首先,明确两点:

1.http连接池不是万能的,过多的长连接会占用服务器资源,导致其他服务受阻
2.http连接池只适用于请求是经常访问同一主机(或同一个接口)的情况下
3.并发数不高的情况下资源利用率低下

那么,当你的业务符合上面3点,那么你可以考虑使用http连接池来提高服务器性能

使用http连接池的优点:

1.复用http连接,省去了tcp的3次握手和4次挥手的时间,极大降低请求响应的时间
2.自动管理tcp连接,不用人为地释放/创建连接

使用http连接池的大致流程 :

1.创建PoolingHttpClientConnectionManager实例
2.给manager设置参数
3.给manager设置重试策略
4.给manager设置连接管理策略
5.开启监控线程,及时关闭被服务器单向断开的连接
6.构建HttpClient实例
7.创建HttpPost/HttpGet实例,并设置参数
8.获取响应,做适当的处理
9.将用完的连接放回连接池

public class HttpConnectionPoolUtil {

    private static Logger logger = LoggerFactory.getLogger(HttpConnectionPoolUtil.class);

    private static final int CONNECT_TIMEOUT = Config.getHttpConnectTimeout();// 设置连接建立的超时时间为10s
    private static final int SOCKET_TIMEOUT = Config.getHttpSocketTimeout();
    private static final int MAX_CONN = Config.getHttpMaxPoolSize(); // 最大连接数
    private static final int Max_PRE_ROUTE = Config.getHttpMaxPoolSize();
    private static final int MAX_ROUTE = Config.getHttpMaxPoolSize();
    private static CloseableHttpClient httpClient; // 发送请求的客户端单例
    private static PoolingHttpClientConnectionManager manager; //连接池管理类
    private static ScheduledExecutorService monitorExecutor;

    private final static Object syncLock = new Object(); // 相当于线程锁,用于线程安全

    /**
     * 对http请求进行基本设置
     * @param httpRequestBase http请求
     */
    private static void setRequestConfig(HttpRequestBase httpRequestBase){
        RequestConfig requestConfig = RequestConfig.custom().setConnectionRequestTimeout(CONNECT_TIMEOUT)
                .setConnectTimeout(CONNECT_TIMEOUT)
                .setSocketTimeout(SOCKET_TIMEOUT).build();

        httpRequestBase.setConfig(requestConfig);
    }

    public static CloseableHttpClient getHttpClient(String url){
        String hostName = url.split("/")[2];
        System.out.println(hostName);
        int port = 80;
        if (hostName.contains(":")){
            String[] args = hostName.split(":");
            hostName = args[0];
            port = Integer.parseInt(args[1]);
        }

        if (httpClient == null){
            //多线程下多个线程同时调用getHttpClient容易导致重复创建httpClient对象的问题,所以加上了同步锁
            synchronized (syncLock){
                if (httpClient == null){
                    httpClient = createHttpClient(hostName, port);
                    //开启监控线程,对异常和空闲线程进行关闭
                    monitorExecutor = Executors.newScheduledThreadPool(1);
                    monitorExecutor.scheduleAtFixedRate(new TimerTask() {
                        @Override
                        public void run() {
                            //关闭异常连接
                            manager.closeExpiredConnections();
                            //关闭5s空闲的连接
                            manager.closeIdleConnections(Config.getHttpIdelTimeout(), TimeUnit.MILLISECONDS);
                            logger.info("close expired and idle for over 5s connection");
                        }
                    }, Config.getHttpMonitorInterval(), Config.getHttpMonitorInterval(), TimeUnit.MILLISECONDS);
                }
            }
        }
        return httpClient;
    }

    /**
     * 根据host和port构建httpclient实例
     * @param host 要访问的域名
     * @param port 要访问的端口
     * @return
     */
    public static CloseableHttpClient createHttpClient(String host, int port){
        ConnectionSocketFactory plainSocketFactory = PlainConnectionSocketFactory.getSocketFactory();
        LayeredConnectionSocketFactory sslSocketFactory = SSLConnectionSocketFactory.getSocketFactory();
        Registry<ConnectionSocketFactory> registry = RegistryBuilder.<ConnectionSocketFactory> create().register("http", plainSocketFactory)
                .register("https", sslSocketFactory).build();

        manager = new PoolingHttpClientConnectionManager(registry);
        //设置连接参数
        manager.setMaxTotal(MAX_CONN); // 最大连接数
        manager.setDefaultMaxPerRoute(Max_PRE_ROUTE); // 路由最大连接数

        HttpHost httpHost = new HttpHost(host, port);
        manager.setMaxPerRoute(new HttpRoute(httpHost), MAX_ROUTE);

        //请求失败时,进行请求重试
        HttpRequestRetryHandler handler = new HttpRequestRetryHandler() {
            @Override
            public boolean retryRequest(IOException e, int i, HttpContext httpContext) {
                if (i > 3){
                    //重试超过3次,放弃请求
                    logger.error("retry has more than 3 time, give up request");
                    return false;
                }
                if (e instanceof NoHttpResponseException){
                    //服务器没有响应,可能是服务器断开了连接,应该重试
                    logger.error("receive no response from server, retry");
                    return true;
                }
                if (e instanceof SSLHandshakeException){
                    // SSL握手异常
                    logger.error("SSL hand shake exception");
                    return false;
                }
                if (e instanceof InterruptedIOException){
                    //超时
                    logger.error("InterruptedIOException");
                    return false;
                }
                if (e instanceof UnknownHostException){
                    // 服务器不可达
                    logger.error("server host unknown");
                    return false;
                }
                if (e instanceof ConnectTimeoutException){
                    // 连接超时
                    logger.error("Connection Time out");
                    return false;
                }
                if (e instanceof SSLException){
                    logger.error("SSLException");
                    return false;
                }

                HttpClientContext context = HttpClientContext.adapt(httpContext);
                HttpRequest request = context.getRequest();
                if (!(request instanceof HttpEntityEnclosingRequest)){
                    //如果请求不是关闭连接的请求
                    return true;
                }
                return false;
            }
        };

        CloseableHttpClient client = HttpClients.custom().setConnectionManager(manager).setRetryHandler(handler).build();
        return client;
    }

    /**
     * 设置post请求的参数
     * @param httpPost
     * @param params
     */
    private static void setPostParams(HttpPost httpPost, Map<String, String> params){
        List<NameValuePair> nvps = new ArrayList<NameValuePair>();
        Set<String> keys = params.keySet();
        for (String key: keys){
            nvps.add(new BasicNameValuePair(key, params.get(key)));
        }
        try {
            httpPost.setEntity(new UrlEncodedFormEntity(nvps, "utf-8"));
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
    }

    public static JsonObject post(String url, Map<String, String> params){
        HttpPost httpPost = new HttpPost(url);
        setRequestConfig(httpPost);
        setPostParams(httpPost, params);
        CloseableHttpResponse response = null;
        InputStream in = null;
        JsonObject object = null;
        try {
            response = getHttpClient(url).execute(httpPost, HttpClientContext.create());
            HttpEntity entity = response.getEntity();
            if (entity != null) {
                in = entity.getContent();
                String result = IOUtils.toString(in, "utf-8");
                Gson gson = new Gson();
                object = gson.fromJson(result, JsonObject.class);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try{
                if (in != null) in.close();
                if (response != null) response.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return object;
    }

    /**
     * 关闭连接池
     */
    public static void closeConnectionPool(){
        try {
            httpClient.close();
            manager.close();
            monitorExecutor.shutdown();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

关键的地方有以下几点:
1.httpclient实例必须是单例,且该实例必须使用HttpClients.custom().setConnectionManager()来绑定一个PollingHttpClientConnectionManager,这样该client每次发送请求都会通过manager来获取连接,如果连接池中没有可用连接的话,则该会阻塞线程,直到有可用的连接

2.httpclients4.5.x版本直接调用ClosableHttpResponse.close()就能直接把连接放回连接池,而不是关闭连接,以前的版本貌似要调用其他方法才能把连接放回连接池

3.由于服务器一般不会允许无限期的长连接,所以需要开启监控线程,每隔一段时间就检测一下连接池中连接的情况,及时关闭异常连接和长时间空闲的连接,避免占用服务器资源.