Java

List of currently running java process

jps -l

Check heap of java process

jmap -heap PID

Installation

Change java version

# List Java versions installed
/usr/libexec/java_home -V

export JAVA_HOME=$(/usr/libexec/java_home -v 11) # Java 11
export JAVA_HOME=$(/usr/libexec/java_home -v 1.8) # Java 1.8
export JAVA_HOME=$(/usr/libexec/java_home -v 1.7) # Java 1.7
export JAVA_HOME=$(/usr/libexec/java_home -v 1.6) # Java 1.6

cd ~/Downloads
curl -v -j -k -L -H "Cookie: oraclelicense=accept-securebackup-cookie" http://download.oracle.com/otn-pub/java/jdk/8u112-b15/jre-8u112-macosx-x64.dmg > jre-8u112-macosx-x64.dmg
hdiutil attach jre-8u112-macosx-x64.dmg
sudo installer -pkg /Volumes/Java\ 8\ Update\ 112/Java\ 8\ Update\ 112.app/Contents/Resources/JavaAppletPlugin.pkg -target /
diskutil umount /Volumes/Java\ 8\ Update\ 112
rm jre-8u112-macosx-x64.dmg

Install java (Ubuntu)

sudo add-apt-repository -y ppa:webupd8team/java
sudo apt-get update
echo debconf shared/accepted-oracle-license-v1-1 select true | sudo debconf-set-selections
echo debconf shared/accepted-oracle-license-v1-1 seen true | sudo debconf-set-selections
sudo aptitude -y install oracle-java8-installer

JVM Tuning

Ref:

Note on non standard option

  • Use java -X to display non standard option
  • option begin with -X are non-standard (not guaranteed to be supported on all VM implementations), and are subject to change without notice
  • option specified with -XX are not stable and are subject to change without notice

Double Curly Brace

Use in test only since the practice can lead memory leak (src)

Map map = new HashMap(){{
    put("firstName", "John");
    put("lastName", "Smith");
    put("organizations", new HashMap(){{
        put("0", new HashMap(){{
            put("id", "1234");
        }});
        put("abc", new HashMap(){{
            put("id", "5678");
        }});
    }});
}};

List<Integer> list = new ArrayList<Integer>() {{
   add(1);
   add(2);
}};