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显然，我们必须在Java还没有准备好支持HTTP2客户端编程的情况下（即使用Java 8及以下版本），提供HTTP2客户端应用的解决方案。目前流行的类库如下：

• OkHttp
• Eclipse Jetty
• Netty
• Apache HttpComponents (Apache HC)

其中，Eclipse Jetty和Netty都同时提供客户端和服务端。而曾经广泛使用的Apache HC已经逐渐掉队，至今没有正式支持HTTP2的版本发布。所以这里特别推荐OkHttp，而且OkHttp还适合在Android应用中作为HTTP客户端。

OkHttp的最新版本3.10.0，2018年2月25日发布，特性如下：

• 支持HTTP2，默认采用TLS 1.2
• GZIP压缩
• 缓存响应对象

在Maven项目中，pom.xml配置如下：

     
    
     com.squareup.okhttp3
      
    
     okhttp
      
    
     3.10.0
    
   

编写HTTP2客户端应用如下：

1. 创建OkHttpClient

1.1 默认配置

OkHttpClient client = new OkHttpClient();

1.2 定制配置

OkHttpClient client = new OkHttpClient	.Builder()	.authenticator(new Authenticator() {...}	.build();

2. 创建请求

2.1 GET请求

Request request = new Request.Builder()        .url("https://www.mydomain.com/")        .build();//GET by default

2.2 POST请求

RequestBody strBody = RequestBody.create(MediaType.parse("text/x-markdown; charset=utf-8"), postBodyStr);RequestBody jsonBody = RequestBody.create(MediaType.parse("application/json; charset=utf-8"), postBodyJsonStr);

Request request = new Request.Builder()        .url("https://www.mydomain.com")        .post(mybody)        .build();

3. 发送请求并处理响应

3.1 同步请求

Response response = client.newCall(request).execute(){//synch call	if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);		System.out.println(response.body().string());}

3.2 异步请求

client.newCall(request).enqueue(new Callback() {//asynch call      @Override public void onFailure(Call call, IOException e) {        e.printStackTrace();      }      @Override public void onResponse(Call call, Response response) throws IOException {        try (ResponseBody responseBody = response.body()) {          if (!response.isSuccessful()) throw new IOException("Unexpected code " + response);          Headers responseHeaders = response.headers();          for (int i = 0, size = responseHeaders.size(); i < size; i++) {            System.out.println(responseHeaders.name(i) + ": " + responseHeaders.value(i));          }          System.out.println(responseBody.string());        }      }});

4. OkHttp访问HTTPS服务

final TrustManager[] trustAllCerts = new TrustManager[]{    new X509TrustManager() {        @Override        public void checkClientTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {}        @Override        public void checkServerTrusted(java.security.cert.X509Certificate[] chain, String authType) throws CertificateException {}        @Override        public java.security.cert.X509Certificate[] getAcceptedIssuers() {            return new java.security.cert.X509Certificate[]{};        }    }};final HostnameVerifier verifiedAllHostname = new HostnameVerifier() {    @Override    public boolean verify(String hostname, SSLSession session) {        return true;    }};SSLContext sslContext = SSLContext.getInstance("TLS");sslContext.init(null, trustAllCerts, new java.security.SecureRandom());SSLSocketFactory sslSocketFactory = sslContext.getSocketFactory();OkHttpClient client = new OkHttpClient.Builder()    .sslSocketFactory(sslSocketFactory, (X509TrustManager) trustAllCerts[0]);    .hostnameVerifier(verifiedAllHostname)    .connectTimeout(60, TimeUnit.SECONDS)    .readTimeout(60, TimeUnit.SECONDS)    .writeTimeout(60, TimeUnit.SECONDS)    .retryOnConnectionFailure(true)    .build();Request request = new Request.Builder()    .url("https://localhost:9443/hello")    .build();Response response = client.newCall(request).execute();System.out.println(response.body().string());

这里我们只使用OkHttp原生API编写HTTP2客户端应用。在Spring Boot 2.0版本中，我们还可以使用Spring Boot starters for OkHttp，辅助编写HTTP2客户端。

参考链接：

http://square.github.io/okhttp/

https://www.eclipse.org/jetty

http://netty.io/

https://hc.apache.org/

https://github.com/freefair/okhttp-spring-boot

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