Debugging

This page describes interesting tools and articles to fix bugs or improve the web performance.

Java memory

• 10 points about Java Heap Space or Java Heap Memory
• OutOfMemory
• Simple procedure to debug memory leaks
• Book recommended in different articles

jmap and jhat

They both come with new JDKs.

• Make heap snapshot with jmap:

 $ ps -ax | grep "tomcat"
 $ # Let us imagine that it is running in PID 29868
 $ sudo -u tomcat jmap -dump:file=dump.txt,format=b 29868

• Analyse it with jhat:

 $ # change the permissions of the file if necessary
 $ jhat -port 7000 /tmp/dump.txt

1. Open the host with the port you specified (by default, 7000) in the browser and explore data.

VisualVM

VisualVM comes with new JDKs.

To enable remote connection, I have done the following in the web server:

• Created a jstatd.all.policy file:

grant codebase "file:[path_to_jvm]/lib/tools.jar" {
   permission java.security.AllPermission;
};

• Configure CATALINA_OPTS in tomcat.conf with something like this:

CATALINA_OPTS="-Dcom.sun.management.jmxremote=true -Dcom.sun.management.jmxremote.port=5555 -Dcom.sun.management.jmxremote.ssl=false -Dcom.sun.management.jmxremote.authenticate=false -Djava.rmi.server.hostname=192.168.34.202

• Restart tomcat: systemctl restart tomcat
• Run jstatd: jstatd -p 12345 -J-Djava.security.policy=[path]/jstatd.all.policy

Visual GC

1. Open VisualVM
2. Go to Tools > Plugins and install Visual GC
3. You might need to reestart VisualVM