Java Tools

• The jvm comes with a lot of tooling apart from compling (javac, javac -jar) and running the class/jar (java ...)

In the JDK /bin

• jps

  • To find running JVM, use jps

    • lists the process ID and the application’s main class name, making it far easier to figure out which process is which.

  • Can use ps aux | grep java

  • jps -l will list the fully qualified main class name

  • jps -l <remote> will list jvm on remote server

  • jps -mwill show the arguments passed to the main method

  • jps -v will show all the arguments passed to the JVM

• javap

  • Java class file disassembler

  • Run javap <class file> to see that class file’s fields and methods,

    • which can often be very enlightening for understanding what code written in JVM-based languages such as Scala, Clojure, or Groovy

  • javap -c <class file> to see the complete bytecode of those methods.

• jmap

  • jmap -heap <process id> will print a summary of the JVM process’s memory space, such as

    • how much memory is being used in each of the JVM’s memory generations,

    • the heap configuration

    • type of GC being used

  • jmap -histo <process id> will print a histogram

    • of each class in the heap

    • how many instances there are of that class

    • how many bytes of memory are consumed

  • jmap -dump:format=b,file=<filename> <process id> will dump a snapshot of the entire heap to a file

• jhat

  • jhat <heap dump file> will take the file generated by jmap and run a local web server.

  • You can connect to this server in a browser to explore the heap space interactively, grouped by package name.

  • The “Show instance counts for all classes (excluding platform)” link shows only instances of classes outside of Java itself.

  • You can also run “OQL” queries, allowing you to query the heap space via SQL-esque syntax.

• jinfo

  • jinfo <process id> to see all system properties the JVM loaded with and JVM command-line flags

• jstack

  • jstack <process id> will print stack traces for all current Java threads running in a JVM

• jconsole and jvisualvm

  • These are graphical tools that allow connecting to JVMs and interactively monitoring running JVMs.

  • They offer visual graphs and histograms of various aspects of a running process and are a mouse-friendly alternative to many of the tools listed above.

  • These can be process/memory heavy for your local setup

• Jshell

  • command line repl