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Package Documentation for org.apache.commons.jexl3 Package
Provides a framework for evaluating JEXL expressions.
Introduction
JEXL is a library intended to facilitate the implementation of dynamic and scripting features in applications
and frameworks.
A Brief Example
In its simplest form, JEXL merges an
{@link org.apache.commons.jexl3.JexlExpression}
with a
{@link org.apache.commons.jexl3.JexlContext} when evaluating expressions.
An Expression is created using
{@link org.apache.commons.jexl3.JexlEngine#createExpression(java.lang.String)},
passing a String containing valid JEXL syntax. A simple JexlContext can be created using
a {@link org.apache.commons.jexl3.MapContext} instance;
a map of variables that will be internally wrapped can be optionally provided through its constructor.
The following example, takes a variable named 'car', and
invokes the checkStatus() method on the property 'engine'
// Create a JexlEngine (could reuse one instead)
JexlEngine jexl = new JexlBuilder().create();
// Create an expression object equivalent to 'car.getEngine().checkStatus()':
String jexlExp = "car.engine.checkStatus()";
Expression e = jexl.createExpression( jexlExp );
// The car we have to handle coming as an argument...
Car car = theCarThatWeHandle;
// Create a context and add data
JexlContext jc = new MapContext();
jc.set("car", car );
// Now evaluate the expression, getting the result
Object o = e.evaluate(jc);
Using JEXL
The API is composed of three levels addressing different functional needs:
- Dynamic invocation of setters, getters, methods and constructors
- Script expressions known as JEXL expressions
- JSP/JSF like expression known as JXLT expressions
Important note
The public API classes reside in the 2 packages:
- org.apache.commons.jexl3
- org.apache.commons.jexl3.introspection
The following packages follow a "use at your own maintenance cost" policy; these are only intended to be used
for extending JEXL.
Their classes and methods are not guaranteed to remain compatible in subsequent versions.
If you think you need to use directly some of their features or methods, it might be a good idea to check with
the community through the mailing list first.
- org.apache.commons.jexl3.parser
- org.apache.commons.jexl3.scripting
- org.apache.commons.jexl3.internal
- org.apache.commons.jexl3.internal.introspection
Dynamic Invocation
These functionalities are close to the core level utilities found in
BeanUtils.
For basic dynamic property manipulations and method invocation, you can use the following
set of methods:
- {@link org.apache.commons.jexl3.JexlEngine#newInstance}
- {@link org.apache.commons.jexl3.JexlEngine#setProperty}
- {@link org.apache.commons.jexl3.JexlEngine#getProperty}
- {@link org.apache.commons.jexl3.JexlEngine#invokeMethod}
The following example illustrate their usage:
// test outer class
public static class Froboz {
int value;
public Froboz(int v) { value = v; }
public void setValue(int v) { value = v; }
public int getValue() { return value; }
}
// test inner class
public static class Quux {
String str;
Froboz froboz;
public Quux(String str, int fro) {
this.str = str;
froboz = new Froboz(fro);
}
public Froboz getFroboz() { return froboz; }
public void setFroboz(Froboz froboz) { this.froboz = froboz; }
public String getStr() { return str; }
public void setStr(String str) { this.str = str; }
}
// test API
JexlEngine jexl = new JexlBuilder().create();
Quux quux = jexl.newInstance(Quux.class, "xuuq", 100);
jexl.setProperty(quux, "froboz.value", Integer.valueOf(100));
Object o = jexl.getProperty(quux, "froboz.value");
assertEquals("Result is not 100", new Integer(100), o);
jexl.setProperty(quux, "['froboz'].value", Integer.valueOf(1000));
o = jexl.getProperty(quux, "['froboz']['value']");
assertEquals("Result is not 1000", new Integer(1000), o);
Expressions and Scripts
If your needs require simple expression evaluation capabilities, the core JEXL features
will most likely fit.
The main methods are:
- {@link org.apache.commons.jexl3.JexlEngine#createScript}
- {@link org.apache.commons.jexl3.JexlScript#execute}
- {@link org.apache.commons.jexl3.JexlEngine#createExpression}
- {@link org.apache.commons.jexl3.JexlExpression#evaluate}
The following example illustrates their usage:
JexlEngine jexl = new JexlBuilder().create();
JexlContext jc = new MapContext();
jc.set("quuxClass", quux.class);
JexlExpression create = jexl.createExpression("quux = new(quuxClass, 'xuuq', 100)");
JelxExpression assign = jexl.createExpression("quux.froboz.value = 10");
JexlExpression check = jexl.createExpression("quux[\"froboz\"].value");
Quux quux = (Quux) create.evaluate(jc);
Object o = assign.evaluate(jc);
assertEquals("Result is not 10", new Integer(10), o);
o = check.evaluate(jc);
assertEquals("Result is not 10", new Integer(10), o);
Unified Expressions and Templates
If you are looking for JSP-EL like and basic templating features, you can
use Expression from a JxltEngine.
The main methods are:
- {@link org.apache.commons.jexl3.JxltEngine#createExpression}
- {@link org.apache.commons.jexl3.JxltEngine.Expression#prepare}
- {@link org.apache.commons.jexl3.JxltEngine.Expression#evaluate}
- {@link org.apache.commons.jexl3.JxltEngine#createTemplate}
- {@link org.apache.commons.jexl3.JxltEngine.Template#prepare}
- {@link org.apache.commons.jexl3.JxltEngine.Template#evaluate}
The following example illustrates their usage:
JexlEngine jexl = new JexlBuilder().create();
JxltEngine jxlt = jexl.createJxltEngine();
JxltEngine.Expression expr = jxlt.createExpression("Hello ${user}");
String hello = expr.evaluate(context).toString();
JexlExpression, JexlScript, Expression and Template: summary
JexlExpression
These are the most basic form of JexlEngine expressions and only allow for a single command
to be executed and its result returned. If you try to use multiple commands, it ignores
everything after the first semi-colon and just returns the result from
the first command.
Also note that expressions are not statements (which is what scripts are made of) and do not allow
using the flow control (if, while, for), variables or lambdas syntactic elements.
JexlScript
These allow you to use multiple statements and you can
use variable assignments, loops, calculations, etc. More or less what can be achieved in Shell or
JavaScript at its basic level. The result from the last command is returned from the script.
JxltEngine.Expression
These are ideal to produce "one-liner" text, like a 'toString()' on steroids.
To get a calculation you use the EL-like syntax
as in ${someVariable}. The expression that goes between the brackets
behaves like a JexlScript, not an expression. You can use semi-colons to
execute multiple commands and the result from the last command is
returned from the script. You also have the ability to use a 2-pass evaluation using
the #{someScript} syntax.
JxltEngine.Template
These produce text documents. Each line beginning with '$$' (as a default) is
considered JEXL code and all others considered as JxltEngine.Expression.
Think of those as simple Velocity templates. A rewritten MudStore initial Velocity sample looks like this:
<html>
<body>
Hello ${customer.name}!
<table>
$$ for(var mud : mudsOnSpecial ) {
$$ if (customer.hasPurchased(mud) ) {
<tr>
<td>
${flogger.getPromo( mud )}
</td>
</tr>
$$ }
$$ }
</table>
</body>
</html>
JEXL Configuration
The JexlEngine can be configured through a few parameters that will drive how it reacts
in case of errors.
These configuration methods are embedded through a {@link org.apache.commons.jexl3.JexlBuilder}.
Static & Shared Configuration
Both JexlEngine and JxltEngine are thread-safe, most of their inner fields are final; the same instance can be shared between different
threads and proper synchronization is enforced in critical areas (introspection caches).
Of particular importance is {@link org.apache.commons.jexl3.JexlBuilder#loader} which indicates to the JexlEngine
being built which class loader to use to solve a class name; this directly affects how JexlEngine.newInstance and the 'new' script method operates.
This can also be very useful in cases where you rely on JEXL to dynamically load and call plugins for your application.
To avoid having to restart the server in case of a plugin implementation change, you can call
{@link org.apache.commons.jexl3.JexlEngine#setClassLoader} and all the scripts created through this engine instance
will automatically point to the newly loaded classes.
You can state what can be manipulated through scripting by the {@link org.apache.commons.jexl3.annotations.NoJexl}
annotation that completely shield classes and methods from JEXL introspection.
The other configurable way to restrict JEXL is by using a
{@link org.apache.commons.jexl3.introspection.JexlSandbox} which allows finer control over what is exposed; the sandbox
can be set through {@link org.apache.commons.jexl3.JexlBuilder#sandbox}.
{@link org.apache.commons.jexl3.JexlBuilder#namespaces} extends JEXL scripting by registering your own classes as
namespaces allowing your own functions to be exposed at will.
This can be used as in:
public static MyMath {
public double cos(double x) {
return Math.cos(x);
}
}
Map<String, Object> funcs = new HashMap<String, Object>();
funcs.put("math", new MyMath());
JexlEngine jexl = new JexlBuilder().namespaces(funcs).create();
JexlContext jc = new MapContext();
jc.set("pi", Math.PI);
JexlExpression e = JEXL.createExpression("math:cos(pi)");
o = e.evaluate(jc);
assertEquals(Double.valueOf(-1),o);
If the namespace is a Class and that class declares a constructor that takes a JexlContext (or
a class extending JexlContext), one namespace instance is created on first usage in an
expression; this instance lifetime is limited to the expression evaluation.
JexlEngine and JxltEngine expression caches can be configured as well. If you intend to use JEXL
repeatedly in your application, these are worth configuring since expression parsing is quite heavy.
Note that all caches created by JEXL are held through SoftReference; under high memory pressure, the GC will be able
to reclaim those caches and JEXL will rebuild them if needed. By default, a JexlEngine does create a cache for "small" expressions
and a JxltEngine does create one for Expression .
{@link org.apache.commons.jexl3.JexlBuilder#cache} will set how many expressions can be simultaneously cached by the
JEXL engine. JxltEngine allows to define the cache size through its constructor.
{@link org.apache.commons.jexl3.JexlBuilder#debug}
makes stacktraces carried by JExlException more meaningful; in particular, these
traces will carry the exact caller location the Expression was created from.
Dynamic Configuration
Those configuration options can be overridden during evaluation by implementing a
{@link org.apache.commons.jexl3.JexlContext}
that also implements {@link org.apache.commons.jexl3.JexlEngine$Options} to carry evaluation options.
An example of such a class exists in the test package.
{@link org.apache.commons.jexl3.JexlBuilder#strict} or {@link org.apache.commons.jexl3.JexlEngine$Options#isStrict}
configures when JEXL considers 'null' as an error or not in various situations;
when facing an unreferenceable variable, using null as an argument to an arithmetic operator or failing to call
a method or constructor. The lenient mode is close to JEXL-1.1 behavior.
{@link org.apache.commons.jexl3.JexlBuilder#silent} or {@link org.apache.commons.jexl3.JexlEngine$Options#isSilent}
configures how JEXL reacts to errors; if silent, the engine will not throw exceptions
but will warn through loggers and return null in case of errors. Note that when non-silent, JEXL throws
JexlException which are unchecked exception.
Implementing a {@link org.apache.commons.jexl3.JexlContext$NamespaceResolver} through a JexlContext - look at
JexlEvalContext
as an example - allows to override the namespace resolution and the default namespace map defined
through {@link org.apache.commons.jexl3.JexlBuilder#namespaces}.
JEXL Customization
The {@link org.apache.commons.jexl3.JexlContext}, {@link org.apache.commons.jexl3.JexlBuilder} and
{@link org.apache.commons.jexl3.JexlEngine$Options} are
the most likely interfaces you'll want to implement for customization. Since they expose variables and options,
they are the primary targets. Before you do so, have a look at JexlEvalContext in the test directory
and {@link org.apache.commons.jexl3.ObjectContext} which may already cover some of your needs.
{@link org.apache.commons.jexl3.JexlArithmetic}
is the class to derive if you need to change how operators behave or add types upon which they
operate.
There are 3 entry points that allow customizing the type of objects created:
- array literals: {@link org.apache.commons.jexl3.JexlArithmetic#arrayBuilder}
- map literals: {@link org.apache.commons.jexl3.JexlArithmetic#mapBuilder}
- set literals: {@link org.apache.commons.jexl3.JexlArithmetic#setBuilder}
- range objects: {@link org.apache.commons.jexl3.JexlArithmetic#createRange}
You can also overload operator methods; by convention, each operator has a method name associated to it.
If you overload some in your JexlArithmetic derived implementation, these methods will be called when the
arguments match your method signature.
For example, this would be the case if you wanted '+' to operate on arrays; you'd need to derive
JexlArithmetic and implement 'public Object add(Set x, Set y)' method.
Note however that you can not change the operator precedence.
The list of operator / method matches is the following:
Operator
Method Name
Example
+
add
add(x, y)
-
subtract
subtract(x, y)
*
multiply
multiply(x, y)
/
divide
divide(x, y)
%
mod
mod(x, y)
&
bitwiseAnd
bitwiseAnd(x, y)
|
bitwiseOr
bitwiseOr(x, y)
^
bitwiseXor
bitwiseXor(x, y)
!
logicalNot
logicalNot(x)
-
bitwiseComplement
bitiwiseComplement(x)
==
equals
equals(x, y)
<
lessThan
lessThan(x, y)
<=
lessThanOrEqual
lessThanOrEqual(x, y)
>
greaterThan
greaterThan(x, y)
>=
greaterThanOrEqual
greaterThanOrEqual(x, y)
-
negate
negate(x)
size
size
size(x)
empty
empty
empty(x)
You can also add methods to overload property getters and setters operators behaviors.
Public methods of the JexlArithmetic instance named propertyGet/propertySet/arrayGet/arraySet are potential
overrides that will be called when appropriate.
The following table is an overview of the relation between a syntactic form and the method to call
where V is the property value class, O the object class and P the property identifier class (usually String or Integer).
Expression
Method Template
foo.property
public V propertyGet(O obj, P property);
foo.property = value
public V propertySet(O obj, P property, V value);
foo[property]
public V arrayGet(O obj, P property, V value);
foo[property] = value
public V arraySet(O obj, P property, V value);
You can also override the base operator methods, those whose arguments are Object which gives you total
control.
Extending JEXL
If you need to make JEXL treat some objects in a specialized manner or tweak how it
reacts to some settings, you can derive most of its inner-workings. The classes and methods are rarely private or
final - only when the inner contract really requires it. However, using the protected methods
and internal package classes imply you might have to re-adapt your code when new JEXL versions are released.
{@link org.apache.commons.jexl3.internal.Engine} can be
extended to let you capture your own configuration defaults wrt cache sizes and various flags.
Implementing your own cache - instead of the basic LinkedHashMap based one - would be
another possible extension.
{@link org.apache.commons.jexl3.internal.Interpreter}
is the class to derive if you need to add more features to the evaluation
itself; for instance, you want pre- and post- resolvers for variables or nested scopes for
for variable contexts.
{@link org.apache.commons.jexl3.internal.introspection.Uberspect}
is the class to derive if you need to add introspection or reflection capabilities for some objects, for
instance adding factory based support to the 'new' operator.
The code already reflects public fields as properties on top of Java-beans conventions.