| 1 | <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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| 2 | <html><head><title>XmlNamespaceSupport</title>
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| 3 | <link rel="stylesheet" type="text/css" href="../stylesheets/antmanual.css">
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| 4 | </head>
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| 5 | <body>
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| 6 | <h2><a name="namespace">XML Namespace Support</a></h2>
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| 7 | Ant 1.6 introduces support for XML namespaces.
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| 8 | <h3>History</h3>
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| 9 |
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| 10 | <p>
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| 11 | All releases of Ant prior to Ant 1.6 do not support XML namespaces.
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| 12 | No support basically implies two things here:
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| 13 | </p>
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| 14 | <ul>
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| 15 | <li> Element names correspond to the "qname" of the tags, which is
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| 16 | usually the same as the local name. But if the build file writer uses
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| 17 | colons in names of defined tasks/types, those become part of the
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| 18 | element name. Turning on namespace support gives colon-separated
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| 19 | prefixes in tag names a special meaning, and thus build files using
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| 20 | colons in user-defined tasks and types will break.
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| 21 | </li>
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| 22 | <li> Attributes with the names 'xmlns' and 'xmlns:<code><prefix></code>'
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| 23 | are not treated specially, which means that custom tasks and types have
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| 24 | actually been able to use such attributes as parameter names. Again,
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| 25 | such tasks/types are going to break when namespace support is enabled
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| 26 | on the parser.
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| 27 | </li>
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| 28 | </ul>
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| 29 | <p>Use of colons in element names has been discouraged in the past
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| 30 | IIRC, and using any attribute starting with "xml" is actually strongly
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| 31 | discouraged by the XML spec to reserve such names for future use.
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| 32 | </p>
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| 33 | <h3>Motivation</h3>
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| 34 |
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| 35 | <p>In build files using a lot of custom and third-party tasks, it is
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| 36 | easy to get into name conflicts. When individual types are defined, the
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| 37 | build file writer can do some name-spacing manually (for example, using
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| 38 | "tomcat-deploy" instead of just "deploy"). But when defining whole
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| 39 | libraries of types using the <code><typedef></code> 'resource' attribute, the
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| 40 | build file writer has no chance to override or even prefix the names
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| 41 | supplied by the library. </p>
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| 42 | <h3>Assigning Namespaces</h3>
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| 43 |
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| 44 | <p>
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| 45 | Adding a 'prefix' attribute to <code><typedef></code> might have been enough,
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| 46 | but XML already has a well-known method for name-spacing. Thus, instead
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| 47 | of adding a 'prefix' attribute, the <code><typedef></code> and <code><taskdef></code>
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| 48 | tasks get a 'uri' attribute, which stores the URI of the XML namespace
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| 49 | with which the type should be associated:
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| 50 | </p><pre> <typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/>
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| 51 | <my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 52 | ...
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| 53 | </my:task>
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| 54 | </pre>
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| 55 | <p>As the above example demonstrates, the namespace URI needs to be
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| 56 | specified at least twice: one time as the value of the 'uri' attribute,
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| 57 | and another time to actually map the namespace to occurrences of
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| 58 | elements from that namespace, by using the 'xmlns' attribute. This
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| 59 | mapping can happen at any level in the build file:
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| 60 | </p><pre> <project name="test" xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 61 | <typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/>
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| 62 | <my:task>
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| 63 | ...
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| 64 | </my:task>
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| 65 | </project>
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| 66 | </pre>
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| 67 | <p>
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| 68 | Use of a namespace prefix is of course optional. Therefore
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| 69 | the example could also look like this:
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| 70 | </p><pre> <project name="test">
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| 71 | <typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/>
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| 72 | <task xmlns="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 73 | ...
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| 74 | </task>
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| 75 | </project>
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| 76 | </pre>
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| 77 | <p>
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| 78 | Here, the namespace is set as the default namespace for the <code><task></code>
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| 79 | element and all its descendants.
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| 80 | </p>
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| 81 | <h3>Default namespace</h3>
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| 82 | <p>
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| 83 | The default namespace used by Ant is "antlib:org.apache.tools.ant".
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| 84 | </p>
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| 85 | <pre>
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| 86 | <typedef resource="org/example/tasks.properties" uri="antlib:org.apache.tools.ant"/>
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| 87 | <task>
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| 88 | ....
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| 89 | </task>
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| 90 | </pre>
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| 91 |
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| 92 |
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| 93 |
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| 94 | <h3>Namespaces and Nested Elements</h3>
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| 95 |
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| 96 | <p>
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| 97 | Almost always in Ant 1.6, elements nested inside a namespaced
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| 98 | element have the same namespace as their parent. So if 'task' in the
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| 99 | example above allowed a nested 'config' element, the build file snippet
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| 100 | would look like this:
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| 101 | </p><pre> <typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/>
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| 102 | <my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 103 | <my:config a="foo" b="bar"/>
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| 104 | ...
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| 105 | </my:task>
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| 106 | </pre>
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| 107 | <p>If the element allows or requires a lot of nested elements, the
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| 108 | prefix needs to be used for every nested element. Making the namespace
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| 109 | the default can reduce the verbosity of the script:
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| 110 | </p><pre> <typedef resource="org/example/tasks.properties" uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/>
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| 111 | <task xmlns="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 112 | <config a="foo" b="bar"/>
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| 113 | ...
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| 114 | </task>
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| 115 | </pre>
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| 116 | <p>
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| 117 | From Ant 1.6.2, elements nested inside a namespaced element may also be
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| 118 | in Ant's default namespace. This means that the following is now allowed:
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| 119 | </p>
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| 120 | </p><pre> <typedef resource="org/example/tasks.properties"
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| 121 | uri="<a href="http://example.org/tasks">http://example.org/tasks</a>"/>
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| 122 | <my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 123 | <config a="foo" b="bar"/>
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| 124 | ...
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| 125 | </my:task>
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| 126 | </pre>
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| 127 |
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| 128 | <h3>Namespaces and Attributes</h3>
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| 129 |
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| 130 | <p>
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| 131 | Attributes are only used to configure the element they belong to if:
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| 132 | </p>
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| 133 | <ul>
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| 134 | <li> they have no namespace (note that the default namespace does *not* apply to attributes)
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| 135 | </li>
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| 136 | <li> they are in the same namespace as the element they belong to
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| 137 | </li>
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| 138 | </ul>
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| 139 | <p>
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| 140 | Other attributes are simply ignored.
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| 141 | </p>
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| 142 | <p>
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| 143 | This means that both:
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| 144 | </p>
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| 145 | <p>
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| 146 | </p><pre> <my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 147 | <my:config a="foo" b="bar"/>
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| 148 | ...
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| 149 | </my:task>
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| 150 | </pre>
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| 151 | <p>
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| 152 | and
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| 153 | </p>
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| 154 | <pre> <my:task xmlns:my="<a href="http://example.org/tasks">http://example.org/tasks</a>">
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| 155 | <my:config my:a="foo" my:b="bar"/>
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| 156 | ...
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| 157 | </my:task>
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| 158 | </pre>
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| 159 | <p>
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| 160 | result in the parameters "a" and "b" being used as parameters to configure the nested "config" element.
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| 161 | </p>
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| 162 | <p>It also means that you can use attributes from other namespaces
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| 163 | to markup the build file with extra meta data, such as RDF and
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| 164 | XML-Schema (whether that's a good thing or not). The same is not true
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| 165 | for elements from unknown namespaces, which result in a error.
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| 166 | </p>
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| 167 | <h3>Mixing Elements from Different Namespaces</h3>
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| 168 |
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| 169 | <p>Now comes the difficult part: elements from different namespaces can
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| 170 | be woven together under certain circumstances. This has a lot to do
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| 171 | with the Ant 1.6
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| 172 | <a href="../develop.html#nestedtype">add type introspection rules</a>:
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| 173 | Ant types and tasks are now free to accept arbritrary named types as
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| 174 | nested elements, as long as the concrete type implements the interface
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| 175 | expected by the task/type. The most obvioius example for this is the
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| 176 | <code><condition></code> task, which supports various nested conditions, all
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| 177 | of which extend the interface <tt>Condition</tt>. To integrate a
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| 178 | custom condition in Ant, you can now simply <code><typedef></code> the
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| 179 | condition, and then use it anywhere develop.html#nestedtypwhere conditions are allowed
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| 180 | (assuming the containing element has a generic <tt>add(Condition)</tt> or <tt>addConfigured(Configured)</tt>method):
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| 181 | </p><pre> <typedef resource="org/example/conditions.properties" uri="<a href="http://example.org/conditions">http://example.org/conditions</a>"/>
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| 182 | <condition property="prop" xmlns="<a href="http://example.org/conditions">http://example.org/conditions</a>">
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| 183 | <and>
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| 184 | <available file="bla.txt"/>
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| 185 | <my:condition a="foo"/>
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| 186 | </and>
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| 187 | </condition>
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| 188 | </pre>
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| 189 | <p>
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| 190 | In Ant 1.6, this feature cannot be used as much as we'd all like to: a
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| 191 | lot of code has not yet been adapted to the new introspection rules,
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| 192 | and elements like the builtin Ant conditions and selectors are not
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| 193 | really types in 1.6. This is expected to change in Ant 1.7.
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| 194 | </p>
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| 195 | <h3>Namespaces and Antlib</h3>
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| 196 |
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| 197 | <p>
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| 198 | The new <a href="antlib.html">AntLib</a>
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| 199 | feature is also very much integrated with the namespace support in Ant
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| 200 | 1.6. Basically, you can "import" Antlibs simply by using a special
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| 201 | scheme for the namespace URI: the <tt>antlib</tt> scheme, which expects the package name in which a special <tt>antlib.xml</tt> file is located.
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| 202 | </p>
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| 203 | <hr>
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| 204 | <p align="center">Copyright © 2003-2004 The Apache Software
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| 205 | Foundation. All rights Reserved.</p>
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| 206 |
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| 207 | </body>
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| 208 | </html>
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