In this reference you will find the following topics to help with your use of JEXL.
You can find two sample programs in JEXL's CVS repository:
As well, JEXL's Unit Tests provide handy examples of expressions. The test code also contains a simple class that evaluates its command line arguments as JEXL expressions when run.
To evaluate expressions using JEXL, you need two things:
The easiest way of obtaining a a context is to use the JexlHelper.createContext() method. This creates a context which is simply an extension of a HashMap
Expressions are created using the ExpressionFactory.createExpression(String) method.
Once you have your expression, you can then use use the evaluate to execute it and obtain a result.
Here's a typical scenario:
// Create an expression object for our calculation String calculateTax = taxManager.getTaxCalc(); //e.g. "((G1 + G2 + G3) * 0.1) + G4"; Expression e = ExpressionFactory.createExpression( calculateTax ); // populate the context JexlContext context = JexlHelper.createContext(); context.getVars().put("G1", businessObject.getTotalSales()); context.getVars().put("G2", taxManager.getTaxCredit(businessObject.getYear())); context.getVars().put("G3", businessObject.getIntercompanyPayments()); context.getVars().put("G4", -taxManager.getAllowances()); // ... // work it out Float result = (Float)e.evaluate(context);
Often you have the objects and values that are needed in the context available elsewhere, and instead of creating the default context and populating it manually in the code, it may be simpler to create a context implementation of your own.
The JexlContext interface is very simple with only two methods, one to get the variables of the context as a Map and another to set the variables of the context from a Map.
Here's a simple context that wraps the JVM's system properties:
JexlContext context = new JexlContext() { public Map getVars() { return System.getProperties(); } public void setVars(Map map) { } };
JEXL allows you to add custom expression resolvers that will get called with before or after expression evaluation.
If pre-resolvers
is added to an expression, any value it returns will be used instead of that normally provided by JEXL. If JEXL evaluates
an expression to null
post-resolvers of an expression are called in turn in an attempt to get a value.
Expression resolvers are called in the order that they are added.
Most valid arithmetic expressions in Java are also valid in Jexl.
1 + 2 12.0 - 5.2 6 * 12 + 5 / 2.6 12 % 2 6 div 4 -12 + 77.2 x * 1.1 + y
Arithmetic expressions can use variables. null
is treated as a zero for arithmetic.
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 aString
,
you could call it's length
method like this:
aString.length() aString.substring(6)
Often the values you want to pass to a method are other variables or expressions.
If you have a number in the context, named i
, you could use it
in a method call:
aString.substring(i)
JEXL provides a shorthand syntax to access methods that follow the JavaBean naming convention for properties, i.e. setters and getters.
If you have some object foo in the context and it has a
method getBar()
, you can call that method using
the following syntax:
foo.bar
Since java.lang.Object
has a getClass()
method
that returns a java.lang.Class
object, and the
class has a getName()
method, the following is a shorthand
for obtaining the class name of an object foo
in the context:
foo.class.name
Array elements can be accessed using either square brackets or a dotted index notation, e.g. the following are equivalent
arr[0] arr.0
For a map, the syntax is very similar, except the 'index' is an object, e.g. the following are equivalent.
aMap['key'] aMap.get('key')