RestTemplate详解

概述

        RestTemplate与httpClient类似，都是java中可以模拟http请求的封装。httpClient的使用，已经在另一篇文章中有所论述，但是RestTemplate比httpClient更优雅，它是spring中的一个封装功能。

        RestTemplate也是java中的模板类。采用的设计模式中的模板模式。

源码解析

        RestTemplate继承InterceptingHttpAccessor抽象类，实现RestOperation接口

public class RestTemplate extends InterceptingHttpAccessor implements RestOperations

        在RestOperations定义了基本的Rest操作集合。RestTemplate实现了这个接口

public interface RestOperations {

	// GET

	 T getForObject(String url, Class responseType, Object... uriVariables) throws RestClientException;

	 T getForObject(String url, Class responseType, Map uriVariables) throws RestClientException;

	 T getForObject(URI url, Class responseType) throws RestClientException;

	 ResponseEntity getForEntity(String url, Class responseType, Object... uriVariables) throws RestClientException;

	 ResponseEntity getForEntity(String url, Class responseType, Map uriVariables) throws RestClientException;

	 ResponseEntity getForEntity(URI url, Class responseType) throws RestClientException;


	// HEAD

	HttpHeaders headForHeaders(String url, Object... uriVariables) throws RestClientException;

	HttpHeaders headForHeaders(String url, Map uriVariables) throws RestClientException;

	HttpHeaders headForHeaders(URI url) throws RestClientException;


	// POST

	URI postForLocation(String url, Object request, Object... uriVariables) throws RestClientException;

	URI postForLocation(String url, Object request, Map uriVariables) throws RestClientException;

	URI postForLocation(URI url, Object request) throws RestClientException;

	 T postForObject(String url, Object request, Class responseType, Object... uriVariables)
			throws RestClientException;

	 T postForObject(String url, Object request, Class responseType, Map uriVariables)
			throws RestClientException;

	 T postForObject(URI url, Object request, Class responseType) throws RestClientException;

	 ResponseEntity postForEntity(String url, Object request, Class responseType, Object... uriVariables)
			throws RestClientException;

	 ResponseEntity postForEntity(String url, Object request, Class responseType, Map uriVariables)
			throws RestClientException;

	 ResponseEntity postForEntity(URI url, Object request, Class responseType) throws RestClientException;


	// PUT

	void put(String url, Object request, Object... uriVariables) throws RestClientException;

	void put(String url, Object request, Map uriVariables) throws RestClientException;

	void put(URI url, Object request) throws RestClientException;


	// PATCH

	 T patchForObject(String url, Object request, Class responseType, Object... uriVariables)
			throws RestClientException;

	 T patchForObject(String url, Object request, Class responseType, Map uriVariables)
			throws RestClientException;

	 T patchForObject(URI url, Object request, Class responseType) throws RestClientException;



	// DELETE

	void delete(String url, Object... uriVariables) throws RestClientException;

	void delete(String url, Map uriVariables) throws RestClientException;

	void delete(URI url) throws RestClientException;


	// OPTIONS

	Set optionsForAllow(String url, Object... uriVariables) throws RestClientException;

	Set optionsForAllow(String url, Map uriVariables) throws RestClientException;

	Set optionsForAllow(URI url) throws RestClientException;


	// exchange

	 ResponseEntity exchange(String url, HttpMethod method, HttpEntity requestEntity,
			Class responseType, Object... uriVariables) throws RestClientException;

	 ResponseEntity exchange(String url, HttpMethod method, HttpEntity requestEntity,
			Class responseType, Map uriVariables) throws RestClientException;

	 ResponseEntity exchange(URI url, HttpMethod method, HttpEntity requestEntity,
			Class responseType) throws RestClientException;

	 ResponseEntity exchange(String url,HttpMethod method, HttpEntity requestEntity,
			ParameterizedTypeReference responseType, Object... uriVariables) throws RestClientException;

	 ResponseEntity exchange(String url, HttpMethod method, HttpEntity requestEntity,
			ParameterizedTypeReference responseType, Map uriVariables) throws RestClientException;

	 ResponseEntity exchange(URI url, HttpMethod method, HttpEntity requestEntity,
			ParameterizedTypeReference responseType) throws RestClientException;

	 ResponseEntity exchange(RequestEntity requestEntity, Class responseType) throws RestClientException;

	 ResponseEntity exchange(RequestEntity requestEntity, ParameterizedTypeReference responseType)
			throws RestClientException;


	// general execution

	 T execute(String url, HttpMethod method, RequestCallback requestCallback,
			ResponseExtractor responseExtractor, Object... uriVariables) throws RestClientException;

	 T execute(String url, HttpMethod method, RequestCallback requestCallback,
			ResponseExtractor responseExtractor, Map uriVariables) throws RestClientException;

	 T execute(URI url, HttpMethod method, RequestCallback requestCallback,
			ResponseExtractor responseExtractor) throws RestClientException;

}

        PostForObject与PostForEntity的区别在于前者请求返回就是响应体，后者返回的是响应行，响应头，响应体。

        这里也有一个基础的http访问类HttpAccessor,里面主要是一个工厂方法，用来构造HttpRequestFactory

public abstract class HttpAccessor {

	protected final Log logger = LogFactory.getLog(getClass());

	private ClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();

	public void setRequestFactory(ClientHttpRequestFactory requestFactory) {
		Assert.notNull(requestFactory, "ClientHttpRequestFactory must not be null");
		this.requestFactory = requestFactory;
	}

	public ClientHttpRequestFactory getRequestFactory() {
		return this.requestFactory;
	}

	protected ClientHttpRequest createRequest(URI url, HttpMethod method) throws IOException {
		ClientHttpRequest request = getRequestFactory().createRequest(url, method);
		if (logger.isDebugEnabled()) {
			logger.debug("Created " + method.name() + " request for \"" + url + "\"");
		}
		return request;
	}

}

springboot对RestTemplate的集成

        首先添加配置;

@Configuration
public class RestTemplateConfig {
	
	@Bean
	public RestTemplate restTemplate(){
		RestTemplate restTemplate = new RestTemplate();
		restTemplate.getMessageConverters().set(1, new StringHttpMessageConverter(StandardCharsets.UTF_8));
		return restTemplate;
	}

}

        不然在service中的autowired会注入不进去

        再设置请求头

		HttpHeaders headers = new HttpHeaders();
		headers.setContentType(MediaType.APPLICATION_JSON);

        请求行，请求头，请求体分装成 一个Entity

HttpEntity entity = new HttpEntity<>(JSONObject.toJSONString(dto),headers );

        接下来就调用出

ResponseEntity r = restTemplate.exchange(ADD_URL, HttpMethod.POST, entity, String.class);

        那就可以请求到结果了。

总结

        整个restTemplate比httpClient使用更加优雅，分装的更好

        restTemplate是org.springframework.web.client包中的类,是spring提供的类。

        httpclient是org.apache.http.client包中的类，是apache提供的类。