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 process
ConnectionRequest connRequest = connMrg.requestConnection(route, null);
// Wait for connection up to 10 sec
HttpClientConnection 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 200
cm.setMaxTotal(200);
// Increase default max connection per route to 20
cm.setDefaultMaxPerRoute(20);
// Increase max connections for localhost:80 to 50
HttpHost 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 on
String[] urisToGet = {
    "http://www.domain1.com/",
    "http://www.domain2.com/",
    "http://www.domain3.com/",
    "http://www.domain4.com/"
};
// create a thread for each URI
GetThread[] 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 threads
for (int j = 0; j < threads.length; j++) {
    threads[j].start();
}
// join the threads
for (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
        }
    }
}