在多线程环境中使用HttpClient的最佳实践

Question

For a while, I have been using HttpClient in a multithreaded environment. For every thread, when it initiates a connection, it will create a completely new HttpClient instance.

Recently, I have discovered that, by using this approach, it can cause the user to have too many ports being opened, and most of the connections are in TIME_WAIT state.

http://www.opensubscriber.com/message/commons-httpclient-dev@jakarta.apache.org/86045.html

Hence, instead of each thread doing :

HttpClient c = new HttpClient();
try {
    c.executeMethod(method);
}
catch(...) {
}
finally {
    method.releaseConnection();
}

We plan to have :

[METHOD A]

// global_c is initialized once through
// HttpClient global_c = new HttpClient(new MultiThreadedHttpConnectionManager());

try {
    global_c.executeMethod(method);
}
catch(...) {
}
finally {
    method.releaseConnection();
}

In a normal situation, global_c will be accessed by 50++ threads concurrently. I was wondering, will this create any performance issues? Is MultiThreadedHttpConnectionManager using a lock-free mechanism to implement its thread safe policy?

If 10 threads are using global_c, will the other 40 threads be locked?

Or would it be better if, in every thread, I create an instance of an HttpClient, but release the connection manager explicitly?

[METHOD B]

MultiThreadedHttpConnectionManager connman = new MultiThreadedHttpConnectionManager();
HttpClient c = new HttpClient(connman);
try {
      c.executeMethod(method);
}
catch(...) {
}
finally {
    method.releaseConnection();
    connman.shutdown();
}

Will connman.shutdown() suffer performance issues?

May I know which method (A or B) is better, for application using an 50++ threads?

Answer 1

Definitely Method A because its pooled and thread safe.

If you are using httpclient 4.x, the connection manager is called ThreadSafeClientConnManager . See this link for further details (scroll down to "Pooling connection manager"). For example:

    HttpParams params = new BasicHttpParams();
    SchemeRegistry registry = new SchemeRegistry();
    registry.register(new Scheme("http", PlainSocketFactory.getSocketFactory(), 80));
    ClientConnectionManager cm = new ThreadSafeClientConnManager(params, registry);
    HttpClient client = new DefaultHttpClient(cm, params);

Answer 2

Method A is recommended by httpclient developer community.

Please refer http://www.mail-archive.com/httpclient-users@hc.apache.org/msg02455.html for more details.

Answer 3

My reading of the docs is that HttpConnection itself is not treated as thread safe, and hence MultiThreadedHttpConnectionManager provides a reusable pool of HttpConnections, you have a single MultiThreadedHttpConnectionManager shared by all threads and initialised exactly once. So you need a couple of small refinements to option A.

MultiThreadedHttpConnectionManager connman = new MultiThreadedHttpConnectionManag

Then each thread should be using the sequence for every request, getting a conection from the pool and putting it back on completion of its work - using a finally block may be good. You should also code for the possibility that the pool has no available connections and process the timeout exception.

HttpConnection connection = null
try {
    connection = connman.getConnectionWithTimeout(
                        HostConfiguration hostConfiguration, long timeout) 
    // work
} catch (/*etc*/) {/*etc*/} finally{
    if ( connection != null )
        connman.releaseConnection(connection);
}

As you are using a pool of connections you won't actually be closing the connections and so this should not hit the TIME_WAIT problem. This approach does assuume that each thread doesn't hang on to the connection for long. Note that conman itself is left open.

Answer 4

I think you will want to use ThreadSafeClientConnManager.

You can see how it works here: http://foo.jasonhudgins.com/2009/08/http-connection-reuse-in-android.html

Or in the AndroidHttpClient which uses it internally.

Answer 5

With HttpClient 4.5 you can do this:

CloseableHttpClient httpClient = HttpClients.custom().setConnectionManager(new PoolingHttpClientConnectionManager()).build();

Note that this one implements Closeable (for shutting down of the connection manager).