pdj allows you to write OS-independent pd externals in Java. It is a clone of the mxj object for Max/MSP, so that the Java code that you write for pdj should(!) also run fine in mxj.

I found personally hard to install in my Dell Latitude E6400 the current pdj version (0.8.7) under Linux Ubuntu 12.04 with pd-extended 0.43.1 beta.

Here is the procedure I followed:

• Assume to have ant and Sun Java(TM) Development Kit (JDK) 7 installed (both by synaptic or similar archive manager)

• download pdj for your OS. Unpack, for example in a separate folder on the ~/Download directory

• download the puredata 0.43.1 source tarball. Unpack, then copy only the src folder into your existing pd-extended folder. Probably like this:

sudo cp -r src /usr/lib/pd-extended/

• configure Linux-build.properties inside pdj-0.8.7/ this way through you preferred text editor (edit accordingly if you have an amd64 equivalent machine):

# property file for Linux, now with default ubuntu packages

# put the path of where the jdk (java sdk) is installed
jdk.home=/usr/lib/jvm/java-7-sun

# put the path of where pd is installed
pd.home=/usr/lib/pd-extended

# common linux properties
isLinux=true
compiler=gcc
linker.type=shared
platform=linux

# 64-bits system ? uncomment this
#jdk.libs=${jdk.home}/jre/lib/amd64

jdk.libs=${jdk.home}/jre/lib/i386
pdj.outfile=${dist.dir}/pdj

• build inside pdj-0.8.7/:

sudo ant package

• copy the folder dist in your additional externals folder of puredata, and rename it as pdj. In my case:

sudo cp -r dist /usr/local/lib/pd-externals/
sudo mv /usr/local/lib/pd-externals/dist /usr/local/lib/pd-externals/pdj

• add needed classes (folder pdj-0.8.7/work/com) inside the new folder. In my case:

cd /usr/local/lib/pd-externals/pdj/classes
sudo cp - r ~/Downloads/temp/pdj-0.8.7/work/classes/com .

• make the new additional externals visible by puredata (Edit -> Preferences -> New... -> /usr/local/lib/pd-externals/pdj)

• configure pdj.properties in (the easiest) way for puredata to run the test classes:

# this is the pdj classpath (dynamic) to set :
pdj.classpath=

# this will print all .jars that are loaded before compiling/loading the 
# user class
pdj.verbose-classloader=true

# the type of compiler to use with the pdj classloader. use only 'javac' 
# or 'jikes'. do not put full path to your compiler
#
# to disable the automatic compilation simply use 
# pdj.compiler=null
#
pdj.compiler=javac

# pdj compiler/classloader directory. by default, ${pdj.home}/classes 
# directory is used if this property is not defined. Before compiling, pdj
# will check if your .java needs to be compiled.
#
#pdj.classes-dir=

# this will redirect the java out/err streams to the pd console. If you need
# to log pdj errors and exceptions to stderr and stdout, set this to false.
#
pdj.redirect-pdio=true
# the jvm to use. If it is not found and the system is on Windows, 
# the jvm installed with the registry will be used. Not used on OS X
#
pdj.JAVA_HOME=/usr/lib/jvm/java-7-sun

# the JVM version to use with OS X; linked to the Java Framework on OS X
#
#pdj.osx.JVM_JAVA_VERSION=1.5.0

# NOTE: pdj.home is set automagicly from where the pdj.pd_linux or pdj.dll 
# is installed (you need to put it in your pd path in OS X)

# =========================================================================
# VM ARGUMENTS
#
# to monitor memory and thread usage with 1.5 (jconsole) use 
#   -Dcom.sun.management.jmxremote
# to do remote debugging at port 8000
#   -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=8000
# to check jni calls
#   -Xcheck:jni -verbose:jni
#
# All arguments on the same line just like $ java -X... -X... myclass
#
pdj.vm_args=-Xincgc -Xms32m -Xmx256m 

# tells pdj what type of jvm it must use: client or server. Not used on 
# OS X. client has lower starting time and can take less memory. server
# starts slower and takes more memory, but once the code is executed, 
# it will be faster than in client mode.
#
pdj.vm_type=server

The test patches should work fine at this point.

The following note, left by Hans, may solve visibility problems encountered by puredata, when running a pdj~ module when it is opened for the first time.

• made a symlink to pdj.pd_linux (GNU/Linux) or pdj.pd_darwin (Mac OS X) and make it called pdj~.pd_linux (GNU/Linux) or pdj~.pd_darwin (Mac OS X) like:

(GNU/Linux)

cd ~/pd-externals
ln -s pdj.pd_linux pdj~.pd_linux

(Mac OS X)

cd ~/Library/Pd
ln -s pdj.pd_darwin pdj~.pd_darwin