wangwei
/
antliscence
forked from youys/ant

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  <title>Properties and PropertyHelpers</title>
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<body>
  <h1>Properties</h1>

  <p>Properties are key-value pairs where Apache Ant tries to
    expand <code>${key}</code> to <code>value</code> at runtime.</p>

  <p>There are many tasks that can set properties, the most common one
    is the <a href="Tasks/property.html">property</a> task.  In
    addition properties can be defined
    via <a href="running.html">command line arguments</a> or similar
    mechanisms from outside of Ant.</p>

  <p>Normally property values can not be changed, once a property is
    set, most tasks will not allow its value to be modified.  In
    general properties are of global scope, i.e. once they have been
    defined they are available for any task or target invoked
    subsequently&mdash;it is not possible to set a property in a child
    build process created via
    the <a href="Tasks/ant.html">ant</a>, <a href="Tasks/antcall.html">antcall</a>
    or <a href="Tasks/subant.html">subant</a> tasks and make it
    available to the calling build process, though.</p>

  <p><em>Since Ant 1.8.0</em> the <a href="Tasks/local.html">local</a>
    task can be used to create properties that are locally scoped to a
    target or a <a href="Tasks/sequential.html">sequential</a> element
    like the one of the <a href="Tasks/macrodef.html">macrodef</a>
    task.</p>

  <h2 id="built-in-props">Built-in Properties</h2>

  <p>Ant provides access to all system properties as if they had been
    defined using a <code>&lt;property&gt;</code> task.  For
    example, <samp>${os.name}</samp> expands to the name of the
    operating system.</p>

  <p>For a list of system properties,
    see <a href="https://docs.oracle.com/javase/8/docs/api/java/lang/System.html#getProperties--">the javadoc of System.getProperties</a>.
  </p>

  <p>In addition, Ant has some built-in properties:</p>
  <dl>
    <dt><code>basedir</code></dt>
    <dd>the absolute path of the project's basedir (as set
      with the <var>basedir</var> attribute of <a href="using.html#projects">&lt;project&gt;</a>).</dd>
    <dt><code>ant.file</code></dt>
    <dd>the absolute path of the buildfile.</dd>
    <dt><code>ant.version</code></dt>
    <dd>the version of Ant</dd>
    <dt><code>ant.project.name</code></dt>
    <dd>the name of the project that is currently executing; it is set
      in the <var>name</var> attribute of &lt;project&gt;.</dd>
    <dt><code>ant.project.default-target</code></dt>
    <dd>the name of the currently executing project's default target;
      it is set via the <var>default</var> attribute
      of <code>&lt;project&gt;</code>.</dd>
    <dt><code>ant.project.invoked-targets</code></dt>
    <dd>a comma separated list of the targets that have been specified
      on the command line (the IDE, an <code>&lt;ant&gt;</code> task
      ...) when invoking the current project.<br/>
      This property is set properly when the first target is executed.
      If you use it in the implicit target (directly under
      the <code>&lt;project&gt;</code> tag) the list will be empty if
      no target has been specified while it will contain the project's
      default target in this case for tasks nested into targets.</dd>
    <dt><code>ant.java.version</code></dt>
    <dd>the JVM version Ant detected; currently it can hold the
      values <q>9</q>, <q>1.8</q>, <q>1.7</q>, <q>1.6</q>, <q>1.5</q>, <q>1.4</q>, <q>1.3</q>
      and <q>1.2</q>.</dd>
    <dt><code>ant.core.lib</code></dt>
    <dd>the absolute path of the <samp>ant.jar</samp> file.</dd>
  </dl>

  <p>There is also another property, but this is set by the launcher
    script and therefore maybe not set inside IDEs:</p>
  <dl>
    <dt><code>ant.home</code></dt>
    <dd>home directory of Ant</dd>
  </dl>

  <p>The following property is only set if Ant is started via the
    Launcher class (which means it may not be set inside IDEs
    either):</p>
  <dl>
    <dt><code>ant.library.dir</code></dt>
    <dd>the directory that has been used to load Ant's
      jars from.  In most cases this is <samp>ANT_HOME/lib</samp>.</dd>
  </dl>

  <h1 id="propertyHelper">PropertyHelpers</h1>

  <p>Ant's property handling is accomplished by an instance
    of <code>org.apache.tools.ant.PropertyHelper</code> associated
    with the current Project.  You can learn more about this class by
    examining Ant's Java API. In Ant 1.8 the PropertyHelper class was
    much reworked and now itself employs a number of helper classes
    (actually instances of
    the <code>org.apache.tools.ant.PropertyHelper$Delegate</code>
    marker interface) to take care of discrete tasks such as property
    setting, retrieval, parsing, etc. This makes Ant's property
    handling highly extensible; also of interest is the
    new <a href="Tasks/propertyhelper.html">propertyhelper</a> task
    used to manipulate the PropertyHelper and its delegates from the
    context of the Ant buildfile.</p>

  <p>There are three sub-interfaces of <code>Delegate</code> that may be
    useful to implement.</p>

  <ul>
    <li><code>org.apache.tools.ant.property.PropertyExpander</code> is
      responsible for finding the property name inside a string in the
      first place (the default extracts <samp>foo</samp>
      from <samp>${foo}</samp>).

      <p>This is the interface you'd implement if you wanted to invent
        your own property syntax&mdash;or allow nested property expansions
        since the default implementation doesn't balance braces
        (see <a href="https://git-wip-us.apache.org/repos/asf?p=ant-antlibs-props.git;a=blob;f=src/main/org/apache/ant/props/NestedPropertyExpander.java;hb=HEAD"><code>NestedPropertyExpander</code>
        in the <samp>props</samp> Antlib</a> for an example).</p>
    </li>

    <li><code>org.apache.tools.ant.PropertyHelper$PropertyEvaluator</code>
      is used to expand <samp>${some-string}</samp> into
      an <code>Object</code>.

      <p>This is the interface you'd implement if you want to provide
        your own storage independent of Ant's project instance&mdash;the
        interface represents the reading end.  An example for this
        would
        be <code>org.apache.tools.ant.property.LocalProperties</code>
        which implements storage
        for <a href="Tasks/local.html">local properties</a>.</p>

      <p>Another reason to implement this interface is if you wanted
        to provide your own "property protocol" like
        expanding <code>toString:foo</code> by looking up the project
        reference <samp>foo</samp> and invoking <code>toString()</code> on it
        (which is already implemented in Ant, see below).</p>
    </li>

    <li><code>org.apache.tools.ant.PropertyHelper$PropertySetter</code>
      is responsible for setting properties.

      <p>This is the interface you'd implement if you want to provide
        your own storage independent of Ant's project instance&mdash;the
        interface represents the reading end.  An example for this
        would
        be <code>org.apache.tools.ant.property.LocalProperties</code>
        which implements storage
        for <a href="Tasks/local.html">local properties</a>.</p>
    </li>

  </ul>

  <p>The default <code>PropertyExpander</code> looks similar to:</p>

<pre>
public class DefaultExpander implements PropertyExpander {
    public String parsePropertyName(String s, ParsePosition pos,
                                    ParseNextProperty notUsed) {
        int index = pos.getIndex();
        if (s.indexOf("${", index) == index) {
            int end = s.indexOf('}', index);
            if (end < 0) {
                throw new BuildException("Syntax error in property: " + s);
            }
            int start = index + 2;
            pos.setIndex(end + 1);
            return s.substring(start, end);
        }
        return null;
    }
}</pre>

  <p>The logic that replaces <samp>${toString:<i>some-id</i>}</samp> with the
    stringified representation of the object with
    <var>id</var> <samp>some-id</samp> inside the current build is contained in a
    PropertyEvaluator similar to the following code:</p>

<pre>
public class ToStringEvaluator implements PropertyHelper.PropertyEvaluator {
    private static final String prefix = "toString:";
    public Object evaluate(String property, PropertyHelper propertyHelper) {
        Object o = null;
        if (property.startsWith(prefix) && propertyHelper.getProject() != null) {
            o = propertyHelper.getProject().getReference(
                    property.substring(prefix.length()));
        }
        return o == null ? null : o.toString();
    }
}</pre>

  <h1>Property Expansion</h1>

  <p>When Ant encounters a construct <samp>${some-text}</samp> the
    exact parsing semantics are subject to the configured property
    helper delegates.</p>

  <h2><code>$$</code> Expansion</h2>

  <p>In its default configuration Ant will expand the text <q>$$</q>
    to a single <q>$</q> and suppress the normal property expansion
    mechanism for the text immediately following it,
    i.e. <samp>$${key}</samp> expands to <samp>${key}</samp> and
    not <code>value</code> even though a property
    named <code>key</code> was defined and had the
    value <samp>value</samp>.  This can be used to escape
    literal <q>$</q> characters and is useful in constructs that only
    look like property expansions or when you want to provide
    diagnostic output like in</p>

<pre>&lt;echo&gt;$${builddir}=${builddir}&lt;/echo&gt;</pre>

  <p>which will echo this message:</p>

<pre>${builddir}=build/classes</pre>

  <p>if the property <code>builddir</code> has the
    value <samp>build/classes</samp>.</p>

  <p>In order to maintain backward compatibility with older Ant
    releases, a single <q>$</q> character encountered apart from a
    property-like construct (including a matched pair of french
    braces) will be interpreted literally; that is, as <q>$</q>.  The
    "correct" way to specify this literal character, however, is by
    using the escaping mechanism unconditionally, so that <q>$$</q> is
    obtained by specifying <q>$$$$</q>.  Mixing the two approaches
    yields unpredictable results, as <q>$$$</q> results
    in <q>$$</q>.</p>

  <h2>Nesting of Braces</h2>

  <p>In its default configuration Ant will not try to balance braces
    in property expansions, it will only consume the text up to the
    first closing brace when creating a property name.  I.e. when
    expanding something like <samp>${a${b}}</samp> it will be
    translated into two parts:</p>

  <ol>
    <li>the expansion of property <samp>a${b</samp>&mdash;likely nothing
      useful.</li>
    <li>the literal text <samp>}</samp> resulting from the second
      closing brace</li>
  </ol>

  <p>This means you can't use easily expand properties whose names are
    given by properties, but there
    are <a href="https://ant.apache.org/faq.html#propertyvalue-as-name-for-property">some
    workarounds</a> for older versions of Ant.  With Ant 1.8.0 and the
    <a href="https://ant.apache.org/antlib/props/">the props Antlib</a>
    you can configure Ant to use
    the <code>NestedPropertyExpander</code> defined there if you need
    such a feature.</p>

  <h2>Expanding a "Property Name"</h2>

  <p>In its most simple form <samp>${key}</samp> is supposed to look
    up a property named <code>key</code> and expand to the value of
    the property.  Additional <code>PropertyEvaluator</code>s may
    result in a different interpretation of <code>key</code>,
    though.</p>

  <p>The <a href="https://ant.apache.org/antlibs/props/">props Antlib</a>
    provides a few interesting evaluators but there are
    also a few built-in ones.</p>

  <h3 id="toString">Getting the value of a Reference with
    <samp>${toString:}</samp></h3>

  <p>Any Ant type which has been declared with a reference can also
    its string value extracted by using the <samp>${toString:}</samp>
    operation, with the name of the reference listed after
    the <code>toString:</code> text.  The <code>toString()</code>
    method of the Java class instance that is referenced is
    invoked&mdash;all built in types strive to produce useful and
    relevant output in such an instance.</p>

  <p>For example, here is how to get a listing of the files in a fileset,<p>

<pre>
&lt;fileset id=&quot;sourcefiles&quot; dir=&quot;src&quot; includes=&quot;**/*.java&quot;/&gt;
&lt;echo&gt; sourcefiles = ${toString:sourcefiles} &lt;/echo&gt;</pre>

  <p>There is no guarantee that external types provide meaningful
    information in such a situation</p>

  <h3 id="ant.refid">Getting the value of a Reference with <samp>${ant.refid:}</samp></h3>

  <p>Any Ant type which has been declared with a reference can also be
    used as a property by using the <samp>${ant.refid:}</samp>
    operation, with the name of the reference listed after
    the <code>ant.refid:</code> text.  The difference between this
    operation and <a href="#toString"><samp>${toString:}</samp></a> is
    that <samp>${ant.refid:}</samp> will expand to the referenced
    object itself.  In most circumstances the <code>toString</code>
    method will be invoked anyway, for example if
    the <samp>${ant.refid:}</samp> is surrounded by other text.</p>

  <p>This syntax is most useful when using a task with attribute
    setters that accept objects other than String.  For example, if
    the setter accepts a Resource object as in</p>

  <pre>public void setAttr(Resource r) { ... }</pre>

  <p>then the syntax can be used to pass in resource subclasses
    previously defined as references like</p>
<pre>
&lt;url url="http://ant.apache.org/" id="anturl"/&gt;
&lt;my:task attr="${ant.refid:anturl}"/&gt;</pre>

  <h2 id="if+unless">If/Unless Attributes</h2>
  <p>
    The <code>&lt;target&gt;</code> element and various tasks (such
    as <code>&lt;fail&gt;</code>) and task elements (such
    as <code>&lt;test&gt;</code> in <code>&lt;junit&gt;</code>)
    support <var>if</var> and <var>unless</var> attributes which can
    be used to control whether the item is run or otherwise takes
    effect.
  </p>
  <p>
    In Ant 1.7.1 and earlier, these attributes could only be property
    names.  The item was enabled if a property with that name was
    defined&mdash;even to be the empty string
    or <q>false</q>&mdash;and disabled if the property was not
    defined. For example, the following works but there is no way to
    override the file existence check negatively (only positively):
  </p>
  <pre>
&lt;target name="-check-use-file"&gt;
    &lt;available property="file.exists" file="some-file"/&gt;
&lt;/target&gt;
&lt;target name="use-file" depends="-check-use-file" <strong>if="file.exists"</strong>&gt;
    &lt;!-- do something requiring that file... --&gt;
&lt;/target>
&lt;target name="lots-of-stuff" depends="use-file,other-unconditional-stuff"/&gt;</pre>
  <p>
    <em>Since Ant 1.8.0</em>, you may instead use property expansion;
    a value of <q>true</q> (or <q>on</q> or <q>yes</q>) will enable
    the item, while <q>false</q> (or <q>off</q> or <q>no</q>) will
    disable it. Other values are still assumed to be property names
    and so the item is enabled only if the named property is defined.
  </p>
  <p>
    Compared to the older style, this gives you additional
    flexibility, because you can override the condition from the
    command line or parent scripts:
  </p>
  <pre>
&lt;target name="-check-use-file" <strong>unless="file.exists"</strong>&gt;
    &lt;available property="file.exists" file="some-file"/&gt;
&lt;/target>
&lt;target name="use-file" depends="-check-use-file" <strong>if="${file.exists}"</strong>&gt;
    &lt;!-- do something requiring that file... --&gt;
&lt;/target>
&lt;target name="lots-of-stuff" depends="use-file,other-unconditional-stuff"/&gt;</pre>
  <p>
    Now <code>ant -Dfile.exists=false lots-of-stuff</code> will
    run <q>other-unconditional-stuff</q> but not <q>use-file</q>, as
    you might expect, and you can disable the condition from another
    script too:
  </p>
  <pre>
&lt;antcall target="lots-of-stuff"&gt;
    &lt;param name="file.exists" value="false"/&gt;
&lt;/antcall&gt;</pre>
  <p>
    Similarly, an <var>unless</var> attribute disables the item if it
    is either the name of property which is defined, or if it
    evaluates to a <q>true</q>-like value. For example, the following
    allows you to define <code>skip.printing.message=true</code>
    in <samp>my-prefs.properties</samp> with the results you might
    expect:
  </p>
  <pre>
&lt;property file="my-prefs.properties"/&gt;
&lt;target name="print-message" <strong>unless="${skip.printing.message}"</strong>&gt;
    &lt;echo>hello!&lt;/echo>
&lt;/target&gt;</pre>

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