HttpClient can't get response from server

Question

This problem has blocked our whole team half a day!

We use apache httpclient 4.3.x to post and get data from an storage server which provides http api. In order to improve performance, we used PoolingHttpClientConnectionManager :

public HttpClient createHttpClient() {
    Registry registry = RegistryBuilder.create()....build();
    PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager(registry);
    connectionManager.setMaxTotal(50);
    connectionManager.setDefaultMaxPerRoute(50);

    CloseableHttpClient httpClient = HttpClients.custom()
       .setConnectionManager(connectionManager)
       .build();
    return httpClient;
}

Then we hold an instance of the httpClient in our program, reuse it with every http request:

Global httpClient:

HttpClient httpClient = createHttpClient();

Post some data:

HttpPost httpPut = new HttpPost("...");
HttpResponse response = httpClient.execute(httpPut);

// Notice we get the response content here!
String content = EntityUtils.toString(response.getEntity());
System.out.println(content);

httpPut.releaseConnection();
response.close();

Then get:

HttpGet httpGet = new HttpGet("...");

// Blocked at this line !!!!
HttpResponse response = httpClient.execute(httpGet);

String content = EntityUtils.toString(response.getEntity());
System.out.println(content);

httpPut.releaseConnection();
response.close();    

Please notice the line: // Blocked at this line !!!!

The program has blocked at that line and never go to next line. In debugging mode, I can see it has been blocked at:

SocketInputStream.socketRead0()

I've searched for a lot of questions and documents, but no lucky.

---

My colleage just fix it by setting NoConnectionReuseStrategy.INSTANCE :

 HttpClients.custom()
       .setConnectionManager(connectionManager)
       // Following line fixed the problem, but why?
       .setConnectionReuseStrategy(NoConnectionReuseStrategy.INSTANCE)
       .build();

Now it doens't blocked, but why?

What does "reuse connection" mean? And is there performance issue by using NoConnectionReuseStrategy ?

Thank you, guys~

Answer 1

I tried to reproduce the blocking http-get (also as an exercise for myself) but even without closing responses I could not get it to block. The ONLY time I managed to make the http-get block is by doing a response.getEntity().getContent() without reading from the returned InputStream and without closing the returned InputStream . For my tests I used Tomcat 7.0.47 with two very simple servlets (one responding "OK" to a get, the other echoing a post) as a server. The client started 50 threads with each thread performing 30 alternating http-get and http-post request (total of 1500 requests). The client did not use the RegistryBuilder , instead the default one is used (created by the PoolingHttpClientConnectionManager itself).

About the NoConnectionReuseStrategy : by default (HttpClient created with HttpClients.createDefault() , I used org.apache.httpcomponents:httpclient:4.3.1 ) a connection pool is used with a maximum of 2 connections to 1 server. Eg even if 5 threads are doing all kinds of requests at the same time to 1 server, the connection pool opens only 2 connections, re-uses them for all requests and ensures that 1 connection is used by 1 thread at any given time. This can have a very positive impact on client performance and significantly reduces load on the server. The only thing you must make sure is to call response.close() in a finally-block (this ensures the connection is returned to the connection pool). By using the NoConnectionReuseStrategy you basically disable the connection pool: for each request a new connection will be created. I recommend you enable debug-logging for category org.apache.http.impl.conn.PoolingHttpClientConnectionManager , it is very informative.

A note about httpPut.releaseConnection() : this does not actually release a connection, it only ensures that you can re-use the "httpPut" object in a next request (see the apidocs , follow the shown link). Also note that in your code for the "httpGet", you call releaseConnection() on "httpPut" instead of "httpGet".

Answer 2

Ran into this problem just a while back. In case someone else comes across this problem, this post might be useful.

I am using a Java Servlet to service my requests. When I wrote to the response stream using the PrintWriter instance my client blocked. Tried writing to the OutputStream directly response.getOutputStream.write("myresponse") and it worked.