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XML Pipelining with Ant

January 28, 2003

Michael Fitzgerald

Ant is an extensible, open-source build tool written in Java and sponsored by Apache's Jakarta project. Ant has developed into something more than a just a build tool, however. It has gone beyond its predecessor make (and make's kin) to become a framework for performing an even larger variety of operations in a single step, not just compiling code or cleaning up after a build.

Ant's build files are written in XML, and Ant takes advantage of XML in a variety of ways. In my opinion, Ant is a suitable if not ideal framework for XML pipelining -- that is, a framework for performing a variety of XML processing, in the desired order and in one fell swoop. The reason why I say ideal is because Ant is open, somewhat mature, reasonably stable, readily available, widely known and used, easily extensible, and already amenable to XML processing. What else could you ask for?

In this article, I'll discuss the XML structures in an Ant build file, named build.xml by default, talk about some common XML-related tasks that Ant can perform, and then finish up with an example of XML pipelining.

I assume that you already know something of Ant and have probably used it. I plan to review the basics of the tool, but I also suggest that you read Tony Coates recent XML.com article ("Running Multiple XSLT Engines with Ant.") Along with an interesting approach to processing multiple XSLT stylesheets with multiple engines, Tony's article also provides good introductory material on Ant.

To get the examples in this piece to work, you'll of course need a recent version of Java on your system. You'll also need to download and install Ant version 1.5.1 (or later) binaries. Because you'll be using a new task that validates with RELAX NG schemas, you'll also need to download and install James Clark's Jing. All the example files discussed in this article are available for download in a ZIP archive and have been tested on the Windows XP Professional platform running Java 2 v1.4.

You can refer to Ant's HTML manual either online or, after installing Ant locally, by bringing up docs/manual/index.html in a browser.

Where Is Ant's DTD?

One of the first things I noticed about Ant was that it didn't have an explicit DTD available in the archives I downloaded, either the binary or source archive. I wanted to see Ant's DTD so I could figure out what went into a build file. Then I discovered the antstructure task. This task in essence extracts a DTD from Ant's source code.

The following snippet is a simple Ant build file that uses the antstructure task (build-dtd.xml in the example archive):

<?xml version="1.0"?>

<project default="dtd">
 <target name="dtd">
  <antstructure output="ant.dtd"/>
 </target>
</project>

Here's a quick review of some basics. The document begins with an optional XML declaration. The root element of an Ant build file is <project>. It has several possible attributes, but only one is required: default. This attribute names the default target for the project, and in this case the only target, dtd. A target represents a way to achieve an expected outcome from an operation, such as a set of compiled Java classes or, in the case of antstructure, a DTD.

The <target> element is a child of <project> and must have a name attribute. The value of this attribute matches the value of the default attribute of <project>. When there is more than one target in a build file, the value of default only matches the value of one name attribute in one <target>. The <target> element also has several other attributes such as depends (which will come to light in later examples).

The <antstructure> task element is empty. One of four possible attributes is output which gives the name of the output file that will contain the DTD that the task produces. This output file is written to the current directory by default; however, if you add a basedir attribute to <project>, you can specify a different output directory than the current one as a value of basedir, such as:

<project default="dtd" basedir="c:/temp">

Now give it a try. The following command presupposes that Ant's bin directory is in the path environment variable, that your working directory is C:\Java\Ant, and that you have unzipped the example archive there:

C:\Java\Ant>ant -f build-dtd.xml

Ant assumes that the build file is named build.xml. If it isn't, you need to use the -f option (or the synonyms -file or -buildfile), followed by a filename. You should see output from this command like this:

Buildfile: build-dtd.xml

dtd:

BUILD SUCCESSFUL
Total time: 2 seconds

The output lists the build filename, the target name dtd, and whether the build was successful. The target produces the file ant.dtd in your current directory. This DTD is straightforward (only three parameter entities), but is quite long (nearly 4000 lines). With this DTD available now, you can see for yourself how a build file is put together. For any element name in the DTD, you are likely to find a corresponding entry in the Ant manual.

At first I wondered how Ant validates build files. The answer lies in the source code, where it is clear that Ant validates build files in its own application-specific, rather than in a general-purpose way. (If you want to see how Ant does this, a good place to start looking is in the Java source of the class org.apache.tools.ant.helper.ProjectHelperImpl.) Ant is in effect self-validating and avoids the use of namespaces.

Validating an XML Document

Ant has a task for validating XML documents called xmlvalidate. By default Ant validates with Xerces version 2.2.0. Consider the small XML document date.xml:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE date SYSTEM "date.dtd">

<date>2003-01-31T00:00:01</date>

And its equally small DTD date.dtd:

<!ELEMENT date (#PCDATA)>

You can validate date.xml with the build file build-valid.xml by using the xmlvalidate task:

<?xml version="1.0"?>

<project default="valid">
 <target name="valid">
  <xmlvalidate file="date.xml"/>
 </target>
</project>

The attribute file specifies the document to validate. Issuing the command

C:\Java\Ant>ant -f build-valid.xml

produces the following output, if successful:

Buildfile: build-valid.xml

valid:
[xmlvalidate] 1 file(s) have been successfully validated.

BUILD SUCCESSFUL
Total time: 2 seconds

In Ant, types are elements that can help performs tasks, such as on groups of files. Using the fileset type as a child of xmlvalidate, you can validate a series of XML documents, as shown in build-fileset.xml:

<?xml version="1.0"?>

<project default="valid">
 <target name="valid">
  <xmlvalidate>
   <fileset file="date*.xml"/>
  </xmlvalidate>
 </target>
</project>

The file attribute of fileset allows you to specify a series of files with wildcards. If you run this build file, you will see that Ant validates six XML documents in one step (all XML documents in the current directory beginning with the name date).

The xmlvalidate task has several other features worth mentioning:

  • An attribute of lenient="true" means that the task will only do well-formedness checking.
  • The classname and classpathref attributes allow you to specify a different XML parser than the default and where to find it.
  • The child element <dtd> lets you indicate a formal public identifier (publicId) attribute as well as the local whereabouts (location attribute) of a DTD.

Validating with Jing

As I mentioned earlier, Ant is extensible. One way that you can extend Ant is by writing your own task ( instructions on how to do this are found in the Ant manual). James Clark has written a task for Jing that allows you to use Ant to validate XML documents against RELAX NG schemas, in both XML and compact syntaxes. Jing's source code is available for download, but for convenience I have included a copy of JingTask.java in the example archive for easy inspection (along with a copy of Jing's license).

The document date.xml is valid with regard to the RELAX NG schema date.rng:

<?xml version="1.0"?>

<element name="date" xmlns="http://relaxng.org/ns/structure/1.0"
 datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
 <data type="dateTime"/>
</element>

RELAX NG supports externally defined datatype libraries, such as W3C XML Schema datatypes. The XML Schema datatype dataTime more precisely defines the valid content of <date> than just #PCDATA in a DTD. To validate date.xml against date.rng with Ant, use the build file build-jing.xml:

<?xml version="1.0"?>

<project default="rng">

 <taskdef name="jing" classname="com.thaiopensource.relaxng.util.JingTask"/>

 <target name="rng">
  <echo message="Validating RELAX NG schema with Jing..."/>
  <jing rngfile="date.rng" file="date.xml"/>
 </target>

</project>

The <taskdef> element defines the jing task, and its classname attribute identifies the class that executes the task. This class is stored in jing.jar, part of the Jing distribution. If you place jing.jar in Ant's lib directory, Ant will be able to find the Jing task.

The echo task echoes the text in message. Jing is silent upon success, as are other tasks. You can throw in an echo task to augment what is normally reported.

The jing task's rngfile identifies a RELAX NG schema, and the file attribute names the instance of the schema. You can also use a fileset type as a child of <jing>, allowing you to validate more than one document at a time.

Jing can also validate against schemas in the compact syntax, RELAX NG's terse, non-XML format. The compact version reduces date.rng to one short line in date.rnc:

element date { xsd:dateTime }

Compact syntax processors automatically declare the XML Schema datatype library with the xsd prefix. The build file build-rnc.xml validates date.xml against date.rnc (note the addition of the compactsyntax attribute):

<?xml version="1.0"?>

<project default="rng">

 <taskdef name="jing" classname="com.thaiopensource.relaxng.util.JingTask"/>

 <target name="rng">
  <echo message="Validating RELAX NG compact syntax schema with Jing..."/>
  <jing compactsyntax="true" rngfile="date.rnc" file="date.xml"/>
 </target>

</project>

Kawaguchi Kohsuke is currently developing an Ant task for validators that support the Java API for Relax Verifiers (JARV). This task will work with Sun's Multi-schema Validator and other JARV validators.

An XML Pipeline Example

This example places targets discussed earlier together into a single build file and adds a few other targets as well. The resulting file, build.xml, is an example of a simple XML pipeline. The basic scenario is that a property is set (the current directory) using a local XML document (properties.xml) and a remote, zipped file (date.zip) is downloaded via the get task. The file, which contains a RELAX NG schema (date.rng), is unzipped and a local document (date.xml) is validated against it. Then the same document is validated against a DTD (date.dtd) and transformed into an HTML document (date.html). Finally, an e-mail is sent, signaling the completion of the process. Granted, this is a rather uncomplicated example, and more complex operations are possible, but this gives you an idea of how you can put your own pipeline together.

Here is the build file:

<?xml version="1.0"?>

<project default="mail">

 <taskdef name="jing" classname="com.thaiopensource.relaxng.util.JingTask"/>

 <target name="init">
  <echo message="Load XML properties..."/>
  <xmlproperty file="properties.xml"/>
 </target>

 <target name="get" depends="init">
  <get src="http://www.wyeast.net/date.zip" dest="date.zip"/>
 </target>

 <target name="unzip" depends="get">
  <unzip src="date.zip" dest="${build.dir}"/>
 </target>

 <target name="rng" depends="unzip">
  <echo message="Jing validating..."/>
  <jing rngfile="date.rng" file="date.xml"/>
 </target>

 <target name="val" depends="rng">
  <xmlvalidate file="date.xml">
   <xmlcatalog>
    <dtd publicId="-//Wy'east Communications//Date DTD//EN"
    location="date.dtd"/>
   </xmlcatalog>
  </xmlvalidate>
 </target>

 <target name="xform" depends="val">
  <xslt in="date.xml" out="date.html"
      style="date.xsl">
   <outputproperty name="method" value="xml"/>
   <outputproperty name="indent" value="yes"/>
  </xslt>
 </target>

 <target name="mail" depends="xform">
  <mail mailhost="mail.example.com" subject="Ant build">
   <to address="schlomo@example.com"/>
   <from address="hermes@example.com"/>
   <message>Complete!</message>
  </mail>
 </target>

</project>

Before running this example, you should change the values of mailhost and both the to and from addresses to something that will work on your own mail server. You will also need to install the JAR files from the JavaMail project in Ant's lib directory (though MIME mail may still not work). To run the build, all you have to do is type:

C:\Java\Ant>ant

Because the build file is named build.xml, Ant automatically picks it up and runs it. The output will look like this, provided you have a live Internet connection (for the get and mail targets), and all files from the example archive are still in place:

Buildfile: build.xml

init:
     [echo] Load XML properties...

get:
      [get] Getting: http://www.wyeast.net/date.zip

unzip:
    [unzip] Expanding: C:\Java\Ant\date.zip into C:\Java\Ant

rng:
     [echo] Jing validating...

val:
[xmlvalidate] 1 file(s) have been successfully validated.

xform:
     [xslt] Processing C:\Java\Ant\date.xml to C:\Java\Ant\date.html
     [xslt] Loading stylesheet C:\Java\Ant\date.xsl

mail:
     [mail] Failed to initialise MIME mail
     [mail] Sending email: Ant build
     [mail] Sent email with 0 attachments

BUILD SUCCESSFUL
Total time: 7 seconds

Each of the targets except the one named init has a depends attribute. The value of this attribute establishes a hierarchy of dependencies between the targets. The default or starting target is mail (identified in the <project> element); in order for it to execute, the xform target must first execute successfully and in order for xform to execute, val must execute, and so forth. So this dependency is not established structurally, as through a parent-child relationship, but rather through attribute values. You can put the targets in any order in the build file. They will be still execute according to the order of the values in the depends and name attributes. These dependencies make up the segments of the pipeline.

The build file has an xslt target that transforms date.xml into date.html according to the XSLT stylesheet date.xsl. The <outputproperty> children contribute values that would normally be supplied by the output element of XSLT. (Tony Coates' article deals with the xslt target extensively, so I'll limit my comments here.)

The xmlvalidate target uses the xmlcatalog type with a <dtd> child to specify a formal public identifier for a DTD and the location of a local copy of that DTD. This type is based on the XML Catalog specification, an entity and URI resolution initiative from OASIS.

The get target gets a URL source, downloading it to a specified location. The xmlproperty target reads the file properties.xml:

<?xml version="1.0"?>

<build>
 <dir>.</dir>
</build>

The arbitrary tags in the properties file determine the name or names for the variable that you can use elsewhere in the build file to reference values, such as ${build.dir}. The first part of the variable name comes from the <build> tag and the second part from <dir>. The content of <dir> becomes the value of the variable. You can also use attributes to create property names.

Running XmlLogger

Ant provides logging and event listening facilities. One such logger-listener is defined in the class org.apache.tools.ant.XmlLogger, which produces XML output. The following command line puts the XML logger to work:

C:\Java\Ant>ant -logger org.apache.tools.ant.XmlLogger -v -l log.xml

The -v (or -verbose) option indicates verbose output, all of which is sent to the log file; the -l option (or -logfile) provides a name for the log file. You can find an XSLT stylesheet for log files in the etc directory called log.xsl. The following figure shows you how log.xml will appear in a browser after it has been transformed by log.xsl.

log.xml
log.xml after being transformed by log.xsl

Conclusion

I realize that Ant was not intended to be a an XML pipeline tool, but it turns out to be a pretty good one anyway. Other tools exist and may eventually do a better job, such as Sean McGrath's XPipes or Eric van der Vlist's XML Validation Interoperability Framework (XVIF). For now, though, Ant remains an attractive option. Like XML, Ant can do things that perhaps it was not originally intended to do. That's a good sign.