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 Debugging

JSP/Servlet
FAQ
Misc
  1. How to get debugging information
  2. Exceptions don't have line numbers
  3. How do I keep the generated .java files?
  4. How do I run Resin with a debugger?
  5. I've got a cryptic ServletException. How do I get the full trace?
    1. PROBLEM:
    2. SOLUTION:
  6. Thread Dumps
  7. Java profiling
    1. java.hprof.txt heap
    2. java.hprof.txt CPU

How to get debugging information

Resin can write debugging information to standard output or to any file. To add debugging information add a log directive to the resin.conf file.

<caucho.com>
  <log id='/' href='stderr:'/>
</caucho.com>

href can be any path. stderr: is perhaps the most useful. id can select which portions of Resin to get logging information. Here are some useful patterns:

/Debug everything
/caucho.com/jspDebug jsp
/caucho.com/jsp/javaSee the JSP generated Java
/caucho.com/jsp/jsSee the JSP generated Java
/caucho.com/xslXSL debugging
/caucho.com/tcp-server See thread creation and deletion
/caucho.com/http HTTP related information

Exceptions don't have line numbers

For debugging, it is crucial to get proper line numbers. Unfortunately, Java JITs (just-in-time compilers) remove the line numbers.

For development we recommend you turn jit compiling off. Start the server with the JIT compiler turned off. On Unix you can use:

unix> httpd.sh -nojit

How do I keep the generated .java files?

The generated .java files are put in <work-dir>. By default, that's /tmp/caucho on Unix and \temp\caucho on Windows.

How do I run Resin with a debugger?

Eric Hansen writes:

A lot of people have been asking about a suitable debugging environment for resin. I have been using JPadPro with the updated version of jdb. (I found that the jdb.exe that ships with the JDKs 1.2.2 do not like all the classloading that happens in a servlet engine.)

Instead, try using the JavaTM Platform Debugger Architecture (JPDA) http://www.javasoft.com/products/jpda/.

Just unzip the executables, dlls, and docs files in some directory. I chose c:\jdb. My project class files are in: c:\local, my resin1.1 files are in: c:\resin1.1, and jdk1.2.2 files are in c:\jdk1.2.2.

To start up the debugger issue the following command (you may need additional classfiles in the classpath, etc.):

C:\jdb\bin\jdb.exe -classpath
"C:\resin1.1\lib\jdk12.jar;C:\resin1.1\lib\jsdk.jar;
C:\resin1.1\lib\jsdk22.jar;C:\resin1.1\lib\resin.jar;
C:\jdk1.2.2\jre\lib\rt.jar;C:\jdk1.2.2\lib\tools.jar;
C:\local" com.caucho.server.http.HttpServer -conf
c:\resin1.1\conf\resin.conf
> run

You can access the webserver at: http://localhost/ or http://localhost:8080/, etc. depending on the port number you picked. You should be able to issue the usual jdb commands. Check the list of commands by doing: help or ? at the jdb> prompt.

I use jdb with JPadPro (http://www.modelworks.com). It has some built-in scripts to work with jdb. It is tailored to work with the jdb that ships with the JDK, which has a slightly different response syntax from the JPDA version of jdb. Since all of JPadPro's commands are written in JavaScript, you can easily modify the commands to parse the different command/response syntax in JPDA. Or you could even modify the jdb code to have it issue similar commands, since JPDA ships with source code for jdb. However, it's probably easiest to modify the JavaScript. (Check in Tools/Java/JDK/Debugger/*, including onBreakPointHit, setBreakPoint.) I modified setBreakPoint, for example, to look like this:

function BreakpointResult(data, breakpoint)
{
   var index = data.indexOf("Breakpoint set");
    var index2 = data.indexOf("Set breakpoint request");

    if ((index == -1) && (index2 == -1))
   {
      breakpoint.active = false;
      gOutput.writeMessage(breakpoint.getPath(),
                           breakpoint.getLineIndex(),
         "Setting breakpoint to inactive - breakpoint is " +
         "not positioned on a valid line");
   }
}

This is because the old version of jdb issued the syntax: "Breakpoint set...", and the new one issues the command "Set breakpoint request...". You get the picture.

A similar formula should bear fruit for Visual Cafe, although they have their own proprietary debugger.

Hope that's useful.

Eric

I've got a cryptic ServletException. How do I get the full trace?

Eric Hansen writes:

PROBLEM:

Have you ever received a cryptic Exception message like this in your error logging due to an error in a JSP page: ???

javax.servlet.ServletException
 at com.caucho.jsp.QPageContext.handlePageException(QPageContext.java:371)
 at work.ProgramOverview__jsp._jspService(ProgramOverview__jsp.java:115)
 at com.caucho.jsp.JavaPage.subservice(JavaPage.java, Compiled Code)
 at com.caucho.jsp.Page.service(Page.java, Compiled Code)
 at com.caucho.jsp.QServlet.service(QServlet.java, Compiled Code)
 at com.caucho.server.http.AbstractRequest.service(AbstractRequest.java, Compiled Code)
 at com.caucho.server.http.VirtualHost.service(VirtualHost.java, Compiled Code)
 at com.caucho.server.http.QRequestDispatcher.forward(QRequestDispatcher.java, Compiled Code)
 at com.caucho.server.http.QRequestDispatcher.forward(QRequestDispatcher.java, Compiled Code)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)
 at com.caucho.server.http.AbstractRequest.service(AbstractRequest.java, Compiled Code)
 at com.caucho.server.http.VirtualHost.service(VirtualHost.java, Compiled Code)
 at com.caucho.server.http.Request.dispatch(Request.java, Compiled Code)
 at com.caucho.server.http.HttpRequest.handleRequest(HttpRequest.java, Compiled Code)
 at com.caucho.server.http.HttpRequest.handleConnection(HttpRequest.java, Compiled Code)
 at com.caucho.server.TcpConnection.run(TcpConnection.java, Compiled Code)
 at java.lang.Thread.run(Thread.java:479)

SOLUTION:

You need to change your catch routine. There is a method in ServletException called getRootCause(). This allows the developer of the servlet engine/JSP engine to embed a regular Exception inside a ServletException, which gets carried along for the ride.

catch(Exception e)
{
  logMessage(e,"",-1);
  if(e instanceof javax.servlet.ServletException)
  {
    Throwable et;
    et = ((javax.servlet.ServletException)e).getRootCause();
    if(et!=null&&et instanceof Exception)
    {
      logMessage(((Exception)et),"ROOT EXCEPTION",-1);
    }
  }
}

(logMessage is some logging routine you write to do logging... Could also be System.err.println...) I was finding it quite difficult to debug JSP pages because I couldn't see what the underlying Exception was, just the ServletException which gets thrown from the JSP. The above modification (or something similar) exposes it. I figure other people might have the same problem.

Hope that helps,

Eric

Thread Dumps

Java's thread dumps are a vital tool for server debugging. Because servlets are intrinsically multithreaded, it's very possible to create deadlocks without realizing it. That's especially true when you start adding third-part software like databases, EJB, and Corba ORBs.

On Windows, you'll need to start Resin with bin/httpd. Then ctrl-break will 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.

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 NameDescription
tcp-accept-8080Resin thread listening for new connections on port 8080.
tcpConnection-8080-3Resin servlet thread handling a connection from port 8080.
tcp-cronResin's run-at thread
tcp-alarmResin'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.

Java profiling

What you really need to do is get a good CPU and heap profile. The JDK 1.2 has a simple one (but not very user-friendly), so you don't absolutely need to buy a profiler.

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.

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.

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)

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