TimeoutException for CloseableHttpAsyncClient

i am trying to connect consume api using CloseableHttpAsyncClient. I making call to the api with connection pool of 45 and timeout of 5 minutes. However, i get the following error:

java.util.concurrent.TimeoutException: null
    at org.apache.http.nio.pool.AbstractNIOConnPool.processPendingRequest(AbstractNIOConnPool.java:364)
    at org.apache.http.nio.pool.AbstractNIOConnPool.processNextPendingRequest(AbstractNIOConnPool.java:344)
    at org.apache.http.nio.pool.AbstractNIOConnPool.release(AbstractNIOConnPool.java:318)
    at org.apache.http.impl.nio.conn.PoolingNHttpClientConnectionManager.releaseConnection(PoolingNHttpClientConnectionManager.java:303)
    at org.apache.http.impl.nio.client.AbstractClientExchangeHandler.releaseConnection(AbstractClientExchangeHandler.java:239)
    at org.apache.http.impl.nio.client.MainClientExec.responseCompleted(MainClientExec.java:387)
    at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.responseCompleted(DefaultClientExchangeHandlerImpl.java:168)
    at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.processResponse(HttpAsyncRequestExecutor.java:436)
    at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.inputReady(HttpAsyncRequestExecutor.java:326)
    at org.apache.http.impl.nio.DefaultNHttpClientConnection.consumeInput(DefaultNHttpClientConnection.java:265)
    at org.apache.http.impl.nio.client.InternalIODispatch.onInputReady(InternalIODispatch.java:81)
    at org.apache.http.impl.nio.client.InternalIODispatch.onInputReady(InternalIODispatch.java:39)
    at org.apache.http.impl.nio.reactor.AbstractIODispatch.inputReady(AbstractIODispatch.java:114)
    at org.apache.http.impl.nio.reactor.BaseIOReactor.readable(BaseIOReactor.java:162)
    at org.apache.http.impl.nio.reactor.AbstractIOReactor.processEvent(AbstractIOReactor.java:337)
    at org.apache.http.impl.nio.reactor.AbstractIOReactor.processEvents(AbstractIOReactor.java:315)
    at org.apache.http.impl.nio.reactor.AbstractIOReactor.execute(AbstractIOReactor.java:276)
    at org.apache.http.impl.nio.reactor.BaseIOReactor.execute(BaseIOReactor.java:104)
    at org.apache.http.impl.nio.reactor.AbstractMultiworkerIOReactor$Worker.run(AbstractMultiworkerIOReactor.java:588)
    at java.lang.Thread.run(Thread.java:745)

Then on reducing the connection pool size to 10, the number of times error is thrown is down.

i am using singleton instance of CloseableHttpAsyncClient, and do not close it for faster call.

This is how i am calling it:

httpclient.execute(post, new FutureCallback<HttpResponse>(....));

i think, it is NOT from the api side. any idea which this exception occurs and does it has any connection from connection pool?

By looking at the source code, this happens when you set connectionRequestTimeout to a value different than the default (-1) in the connection manager (e.g. PoolingNHttpClientConnectionManager). If the pool is very busy and timeout is not enough, it will throw this exception.

To solve it, either increase the connectionRequestTimeout of the connection manager pool or set it to -1 (for indefinite wait).