infos = Information about the jni plugin is in keys below
infos/author = Name name@.nosp@m.libe.nosp@m.lektr.nosp@m.a.or.nosp@m.g
infos/licence = BSD
infos/provides =
infos/needs =
infos/placements =
infos/status = maintained configurable experimental -500 memleak
infos/description =

Generic Java plugin

Introduction

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
- a constructor that takes a C-Pointer as long (J)
- a method "release" that gives up ownership (set internal pointer to NULL)

The java plugin itself needs to have the following methods:

constructor without arguments (i.e. 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

Installation

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/ld.so.conf.d/java-8-oracle.conf with the content (for amd64):

/usr/lib/jvm/default-java/jre/lib/amd64
/usr/lib/jvm/default-java/lib/amd64
/usr/lib/jvm/default-java/jre/lib/amd64/server

and run:

sudo ldconfig

Then enable the plugin using:

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

Running

kdb-full

should work then, if you get one of these:

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

E.g.

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= file.properties /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= file.properties /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.

Development

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

javap -s YourClass

Also explained here

JNI Functions Invocation

Issues

(Argumentation for -500 in status)

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.