首先,明确三点:
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10sprivate 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() {@Overridepublic 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() {@Overridepublic 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.