First let's look at using JStack for a basic Java app, then look at the same app written in Ruby and running on JRuby.
Example taken from http://www.herongyang.com/Java-Tools/jstack-JVM-Thread-Dump-Stack-Strace.html
/**
* LongSleep.java
* Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
*/
class LongSleep {
public static void main(String[] a) {
Runtime rt = Runtime.getRuntime();
System.out.println(" Free memory: " + rt.freeMemory());
System.out.println("Total memory: " + rt.totalMemory());
try {Thread.sleep(1000*60*60);}
catch (InterruptedException e) {}
}
}Then run:
$ javac LongSleep.java
$ java LongSleep
Free memory: 83588920
Total memory: 85000192
* pauses unexplainably!! *
Find all running Java processes in another terminal:
$ jps
47521 Jps
30447 LongSleep
Now we can view all the threads of our LongSleep application. We see that one of the threads is in a TIMED_WAITING state and it came from our application.
$ jstack -l 30447
...
"main" prio=5 tid=7fdec4000800 nid=0x110535000 waiting on condition [110534000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at LongSleep.main(LongSleep.java:10)
Locked ownable synchronizers:
- None
Ahh, its sleeping. Great, we could now go and fix that in LongSleep.main(LongSleep.java:10).
Awesome.
Let's rewrite LongSleep as a Ruby app for JRuby.
# long_sleep.rb
require 'java'
class RuntimeView
def display
rt = java.lang.Runtime.getRuntime
puts " Free memory: #{rt.freeMemory}"
puts "Total memory: #{rt.totalMemory}"
sleep 1000 * 60 * 60
end
end
RuntimeView.new.displayRun it:
$ jruby long_sleep.rb
Free memory: 73802224
Total memory: 85000192
Find the process and look for our stuck thread:
$ jps
47521 Jps
30447 LongSleep
$ jstack -l 30447
...
"main" prio=5 tid=7fcfe3000800 nid=0x101bb4000 in Object.wait() [101bb2000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <7df9e8d60> (a org.jruby.RubyThread)
at org.jruby.RubyThread.sleep(RubyThread.java:864)
- locked <7df9e8d60> (a org.jruby.RubyThread)
- locked <7df9e8d60> (a org.jruby.RubyThread)
at org.jruby.RubyKernel.sleep(RubyKernel.java:792)
at org.jruby.RubyKernel$INVOKER$s$0$1$sleep.call(RubyKernel$INVOKER$s$0$1$sleep.gen)
at org.jruby.internal.runtime.methods.JavaMethod$JavaMethodN.call(JavaMethod.java:642)
at org.jruby.internal.runtime.methods.DynamicMethod.call(DynamicMethod.java:204)
at org.jruby.runtime.callsite.CachingCallSite.cacheAndCall(CachingCallSite.java:326)
at org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:170)
at long_sleep.method__1$RUBY$display(long_sleep.rb:9)
at long_sleep$method__1$RUBY$display.call(long_sleep$method__1$RUBY$display)
at long_sleep$method__1$RUBY$display.call(long_sleep$method__1$RUBY$display)
at org.jruby.runtime.callsite.CachingCallSite.cacheAndCall(CachingCallSite.java:306)
at org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:136)
at long_sleep.__file__(long_sleep.rb:13)
at long_sleep.load(long_sleep.rb)
at org.jruby.Ruby.runScript(Ruby.java:770)
at org.jruby.Ruby.runScript(Ruby.java:763)
at org.jruby.Ruby.runNormally(Ruby.java:640)
at org.jruby.Ruby.runFromMain(Ruby.java:489)
at org.jruby.Main.doRunFromMain(Main.java:375)
at org.jruby.Main.internalRun(Main.java:264)
at org.jruby.Main.run(Main.java:230)
at org.jruby.Main.run(Main.java:214)
at org.jruby.Main.main(Main.java:194)
Locked ownable synchronizers:
- None
Looking through the list of threads (not shown above) we again find this thread in a TIMED_WAITING state. The first thing I notice is that the JRuby stack trace is a lot longer than the pure Java one.
After finding the TIMED_WAITING state thread, it is clear that this thread is locked by RubyKernel.sleep.
Looking down the stack trace, it is not as clear, but you can see that my application is involved at long_sleep.rb:9:
long_sleep.method__1$RUBY$display(long_sleep.rb:9)
Slightly hard to read, but the blocking occurs within a #display method (on an unspecified class?) on line 9 in long_sleep.rb.
Again, wonderful. I can now start debugging!
I definitely look forward to using jstack in future as part of debugging applications in development and in production.
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