Purpose of this repository is to provide materials allowing understand basic JVM features. Below is a list of subjects.

• installed jdk
• JAVA_HOME is set in system environment variables
• %JAVA_HOME/bin% is added to PATH system environment variable - without it java commandline utils may not be visible in terminal
• in console/terminal try javac if do not working something is wrong :(
• check version of java to confirm that you use desired one
• check project setting in IntellijJ that you use desired one

• examples are prepared to be run in windows, to start them on linux there maybe required some changes with slashes and replacement of ":" to ";" in class path values

• Write program that will show sequence of loading and initialization of classes using static initializer section and constructors.
• For this purpose create three classes ClassB, ClassA and Main. Each class should have static initialization block, moreover ClassA should extend ClassB. Instantiate ClassA in Main class main method.
• Both static initialization block and constructors should print suitable message to console to make it possible to track loading and execution sequence.

cd compiling - go to submodule in terminal (git bash or intelliJ)

javac -d dist src\main\java\HelloWorld.java - compile class and put it in dist directory

java -cp dist HelloWorld - set class path to dist folder and run class

javap -v dist/HelloWorld.class - shows generated byte code with details (look at the same code in IntelliJ)

javac -cp lib\commons-lang3-3.8.1.jar -d dist src\main\java\HelloWorld.java - compile class with dependency to library and put it in dist directory

java -cp dist;lib\commons-lang3-3.8.1.jar HelloWorld - set class path to dist folder and library , then run class

jar -cf simple-compile.jar -C dist . - builds jar with all files in dist

jar -tf simple-compile.jar - list content of jar

java -cp simple-compile.jar HelloWorld - run class from jar

jar -cfe simple-compile.jar HelloWorld -C dist . - builds jar with all files in dist and Manifest that include main class

jar -cfm simple-compile.jar etc\MANIFEST.MF -C dist . - builds jar with all files in dist and custom Manifest that include main class and class path

java -jar simple-compile.jar - run class configured in manifest

• reduce boilerplate code with lombok https://projectlombok.org/contributing/lombok-execution-path
• Intellij needs "Lombok Plugin", and "annotation processing" enabled by compiler

javac -cp lib\lombok.jar -d dist src\main\java\HelloWorld.java - compiles class with lombok compile time preprocessor

• add remote debug config in IntellijJ
• set brake point in HelloWorld class
• java -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=y,address=1044 -cp dist HelloWorld - runs application in debug mode waiting for debugger

• write program with method that recursively call itself (StackOverflow)
• write program with two object that call each other recursively (StackOverflow2)

• write program that will reserve all heap memory - by collecting large objects in list

node: java there are possible memory leaks when objects references are stored in some application scope collection.

• java -X help - jvm options
• commandline tools:
  • jshell
  • jps
  • jinfo
  • jstack
• jvisualvm
  • https://visualvm.github.io/download.html
  • unpack and install all plugins
  • -Xmx100m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=./java_pid.hprof - heap dump on error
  • -XX:+UnlockCommercialFeatures -XX:+FlightRecorder - profiling
• play with memory:
  • -Xmx10m
  • -Xmx1g
• play garbage collectors:
  • -XX:+UseSerialGC
  • -XX:+USeParNewGC
  • -XX:+UseG1GC
• see MBean in action

• https://www.baeldung.com/jvm-parameters
• https://www.baeldung.com/graal-java-jit-compiler
• https://www.baeldung.com/jvm-garbage-collectors
• https://bottega.com.pl/pdf/materialy/jvm/jvm1.pdf
• https://bottega.com.pl/pdf/materialy/jvm/jvm2.pdf
• https://blog.sokolenko.me/2014/11/javavm-options-production.html
• https://www.baeldung.com/java-management-extensions
• https://raygun.com/blog/java-performance-optimization-tips/