md_src_plugins_jni_README(3) README

Generic Java plugin


Allows you to write plugins in java.

Needs Java 8 or later. While the plugin internally uses JNI, the Java binding for your java-plugin may use something different, e.g. JNA. The requirements for the java bindings are:

needs to have the classes elektra/Key and elektra/KeySet with
an constructor that takes a C-Pointer as long (J)
an method 'release' that gives up ownership (set internal pointer to NULL)

The java plugin itself needs to have following methods:

  • constructor without arguments (e.g. default constructor)
  • open with argument elektra/KeySet (the plugin's conf) and elektra/Key
  • close with argument elektra/Key
  • get with arguments elektra/KeySet and elektra/Key
  • set with arguments elektra/KeySet and elektra/Key
  • error with arguments elektra/KeySet and elektra/Key


Please install java8 as package, e.g. for debian and then let cmake actually find jdk8:

cd /usr/lib/jvm/ && sudo rm -f default-java && sudo ln -s java-8-oracle default-java

and for the runtime, create the file /etc/ with the content (for amd64):


and run:

sudo ldconfig

Then enable the plugin using:

cmake -DPLUGINS="...;jni" /path/to/libelektra



should work then, if you get one of these:

kdb-full: error while loading shared libraries: cannot open shared object file: No such file or directory
kdb-full: error while loading shared libraries: cannot open shared object file: No such file or directory

You missed one of the ldconfig steps.

Plugin Config

You need to pass :

  • classname the classname to use as plugin, e.g. elektra/plugin/Echo
  • classpath the classpath where to find JNA, the package elektra and other classes needed

Additionally, you can set:

  • option allows you to pass a option to the jvm, default: -verbose:gc,class,jni
  • ignore allows you to ignore broken options, default: false
  • print allows you to print java exceptions for debugging purposes


kdb info -c classname=elektra/plugin/PropertiesStorage,classpath=.:/usr/share/java/jna.jar:/usr/lib/java:/path/to/libelektra/src/bindings/jna,print= jni
kdb check -c classname=elektra/plugin/PropertiesStorage,classpath=.:/usr/share/java/jna.jar:/usr/lib/java:/path/to/libelektra/src/bindings/jna,print= jni
kdb mount -c classname=elektra/plugin/PropertiesStorage,classpath=.:/usr/share/java/jna.jar:/usr/lib/java:/path/to/src/bindings/jna,print= /jni jni classname=elektra/plugin/PropertiesStorage,classpath=.:/usr/share/java/jna.jar:/usr/lib/java:/path/to/libelektra/src/bindings/jna,print=

Or if .jar is already installed:

bin/kdb mount -c classname=elektra/plugin/PropertiesStorage,classpath=.:/usr/share/java/jna.jar:/usr/share/java/elektra.jar,print= /jni jni classname=elektra/plugin/PropertiesStorage,classpath=.:/usr/share/java/jna.jar:/usr/share/java/elektra.jar,print=

Additionally, the java implementation can request any other additional configuration, read about it below in the section (specific java plugin). If you are reading this page on github, you won't see it, because the plugins dynamically append text after the end of this page.


To know how the methods of your class are called, use:

javap -s YourClass

Also explained here

JNI Functions Invocation


  • In Debian Wheezy you cannot use openjdk: you get a linker error because of some missing private SUN symbols. Maybe just the cmake mechanism to find java is broken.
  • Only a single java plugin can be loaded
  • when this plugin is enabled, valgrind detects memory problems even if the plugin is not mounted.

Specific Java Plugin