☕ Java

JAVA_HOME

macOS

export JAVA_HOME="/Library/Java/Home"

# Override current JDK version
function jdk() {
  sudo ln -nsf "$(/usr/libexec/java_home -v $1)" $JAVA_HOME
  java -version
}

# Configure default JDK
if [ ! -L "$JAVA_HOME" ]
then
     echo "JAVA_HOME symlink is missing, let's configure it" >&2
     jdk 17
fi

JVM flags

java -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+PrintFlagsFinal -version

JVM utilities

• Heap info

  jps -l `: # list VMs` |
    grep -E "*" `: # optional filter` |
    cut -d " " -f1 `: # extract pid` |
    xargs -t -n1 -I % jcmd % GC.heap_info
  

• GC

  jps -l `: # list VMs` |
    grep -E "*" `: # optional filter` |
    cut -d " " -f1 `: # extract pid` |
    xargs -t -n1 -I % jcmd % GC.run
  

• Kill

  jps -l `: # list VMs` |
    grep -E "*" `: # optional filter` |
    cut -d " " -f1 `: # extract pid` |
    xargs -t -n1 kill -9
  

UncaughtExceptionHandler

Quote

When a thread is about to terminate due to an uncaught exception the Java Virtual Machine will query the thread for its UncaughtExceptionHandler using Thread.getUncaughtExceptionHandler() and will invoke the handler's uncaughtException method, passing the thread and the exception as arguments.

// Keep a reference to the default handler
val default = Thread.getDefaultUncaughtExceptionHandler()

// Override the default handler
Thread.setDefaultUncaughtExceptionHandler { thread: Thread, throwable: Throwable ->
    when (throwable) {
        /* Check if it should be swallowed */
        is MyUncaughtException -> Unit
        /* Otherwise rethrow to the default handler */
        else -> default?.uncaughtException(thread, throwable)
    }
}