SpringBoot RestTemplate源码分析

类定义为public class RestTemplate extends InterceptingHttpAccessor implements RestOperations，RestOperations实现了RestOperations接口，也是RestOperations的唯一实现类，具体定义方法不再阐述。

看下RestTemplate 的构造函数，

public RestTemplate() {
		this.messageConverters.add(new ByteArrayHttpMessageConverter());
		this.messageConverters.add(new StringHttpMessageConverter());
		this.messageConverters.add(new ResourceHttpMessageConverter());
		this.messageConverters.add(new SourceHttpMessageConverter<Source>());
		this.messageConverters.add(new AllEncompassingFormHttpMessageConverter());

		if (romePresent) {
			this.messageConverters.add(new AtomFeedHttpMessageConverter());
			this.messageConverters.add(new RssChannelHttpMessageConverter());
		}

		if (jackson2XmlPresent) {
			this.messageConverters.add(new MappingJackson2XmlHttpMessageConverter());
		}
		else if (jaxb2Present) {
			this.messageConverters.add(new Jaxb2RootElementHttpMessageConverter());
		}

		if (jackson2Present) {
			this.messageConverters.add(new MappingJackson2HttpMessageConverter());
		}
		else if (gsonPresent) {
			this.messageConverters.add(new GsonHttpMessageConverter());
		}
	}

RestTemplate 默认构造函数初始化添加了很多对象读写转换器。

@Override
	public <T> T getForObject(String url, Class<T> responseType, Object... urlVariables) throws RestClientException {
		RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
		HttpMessageConverterExtractor<T> responseExtractor =
				new HttpMessageConverterExtractor<T>(responseType, getMessageConverters(), logger);
		return execute(url, HttpMethod.GET, requestCallback, responseExtractor, urlVariables);
	}

	@Override
	public <T> T getForObject(String url, Class<T> responseType, Map<String, ?> urlVariables) throws RestClientException {
		RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
		HttpMessageConverterExtractor<T> responseExtractor =
				new HttpMessageConverterExtractor<T>(responseType, getMessageConverters(), logger);
		return execute(url, HttpMethod.GET, requestCallback, responseExtractor, urlVariables);
	}

	@Override
	public <T> T getForObject(URI url, Class<T> responseType) throws RestClientException {
		RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
		HttpMessageConverterExtractor<T> responseExtractor =
				new HttpMessageConverterExtractor<T>(responseType, getMessageConverters(), logger);
		return execute(url, HttpMethod.GET, requestCallback, responseExtractor);
	}

	@Override
	public <T> ResponseEntity<T> getForEntity(String url, Class<T> responseType, Object... urlVariables)
			throws RestClientException {

		RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
		ResponseExtractor<ResponseEntity<T>> responseExtractor = responseEntityExtractor(responseType);
		return execute(url, HttpMethod.GET, requestCallback, responseExtractor, urlVariables);
	}

此处代码提供给外部调用核心方法实现，在RestOperations中定义，可进行http相关get，post等操作。利用回调模式，实现构造RequestCallback接口的相关实现类，excute方法执行回调操作。ResponseExtractor接口定义了解析http返回的数据封装操作。

protected <T> T doExecute(URI url, HttpMethod method, RequestCallback requestCallback,
			ResponseExtractor<T> responseExtractor) throws RestClientException {

		Assert.notNull(url, "'url' must not be null");
		Assert.notNull(method, "'method' must not be null");
		ClientHttpResponse response = null;
		try {
			ClientHttpRequest request = createRequest(url, method);
			if (requestCallback != null) {
				requestCallback.doWithRequest(request);
			}
			response = request.execute();
			handleResponse(url, method, response);
			if (responseExtractor != null) {
				return responseExtractor.extractData(response);
			}
			else {
				return null;
			}
		}
		catch (IOException ex) {
			throw new ResourceAccessException("I/O error on " + method.name() +
					" request for \"" + url + "\": " + ex.getMessage(), ex);
		}
		finally {
			if (response != null) {
				response.close();
			}
		}
	}

简单名了的一个http请求操作，其中请求操作封装到了response = request.execute();中，这里引出了一个封装请求接口ClientHttpRequest，该接口各个实现类因为http请求结构方式的差异而不同。

public interface ClientHttpRequestFactory {

	/**
	 * Create a new {@link ClientHttpRequest} for the specified URI and HTTP method.
	 * <p>The returned request can be written to, and then executed by calling
	 * {@link ClientHttpRequest#execute()}.
	 * @param uri the URI to create a request for
	 * @param httpMethod the HTTP method to execute
	 * @return the created request
	 * @throws IOException in case of I/O errors
	 */
	ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException;

}

分别为每种requet提供了自己的ClientHttpRequestFactory工厂类，定义了生产request的接口方法createRequest。