Wednesday, May 25, 2011

Cannot get GATE Home. Pease set it manually!

When you run your application using GATE Embedded, you often run into an error:
  • Cannot get GATE Home. Pease set it manually!
This means that you need to set gate.home property before calling Gate.init(). You can do that in two ways:
  1. In your Java code
    • Gate.setGateHome(File)
  2. In the Java command that launches your program
    • -Dgate.home=path/to/gate/home
GATE also needs to initialize the paths to local files of interest like:
  • Installed plugins home
  • Site configuration file
  • User configuration file
if these are not at their default locations. To help configure these paths, you can use the following system properties:
sets the location of the GATE install directory. This should point to the top level directory of your GATE installation. This is the only property that is required. If this is not set, the system will display an error message and them it will attempt to guess the correct value.
points to the location of the directory containing installed plugins (a.k.a. CREOLE directories). If this is not set then the default value of {gate.home}/plugins is used.
points to the location of the configuration file containing the site-wide options. If not set this will default to {gate.home}/gate.xml. The site configuration file must exist!
points to the file containing the user’s options. If not specified, or if the specified file does not exist at startup time, the default value of gate.xml (.gate.xml on Unix platforms) in the user’s home directory is used.
is a path-like structure, i.e. a list of URLs separated by ‘;’. All directories listed here will be loaded as CREOLE plugins during initialisation. This has similar functionality with the the -d command line option.
is a URL pointing to the location of GATE’s built-in CREOLE directory. This is the location of the creole.xml file that defines the fundamental GATE resource types, such as documents, document format handlers, controllers and the basic visual resources that make up GATE. The default points to a location inside gate.jar and should not generally need to be overridden.
As described above, the only property that is required is gate.home if you lay out other resources at their default locations.

In this article, we will show you one way to run your GATE application in Oracle WebLogic Server (WLS). This allows you to test your deployed application quickly.

Classloading in Java Platform and Oracle WebLogic Server

If the application you are creating has dependencies on some third-party code (for example, gate.jar), what is the proper way to package these libraries so that they can be used by a portable J2EE application?

In the J2EE platform, there are mechanisms[4] available for including libraries in a portable application:
  1. The WEB-INF/lib Directory
  2. Bundled Optional Classes
  3. Installed Packages (or installed optional packages mechanism)
Since these mechanisms are well-documented, they will not be repeated here.

To use these third-party libraries along with your application code, you face the decision of which packaging mechanism to choose. The decision you make can have major effects on the following:
  • The portability of your application
  • The size of your WAR and EAR files
  • The maintenance of the application
  • Version control as libraries and application servers are updated
Some solutions for packaging library JAR files are specific to a particular application server: for example, placing a library JAR file in an application server's classpath so that applications can use the APIs in that JAR file. Some application servers have container-specific locations where you can place JAR files to be shared by applications and modules. But these mechanisms are not portable, unlike the mechanisms provided by the J2EE platform.

In this article, we will introduce one WLS-specific mechanism to use for the GATE installation. This will allow you to quick-test your GATE application.

In WLS, you can place JAR files to be shared by applications and modules at the following location:
  • $DOMAIN_DIR/lib
This is the domain library directory. The domain library directory is one mechanism that can be used for adding application libraries to the server classpath. The jars located in this directory will be picked up and added dynamically to the end of the server classpath at server startup. The jars will be ordered lexically in the classpath.

It is possible to override the $DOMAIN_DIR/lib directory using the -Dweblogic.ext.dirs system property during startup. This property specifies a list of directories to pick up jars from and dynamically append to the end of the server classpath using as the delimiter between path entries.

Default GATE Installation Layout

The GATE architecture is based on components. Each component (i.e., a Java Beans), is a reusable chunks of software with well-defined interfaces that may be deployed in a variety of contexts.

You can define applications with processing pipelines using these reusable components. In GATE, these resources are officially named CREOLE (i.e., Collection of REusable Objects for Language Engineering). You can read this article to understand how GATE plugins and CREOLE resources are configured.

In the following, we show how GATE's resources are laid out in the WLS' domain library directory:
/wls_domain/lib/gatehome (i.e., GATE's home directory)
+-- lib/
+-- Bib2HTML.jar
+-- GnuGetOpt.jar
+-- ...
+-- plugins/
+-- ANNIE/
+-- ANNIE_with_defaults.gapp
+-- build.xml
+-- creole.xml
+-- resources/
+-- Tools/
+-- build.xml
+-- creole.xml
+-- doc/
+-- resources/
+-- src/
+-- tools.jar
+-- gate .xml

After you've installed GATE's libraries and resources in the domain library directory. The next step you need to do is setting gate.home property in wls_domain/bin/


Final Words

As mentioned before, this is not the best way to configure GATE's installation in a WLS. However, this approach will allow you to test your deployed GATE application quickly on it.

The domain library directory in WLS is intended for JAR files that change infrequently and are required by all or most applications deployed in the server, or by WebLogic Server itself. For example, you might use the lib directory to store third-party utility classes that are required by all deployments in a domain. You can also use it to apply patches to WebLogic Server.

The domain library directory is not recommended as a general-purpose method for sharing a JARs between one or two applications deployed in a domain, or for sharing JARs that need to be updated periodically. If you update a JAR in the lib directory, you must reboot all servers in the domain in order for applications to realize the change. If you need to share a JAR file or Java EE modules among several applications, use the Java EE libraries feature here. Alternatively, you can write custom class loaders to better fit your application's needs.

  1. Packaging Utility Classes or Library JAR Files in a Portable J2EE Application
  2. Understanding WebLogic Server Application Classloading
  3. Overview of WebLogic Server Application Classloading
  4. Mechanisms for Using Libraries in J2EE Applications
  5. Class Gate
  6. GATE Embedded
  7. Using System Properties with GATE
  8. GATE Plugins and CREOLE Resources

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