有一天，客户告诉我他们的HTTPS服务，采用了gzip方式进行压缩，因此我理所当然认为我这边需要对gzip的数据进行解压。

方案如下：

1，判断response header中是否有Content-Encoding 。如果为 gzip，则对其进行解压处理。

/**
* Get 请求
*/
public static void sendGet(String url) throws IOException {
        SSLClient httpClient = null;
        HttpGet httpGet = null;
        CloseableHttpResponse response = null;
        String result = null;
        try {
           //这里因为是HTTPS，所以这里HttpClient有特殊处理
            httpClient = new SSLClient();
            httpGet = new HttpGet(url);

            Map<String, String> params = new HashMap<>(4);
            params.put("Accept-Encoding", "gzip");
            //设置header，告知服务端，本客户端支持gzip压缩格式
            setHeader(httpGet, params);

            response = httpClient.execute(httpGet);

            HttpEntity resEntity = response.getEntity();

            Header contentEncoding = response.getFirstHeader("Content-Encoding");

            if (contentEncoding != null && contentEncoding.getValue().equalsIgnoreCase("gzip")) {
                //如果返回的response header中有Content-Encoding项，且值为gzip，则进行解压处理
                InputStream is = new GZIPInputStream(new BufferedInputStream(resEntity.getContent()));
                StringBuffer out = new StringBuffer();
                byte[] b = new byte[4096];
                for (int n; (n = is.read(b)) != -1; ) {
                    out.append(new String(b, 0, n));
                }
                String strResult = out.toString();
                System.out.println(strResult);
            } else {
                if (resEntity != null) {
                    result = EntityUtils.toString(resEntity, "utf-8");
                    System.out.println(result);
                }
                EntityUtils.consume(resEntity);
            }
        } catch (Exception ex) {
            ex.printStackTrace();
        } finally {
            try {
                response.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            httpClient.close();
        }
    }


/**
     * set HttpGet method header
     *
     * @param httpGet
     * @param params
     */
    private static void setHeader(HttpGet httpGet, Map<String, String> params) {
        if (null == httpGet) {
            throw new IllegalArgumentException("httpGet is null.");
        }
        if (!MapUtils.isEmpty(params)) {
            for (Map.Entry<String, String> entry : params.entrySet()) {
                httpGet.addHeader(entry.getKey(), entry.getValue());
            }
        }
    }

SSLClient:

package com.hello.https;

import java.security.cert.CertificateException;
import java.security.cert.X509Certificate;
import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;
import org.apache.http.conn.ClientConnectionManager;
import org.apache.http.conn.scheme.Scheme;
import org.apache.http.conn.scheme.SchemeRegistry;
import org.apache.http.conn.ssl.SSLSocketFactory;
import org.apache.http.impl.client.DefaultHttpClient;

public class SSLClient extends DefaultHttpClient {
    public SSLClient() throws Exception{
        super();
        SSLContext ctx = SSLContext.getInstance("TLS");

        X509TrustManager tm = new X509TrustManager() {
            @Override
            public void checkClientTrusted(X509Certificate[] chain,
                                           String authType) throws CertificateException {
            }
            @Override
            public void checkServerTrusted(X509Certificate[] chain,
                                           String authType) throws CertificateException {
            }
            @Override
            public X509Certificate[] getAcceptedIssuers() {
                return null;
            }
        };

        ctx.init(null, new TrustManager[]{tm}, null);
        SSLSocketFactory ssf = new SSLSocketFactory(ctx,SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER);
        ClientConnectionManager ccm = this.getConnectionManager();
        SchemeRegistry sr = ccm.getSchemeRegistry();
        sr.register(new Scheme("https", 443, ssf));
    }
}

2，可以利用HttpClient中的ResponseContentEncoding进行自动解压

/*
 * ====================================================================
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 *
 */
package org.apache.http.client.protocol;

import java.io.IOException;
import java.io.InputStream;
import java.util.Locale;
import java.util.zip.GZIPInputStream;

import org.apache.http.Header;
import org.apache.http.HeaderElement;
import org.apache.http.HttpEntity;
import org.apache.http.HttpException;
import org.apache.http.HttpResponse;
import org.apache.http.HttpResponseInterceptor;
import org.apache.http.annotation.Immutable;
import org.apache.http.client.config.RequestConfig;
import org.apache.http.client.entity.DecompressingEntity;
import org.apache.http.client.entity.DeflateInputStream;
import org.apache.http.client.entity.InputStreamFactory;
import org.apache.http.config.Lookup;
import org.apache.http.config.RegistryBuilder;
import org.apache.http.protocol.HttpContext;

/**
 * {@link HttpResponseInterceptor} responsible for processing Content-Encoding
 * responses.
 * <p>
 * Instances of this class are stateless and immutable, therefore threadsafe.
 *
 * @since 4.1
 *
 */
@Immutable
public class ResponseContentEncoding implements HttpResponseInterceptor {

    public static final String UNCOMPRESSED = "http.client.response.uncompressed";

    private final static InputStreamFactory GZIP = new InputStreamFactory() {

        @Override
        public InputStream create(final InputStream instream) throws IOException {
            return new GZIPInputStream(instream);
        }
    };

    private final static InputStreamFactory DEFLATE = new InputStreamFactory() {

        @Override
        public InputStream create(final InputStream instream) throws IOException {
            return new DeflateInputStream(instream);
        }

    };

    private final Lookup<InputStreamFactory> decoderRegistry;
    private final boolean ignoreUnknown;

    /**
     * @since 4.5
     */
    public ResponseContentEncoding(final Lookup<InputStreamFactory> decoderRegistry, final boolean ignoreUnknown) {
        this.decoderRegistry = decoderRegistry != null ? decoderRegistry :
            RegistryBuilder.<InputStreamFactory>create()
                    .register("gzip", GZIP)
                    .register("x-gzip", GZIP)
                    .register("deflate", DEFLATE)
                    .build();
        this.ignoreUnknown = ignoreUnknown;
    }

    /**
     * @since 4.5
     */
    public ResponseContentEncoding(final boolean ignoreUnknown) {
        this(null, ignoreUnknown);
    }

    /**
     * @since 4.4
     */
    public ResponseContentEncoding(final Lookup<InputStreamFactory> decoderRegistry) {
        this(decoderRegistry, true);
    }

    /**
     * Handles {@code gzip} and {@code deflate} compressed entities by using the following
     * decoders:
     * <ul>
     * <li>gzip - see {@link GZIPInputStream}</li>
     * <li>deflate - see {@link DeflateInputStream}</li>
     * </ul>
     */
    public ResponseContentEncoding() {
        this(null);
    }

    @Override
    public void process(
            final HttpResponse response,
            final HttpContext context) throws HttpException, IOException {
        final HttpEntity entity = response.getEntity();

        final HttpClientContext clientContext = HttpClientContext.adapt(context);
        final RequestConfig requestConfig = clientContext.getRequestConfig();
        // entity can be null in case of 304 Not Modified, 204 No Content or similar
        // check for zero length entity.
        if (requestConfig.isContentCompressionEnabled() && entity != null && entity.getContentLength() != 0) {
            final Header ceheader = entity.getContentEncoding();
            if (ceheader != null) {
                final HeaderElement[] codecs = ceheader.getElements();
                for (final HeaderElement codec : codecs) {
                    final String codecname = codec.getName().toLowerCase(Locale.ROOT);
                    final InputStreamFactory decoderFactory = decoderRegistry.lookup(codecname);
                    if (decoderFactory != null) {
                        response.setEntity(new DecompressingEntity(response.getEntity(), decoderFactory));
                        response.removeHeaders("Content-Length");
                        response.removeHeaders("Content-Encoding");
                        response.removeHeaders("Content-MD5");
                    } else {
                        if (!"identity".equals(codecname) && !ignoreUnknown) {
                            throw new HttpException("Unsupported Content-Encoding: " + codec.getName());
                        }
                    }
                }
            }
        }
    }
}

我们在获取HttpClient对象时，通过如下方式即可创建能自动处理服务器端进行压缩过的数据的HttpClient

CloseableHttpClient httpclient = HttpClients.custom()
                .setSSLSocketFactory(sslsf) //这里因为要请求的是https服务，所以这样处理，后面会写文介绍此处
                .build();

为什么要这样操作呢？那是因为HttpClients.custom()方法会返回一个HttpClientBuilder对象，这个HttpClientBuilder对象用于构建httpclient 对象。我们不妨看看HttpClientBuilder类的build方法：

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,
                    null,
                    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 = DefaultConnectionReuseStrategy.INSTANCE;
                } else {
                    reuseStrategyCopy = NoConnectionReuseStrategy.INSTANCE;
                }
            } else {
                reuseStrategyCopy = DefaultConnectionReuseStrategy.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 (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);
            }
        }
        // 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 service unavailable retry executor
        final ServiceUnavailableRetryStrategy serviceUnavailStrategyCopy = this.serviceUnavailStrategy;
        if (serviceUnavailStrategyCopy != null) {
            execChain = new ServiceUnavailableRetryExec(execChain, serviceUnavailStrategyCopy);
        }
        // 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);
    }

其中关键点是 image.png

我们在看看new ResponseContentEncoding()：

会发现这里： image.png
这里有对gzip压缩格式进行处理。
HttpClientBuilder中 image.png
HttpProcessorBuilder image.png
我们可以看到其有两种类型的拦截器链，一个是用于请求的request，一个是用于响应的response，而ResponseContentEncoding就是response中的一个拦截器，用于处理压缩数据的。
综上，我自己推测，HttpClient通过这种方法自己设置了用于解压服务器端压缩过的gzip数据。