持久连接

通常一次连接之间的握手还是很耗费时间的，Http1.1提供了持久连接，可以在一次连接期间，发送多次请求。

HttpClientConnectionManager

Http连接不是线程安全的，每次只能在一个线程里头使用，HttpClient通过HttpConnectionManager来管理。主要作为http connection的工厂，管理它的生命周期，确保每次只被一个线程使用。主要通过ManagedHttpClientConnection来作为代理类，管理连接的状态和I/O操作。如果底层的连接被关闭了，则它会归还到manager。

HttpClientContext context = HttpClientContext.create();HttpClientConnectionManager connMrg = new BasicHttpClientConnectionManager();HttpRoute route = new HttpRoute(new HttpHost("localhost", 80));// Request new connection. This can be a long processConnectionRequest connRequest = connMrg.requestConnection(route, null);// Wait for connection up to 10 secHttpClientConnection conn = connRequest.get(10, TimeUnit.SECONDS);try {    // If not open    if (!conn.isOpen()) {        // establish connection based on its route info        connMrg.connect(conn, route, 1000, context);        // and mark it as route complete        connMrg.routeComplete(conn, route, context);    }    // Do useful things with the connection.} finally {    connMrg.releaseConnection(conn, null, 1, TimeUnit.MINUTES);}

BasicHttpClientConnectionManager

BasicHttpClientConnectionManager是一个简单的连接管理器，每次只维持一个连接，它会试图在同一个route下的一系列请求之间重用这个连接。

PoolingHttpClientConnectionManager

PoolingHttpClientConnectionManager是一个相对复杂的管理器，可以在多线程中使用。

PoolingHttpClientConnectionManager cm = new PoolingHttpClientConnectionManager();// Increase max total connection to 200cm.setMaxTotal(200);// Increase default max connection per route to 20cm.setDefaultMaxPerRoute(20);// Increase max connections for localhost:80 to 50HttpHost localhost = new HttpHost("locahost", 80);cm.setMaxPerRoute(new HttpRoute(localhost), 50);CloseableHttpClient httpClient = HttpClients.custom()        .setConnectionManager(cm)        .build();

如果对于同一个route的所有连接都被租用了，那么新的请求会被阻塞住，直到该route的连接被归还。

注意设置http.conn-manager.timeout，避免一个连接被占用过长时间。

PoolingHttpClientConnectionManager cm = new PoolingHttpClientConnectionManager();CloseableHttpClient httpClient = HttpClients.custom()        .setConnectionManager(cm)        .build();// URIs to perform GETs onString[] urisToGet = {    "http://www.domain1.com/",    "http://www.domain2.com/",    "http://www.domain3.com/",    "http://www.domain4.com/"};// create a thread for each URIGetThread[] threads = new GetThread[urisToGet.length];for (int i = 0; i < threads.length; i++) {    HttpGet httpget = new HttpGet(urisToGet[i]);    threads[i] = new GetThread(httpClient, httpget);}// start the threadsfor (int j = 0; j < threads.length; j++) {    threads[j].start();}// join the threadsfor (int j = 0; j < threads.length; j++) {    threads[j].join();}

建议每个线程维护自己的context：

static class GetThread extends Thread {    private final CloseableHttpClient httpClient;    private final HttpContext context;    private final HttpGet httpget;    public GetThread(CloseableHttpClient httpClient, HttpGet httpget) {        this.httpClient = httpClient;        this.context = HttpClientContext.create();        this.httpget = httpget;    }    @Override    public void run() {        try {            CloseableHttpResponse response = httpClient.execute(                    httpget, context);            try {                HttpEntity entity = response.getEntity();            } finally {                response.close();            }        } catch (ClientProtocolException ex) {            // Handle protocol errors        } catch (IOException ex) {            // Handle I/O errors        }    }}