Repository

This article is part of the XQuery Portal. It describes how external XQuery packages can be installed in the system's repository and used from XQuery, and how new packages can be built.

Motivation

One of the reasons why languages such as Java or Perl have been so successful is the vast amount of libraries that are available to developers. XQuery is a Turing complete language, but it just provides around 100 pre-defined functions, which cannot live up to all expectations. This is why more and more libraries arise – such as FunctX – that extend the language with new features.

BaseX offers two mechanisms to make new packages accessible to the XQuery processor:

• With Version 7.2.1, we offer a simple packaging mechanism to directly install XQuery archives and JAR files to the repository.

• The EXPath Packaging system provides a generic mechanism for extending XQuery processors with new packages.

A package is defined as a .xar archive, which encapsulates one or more extension libraries.

Usage

All packages are stored in the package repository. The repository is a directory named BaseXRepo or repo, which resides in your home directory.

BaseX provides three commands for interaction with the package repository – REPO INSTALL, REPO DELETE and REPO LIST. The syntax of these commands is described in Commands. Since Template:Mark, a Repository Module exists, which can also be used to manage packages via XQuery.

Here we give simple examples of their usage and the usage of a package after it is installed.

Installation

A package can be installed using the REPO INSTALL command. The path to the package has to be given as a parameter. An example:

REPO INSTALL http://files.basex.org/modules/functx-1.0.xar

The installation will only succeed if the package fully conforms to the specification. If you know that your package is valid, you may as well unzip and copy its files directly to the repository, or edit its contents without reinstalling the package.

Querying

Installed packages can be addressed by importing them as modules. Since we have the package repository in which all packages are located, it is sufficient to just specify the namespace of a module:

import module namespace functx = "http://www.functx.com";

When this statement is parsed, the query processor will check if the namespace "http://www.functx.com" is used in any of the installed packages and, if yes, will load and parse the modules. In the remaining query, you can call the parsed module functions in the standard way, e.g.:

functx:capitalize-first("test")

Package encapsulating Java archives can be imported in the same way as pure XQuery modules (see below).

Listing

All currently installed packages can be listed with the REPO LIST command. It will return the names of all packages, their version, and the directory in which they are installed:

URI                    Version  Directory
-------------------------------------------------------
http://www.functx.com  1.0      http-www.functx.com-1.0

1 package(s).

Removal

A package can be deleted with the command REPO DELETE and by specifying either its name or the name of its directory:

REPO DELETE http://www.functx.com  ...or...
REPO DELETE functx-1.0

Simple Packaging

Template:Mark, simple JAR archives can be treated and installed as XQuery modules. The article on Java Bindings gives more insight on how Java code is handled in XQuery. The following steps are to be observed when preparing a JAR archive:

• The installed JAR archive must contain a manifest (META-INF/MANIFEST.MF) with a Main-Class: entry, which will be bound to the namespace.
• All public functions of this class can then be addressed from XQuery.
• The repository path to the JAR file can be directly used as namespace URI.

EXPath Packaging

The EXPath specification defines how the structure of a .xar archive shall look like. The package contains at its root a package descriptor named expath-pkg.xml. This descriptor presents some meta data about the package as well as the libraries which it contains and their dependencies on other libraries or processors.

XQuery

Apart from the package descriptor, a .xar archive contains a directory which includes the actual XQuery modules. For example, the FunctX XQuery Library is packaged as follows:

expath-pkg.xml
functx/
  functx.xql
  functx.xsl

Java

In case you want to extend BaseX with a Java archive, some additional requirements have to be fulfilled:

• Apart from the package descriptor expath-pkg.xml, the package has to contain a descriptor file at its root, defining the included jars and the binary names of their public classes. It must be named basex.xml and must conform to the following structure:

<package xmlns="http://expath.org/ns/pkg">
  <jar>...</jar>
    ....
    <class>...</class>
    <class>...</class>
    ....
</package>

• The jar file itself along with an XQuery file defining wrapper functions around the java methods has to reside in the module directory. The following example illustrates how java methods are wrapped with XQuery functions:

Example:
Suppose we have a simple class Printer having just one public method print():

package test;

public final class Printer {
  public String print(final String s) {
    return new Writer(s).write();
  }
}

We want to extend BaseX with this class and use its method. In order to make this possible we have to define an XQuery function which wraps the print method of our class. This can be done in the following way:

import module namespace j="http://basex.org/lib/testJar";

declare namespace p="java:test.Printer";

declare function j:print($str as xs:string) as xs:string {
  let $printer := p:new()
  return p:print($printer, $str)
};

As it can be seen, the class Printer is declared with its binary name as a namespace prefixed with "java" and the XQuery function is implemented using the Java Bindings offered by BaseX.

On our file server, you can find some example libraries packaged as XML archives (xar files). You can use them to try our packaging API or just as a reference for creating your own packages.

Changelog

Version 7.2.1

• Added: simple packaging