how to keep multiple Java HttpConnections open to same destination

We are using HttpURLConnection API to invoke a REST API to the same provider often (kind of an aggregation usecase). We want to keep a pool of 5 connections always open to the provider host (always the same IP).

What is the proper solution? Here is what we tried:

System.setProperty("http.maxConnections", 5);  // set globally only once
...
// everytime we need a connection, we use the following
HttpURLConnection conn = (HttpURLConnection) (new URL(url)).openConnection();
conn.setRequestMethod("GET");
conn.setDoInput(true);
conn.setDoOutput(false);
conn.setUseCaches(true);
...
BufferedReader reader = new BufferedReader(new 开发者_Python百科InputStreamReader(conn.getInputStream()));
...

At this point we read the input stream until the BufferedReader returns no more bytes. What do we do after that point if we want to reuse the underlying connection to the provider? We were under the impression that if the input stream is completely read, the connection is then added back to the pool.

It's been working for several weeks this way, but today it stopped working producing this exception: java.net.SocketException: Too many open files

We found numerous sockets in the CLOSE_WAIT state like this (by running lsof): java 1814 root 97u IPv6 844702 TCP colinux:58517->123.123.254.205:www (CLOSE_WAIT)

Won't either conn.getInputStream().close() or conn.disconnect() completely close the connection and remove it from the pool?

We had this problem also on Java 5 and our solution is to switch to Apache HttpClient with pooled connection manager.

The keepalive implementation of Sun's URL handler for HTTP is very buggy. There is no maintenance thread to close idle connections.

Another bigger problem with keepalive is that you need to delete responses. Otherwise, the connection will be orphaned also. Most people don't handle error stream correctly. Please see my answer to this question for an example on how to read error responses correctly,

HttpURLConnection.getResponseCode() returns -1 on second invocation

From here:

The current implementation doesn't buffer the response body. Which means that the application has to finish reading the response body or call close() to abandon the rest of the response body, in order for that connection to be reused. Furthermore, current implementation will not try block-reading when cleaning up the connection, meaning if the whole response body is not available, the connection will not be reused.

I read this as if your solution should work, but that you are also free to call close and the connection will still be reused.

The reference cited by disown was what really helped.

We know Apache HttpClient is better, but that would require another jar and we might use this code in an applet.

Calling HttpURLConnection.connect() was unnecessary. I'm not sure if it prevents connection reuse, but we took it out. It is safe to close the stream, but calling disconnect() on the connection will prevent reuse. Also, setting sun.net.http.errorstream.enableBuffering=true helps.

Here is what we ended up using:

System.setProperty("http.maxConnections", String.valueOf(CONST.CONNECTION_LIMIT));
System.setProperty("sun.net.http.errorstream.enableBuffering", "true");

...

int responseCode = -1;
HttpURLConnection conn = null;
BufferedReader reader = null;
try {
 conn = (HttpURLConnection) (new URL(url)).openConnection();
 conn.setRequestProperty("Accept-Encoding", "gzip");

 // this blocks until the connection responds
 InputStream in = new GZIPInputStream(conn.getInputStream());

 reader = new BufferedReader(new InputStreamReader(in));
 StringBuffer sb = new StringBuffer();
 char[] buff = new char[CONST.HTTP_BUFFER_SIZE];
 int cnt;

 while((cnt = reader.read(buff)) > 0) sb.append(buff, 0, cnt);

 reader.close();

 responseCode = conn.getResponseCode();
 if(responseCode != HttpURLConnection.HTTP_OK) throw new IOException("abnormal HTTP response code:"+responseCode);

 return sb.toString();

} catch(IOException e) {
    // consume error stream, otherwise, connection won't be reused
    if(conn != null) {
     try {
         InputStream in = ((HttpURLConnection)conn).getErrorStream();
         in.close();
         if(reader != null) reader.close();
     } catch(IOException ex) {
         log.fine(ex);
     }
    }

    // log exception    
    String rc = (responseCode == -1) ? "unknown" : ""+responseCode;
    log.severe("Error for HttpUtil.httpGet("+url+")\nServer returned an HTTP response code of '"+rc+"'");
    log.severe(e);
}