High tomcat thread count caused by threads waiting on MultiThreadedHttpConnectionManager ConnectionPool - httpclient

Question

We've recently started seeing spikes in the thread counts on our tomcat servers (peaking at over 1000 when normally at around 100). We performed a thread dump on one of the tomcat servers whilst it's thread count was high and found that a large number of the threads were waiting on MultiThreadedHttpConnectionManager$ConnectionPool, stack trace as follows:
"TP-Processor21700" daemon prio=10 tid=0x4a0b3400 nid=0x2091 in Object.wait() [0x399f3000..0x399f4004]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x58ee5030> (a org.apache.commons.httpclient.MultiThreadedHttpConnectionManager$ConnectionPool)
at org.apache.commons.httpclient.MultiThreadedHttpConnectionManager.doGetConnection(MultiThreadedHttpConnectionManager.java:518)
- locked <0x58ee5030> (a org.apache.commons.httpclient.MultiThreadedHttpConnectionManager$ConnectionPool)
at org.apache.commons.httpclient.MultiThreadedHttpConnectionManager.getConnectionWithTimeout(MultiThreadedHttpConnectionManager.java:416)
at org.apache.commons.httpclient.HttpMethodDirector.executeMethod(HttpMethodDirector.java:153)
at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:397)
at org.apache.commons.httpclient.HttpClient.executeMethod(HttpClient.java:323)
...
There are 3 points in our code where httpClient.executeMethod() is called (to obtain info via an http request to another tomcat server). In each case the GetMethod object passed to it has had its socket timeout value set (i.e. via getMethod.getParams().setSoTimeout();) before hand, and the MultiThreadedConnectionManager is configured in spring to have a connectionTimeout value of 10 seconds. One thing I have noticed is that only 2 of the 3 httpClient.executeMethod() invocations are followed by a call to getMethod.releaseConnection(), so I'm wondering if this may be the cause of the problem (i.e. connections not being explicitly released). However what's strange is that
the problem has only started occurring in the last few days and the source code has not been modified for over a year, plus the fact that there has been no recent surge in requests coming through to the tomcat servers. One change that did occur a couple of days before the problem started to occur was that we upgraded the JVM used by the tomcat server from Java 5 (1.5 update 14) to Java 6 (1.6 update 25). We have tried temporarily reverting the JVM version to Java 5 to see if the problem stopped occurring but it did not. Another point to note is that in most cases the tomcat server eventually recovers and
the thread count drops back to normal - we've only had one instance where a tomcat process appears to have crashed because of the thread count increase.
We are running Tomcat 5.5 with commons-httpclient-3.1.jar running against a Java 1.6 update 25 on a Red Hat linux environment.
Please let me know if you can suggest any ideas as to what may be the cause of this issue.
Thanks.

Answer 1

The problem was indeed caused by the fact that only 2 of the 3 httpClient.executeMethod(getMethod) invocations were followed by a call to getMethod.releaseConnection(). Ensuring all 3 httpClient.executeMethod(getMethod) invocations were inside a try/catch block followed by a finally block containing a call to getMethod.releaseConnection() prevented the high thread counts from occurring. Although this code had been in our live system for over a year, it appears that the reason the high thread count issue only recently started occurring was because various search engine crawlers had started hitting the site with lots of URL requests that caused the code where the connection was being used but not subsequently released to execute. Problem solved.