troubleshooting

A list of symptoms and their possible resolution.

Symptoms
Techniques
Common Problems
1. Specification version 1.3 of package javax.servlet, J2EE Specification, version 1.3 is not compatible with Resin Resin-3.0.x. Resin-3.0.x requires version 2.4.
2. class file has wrong version 48.0, should be 47.0

Symptoms

Resin stops responding

There may be a thread deadlock problem, obtain a thread dump.
Enable full debug logging and check the last log entries for hints about what the last thing to happen was.

Resin keeps restarting

Enable full debug logging and check the entries for an explanation of why Resin thinks it should restart.

No line numbers in exception stack traces

In resin.conf specify -g as an option for Resin to pass to the java compiler:
-g compiler option
```
<java compiler='javac' compiler-args='-g'/>
```
Specify -nojit as a command-line option when starting Resin:
-nojit command-line options
```
win> bin/httpd.exe -nojit
unix> bin/httpd.sh -nojit
```

java.lang.OutOfMemoryError exception, application runs out of memory

Use JVM startup arguments to increase heap memory.
Obtain a heap dump to determine which objects are not getting garbage collected.
Obtain a thread dump and check for unreleased threads that might be holding onto objects.

An OutOfMemoryError exception is usually an indication that heap memory is being used up. Often this is from application code keeping references to objects that are no longer needed, and the garbage collector does not free them. A heap dump can help determine which code and which kinds of objects are being held.

If the heap dump or other monitoring tools reveal that the server and your appliciation(s) actually are not running out of heap memory then an OutOfMemoryError indicates that then the JVM is running out of virtual memory, i.e. an underlying malloc() call is failing.

Often this situation is indicated by using OS tools to show memory usage, the JVM itself indicates it has heap memory but the OS tool shows that the process is using a very large amount of memory. On Windows the task manager is used, on Unix top or ps.

The only non-heap allocations by the JVM (and therefore Resin and your application) are the following:

threads and particularly the thread stack takes up virtual memory
any JNI libraries might be calling malloc or mmap (or the windows equivalent of mmap) and taking virtual memory. This includes many database drivers, and also some JNI code that Resin uses.
for .jar/.zip files (specifically ZipFile), the JDK allocates virtual memory. If you have a large number of jars open, you can run into problems. It's conceivable that a getResourceAsStream for a jar file that wasn't closed would use up .jar memory.

After a while the server starts to go very slow

This may be a garbage collection issue. If you have a memory leak, then an excessive number of objects may get created, causing the garbage collector to use up all of your CPU. If you're running out of memory, then the JVM will slowly halt (and continually GC) until it dies.
- Monitor garbage collection.
- Obtain a heap dump to determine if objects are not getting garbage collected.
- See Garbage Collection in the JVM Tuning section for more about garbage collecting.
This may be a runaway thread or a request that is tying up resources. If a thread does not return from a request, Resin will not be able to reuse the thread and will have less threads to service new requests.
- Obtain a thread dump and check for unreleased threads that might be holding onto objects.

CPU spikes, excessive CPU usage

Obtain a thread dump and check for threads that are caught in tight loops.
Check for garbage collection issues.

Sessions become null, losing session

Debug logging

A first step is to enable debug logging. In particular the headers submitted by the browser for a request can reveal the state of the JSESSIONID for a client, and logging will also show when Resin recognizes, creates, and/or invalidates a session.

Resin configuration for sessions

Another possiblity is that the <session-max> setting is too low, and you are getting more users establishing sessions than you have configured Resin for.

Yet another possibility is that the session is timing out, you can use the <session-timeout> tag to configure this.

<web-app id='/'>
  ...
  <session-config>
    <!-- timeout after 120 minutes -->
    <session-timeout>120</session-timeout>
    <!-- up to 4096 sessions at once -->
    <session-max>4096</session-max>
  </session-config>
  ...
</web-app>

Application reloading

Whenever a java source file, web.xml, or resin.conf changes then Resin will restart the application. If this happens, your current sessions will be lost unless you have configured a persistent session store.

Browser cookie limitations

Some users have reported that if their application uses a lot of cookies, the browser will start to discard older cookies to make room for the new. This may result in the browser discarding the cookie that Resin is using to keep track of the session.

Internet Explorer users are particularily prone to encountering this problem.

If your application uses a lot of cookies, the best solution is to reduce the number of cookies and size of cookie data. Resin uses a single cookie with a relatively small amount of data for tracking the session id of the user. Information that the application stores in cookies can be stored in the HttpSession object instead.

As a last resort, you may need to configure Resin to always use URL rewriting by setting <enable-cookies> to false. However URL rewriting is not recommended because of security issues, the high probabilty that the application is missing calls to cause the rewriting on some pages, and because of the effect that rewriting has on the url.

<web-app id='/'>
  ...
  <session-config>
    <enable-cookies>false</enable-cookies>
    <enable-url-rewriting>true</enable-url-rewriting>
  </session-config>
  ...
</web-app>

Problems with cookie domains

You may also lose your sessions if your cookie domains are incompatible. For example, if you have one server that uses cookie domain "hogwarts.com" and another that uses "qa.hogwarts.com", the cookie in the browser for "hogwarts.com" will interfere with sessions on "qa.hogwarts.com". The solution is to change the cookie domain "hogwarts.com" to "www.hogwarts.com".

You set the cookie domain in <session-config>. (Thanks Laura for providing this)

Conflicting cookie names

If you are using Resin and another application server (such as Tomcat), you may encounter a conflict because both of them are using the same cookie name (usually JSESSIONID) for the session tracking cookie.

Resin provides <session-cookie> and <ssl-session-cookie> to let you change the name of the cookie that Resin uses.

Change the name of the cookie used for session tracking

  <cluster>

    ...

    <session-cookie>RJESSESSIONID</session-cookie>

URL rewriting

If you forget to rewrite a URL, a user who requires rewriting will lose their session as soon as they follow that URL.

Resin establishes an association between a session and a user's browser by establishing a unique id that is returned back with each new request. This is accomplished in one of two ways: using cookies or URL rewriting.

Resin first attempts to track the session of a user by sending the user's browser a cookie containing the unique session id.

Sometimes Resin cannot establish a cookie, either because the user has disabled cookies in their browser or because the browser does not support them (as is the case with some HDML and WML browsers). If the cookie cannot be established then something called URL rewriting is used.

In this case, Resin rewrites every URL that it submits to the user so that it contains a parameter named _jsessionid. Then for each incoming request the first thing it does is look for this parameter, and if it is available it knows a session has been established and it removes the parameter and uses it to find the users session object.

Rewriting requires the cooperation of the developer. The developer must encode every URL reference so that Resin has an opportunity to put in the _jsessionid parameter.

URL rewriting using JSTL

<%@ taglib prefix='c' uri='http://java.sun.com/jstl/core' %>

Time to go <a href="<c:url value='home.jsp'/>">Home</a>!

URL rewriting using Java scriptlet

<%
String homeUrl = response.encodeURL("home.jsp");
%>

<%-- the presentation --%>

Time to go <a href="<%= homeUrl %>">Home</a>!

Specification version 1.3 of package javax.servlet, J2EE Specification, version 1.3 is not compatible with Resin

See Clean up the classpath

Unsupported major.minor version 48.0

This error usually happens when a conflicting jar is found, see Clean up the classpath

If the classpath is completely cleaned up, as suggested in the link, then a jar or some other component of a jdk installation or older Resin installation is coming from somewhere else.

There may be a problem with some jar in your JAVA_HOME tree, if you have added anything there. There may be a conflicting jar in WEB-INF/lib/ of your webapp.

Another possibility is that you have not set JAVA_HOME, or if you have then some component of a conflicting jdk installation (javac for example, or the java executable itself) is getting used.

If on windows, check for stray copies of java.exe outside of JAVA_HOME, for example in C:/WINDOWS/java.exe or anywhere else in your PATH.

Problems reading POST data

A first step is to enable debug logging. Debug logging will show the request coming into to Resin, and provide some information on how Resin is dealing with the post.

It is important to make sure the encoding is set properly before reading POST parameters.

The browser will always send back parameters in the same encoding as the output page. Since the request doesn't include the encoding (an oversight in the HTTP spec), the application code needs to make sure the encoding matches.

So the first thing to do is to determine that encoding your form page has when it is sent to the browser. Your application should always specify that. Having established that, you will know what encoding the browser is going to use for the POST. (utf-8 is a natural choice for encoding here, I'm not sure why you would use anything else).

Then before reading the POST parameters make sure you set the encoding to match. You can call request.setCharacterEncoding(encoding) to do that.

Techniques

Enable debug logging

Resin provides a number of diagnostic messages using the JDK logging facility. Full debug logging is enabled with an empty name (meaning match all names) and a level of 'all'. Since this will produce many messages, it is useful to put the messages in a a seperate file.

Full debug logging of the server and all applications

The following configuration creates a debug log for each day, and when a problem occurs the debug log is used to help find out what the problem is. Since the <log> entry is in resin.conf, the log will capture messages from the server itself and all applications. The resulting log messages are found in the file $RESIN_HOME/log/debug.log.

Full debug logging of the server to $RESIN_HOME/log/debug.log

<!-- resin.conf -->
<resin>
  <log name="" path="log/debug.log" timestamp="[%H:%M:%S.%s] {%{thread}} "/>

  <logger name="" level="finer"/>

</resin>

More specific areas can be targetted with appropriate values for name. The documentation for <log> provides a table of useful values for name.

Full debug logging of one web application

Often you can narrow down the logging requirements to a single web application. The log entry can be placed in <web-app-root>/WEB-INF/web.xml and only log messages from that web application will go to the log file.

The following configuration creates a debug log for each day in <web-app-root>/WEB-INF/work/debug.log.

Full debug logging of one application to <web-app-root>/WEB-INF/work/debug.log

<!-- <web-app-root>/WEB-INF/web.xml -->

<web-app>
  ...
  <log name="" path="WEB-INF/work/debug.log" timestamp="[%H:%M:%S.%s] {%{thread}} "/>

  <logger name="" level="finer"/>
  ...
</web-app>

Thread dump

If an application seems stuck, or is running out of resources, a thread dump will reveal the state of the server.

Java's thread dumps are a vital tool for server debugging. Because servlets are intrinsically multithreaded, it is very possible to create deadlocks without realizing it, or to have runaway threads that consume resources and cause OutOfMemory exceptions. That's especially true when you start adding third-party software like databases, EJB, and Corba ORBs.

Thread dump using JDK 5 tools

jps and jstack are useful tools included in JDK 5, providing a quick command line method for obtaining stack traces of all current threads.

jps and jstack

# jps
12903 Jps
20087 Resin
# jstack 20087
Attaching to process ID 20087, please wait...
Debugger attached successfully.
Client compiler detected.
JVM version is 1.5.0-beta2-b51
Thread 12691: (state = BLOCKED)
 - java.lang.Object.wait(long) (Compiled frame; information may be imprecise)
 - com.caucho.util.ThreadPool.runTasks() @bci=111, line=474 (Compiled frame)
 - com.caucho.util.ThreadPool.run() @bci=85, line=423 (Interpreted frame)
 - java.lang.Thread.run() @bci=11, line=595 (Interpreted frame)


Thread 12689: (state = BLOCKED)
 - java.lang.Object.wait(long) (Compiled frame; information may be imprecise)
 - com.caucho.util.ThreadPool.runTasks() @bci=111, line=474 (Compiled frame)
 - com.caucho.util.ThreadPool.run() @bci=85, line=423 (Interpreted frame)
 - java.lang.Thread.run() @bci=11, line=595 (Interpreted frame)

...

Thread dump by sending a signal

On Windows, ctrl-break may produce a thread dump.

On Unix, "kill -QUIT" will produce a thread dump. On the Linux IBM JDKs, you'll need to signal one of the child processes. On the Linux Sun JDK, you'll need to kill the grandparent process of most of the threads:

unix> ps -ef | grep java
ferg     14243 10792  0 Jan13 pts/0    00:00:00 sh -c /usr/java/bin/java
ferg     14244 14243  0 Jan13 pts/0    00:00:00 java -Dres
ferg     14253 14244  0 Jan13 pts/0    00:00:01 java -Dres
ferg     14254 14253  0 Jan13 pts/0    00:00:00 java -Dres
ferg     14255 14253  0 Jan13 pts/0    00:00:00 java -Dres
ferg     14256 14253  0 Jan13 pts/0    00:00:00 java -Dres
ferg     14257 14253  0 Jan13 pts/0    00:00:00 java -Dres
ferg     14258 14253  0 Jan13 pts/0    00:00:00 java -Dres
unix> kill -QUIT 14244

The Solaris 1.3 JDK has a single process, so it's easy.

The Solaris 1.2 JDK has an extra quirk. You need to run Resin in the foreground (i.e. not using "start".) Then you'll need to answer some questions on the console.

Thread dump if signalling doesn't work

You get a thread dump without signalling the process by starting the JVM with some extra arguments to allow a debugger to attach. You can then attach with the debugger at any time to get a thread dump. This technique works on all operating systems.

Here are some step by step instructions:

Start Resin with some extra arguments that allow a debugger to attach:

$RESIN_HOME/bin/httpd -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=5432

Wait until you believe the application is in a state of deadlock or has runaway threads.

In another terminal (window), use jdb to connect to the running instance of Resin:

$JAVA_HOME/bin/jdb -connect com.sun.jdi.SocketAttach:hostname=localhost,port=5432

jdb will show something like:

Set uncaught java.lang.Throwable
Set deferred uncaught java.lang.Throwable
Initializing jdb ...
>

Use the "suspend" command and then the "where all" command to get a thread dump:

> suspend

All threads suspended.
> where all

tcpConnection-6862-3:

 [1] java.lang.Object.wait (native method)
 [2] com.caucho.server.TcpServer.accept (TcpServer.java:650)
 [3] com.caucho.server.TcpConnection.accept
(TcpConnection.java:208)
 [4] com.caucho.server.TcpConnection.run (TcpConnection.java:131)
 [5] java.lang.Thread.run (Thread.java:536)
tcpConnection-543-2:
 [1] java.lang.Object.wait (native method)
 [2] com.caucho.server.TcpServer.accept (TcpServer.java:650)
 [3] com.caucho.server.TcpConnection.accept
(TcpConnection.java:208)
 [4] com.caucho.server.TcpConnection.run (TcpConnection.java:131)
 [5] java.lang.Thread.run (Thread.java:536)

..

Use the "resume" command to resume the process
```
> resume
```

Unix users (and Cygwin users on Windows) will recognize the opportunity to make a script:

resin-thread-dump.sh

#!/bin/sh
echo -e "suspend\nwhere all\nresume\nquit" | $JAVA_HOME/bin/jdb -connect \
  com.sun.jdi.SocketAttach:hostname=localhost,port=5432

Although no rigorous benchmarking has been done, there appears to be little overhead or performance penalties involved in having the JVM start with the options that allow a debugger to attach.

Understanding the thread dump

In any case, you'll eventually get a trace that looks something like the following (each JDK is slightly different):

Full thread dump:

"tcpConnection-8080-2" daemon waiting on monitor [0xbddff000..0xbddff8c4]
        at java.lang.Object.wait(Native Method)
        at com.caucho.server.TcpServer.accept(TcpServer.java:525)
        at com.caucho.server.TcpConnection.accept(TcpConnection.java:190)
        at com.caucho.server.TcpConnection.run(TcpConnection.java:136)
        at java.lang.Thread.run(Thread.java:484)

"tcpConnection-8080-1" daemon waiting on monitor [0xbdfff000..0xbdfff8c4]
        at java.lang.Object.wait(Native Method)
        at com.caucho.server.TcpServer.accept(TcpServer.java:525)
        at com.caucho.server.TcpConnection.accept(TcpConnection.java:190)
        at com.caucho.server.TcpConnection.run(TcpConnection.java:136)
        at java.lang.Thread.run(Thread.java:484)

"tcpConnection-8080-0" daemon waiting on monitor [0xbe1ff000..0xbe1ff8c4]
        at java.lang.Object.wait(Native Method)
        at com.caucho.server.TcpServer.accept(TcpServer.java:525)
        at com.caucho.server.TcpConnection.accept(TcpConnection.java:190)
        at com.caucho.server.TcpConnection.run(TcpConnection.java:136)
        at java.lang.Thread.run(Thread.java:484)

"tcp-accept-8080" runnable [0xbe7ff000..0xbe7ff8c4]
        at java.net.PlainSocketImpl.socketAccept(Native Method)
        at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:413)
        at java.net.ServerSocket.implAccept(ServerSocket.java:243)
        at java.net.ServerSocket.accept(ServerSocket.java:222)
        at com.caucho.server.TcpServer.run(TcpServer.java:415)
        at java.lang.Thread.run(Thread.java:484)

"resin-cron" daemon waiting on monitor [0xbe9ff000..0xbe9ff8c4]
        at java.lang.Thread.sleep(Native Method)
        at com.caucho.util.Cron$CronThread.run(Cron.java:195)

"resin-alarm" daemon waiting on monitor [0xbebff000..0xbebff8c4]
        at java.lang.Thread.sleep(Native Method)
        at com.caucho.util.Alarm$AlarmThread.run(Alarm.java:268)

"Signal Dispatcher" runnable [0..0]

"Finalizer" daemon waiting on monitor [0xbf3ff000..0xbf3ff8c4]
        at java.lang.Object.wait(Native Method)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:108)
        at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:123)
        at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:162)

"Reference Handler" daemon waiting on monitor [0xbf5ff000..0xbf5ff8c4]
        at java.lang.Object.wait(Native Method)
        at java.lang.Object.wait(Object.java:420)
        at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:110)

"main" waiting on monitor [0xbfffd000..0xbfffd210]
        at java.lang.Thread.sleep(Native Method)
        at com.caucho.server.http.ResinServer.waitForExit(ResinServer.java:674)
        at com.caucho.server.http.ResinServer.main(ResinServer.java:821)
        at com.caucho.server.http.HttpServer.main(HttpServer.java:95)

Each thread is named. Here are some of the common names:

THREAD NAME	DESCRIPTION
tcp-accept-8080	Resin thread listening for new connections on port 8080.
tcpConnection-8080-3	Resin servlet thread handling a connection from port 8080.
tcp-cron	Resin's run-at thread
tcp-alarm	Resin's alarm thread

There should be one tcp-accept-xxx thread for each <http> and <srun> that Resin's listening for. The tcp-accept-xxx thread should almost always be in socketAccept.

There should be several tcpConnection-xxx-n threads. Each of these is the servlet thread. On a busy server, these can appear anywhere in your code. If several appear in one location, you've likely got some sort of deadlock or at least a slow lock. Idle threads are either in tcpAccept or httpRequest or runnerRequest (for keepalive threads.)

For deadlocks, you'll want to look at the "waiting on monitor" threads and any case where lots of threads are stuck at the same location.

Out of Memory and Garbage Collection

Most memory problems are due to memory leaks in the application program. For example, a cache or a vector that fills up with stale data, or a singleton or static variable which doesn't properly detect a web-app reload. Some more exotic memory issues relate to running out of heap memory or virtual memory when using a large number of threads (> 256).

The steps to track down a memory problem are:

Enable -J-verbosegc with the httpd.sh start or httpd -install. The -verbosegc flag logs the garbage collection of the heap, which will let you know if you're running out of heap memory (the most common case).
Get a heap profiler or use the heap dump in the JVM. JProfiler is an inexpensive commercial heap profiler. Although the JVM's heap dump is not very user friendly, it's always available. You should be using a heap profiler as part of your development process and certainly use one before any production launch.
With the heap profiler, find the 2-3 top memory users and fix those memory leaks.
Common application errors include:
- ThreadLocal variables that are not properly cleared at the end of each request.
- Singleton or static hash maps and caches, esp check for clearing web-app restarts.
- Spawned threads which are not stopped on web-app restart.
- web-app variables (like the "application" variable), stored in a static variable.
If the heap is clean, i.e. -verbosegc shows a steady heap, you'll need to look at non-heap memory:
- Thread stack usage (-Xss2m). Each thread takes up some non-heap memory. The default on some systems is 8 meg. Because of 32-bit virtual memory limits of about 2G on some systems, even 256 threads with 8 meg stacks can chew up the virtual memory. You can drop the stack size with the -Xss directive.
- JNI memory. If you're using JNI libraries or drivers that use JNI, it's possible that the JNI can allocate more memory than is available.
- fork/exec and OS limits. If the OS does not have enough available swap space, for example, the OS might refuse a "jikes" compilation.
- NIO, memory mapping, and .jar files. The JDK memory-maps .jar files. In some cases with very large numbers of jars, this can result in running out of virtual memory. The same issue can appear for NIO memory mapping.
If all of these are eliminated, it might be a Resin bug. However, you will need to have located the memory leak as Resin-related before any memory-related bug report, i.e. it's necessary to go through the above steps before reporting a bug. Bug reports that complain about OutOfMemory errors, but have not even gotten a JDK memory dump are most likely application errors. You must provide a heap dump when reporting any potential Resin memory problems.

-verbosegc

-verbosegc is a debugging output from the JVM. It's very convenient for checking basic memory usage and garbage collection time. It's a good idea to use -verbosegc for any production system. When starting Resin, you'll use -J-verbosegc.

The specific output depends on the JVM, and looks something like the following:

-verbosegc output

[GC 9176K->8647K(9768K), 0.0014790 secs]
[GC 9287K->8668K(9768K), 0.0011120 secs]
[GC 9308K->8668K(9768K), 0.0007810 secs]

The "(9768K)" is the most important number, showing that the maximum allocated heap is about 10 meg. The other numbers show the actual heap used before and after garbage collection.

Check memory usage with a heap dump

If an application gobbles up memory until it throws an OutOfMemory exception, or seems to be spending an excessive amount of time doing garbage collection, a heap dump can help track down the source of the problem.

What you really need to do is get a good CPU and heap profile. The JDK has a simple one (but not very user-friendly), so you don't absolutely need to buy a profiler.

An alternative for simple heap monitoring (garbage collection issues), are the jvmstat tools.

It's a standard java argument, so "java -Xrunhprof:help" will tell you how to start it. For example, you could start Resin (unix or windows) as

> httpd.sh -J-Xrunhprof:heap=sites,cpu=samples

(On Unix, Resin's startup script has a -cpuprof-ascii argument that sets it for you automatically.)

Run a load against it for a decent amount of time (you can even use something like Apache's "ab" for 10 minutes), and then stop the server nicely. i.e. you can't just kill it with ctrl-C, but you need to make it do a clean exit.

That will dump a java.hprof.txt. Skip to the bottom to get the trace.

Understanding java.hprof.txt heap

Assuming you're being cheap, and using the JDK's heap profiling instead of buying a profiler, you'll need a bit of help interpreting it.

The following is an example of a heap dump running Resin. Here's what you might see skipping down to the "SITES BEGIN" section. (I did mention this is the cheap way of doing things.) As with most profiling, you only want to pay attention to the top 20. Everything else is in the noise. Ignore it.

SITES BEGIN (ordered by live bytes) Tue Jan  9 17:44:33 2001
          percent         live       alloc'ed  stack class
 rank   self  accum    bytes objs   bytes objs trace name
    1 11.87% 11.87%   983520  120 1393320  170  2553 [B
    2  9.89% 21.76%   819600  100 1286772  157  4070 [B
    3  9.09% 30.85%   753756   23 3539376  108  4970 [L<Unknown>;
    4  5.83% 36.68%   483564   59  778620   95  7180 [B
    5  5.74% 42.42%   475368   58  745836   91  7178 [B
    6  4.35% 46.77%   360624   44  696660   85  7182 [B
    7  2.97% 49.74%   245880   30  450780   55  7176 [B
    8  2.37% 52.11%   196704   24  352428   43  7254 [B
    9  1.88% 53.99%   155724   19  262272   32  7174 [B
   10  1.78% 55.77%   147528   18  245880   30  7137 [B
   11  1.53% 57.30%   126988 1063 16973092 129113  3271 [C
   12  1.34% 58.64%   110684 3953 20362832 727244  1213 sun/io/CharToByteISO8859_1
   13  1.25% 59.88%   103320  738  141820 1013  5942 java/lang/Class
   14  1.21% 61.10%   100548   49  221616  108  5003 [L<Unknown>;
   15  1.21% 62.31%   100548   49  221616  108  5005 [L<Unknown>;
   16  1.07% 63.38%    89080 1532 18393580 317347  1340 [B
   17  0.79% 64.18%    65568    8   81960   10  8408 [B
   18  0.79% 64.97%    65552    4   65552    4 27630 [C
   19  0.70% 65.67%    58232   24 1110128  386  5038 [C
   20  0.68% 66.35%    56200  450  116816  980  7186 [C

There are two things to look for. First, if any one class starts chewing up a large number in the "live objs" column, you'll need to take a look. That might be a memory leak.

Second, if some class has a huge number in the "alloc'ed objs" column, you may be wasting lots of garbage collection time that could be easily solved with some caching.

The [C in the class name means a character array. To see what that really means, you'll need to look at the stack trace (3271):

TRACE 3271:
	java/lang/String.<init>(String.java:244)
	com/caucho/util/CharBuffer.close(CharBuffer.java:714)
	com/caucho/vfs/FilesystemPath.normalizePath(FilesystemPath.java:162)
	com/caucho/vfs/FilesystemPath.schemeWalk(FilesystemPath.java:127)

That's part of Resin's VFS code. Maybe in the future it'll make sense to try to reduce that.

You can get a longer trace using the "-prof-depth 10" argument to Resin. (Or the corresponding depth in -Xrunhprof.) That will often give more information.

Understanding java.hprof.txt CPU

The CPU profiling is a little easier to read. On some JDK's you'll need to run it with the JIT disabled.

CPU SAMPLES BEGIN (total = 424614) Tue Jan  9 17:44:33 2001
rank   self  accum   count trace method
   1 21.36% 21.36%   90704  7266 com/caucho/server/http/VirtualHost.logAccess
   2 10.84% 32.20%   46041  7269 java/net/SocketInputStream.socketRead
   3  5.99% 38.19%   25428  1213 java/lang/Class.newInstance0
   4  5.11% 43.31%   21715  7896 com/caucho/util/CharBuffer.toString
   5  4.82% 48.13%   20463  1286 sun/io/CharToByteISO8859_1.convert
   6  3.54% 51.66%   15018  1242 sun/io/CharToByteConverter.<init>
   7  2.68% 54.35%   11388  7241 java/io/PrintWriter.<init>
   8  2.47% 56.82%   10508  7748 com/caucho/server/http/Request.fillCookies
   9  2.27% 59.09%    9650  1214 sun/io/ByteToCharConverter.<init>
  10  1.85% 60.94%    7857  5097 java/lang/String.<init>
  11  1.59% 62.53%    6754  1341 java/lang/String.substring
  12  1.57% 64.10%    6650  1340 java/lang/String.getBytes
  13  0.92% 65.02%    3907  7897 java/lang/String.<init>
  14  0.76% 65.78%    3227  3259 com/caucho/vfs/FilePath.fsWalk
  15  0.75% 66.53%    3195  7895 com/caucho/server/http/Request.fillCookie
  16  0.71% 67.25%    3031  7321 java/lang/String.getBytes
  17  0.71% 67.95%    2996  3270 com/caucho/util/CharBuffer.close
  18  0.68% 68.63%    2892  3271 java/lang/String.<init>
  19  0.66% 69.29%    2782  7318 com/caucho/vfs/FilePath.openWriteImpl
  20  0.61% 69.90%    2604  7320 java/io/FileOutputStream.<init>

You should only pay attention to the top 20 or so. You'll probably need to ignore a few of the top 10, because they're just waiting for a user response. The SocketInputStream.socketRead is an example.

You can use the trace number to get a call trace:

TRACE 7266:
	com/caucho/server/http/VirtualHost.logAccess(VirtualHost.java:487)
	com/caucho/server/http/Application.logAccess(Application.java:1846)
	com/caucho/server/http/Response.finish(Response.java:1345)
	com/caucho/server/http/Request.finish(Request.java:416)

Monitor garbage collection

(thanks to Rob Lockstone for his comments)

Run Resin with the additional argument -Xloggc:gc.log where "gc.log" is the name of the log file which will be created in the root Resin directory, e.g. /resin/gc.log.

Once the server has run under load for a while, or when problems start appearing, look in the gc.log file and search for "Full". They will happen fairly infrequently at first, and then towards the end of the file, they will become progressively more frequent until they are happening almost continuously (with little or no change in the size of the heap). Note that the "timestamp" in the first column is the number of seconds since the process started.

Garbage collection logging will have a small imapct on performance but is invaluable in diagnosing garbe collection related problems.

Possible reasons for excessive garbage collection are memory leaks and inadequate heap memory

Increase heap memory

See Performance : JVM Tuning : Memory.

Clean up the classpath

Conflicts often arise with older or incompatible versions of classes.

Starting Resin with an empty CLASSPATH environmental variable is a first step to eliminating these classes.

Starting Resin with an empty CLASSPATH

win> set CLASSPATH=
win> bin/httpd.exe

unix.sh> export CLASSPATH=""
unix.sh> bin/httpd.sh

If you have manually placed any jars in $RESIN_HOME/lib or you jdk directories, those are possible conflicts as well.

If a new version of Resin s installed overtop of an older version of Resin (i.e. in the same directory), some old jars might be left. It is best to give each version it's own directory.

If the RESIN_HOME environment variable is not set, Resin may be picking up an older version.

You can start httpd with the -verbose option to see the CLASSPATH that is being used when Resin is started.

Monitor the HTTP communication

To monitor just the HTTP headers, the following debug logging can be enabled in $RESIN_HOME/resin.conf:

debug logging for HTTP headers

<resin xmlns="http://caucho.com/ns/resin">

  ...

  <log name='com.caucho.server.http' level='finer' path='log/http.log'/>
  <log name='com.caucho.server.connection' level='finer' path='log/http.log'/>

  ...

</resin>

Intercepting and monitoring the raw data passed between a web browser and Resin can provide valuable insight. This raw data reveals the headers and content that are submitted by the browser, and the headers and content that are returned by Resin.

The Apache Axis jar includes a utility "tcpmon" that can be used to intercept and monitor the communication between a browser and Resin. Using tcpmon, you specify a "listen port" and a "target host" and a "target port". For example, if you usually have Resin running on port 8080, you can start tcpmon with a "listen port" os 9090, and a target host of localhost and a target port of 8080. Now you will use a url http://localhost:9090 in your browser. This causes your browser to use tcpmon. tcpmon will log the request, pass it through to Resin (on port 8080), and log the response before returning it to the browser.

Use an external compiler

Resin's default is to use the `internal' compiler because it is the most likely to be available. Sometimes the internal compiler will start to cause errors, causing exceptions or simply hanging and taking up a thread. The solution is to change the compiler to `javac' in resin.conf:

Using the external javac java compiler

  <javac compiler="javac" args=""/>

Jikes is another option for the compiler, as discussed in the documentation for <javac>.

Increase stack size to avoid thread limitations

Each thread is allocated a stack, if the size of the stack is too large, then you may run out of memory when a higher number of threads is reached. Techniques to adress this are discussed in JVM Tuning.

Use netstat to obtain an OS level view of current TCP/IP port usage

The netstat utility is useful for obtaining information from the operating system about the current state of network usage.

unix$ netstat -anp
win> netstat -an

The -a indicates that both listening (waiting for a connection) and non-listening (active) sockets should be displayed. The -n causes the display of port numbers (i.e. 80) instead of port names (i.e. http). The -p causes the display of the process that is using the socket. Since the windows netstat is different than the Unix one, -p is not used on Windows.

Connection state

The connection state is probably the most important information. The man page for netstat has a full description of each state that is displayed.

"LISTEN" or "LISTENING" is an indication that there is a process waiting for connections on the socket.

"TIME_WAIT" indicates that the socket is waiting after close to handle packets still in the network. After a connection is closed, the socket is kept open by the operating system for a short period of time. Even after the close is done, there may be some stray packets on the network that are destined for the connection. The idea of TIME_WAIT is that it keeps the socket open long enough to catch those stray packets, so that they don't get erronously delivered to a new connection opened on the same socket.

Port usage

If Resin is indicating that it cannot bind to a port, it may be an indication that another process is using the port. netstat may help determine the process that is using the port.

Since netstat produces a lot of information, it is helpful to filter the output to limit it to the port of interest. The following example is for port 80:

netstat -anp

unix$ netstat -anp | grep ":80" | less

Common Problems

Specification version 1.3 of package javax.servlet, J2EE Specification, version 1.3 is not compatible with Resin Resin-3.0.x. Resin-3.0.x requires version 2.4.

This error occurs when you have an old or conflicting jar containing the servlet api in your CLASSPATH. The solution is to clean up your classpath.

class file has wrong version 48.0, should be 47.0

This error occurs when you have an old or conflicting jar in your CLASSPATH. The solution is to clean up your classpath.

If it is a Resin jar that is causing the error, you may have jars from a previous installation that are causing the error. This occasionally occurs when people try to copy a new installation over an old one, and due to write permissions etc. some of the older jars are not replaced.

If the cleaning of the CLASSPATH (above) does not work, you may need to completely remove Resin from your system and re-install.

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