HttpClient Get请求实例

Httpclient是我们平时中用的比较多的，但是一般用的时候都是去网上百度一下，把demo直接拿过来改一下用就行了，接下来我们来看他的一些具体的用法。Apache HttpComponents™项目负责创建和维护一个专注于HTTP和相关协议的低级Java组件工具集。该项目在Apache软件基金会下运行，并且是更大的开发人员和用户社区的一部分。

很多情况下我们都会用到HttpClient来发送Get请求，下面我来看下具体的代码：

CloseableHttpClient httpclient = HttpClients.createDefalt();
HttpGet httpget = new HttpGet("http://www.baidu.com");

RequestConfig requestConfig = RequestConfig.custom().setConnectTimeout(50000).setSocketTimeout(50000)
				.setConnectionRequestTimeout(3000).build();
httpget.setConfig(requestConfig);
CloseableHttpResponse response = httpClient.execute(httpget);
if (response != null) {
	HttpEntity entity = response.getEntity(); // 获取返回实体
	if (entity != null) {
	    EntityUtils.toString(entity)
	}
}

这是CloseableHttpClient的源码可知：

这个类继承自HttpClient，我们继续分析HttpClient类，可以看到他是顶级的接口类。

HttpClients的一些工具类，我们来看：

继续看build是个什么东西：

public CloseableHttpClient build() {
        // Create main request executor
        // We copy the instance fields to avoid changing them, and rename to avoid accidental use of the wrong version
        PublicSuffixMatcher publicSuffixMatcherCopy = this.publicSuffixMatcher;
        if (publicSuffixMatcherCopy == null) {
            publicSuffixMatcherCopy = PublicSuffixMatcherLoader.getDefault();
        }

        HttpRequestExecutor requestExecCopy = this.requestExec;
        if (requestExecCopy == null) {
            requestExecCopy = new HttpRequestExecutor();
        }
        HttpClientConnectionManager connManagerCopy = this.connManager;
        if (connManagerCopy == null) {
            LayeredConnectionSocketFactory sslSocketFactoryCopy = this.sslSocketFactory;
            if (sslSocketFactoryCopy == null) {
                final String[] supportedProtocols = systemProperties ? split(
                        System.getProperty("https.protocols")) : null;
                final String[] supportedCipherSuites = systemProperties ? split(
                        System.getProperty("https.cipherSuites")) : null;
                HostnameVerifier hostnameVerifierCopy = this.hostnameVerifier;
                if (hostnameVerifierCopy == null) {
                    hostnameVerifierCopy = new DefaultHostnameVerifier(publicSuffixMatcherCopy);
                }
                if (sslContext != null) {
                    sslSocketFactoryCopy = new SSLConnectionSocketFactory(
                            sslContext, supportedProtocols, supportedCipherSuites, hostnameVerifierCopy);
                } else {
                    if (systemProperties) {
                        sslSocketFactoryCopy = new SSLConnectionSocketFactory(
                                (SSLSocketFactory) SSLSocketFactory.getDefault(),
                                supportedProtocols, supportedCipherSuites, hostnameVerifierCopy);
                    } else {
                        sslSocketFactoryCopy = new SSLConnectionSocketFactory(
                                SSLContexts.createDefault(),
                                hostnameVerifierCopy);
                    }
                }
            }
            @SuppressWarnings("resource")
            final PoolingHttpClientConnectionManager poolingmgr = new PoolingHttpClientConnectionManager(
                    RegistryBuilder.<ConnectionSocketFactory>create()
                        .register("http", PlainConnectionSocketFactory.getSocketFactory())
                        .register("https", sslSocketFactoryCopy)
                        .build(),
                    null,
                    null,
                    dnsResolver,
                    connTimeToLive,
                    connTimeToLiveTimeUnit != null ? connTimeToLiveTimeUnit : TimeUnit.MILLISECONDS);
            if (defaultSocketConfig != null) {
                poolingmgr.setDefaultSocketConfig(defaultSocketConfig);
            }
            if (defaultConnectionConfig != null) {
                poolingmgr.setDefaultConnectionConfig(defaultConnectionConfig);
            }
            if (systemProperties) {
                String s = System.getProperty("http.keepAlive", "true");
                if ("true".equalsIgnoreCase(s)) {
                    s = System.getProperty("http.maxConnections", "5");
                    final int max = Integer.parseInt(s);
                    poolingmgr.setDefaultMaxPerRoute(max);
                    poolingmgr.setMaxTotal(2 * max);
                }
            }
            if (maxConnTotal > 0) {
                poolingmgr.setMaxTotal(maxConnTotal);
            }
            if (maxConnPerRoute > 0) {
                poolingmgr.setDefaultMaxPerRoute(maxConnPerRoute);
            }
            connManagerCopy = poolingmgr;
        }
        ConnectionReuseStrategy reuseStrategyCopy = this.reuseStrategy;
        if (reuseStrategyCopy == null) {
            if (systemProperties) {
                final String s = System.getProperty("http.keepAlive", "true");
                if ("true".equalsIgnoreCase(s)) {
                    reuseStrategyCopy = DefaultClientConnectionReuseStrategy.INSTANCE;
                } else {
                    reuseStrategyCopy = NoConnectionReuseStrategy.INSTANCE;
                }
            } else {
                reuseStrategyCopy = DefaultClientConnectionReuseStrategy.INSTANCE;
            }
        }
        ConnectionKeepAliveStrategy keepAliveStrategyCopy = this.keepAliveStrategy;
        if (keepAliveStrategyCopy == null) {
            keepAliveStrategyCopy = DefaultConnectionKeepAliveStrategy.INSTANCE;
        }
        AuthenticationStrategy targetAuthStrategyCopy = this.targetAuthStrategy;
        if (targetAuthStrategyCopy == null) {
            targetAuthStrategyCopy = TargetAuthenticationStrategy.INSTANCE;
        }
        AuthenticationStrategy proxyAuthStrategyCopy = this.proxyAuthStrategy;
        if (proxyAuthStrategyCopy == null) {
            proxyAuthStrategyCopy = ProxyAuthenticationStrategy.INSTANCE;
        }
        UserTokenHandler userTokenHandlerCopy = this.userTokenHandler;
        if (userTokenHandlerCopy == null) {
            if (!connectionStateDisabled) {
                userTokenHandlerCopy = DefaultUserTokenHandler.INSTANCE;
            } else {
                userTokenHandlerCopy = NoopUserTokenHandler.INSTANCE;
            }
        }

        String userAgentCopy = this.userAgent;
        if (userAgentCopy == null) {
            if (systemProperties) {
                userAgentCopy = System.getProperty("http.agent");
            }
            if (userAgentCopy == null) {
                userAgentCopy = VersionInfo.getUserAgent("Apache-HttpClient",
                        "org.apache.http.client", getClass());
            }
        }

        ClientExecChain execChain = createMainExec(
                requestExecCopy,
                connManagerCopy,
                reuseStrategyCopy,
                keepAliveStrategyCopy,
                new ImmutableHttpProcessor(new RequestTargetHost(), new RequestUserAgent(userAgentCopy)),
                targetAuthStrategyCopy,
                proxyAuthStrategyCopy,
                userTokenHandlerCopy);

        execChain = decorateMainExec(execChain);

        HttpProcessor httpprocessorCopy = this.httpprocessor;
        if (httpprocessorCopy == null) {

            final HttpProcessorBuilder b = HttpProcessorBuilder.create();
            if (requestFirst != null) {
                for (final HttpRequestInterceptor i: requestFirst) {
                    b.addFirst(i);
                }
            }
            if (responseFirst != null) {
                for (final HttpResponseInterceptor i: responseFirst) {
                    b.addFirst(i);
                }
            }
            b.addAll(
                    new RequestDefaultHeaders(defaultHeaders),
                    new RequestContent(),
                    new RequestTargetHost(),
                    new RequestClientConnControl(),
                    new RequestUserAgent(userAgentCopy),
                    new RequestExpectContinue());
            if (!cookieManagementDisabled) {
                b.add(new RequestAddCookies());
            }
            if (!contentCompressionDisabled) {
                if (contentDecoderMap != null) {
                    final List<String> encodings = new ArrayList<String>(contentDecoderMap.keySet());
                    Collections.sort(encodings);
                    b.add(new RequestAcceptEncoding(encodings));
                } else {
                    b.add(new RequestAcceptEncoding());
                }
            }
            if (!authCachingDisabled) {
                b.add(new RequestAuthCache());
            }
            if (!cookieManagementDisabled) {
                b.add(new ResponseProcessCookies());
            }
            if (!contentCompressionDisabled) {
                if (contentDecoderMap != null) {
                    final RegistryBuilder<InputStreamFactory> b2 = RegistryBuilder.create();
                    for (final Map.Entry<String, InputStreamFactory> entry: contentDecoderMap.entrySet()) {
                        b2.register(entry.getKey(), entry.getValue());
                    }
                    b.add(new ResponseContentEncoding(b2.build()));
                } else {
                    b.add(new ResponseContentEncoding());
                }
            }
            if (requestLast != null) {
                for (final HttpRequestInterceptor i: requestLast) {
                    b.addLast(i);
                }
            }
            if (responseLast != null) {
                for (final HttpResponseInterceptor i: responseLast) {
                    b.addLast(i);
                }
            }
            httpprocessorCopy = b.build();
        }
        execChain = new ProtocolExec(execChain, httpprocessorCopy);

        execChain = decorateProtocolExec(execChain);

        // Add request retry executor, if not disabled
        if (!automaticRetriesDisabled) {
            HttpRequestRetryHandler retryHandlerCopy = this.retryHandler;
            if (retryHandlerCopy == null) {
                retryHandlerCopy = DefaultHttpRequestRetryHandler.INSTANCE;
            }
            execChain = new RetryExec(execChain, retryHandlerCopy);
        }

        HttpRoutePlanner routePlannerCopy = this.routePlanner;
        if (routePlannerCopy == null) {
            SchemePortResolver schemePortResolverCopy = this.schemePortResolver;
            if (schemePortResolverCopy == null) {
                schemePortResolverCopy = DefaultSchemePortResolver.INSTANCE;
            }
            if (proxy != null) {
                routePlannerCopy = new DefaultProxyRoutePlanner(proxy, schemePortResolverCopy);
            } else if (systemProperties) {
                routePlannerCopy = new SystemDefaultRoutePlanner(
                        schemePortResolverCopy, ProxySelector.getDefault());
            } else {
                routePlannerCopy = new DefaultRoutePlanner(schemePortResolverCopy);
            }
        }

        // Optionally, add service unavailable retry executor
        final ServiceUnavailableRetryStrategy serviceUnavailStrategyCopy = this.serviceUnavailStrategy;
        if (serviceUnavailStrategyCopy != null) {
            execChain = new ServiceUnavailableRetryExec(execChain, serviceUnavailStrategyCopy);
        }

        // Add redirect executor, if not disabled
        if (!redirectHandlingDisabled) {
            RedirectStrategy redirectStrategyCopy = this.redirectStrategy;
            if (redirectStrategyCopy == null) {
                redirectStrategyCopy = DefaultRedirectStrategy.INSTANCE;
            }
            execChain = new RedirectExec(execChain, routePlannerCopy, redirectStrategyCopy);
        }

        // Optionally, add connection back-off executor
        if (this.backoffManager != null && this.connectionBackoffStrategy != null) {
            execChain = new BackoffStrategyExec(execChain, this.connectionBackoffStrategy, this.backoffManager);
        }

        Lookup<AuthSchemeProvider> authSchemeRegistryCopy = this.authSchemeRegistry;
        if (authSchemeRegistryCopy == null) {
            authSchemeRegistryCopy = RegistryBuilder.<AuthSchemeProvider>create()
                .register(AuthSchemes.BASIC, new BasicSchemeFactory())
                .register(AuthSchemes.DIGEST, new DigestSchemeFactory())
                .register(AuthSchemes.NTLM, new NTLMSchemeFactory())
                .register(AuthSchemes.SPNEGO, new SPNegoSchemeFactory())
                .register(AuthSchemes.KERBEROS, new KerberosSchemeFactory())
                .build();
        }
        Lookup<CookieSpecProvider> cookieSpecRegistryCopy = this.cookieSpecRegistry;
        if (cookieSpecRegistryCopy == null) {
            cookieSpecRegistryCopy = CookieSpecRegistries.createDefault(publicSuffixMatcherCopy);
        }

        CookieStore defaultCookieStore = this.cookieStore;
        if (defaultCookieStore == null) {
            defaultCookieStore = new BasicCookieStore();
        }

        CredentialsProvider defaultCredentialsProvider = this.credentialsProvider;
        if (defaultCredentialsProvider == null) {
            if (systemProperties) {
                defaultCredentialsProvider = new SystemDefaultCredentialsProvider();
            } else {
                defaultCredentialsProvider = new BasicCredentialsProvider();
            }
        }

        List<Closeable> closeablesCopy = closeables != null ? new ArrayList<Closeable>(closeables) : null;
        if (!this.connManagerShared) {
            if (closeablesCopy == null) {
                closeablesCopy = new ArrayList<Closeable>(1);
            }
            final HttpClientConnectionManager cm = connManagerCopy;

            if (evictExpiredConnections || evictIdleConnections) {
                final IdleConnectionEvictor connectionEvictor = new IdleConnectionEvictor(cm,
                        maxIdleTime > 0 ? maxIdleTime : 10, maxIdleTimeUnit != null ? maxIdleTimeUnit : TimeUnit.SECONDS);
                closeablesCopy.add(new Closeable() {

                    @Override
                    public void close() throws IOException {
                        connectionEvictor.shutdown();
                    }

                });
                connectionEvictor.start();
            }
            closeablesCopy.add(new Closeable() {

                @Override
                public void close() throws IOException {
                    cm.shutdown();
                }

            });
        }

        return new InternalHttpClient(
                execChain,
                connManagerCopy,
                routePlannerCopy,
                cookieSpecRegistryCopy,
                authSchemeRegistryCopy,
                defaultCookieStore,
                defaultCredentialsProvider,
                defaultRequestConfig != null ? defaultRequestConfig : RequestConfig.DEFAULT,
                closeablesCopy);
    }

我们可以看到他这个的实现是非常复杂的，我们来看execute方法：

他是一个抽象类，具体的实现在子类：

@Override
    protected final CloseableHttpResponse doExecute(final HttpHost target, final HttpRequest request,
                                      final HttpContext context)
        throws IOException, ClientProtocolException {

        Args.notNull(request, "HTTP request");
        // a null target may be acceptable, this depends on the route planner
        // a null context is acceptable, default context created below

        HttpContext execContext = null;
        RequestDirector director = null;
        HttpRoutePlanner routePlanner = null;
        ConnectionBackoffStrategy connectionBackoffStrategy = null;
        BackoffManager backoffManager = null;

        // Initialize the request execution context making copies of
        // all shared objects that are potentially threading unsafe.
        synchronized (this) {

            final HttpContext defaultContext = createHttpContext();
            if (context == null) {
                execContext = defaultContext;
            } else {
                execContext = new DefaultedHttpContext(context, defaultContext);
            }
            final HttpParams params = determineParams(request);
            final RequestConfig config = HttpClientParamConfig.getRequestConfig(params);
            execContext.setAttribute(ClientContext.REQUEST_CONFIG, config);

            // Create a director for this request
            director = createClientRequestDirector(
                    getRequestExecutor(),
                    getConnectionManager(),
                    getConnectionReuseStrategy(),
                    getConnectionKeepAliveStrategy(),
                    getRoutePlanner(),
                    getProtocolProcessor(),
                    getHttpRequestRetryHandler(),
                    getRedirectStrategy(),
                    getTargetAuthenticationStrategy(),
                    getProxyAuthenticationStrategy(),
                    getUserTokenHandler(),
                    params);
            routePlanner = getRoutePlanner();
            connectionBackoffStrategy = getConnectionBackoffStrategy();
            backoffManager = getBackoffManager();
        }

        try {
            if (connectionBackoffStrategy != null && backoffManager != null) {
                final HttpHost targetForRoute = (target != null) ? target
                        : (HttpHost) determineParams(request).getParameter(
                                ClientPNames.DEFAULT_HOST);
                final HttpRoute route = routePlanner.determineRoute(targetForRoute, request, execContext);

                final CloseableHttpResponse out;
                try {
                    out = CloseableHttpResponseProxy.newProxy(
                            director.execute(target, request, execContext));
                } catch (final RuntimeException re) {
                    if (connectionBackoffStrategy.shouldBackoff(re)) {
                        backoffManager.backOff(route);
                    }
                    throw re;
                } catch (final Exception e) {
                    if (connectionBackoffStrategy.shouldBackoff(e)) {
                        backoffManager.backOff(route);
                    }
                    if (e instanceof HttpException) {
                        throw (HttpException)e;
                    }
                    if (e instanceof IOException) {
                        throw (IOException)e;
                    }
                    throw new UndeclaredThrowableException(e);
                }
                if (connectionBackoffStrategy.shouldBackoff(out)) {
                    backoffManager.backOff(route);
                } else {
                    backoffManager.probe(route);
                }
                return out;
            } else {
                return CloseableHttpResponseProxy.newProxy(
                        director.execute(target, request, execContext));
            }
        } catch(final HttpException httpException) {
            throw new ClientProtocolException(httpException);
        }
    }

这个是他里边的一个实现，可以看出来，也是非常复杂的。HttpClient Get的请求大概就这么多，想深入的同学可以去仔细研究源码。

有问题可以在下面评论，技术问题可以私聊我。