Yaz4J is a wrapper library over the client-specific parts of YAZ, a C-based Z39.50 toolkit, and allows you to use the ZOOM API directly from Java. Initial version of Yaz4j has been written by Rob Styles from Talis and the project is now developed and maintained at Index Data. ZOOM is a relatively straightforward API and with a few lines of code you can write a basic application that can establish connection to a Z39.50 server. Here we will try to build a very simple HTTP-to-Z3950 gateway using yaz4j and the Java Servlet technology.

COMPILING AND INSTALLING YAZ4J

Yaz4j is still an experimental piece of software and as such is not distributed via Index Data's public Debian Apt repository and there is no Windows build (yet) either. While it is possible to use the pre-built Linux binaries, users of other OSes will have to compile yaz4j from source. No need to worry (yet) - the process of compiling yaz4j is quite simple and we will be up and running in no time :).

As a prerequisite, to complete th build process you will need JDK, Maven, Swig and Yaz (development package) installed on your machine. On Debian/Ubuntu you can get those easily via apt:

apt-get install sun-java6-jdk maven2 libyaz4-dev swig

The Yaz4j's source code can be checked-out out from our Git repository, and assuming you have Git installed on your machine you can do that with:

git clone git://git.indexdata.com/yaz4j

The compilation of both native and Java source code is controlled by Maven2, to build the library, invoke the following commands:

cd yaz4j
mvn install

That's it. If the build has completed successfully you end up with two files: os-independent jar archive with Java ZOOM API classes (yaz4j/any/target/yaz4j-any-VERSION.jar) and os-dependent shared library (yaz4j/linux/target/libyaz4j.so or yaz4j/win32/target/yaz4j.dll) that contains all necessary JNI "glue" to make the native calls possible from Java. If we were writing a command line Java application, like any other external Java library, yaz4j-any-VERSION.jar would have to be placed on your application classpath and the native, shared library would have to be added to your system shared library path (LD_LIBRARY_PATH on linux, PATH on Windows) or specified as a Java system property (namely the java.library.path) just before your application is executed:

java -cp /path/to/yaz4j-*.jar -Djava.library.path=/path/to/libyaz4j.so MyApp

SETTING UP THE DEVELOPMENT ENVIRONMENT

Setting up a development/runtime environment for a web (servlet) application is a bit more complicated. First, you are not invoking the JVM directly, but the servlet container (e.g Tomcat) run-script is doing that for you. At this point the shared library (so or dll) has to be placed on the servlet container's shared libraries load path. Unless your library is deployed to the standard system location for shared libs (/usr/lib on Linux) or it's location is already added to the path, the easiest way to do this in Tomcat is by editing (create it if it does not exist) the CATALINA_HOME/bin/setenv.sh (setenv.bat on Windows) script and putting the following lines in there:

LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/path/to/libyaz4j.so
export LD_LIBRARY_PATH

on Windows (though no Windows build is yet provided)

 set PATH=%PATH;X:\path\to\yaz4j.dll

That's one way of doing it, another would be to alter the standard set of arguments passed to the JVM before the Tomcat starts and add -Djava.library.path=/path/to/lib there. Depending on a situation this might be preferable/easier (on Debian/Ubuntu you can specify JVM arguments in the /etc/default/tomcat6 file).

With the shared library installed we need to install the pure-Java yaz4j-any*jar with ZOOM API classes by placing it in Tomcat's lib directory (CATALINA_HOME/lib). As this library makes the Java System call to load the native library into the JVM you cannot simply package it along with your web application (inside the .war file) - it would try to load the library each time you deploy the webapp and all consecutive deployments would fail.

WRITING A SERVLET-BASED GATEWAY

With your servlet environment set up all that is left is to write the actual application (peanuts :)). At Index Data we use Maven for managing builds of our Java software components but Maven is also a great tool for quickly starting up a project. To generate a skeleton for our webapp use the Maven archetype plugin:

mvn -DarchetypeVersion=1.0.1 -Darchetype.interactive=false \
-DarchetypeArtifactId=webapp-jee5 -DarchetypeGroupId=org.codehaus.mojo.archetypes \ 
-Dpackage=com.indexdata.zgate -DgroupId=com.indexdata -DartifactId=zgate \ 
archetype:generate --batch-mode

This will generate a basic webapp project structure:

Maven has already added basic JEE APIs for web development as the project dependencies, we need to do the same for yaz4j, so edit the pom.xml and add the following lines in the dependencies section:

It's crucial that the scope of this dependency is set to provided otherwise the library would end up packaged in the .war archive and we don't want that.

The implementation of our simple gateway will be contained in a single servlet - ZGateServlet - which we need to place under src/main/webapp/com/indexdata/zgate. The gateway will work by answering HTTP GET requests and will be controlled solely by HTTP parameters, the servlet doGet method is shown below:

With the code in-place we can try to compile the project:

mvn compile

If all is OK, the next step is to register our servlet and map it to an URL in src/main/webapp/WEB-INF/web.xml:

On top of that, we will also make sure that our servlet is automatically triggered when accessing the root path of our application:

Now we are ready to build our webapp:

mvn package

The resulting .war archive is located under target/zgate.war, we can deploy it on tomcat (e.g by using the /admin Tomcat admin console) and test by issuing the following request with your browser or curl (assuming Tomcat is running on localhost:8080):

http://localhost:8080/zgate/?zurl=z3950.loc.gov:7090/voyager...

That's it! You just build yourself a HTTP-to-Z3950 gateway! Just be careful with exposing it to the outside world - it's not very secure and could be easily exploited. The source code and the gateway's Maven project is available in the Yaz4j's Git repository under examples/zgate. In the meantime, Index Data is working on a Debian/Ubuntu package to make the installation of Yaz4j and Tomcat configuration greatly simplified - so stay tuned!. If you are interested in Windows support - e.g. Visual Studio based build or an installer - please let us know.