etc.

Open the source code, we can clearly see this.

Using the exchange method, it can meet the request operation in various scenarios!

3.6. File upload and download

In addition to the frequently used get and post requests, there is another scenario that we often encounter, that is, the upload and download of files. If RestTemplate is used, how to use it?

The case is as follows, the specific implementation details refer to the code comments!

• File Upload

@RestController
public class FileUploadController {


    private static final String UPLOAD_PATH = "/springboot-frame-example/springboot-example-resttemplate/";

    /**
     * 文件上传
     * @param uploadFile
     * @return
     */
    @RequestMapping(value = "upload", method = RequestMethod.POST)
    public ResponseBean upload(@RequestParam("uploadFile") MultipartFile uploadFile,
                               @RequestParam("userName") String userName) {
        // 在 uploadPath 文件夹中通过用户名对上传的文件归类保存
        File folder = new File(UPLOAD_PATH + userName);
        if (!folder.isDirectory()) {
            folder.mkdirs();
        }

        // 对上传的文件重命名，避免文件重名
        String oldName = uploadFile.getOriginalFilename();
        String newName = UUID.randomUUID().toString() + oldName.substring(oldName.lastIndexOf("."));

        //定义返回视图
        ResponseBean result = new ResponseBean();
        try {
            // 文件保存
            uploadFile.transferTo(new File(folder, newName));
            result.setCode("200");
            result.setMsg("文件上传成功，方法：upload，文件名：" + newName);
        } catch (IOException e) {
            e.printStackTrace();
            result.setCode("500");
            result.setMsg("文件上传失败，方法：upload，请求文件：" + oldName);
        }
        return result;
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 文件上传，post请求
 */
@Test
public void upload(){
    //需要上传的文件
    String filePath = "/Users/panzhi/Desktop/Jietu20220205-194655.jpg";

    //请求地址
    String url = "http://localhost:8080/upload";

    // 请求头设置,multipart/form-data格式的数据
    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.MULTIPART_FORM_DATA);

    //提交参数设置
    MultiValueMap<String, Object> param = new LinkedMultiValueMap<>();
    param.add("uploadFile", new FileSystemResource(new File(filePath)));
    //服务端如果接受额外参数，可以传递
    param.add("userName", "张三");

    // 组装请求体
    HttpEntity<MultiValueMap<String, Object>> request = new HttpEntity<>(param, headers);

    //发起请求
    ResponseBean responseBean = restTemplate.postForObject(url, request, ResponseBean.class);
    System.out.println(responseBean.toString());
}

• document dowload

@RestController
public class FileUploadController {


    private static final String UPLOAD_PATH = "springboot-frame-example/springboot-example-resttemplate/";


    /**
     * 带参的get请求(restful风格)
     * @return
     */
    @RequestMapping(value = "downloadFile/{userName}/{fileName}", method = RequestMethod.GET)
    public void downloadFile(@PathVariable(value = "userName") String userName,
                             @PathVariable(value = "fileName") String fileName,
                             HttpServletRequest request,
                             HttpServletResponse response) throws Exception {

        File file = new File(UPLOAD_PATH + userName + File.separator + fileName);
        if (file.exists()) {
            //获取文件流
            FileInputStream fis = new FileInputStream(file);
            //获取文件后缀（.png）
            String extendFileName = fileName.substring(fileName.lastIndexOf('.'));
            //动态设置响应类型，根据前台传递文件类型设置响应类型
            response.setContentType(request.getSession().getServletContext().getMimeType(extendFileName));
            //设置响应头,attachment表示以附件的形式下载，inline表示在线打开
            response.setHeader("content-disposition","attachment;fileName=" + URLEncoder.encode(fileName,"UTF-8"));
            //获取输出流对象（用于写文件）
            OutputStream os = response.getOutputStream();
            //下载文件,使用spring框架中的FileCopyUtils工具
            FileCopyUtils.copy(fis,os);
        }
    }
}

@Autowired
private RestTemplate restTemplate;

/**
 * 小文件下载
 * @throws IOException
 */
@Test
public void downloadFile() throws IOException {
    String userName = "张三";
    String fileName = "c98b677c-0948-46ef-84d2-3742a2b821b0.jpg";
    //请求地址
    String url = "http://localhost:8080/downloadFile/{1}/{2}";

    //发起请求,直接返回对象（restful风格）
    ResponseEntity<byte[]> rsp = restTemplate.getForEntity(url, byte[].class, userName,fileName);
    System.out.println("文件下载请求结果状态码：" + rsp.getStatusCode());

    // 将下载下来的文件内容保存到本地
    String targetPath = "/Users/panzhi/Desktop/"  + fileName;
    Files.write(Paths.get(targetPath), Objects.requireNonNull(rsp.getBody(), "未获取到下载文件"));
}

This download method actually loads the downloaded file once into the client's local memory, and then writes the file from memory to disk. This method is more suitable for downloading small files. If the file is relatively large or the number of concurrent file downloads is relatively large, it is easy to cause a large amount of memory occupation, thereby reducing the operating efficiency of the application.

• large file download

@Autowired
private RestTemplate restTemplate;

/**
 * 大文件下载
 * @throws IOException
 */
@Test
public void downloadBigFile() throws IOException {
    String userName = "张三";
    String fileName = "c98b677c-0948-46ef-84d2-3742a2b821b0.jpg";
    //请求地址
    String url = "http://localhost:8080/downloadFile/{1}/{2}";

    //定义请求头的接收类型
    RequestCallback requestCallback = request -> request.getHeaders()
            .setAccept(Arrays.asList(MediaType.APPLICATION_OCTET_STREAM, MediaType.ALL));

    //对响应进行流式处理而不是将其全部加载到内存中
    String targetPath = "/Users/panzhi/Desktop/"  + fileName;
    restTemplate.execute(url, HttpMethod.GET, requestCallback, clientHttpResponse -> {
        Files.copy(clientHttpResponse.getBody(), Paths.get(targetPath));
        return null;
    }, userName, fileName);
}

The difference between this download method is:

• The request header APPLICATION_OCTET_STREAM is set, indicating that data is loaded in the form of a stream
• RequestCallback combined with File.copy ensures that part of the file content is received, and part of the content is written to the disk. Instead of loading it all into memory and writing it to disk at the end.

Especially useful when downloading large files, such as excel, pdf, zip, etc.

4. Summary

Through the explanation in this chapter, the reader must have a preliminary understanding of how to use RestTemplate to easily and quickly access the restful interface. In fact, the function of RestTemplate is very powerful, and the author has only learned a little bit. If you feel that there is something unclear in this article or if you have anything else you want to know, you can leave a message below, and I will try my best to supplement and improve it in the future!