kagkarlsson / javatools-examples

Crappy webapp used to demonstrate use of jstack, jmap, jhat etc

Building

mvn clean install

Scenarios

Hint: jetty request-threads are on the format qtp611437735-26

Scenario 1

• Start server: ./start_webserver.sh
• Simulate traffic: ./run_requests.sh get_token1 5
• Use jstack to find out why requests are slow

Scenario 2

• Start server: ./start_webserver.sh
• Simulate traffic: ./run_requests.sh get_token2 1/3/10
• Use jstack to find out why requests are slow

Scenario 3

• Start server: ./start_webserver.sh
• Simulate traffic: ./run_requests.sh http://localhost:8787/caching 10
• Run for a while, and there should be an OutOfMemoryError in the webserver log
• Manually generate a heapdump using jmap (jmap -dump:format=b,file=[filename].hprof [pid])
• Analyze heapdump in Eclipse Memory Analyzer (MAT) to find cause of OOM
• View live heap of running JVM using jmap histo:live [pid]
• Use jhat to find the source of the OOM
• Also try to locate heapdump generated automatically on OOM

General tips

Find process:

• jps
• ps aux

Threaddump:

• jstack [pid]
• kill -QUIT [pid]

Thread summary:

• jstack [pid] | grep "^"" | less

One-liners (Funkar lite sämre med exec:java):

• jps | grep WebappMain | cut -d' ' -f1 | xargs jstack
• ps aux | grep WebappMain | grep -v grep | cut -d' ' -f2 | xargs jstack

Heapdump:

• jmap -dump:format=b,file=[filename].hprof [pid]

Heap overview:

• jmap -histo:live [pid] | head -n20

Thread states:

• RUNNABLE - thread is currently executing/can be executed in the jvm
• BLOCKED - thread is blocked indefinately while waiting for a lock (typically synchronized)
• TIMED_WAITING - thread is waiting for a period of time (triggered by e.g Thread.sleep(..))
• WAITING - thread is waiting indefinitely for another thread to perform a certain action (i.e. Object.wait()->Object.notify, ...)