聊聊HttpClient的RedirectStrategy

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本文主要研究一下HttpClient的RedirectStrategy

RedirectStrategy

org/apache/http/client/RedirectStrategy.java

public interface RedirectStrategy {/*** Determines if a request should be redirected to a new location* given the response from the target server.** @param request the executed request* @param response the response received from the target server* @param context the context for the request execution** @return {@code true} if the request should be redirected, {@code false}* otherwise*/boolean isRedirected(HttpRequest request,HttpResponse response,HttpContext context) throws ProtocolException;/*** Determines the redirect location given the response from the target* server and the current request execution context and generates a new* request to be sent to the location.** @param request the executed request* @param response the response received from the target server* @param context the context for the request execution** @return redirected request*/HttpUriRequest getRedirect(HttpRequest request,HttpResponse response,HttpContext context) throws ProtocolException;}

RedirectStrategy接口定义了isRedirected方法用于判断是否需要redirect，还定义了getRedirect方法用于返回redirect的目标地址

DefaultRedirectStrategy

org/apache/http/impl/client/DefaultRedirectStrategy.java

@Contract(threading = ThreadingBehavior.IMMUTABLE)
public class DefaultRedirectStrategy implements RedirectStrategy {private final Log log = LogFactory.getLog(getClass());/*** @deprecated (4.3) use {@link org.apache.http.client.protocol.HttpClientContext#REDIRECT_LOCATIONS}.*/@Deprecatedpublic static final String REDIRECT_LOCATIONS = "http.protocol.redirect-locations";public static final DefaultRedirectStrategy INSTANCE = new DefaultRedirectStrategy();private final String[] redirectMethods;public DefaultRedirectStrategy() {this(new String[] {HttpGet.METHOD_NAME,HttpHead.METHOD_NAME});}/*** Constructs a new instance to redirect the given HTTP methods.** @param redirectMethods The methods to redirect.* @since 4.5.10*/public DefaultRedirectStrategy(final String[] redirectMethods) {super();final String[] tmp = redirectMethods.clone();Arrays.sort(tmp);this.redirectMethods = tmp;}@Overridepublic boolean isRedirected(final HttpRequest request,final HttpResponse response,final HttpContext context) throws ProtocolException {Args.notNull(request, "HTTP request");Args.notNull(response, "HTTP response");final int statusCode = response.getStatusLine().getStatusCode();final String method = request.getRequestLine().getMethod();final Header locationHeader = response.getFirstHeader("location");switch (statusCode) {case HttpStatus.SC_MOVED_TEMPORARILY:return isRedirectable(method) && locationHeader != null;case HttpStatus.SC_MOVED_PERMANENTLY:case HttpStatus.SC_TEMPORARY_REDIRECT:return isRedirectable(method);case HttpStatus.SC_SEE_OTHER:return true;default:return false;} //end of switch}/*** @since 4.2*/protected boolean isRedirectable(final String method) {return Arrays.binarySearch(redirectMethods, method) >= 0;}@Overridepublic HttpUriRequest getRedirect(final HttpRequest request,final HttpResponse response,final HttpContext context) throws ProtocolException {final URI uri = getLocationURI(request, response, context);final String method = request.getRequestLine().getMethod();if (method.equalsIgnoreCase(HttpHead.METHOD_NAME)) {return new HttpHead(uri);} else if (method.equalsIgnoreCase(HttpGet.METHOD_NAME)) {return new HttpGet(uri);} else {final int status = response.getStatusLine().getStatusCode();return status == HttpStatus.SC_TEMPORARY_REDIRECT? RequestBuilder.copy(request).setUri(uri).build(): new HttpGet(uri);}}}    

DefaultRedirectStrategy实现了RedirectStrategy接口，它定义了redirectMethods，默认是Get和Head；isRedirected方法先获取response的statusCode，对于302需要location的header有值且请求method在redirectMethods中(isRedirectable)，对于301及307仅仅判断isRedirectable，对于303返回true，其余的返回false

getRedirect方法先通过getLocationURI获取目标地址，然后针对get或者head分别构造HttpHead及HttpGet，剩下的根据statusCode判断，是307则拷贝原来的request，否则返回HttpGet

RedirectExec

org/apache/http/impl/execchain/RedirectExec.java

/*** Request executor in the request execution chain that is responsible* for handling of request redirects.* <p>* Further responsibilities such as communication with the opposite* endpoint is delegated to the next executor in the request execution* chain.* </p>** @since 4.3*/
@Contract(threading = ThreadingBehavior.IMMUTABLE_CONDITIONAL)
public class RedirectExec implements ClientExecChain {private final Log log = LogFactory.getLog(getClass());private final ClientExecChain requestExecutor;private final RedirectStrategy redirectStrategy;private final HttpRoutePlanner routePlanner;public RedirectExec(final ClientExecChain requestExecutor,final HttpRoutePlanner routePlanner,final RedirectStrategy redirectStrategy) {super();Args.notNull(requestExecutor, "HTTP client request executor");Args.notNull(routePlanner, "HTTP route planner");Args.notNull(redirectStrategy, "HTTP redirect strategy");this.requestExecutor = requestExecutor;this.routePlanner = routePlanner;this.redirectStrategy = redirectStrategy;}@Overridepublic CloseableHttpResponse execute(final HttpRoute route,final HttpRequestWrapper request,final HttpClientContext context,final HttpExecutionAware execAware) throws IOException, HttpException {Args.notNull(route, "HTTP route");Args.notNull(request, "HTTP request");Args.notNull(context, "HTTP context");final List<URI> redirectLocations = context.getRedirectLocations();if (redirectLocations != null) {redirectLocations.clear();}final RequestConfig config = context.getRequestConfig();final int maxRedirects = config.getMaxRedirects() > 0 ? config.getMaxRedirects() : 50;HttpRoute currentRoute = route;HttpRequestWrapper currentRequest = request;for (int redirectCount = 0;;) {final CloseableHttpResponse response = requestExecutor.execute(currentRoute, currentRequest, context, execAware);try {if (config.isRedirectsEnabled() &&this.redirectStrategy.isRedirected(currentRequest.getOriginal(), response, context)) {if (redirectCount >= maxRedirects) {throw new RedirectException("Maximum redirects ("+ maxRedirects + ") exceeded");}redirectCount++;final HttpRequest redirect = this.redirectStrategy.getRedirect(currentRequest.getOriginal(), response, context);if (!redirect.headerIterator().hasNext()) {final HttpRequest original = request.getOriginal();redirect.setHeaders(original.getAllHeaders());}currentRequest = HttpRequestWrapper.wrap(redirect);if (currentRequest instanceof HttpEntityEnclosingRequest) {RequestEntityProxy.enhance((HttpEntityEnclosingRequest) currentRequest);}final URI uri = currentRequest.getURI();final HttpHost newTarget = URIUtils.extractHost(uri);if (newTarget == null) {throw new ProtocolException("Redirect URI does not specify a valid host name: " +uri);}// Reset virtual host and auth states if redirecting to another hostif (!currentRoute.getTargetHost().equals(newTarget)) {final AuthState targetAuthState = context.getTargetAuthState();if (targetAuthState != null) {this.log.debug("Resetting target auth state");targetAuthState.reset();}final AuthState proxyAuthState = context.getProxyAuthState();if (proxyAuthState != null && proxyAuthState.isConnectionBased()) {this.log.debug("Resetting proxy auth state");proxyAuthState.reset();}}currentRoute = this.routePlanner.determineRoute(newTarget, currentRequest, context);if (this.log.isDebugEnabled()) {this.log.debug("Redirecting to '" + uri + "' via " + currentRoute);}EntityUtils.consume(response.getEntity());response.close();} else {return response;}} catch (final RuntimeException ex) {response.close();throw ex;} catch (final IOException ex) {response.close();throw ex;} catch (final HttpException ex) {// Protocol exception related to a direct.// The underlying connection may still be salvaged.try {EntityUtils.consume(response.getEntity());} catch (final IOException ioex) {this.log.debug("I/O error while releasing connection", ioex);} finally {response.close();}throw ex;}}}}

RedirectExec实现了ClientExecChain接口，其构造器要求传入requestExecutor、redirectStrategy、routePlanner，其execute方法会先获取maxRedirects参数，然后执行requestExecutor.execute，接着在config.isRedirectsEnabled()以及redirectStrategy.isRedirected为true时才进入redirect逻辑，它会先判断是否超出maxRedirects，大于等于则抛出RedirectException，否则通过redirectStrategy.getRedirect获取HttpRequest，更新currentRoute，然后清理entity关闭response继续下次循环，即执行redirect逻辑。

小结

HttpClient的RedirectStrategy定义了两个方法，一个是是否需要redirect，一个是获取redirect的请求，DefaultRedirectStrategy的构造器支持传入redirectMethods，默认是Get和Head，isRedirected方法主要是对302,301,307,303进行了判断，getRedirect方法主要是通过location获取目标地址，然后根据原来的method和statusCode构造HttpUriRequest。