RestTemplate工作原理及攔截器的使用

RestTemplate的使用

new RestTemplate().getForObject("http://www.baidu.com", String.class);

源碼分析

getForObject()方法源碼：

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

execute()方法源碼：

	@Override
	@Nullable
	public <T> T execute(String url, HttpMethod method, @Nullable RequestCallback requestCallback,
			@Nullable ResponseExtractor<T> responseExtractor, Object... uriVariables) throws RestClientException {

		URI expanded = getUriTemplateHandler().expand(url, uriVariables);
		return doExecute(expanded, method, requestCallback, responseExtractor);
	}

doExecute()方法源碼：

	@Nullable
	protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback,
			@Nullable 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 {
		    //創建客戶端請求
			ClientHttpRequest request = createRequest(url, method);
			if (requestCallback != null) {
				requestCallback.doWithRequest(request);
			}
			//執行請求
			response = request.execute();
			handleResponse(url, method, response);
			if (responseExtractor != null) {
				return responseExtractor.extractData(response);
			}
			else {
				return null;
			}
		}
		catch (IOException ex) {
			String resource = url.toString();
			String query = url.getRawQuery();
			resource = (query != null ? resource.substring(0, resource.indexOf('?')) : resource);
			throw new ResourceAccessException("I/O error on " + method.name() +
					" request for \"" + resource + "\": " + ex.getMessage(), ex);
		}
		finally {
			if (response != null) {
				response.close();
			}
		}
	}

AbstractClientHttpRequest類execute()方法代碼實現：

	@Override
	public final ClientHttpResponse execute() throws IOException {
		assertNotExecuted();
		ClientHttpResponse result = executeInternal(this.headers);
		this.executed = true;
		return result;
	}

AbstractClientHttpRequest類executeInternal()方法代碼實現

	@Override
	protected ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException {
		byte[] bytes = this.bufferedOutput.toByteArray();
		if (headers.getContentLength() < 0) {
			headers.setContentLength(bytes.length);
		}
		ClientHttpResponse result = executeInternal(headers, bytes);
		this.bufferedOutput = new ByteArrayOutputStream(0);
		return result;
	}
	@Override
	protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {
		InterceptingRequestExecution requestExecution = new InterceptingRequestExecution();
		return requestExecution.execute(this, bufferedOutput);
	}

InterceptingRequestExecution類execute()方法實現：

		@Override
		public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
		    //按個遍歷攔截器
			if (this.iterator.hasNext()) {
				ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
				return nextInterceptor.intercept(request, body, this);
			}
			else {
				HttpMethod method = request.getMethod();
				Assert.state(method != null, "No standard HTTP method");
				ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), method);
				request.getHeaders().forEach((key, value) -> delegate.getHeaders().addAll(key, value));
				if (body.length > 0) {
					if (delegate instanceof StreamingHttpOutputMessage) {
						StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) delegate;
						streamingOutputMessage.setBody(outputStream -> StreamUtils.copy(body, outputStream));
					}
					else {
						StreamUtils.copy(body, delegate.getBody());
					}
				}
				return delegate.execute();
			}
		}

需要注意上面這段代碼中的RestTemplate攔截器的處理動作，我們自定義的RestTemplate攔截器就是在這個過程中發揮作用的。我們來看下ClientHttpRequestInterceptor的源碼：

@FunctionalInterface
public interface ClientHttpRequestInterceptor {

	/**
	 * Intercept the given request, and return a response. The given
	 * {@link ClientHttpRequestExecution} allows the interceptor to pass on the
	 * request and response to the next entity in the chain.
	 * <p>A typical implementation of this method would follow the following pattern:
	 * <ol>
	 * <li>Examine the {@linkplain HttpRequest request} and body</li>
	 * <li>Optionally {@linkplain org.springframework.http.client.support.HttpRequestWrapper
	 * wrap} the request to filter HTTP attributes.</li>
	 * <li>Optionally modify the body of the request.</li>
	 * <li><strong>Either</strong>
	 * <ul>
	 * <li>execute the request using
	 * {@link ClientHttpRequestExecution#execute(org.springframework.http.HttpRequest, byte[])},</li>
	 * <strong>or</strong>
	 * <li>do not execute the request to block the execution altogether.</li>
	 * </ul>
	 * <li>Optionally wrap the response to filter HTTP attributes.</li>
	 * </ol>
	 * @param request the request, containing method, URI, and headers
	 * @param body the body of the request
	 * @param execution the request execution
	 * @return the response
	 * @throws IOException in case of I/O errors
	 */
	ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution)
			throws IOException;

}

我們自己實現一個RestTemplate攔截器：

@Component
public class RestTemplateInterceptor implements ClientHttpRequestInterceptor {

	@Override
	public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution)
			throws IOException {
		request.getHeaders().add("token", String.valueOf(System.currentTimeMillis()));
		return execution.execute(request, body);
	}

}