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<document>
  <properties>
    <title>Commons JEXL Examples</title>
  </properties>

  <body>
    <section name="Overview">
      <p>
        In this reference you will find the following topics to help with your use of JEXL.
        <ul>
          <li><a href="#Evaluating Expressions">Evaluating Expressions</a></li>
          <li><a href="#Custom Contexts">Custom Contexts</a></li>
          <li><a href="#Example Expressions">Example Expressions</a></li>
        </ul>
      </p>
      <p>
        You can find two sample programs in JEXL's CVS repository:
        <ul>
          <li><a href="http://svn.apache.org/viewvc/commons/proper/jexl/trunk/src/test/java/org/apache/commons/jexl2/examples/ArrayTest.java?view=markup">Using arrays</a></li>
          <li><a href="http://svn.apache.org/viewvc/commons/proper/jexl/trunk/src/test/java/org/apache/commons/jexl2/examples/MethodPropertyTest.java?view=markup">Accessing Properties and invoking methods</a></li>
        </ul>
      </p>
      <p>
        As well, <a href="http://svn.apache.org/viewvc/commons/proper/jexl/trunk/src/test/org/apache/commons/jexl2/JexlTest.java?view=markup">JEXL's Unit Tests</a>
        provide handy examples of expressions. The test code also contains a
        <a href="http://svn.apache.org/viewvc/commons/proper/jexl/trunk/src/test/org/apache/commons/jexl2/Jexl.java?view=markup">simple class</a>
        that evaluates its command line arguments as JEXL expressions when run.
      </p>
      <p>
          The <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/package-summary.html#intro">
          package javadoc</a> also contains useful information.
      </p>
    </section>
    <section name="Evaluating Expressions">
      <p>
        To evaluate expressions using JEXL, you need three things:
        <ul>
          <li>An <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/JexlEngine.html">engine</a> to create expressions,</li>
          <li>A <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/JexlContext.html">context</a> containing any variables, and</li>
          <li>An <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/Expression.html">expression</a></li>
        </ul>
      </p>
      <p>
        The common way of using a JEXL engine is to allocate it as a singleton and use this opportunity to tailor its
        behavior and cache.
        <source>
            private static final JexlEngine jexl = new JexlEngine();
            static {
               jexl.setCache(512);
               jexl.setLenient(false);
               jexl.setSilent(false);
            }
        </source>
      </p>
      <p>
        The easiest way of obtaining a a context is to use the
        <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/MapContext.html#MapContext()">new MapContext()</a>
        statement. This creates a context implemented using an underlying
        <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/util/HashMap.html">HashMap</a>
      </p>
      <p>
        <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/Expression.html">Expressions</a> are
        created using the <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/JexlEngine.html#createExpression(java.lang.String)">JexlEngine.createExpression(String)</a>
        method. 
      </p>
      <p>
        Once you have your expression, you can then use use the
        <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/Expression.html#evaluate(org.apache.commons.jexl2.JexlContext)">evaluate</a>
        to execute it and obtain a result.
      </p>
      <p>
        Here's a typical scenario:
      </p>
      <source>
    // Assuming we have a JexlEngine instance initialized in our class named 'jexl':
    // Create an expression object for our calculation
    String calculateTax = taxManager.getTaxCalc(); //e.g. "((G1 + G2 + G3) * 0.1) + G4";
    Expression e = jexl.createExpression( calculateTax );

    // populate the context
    JexlContext context = new MapContext();
    context.set("G1", businessObject.getTotalSales());
    context.set("G2", taxManager.getTaxCredit(businessObject.getYear()));
    context.set("G3", businessObject.getIntercompanyPayments());
    context.set("G4", -taxManager.getAllowances());
    // ...
    
    // work it out
    Float result = (Float)e.evaluate(context);
      </source>
    </section>
    <section name="Custom Contexts">
      <p>
        Often you have the objects and values that are needed in the context available
        elsewhere. If those are already in a Map, instead of creating the default context and populating it
        manually in the code, you can wrap a MapContext around your own map using
        <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/MapContext.html#MapContext()">new MapContext(yourOwnMap)</a>
      </p>
      <p>
        In edge cases, it may be simpler to create a context implementation of your
        own.
        The <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/JexlContext.html">JexlContext</a>
        interface is very simple with only three methods, to set, get and check the existence of variables.
      </p>
    </section>
    <section name="Example Expressions">
      <!--  invoking methods, property access, array access, empty, size etc... -->
      <subsection name="Arithmetic">
        <p>Most valid arithmetic expressions in Java are also valid in Jexl.</p>
        <source>
1 + 2
12.0 - 5.2
6 * 12 + 5 / 2.6
12 % 2
6 / 4
-12 + 77.2
x * 1.1 + y
        </source>
        <p>Arithmetic expressions can use variables. <code>null</code> can be treated as a zero for arithmetic;
        if you need stricter semantics and consider <code>null</code> as an erroneous operand for arithmetic and
        comparisons operations, you should initialize your JEXL engine using
        <a href="http://commons.apache.org/jexl/apidocs/org/apache/commons/jexl2/JexlEngine.html#setLenient(boolean)">JexlEngine.setLenient(false)</a>.
        </p>
      </subsection>
      <subsection name="Calling methods">
        <p>
          JEXL allows you to call any method on a Java object using the same syntax.
          If you have a string in the context under the name <code>aString</code>,
          you could call it's <code>length</code> 
          method like this:
          <source>
aString.length()
aString.substring(6)
          </source>
        </p>
        <p>
          Often the values you want to pass to a method are other variables or expressions.
          If you have a number in the context, named <code>i</code>, you could use it
          in a method call:
          <source>aString.substring(i)</source>
        </p>
      </subsection>
      <subsection name="Accessing properties">
        <p>
          JEXL provides a shorthand syntax to access methods that
          follow the JavaBean naming convention for properties, i.e.
          setters and getters.
        </p>
        <p>
          If you have some object foo in the context and it has a
          method <code>getBar()</code>, you can call that method using
          the following syntax:
          <source>foo.bar</source>
        </p>
        <p>
          Since <code>java.lang.Object</code> has a <code>getClass()</code> method
          that returns a <code>java.lang.Class</code> object, and the
          class has a <code>getName()</code> method, the following is a shorthand
          for obtaining the class name of an object <code>foo</code> in the context: 
          <source>foo.class.name</source>
        </p>
      </subsection>
      <subsection name="Arrays, Lists and Maps">
        <p>
          Array elements can be accessed using either square brackets or a dotted
          index notation, e.g. the following are equivalent
          <source>arr[0]
arr.0</source>
          The same holds true for lists.
        </p>
        <p>
          For a map, the syntax is very similar, except the 'index' is an object, e.g.
          the following are equivalent.
          <source>aMap['key']
aMap.get('key')</source>
          Please note that the index does not have to be a string, and
          that the string usage above is just one possible option.
        </p>
      </subsection>
    </section>
  </body>
</document>
