org.apache.commons.jexl3 (Apache Commons JEXL 3.7.0 API)
Description: declaration: package: org.apache.commons.jexl3

# Package org.apache.commons.jexl3

package org.apache.commons.jexl3
Provides a framework for evaluating JEXL expressions and scripts.

- Introduction
- Brief Example
- Using JEXL
- Configuring JEXL
- Customizing JEXL
- Extending JEXL

## Introduction

JEXL is a library intended to facilitate the implementation of dynamic and scripting features in applications and frameworks.

The entry point is always a `JexlEngine`, created and configured through a `JexlBuilder`. An engine is thread-safe and meant to be created once and shared; it produces `JexlExpression` (single expressions) and `JexlScript` (multi-statement scripts) which are evaluated against a `JexlContext` carrying the variable bindings. A `JxltEngine` adds a JSP/JSF-like templating layer on top of the same engine.

Security disclaimer. Neither `JexlPermissions.RESTRICTED` nor `JexlPermissions.SECURE` is exhaustive, and neither must be considered completely safe or sufficient on its own for executing untrusted user input. They are hardened baselines, not guarantees. Any application that evaluates untrusted scripts must define its own tailored, strict whitelist of exactly the classes, methods and fields its scripts legitimately need - ideally by composing on top of `JexlPermissions.NONE` (which denies everything) via `JexlPermissions.create(java.lang.String...)` - and audit the result with `JexlPermissions.logging()`.

Security note (since 3.7): a bare `new JexlBuilder().create()` engine is hardened by default. Its permissions are `JexlPermissions.SECURE` (only a small, safe allow-list of `java.lang`/`java.math`/`java.util` types is reachable) and its default `JexlFeatures` disable `new(...)`, global side-effects, pragmas and annotations (lexical scoping is on; loops remain available to scripts). Relax these deliberately - see the configuration section - rather than by accident.

Permissions and features can be set programmatically on the `JexlBuilder` or loaded from a YAML document through `JexlConfigLoader`, which keeps security policy out of code and in configuration:

     try (InputStream in = getClass().getResourceAsStream("/jexl.yaml")) {
         JexlEngine jexl = JexlConfigLoader.load(in).create();
     }

A ready-made `jexl.yaml` that restores the pre-3.7 behavior (RESTRICTED permissions, the full feature set) is bundled as a migration aid; see `JexlConfigLoader` for the full schema.

## A Brief Example

In its simplest form, JEXL merges an `JexlExpression` with a `JexlContext` when evaluating expressions. An expression is created using `JexlEngine.createExpression(String)`, passing a String containing valid JEXL syntax. A simple JexlContext can be created using a `MapContext` instance; a map of variables that will be internally wrapped can be optionally provided through its constructor. The following example binds a 'name' variable, calls a String method on it and concatenates the result. It relies only on a `java.lang.String` and arithmetic, so it runs as-is under the default (SECURE) permissions - no configuration needed.

     // Create a JexlEngine (could reuse one instead)
     JexlEngine jexl = new JexlBuilder().create();
     // An expression using a variable, a String method call and concatenation:
     String jexlExp = "'Hello ' + name + ', your name has ' + name.length() + ' letters'";
     JexlExpression e = jexl.createExpression( jexlExp );
     // Create a context and add data
     JexlContext jc = new MapContext();
     jc.set("name", "John");
     // Now evaluate the expression; result is "Hello John, your name has 4 letters"
     Object o = e.evaluate(jc);

To expose your own classes (a `Car` bean, a service, ...) to scripts, you must permit them explicitly: the default SECURE permissions deny application types, and passing an instance through the `JexlContext` does not bypass that gate - the introspector still checks the object's class. Grant access by composing your package into the permission set (e.g. `new JexlBuilder().permissions(JexlPermissions.RESTRICTED.compose("com.example.app +{}"))`) or choose a broader base; see the configuration section on permissions.

## 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

Note that the `org.apache.commons.jexl3.scripting` package implements the JSR-223 (`javax.script`) integration through `JexlScriptEngine`.

### 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:

- `JexlEngine.newInstance(java.lang.Class<? extends T>, java.lang.Object...)`
- `JexlEngine.setProperty(org.apache.commons.jexl3.JexlContext, java.lang.Object, java.lang.String, java.lang.Object)`
- `JexlEngine.getProperty(org.apache.commons.jexl3.JexlContext, java.lang.Object, java.lang.String)`
- `JexlEngine.invokeMethod(java.lang.Object, java.lang.String, java.lang.Object...)`

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 - Quux and Froboz are in your own package; compose it into the permissions.
     JexlEngine jexl = new JexlBuilder()
         .permissions(JexlPermissions.SECURE.compose("com.example +{}"))
         .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", Integer.valueOf(100), o);
     jexl.setProperty(quux, "['froboz'].value", Integer.valueOf(1000));
     o = jexl.getProperty(quux, "['froboz']['value']");
     assertEquals("Result is not 1000", Integer.valueOf(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:

- `JexlEngine.createScript(java.io.File)`
- `JexlScript.execute(org.apache.commons.jexl3.JexlContext)`
- `JexlEngine.createExpression(org.apache.commons.jexl3.JexlInfo, java.lang.String)`
- `JexlExpression.evaluate(org.apache.commons.jexl3.JexlContext)`

The following example illustrates their usage:

     // new(...) needs the newInstance feature; Quux/Froboz need explicit permissions.
     JexlEngine jexl = new JexlBuilder()
         .permissions(JexlPermissions.SECURE.compose("com.example +{}"))
         .features(JexlFeatures.createDefault())
         .create();
     JexlContext jc = new MapContext();
     jc.set("quuxClass", quux.class);
     JexlExpression create = jexl.createExpression("quux = new(quuxClass, 'xuuq', 100)");
     JexlExpression 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", Integer.valueOf(10), o);
     o = check.evaluate(jc);
     assertEquals("Result is not 10", Integer.valueOf(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:

- `JxltEngine.createExpression(org.apache.commons.jexl3.JexlInfo, java.lang.String)`
- `JxltEngine.Expression.prepare(org.apache.commons.jexl3.JexlContext)`
- `JxltEngine.Expression.evaluate(org.apache.commons.jexl3.JexlContext)`
- `JxltEngine.createTemplate(org.apache.commons.jexl3.JexlInfo, java.lang.String)`
- `JxltEngine.Template.prepare(org.apache.commons.jexl3.JexlContext)`
- `JxltEngine.Template.evaluate(org.apache.commons.jexl3.JexlContext, java.io.Writer)`

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

Almost everything is configured through a `JexlBuilder` before the engine is created. The builder controls four largely orthogonal concerns:

- Permissions (`JexlPermissions`) - what classes, methods and fields scripts may reach through reflection.
- Features (`JexlFeatures`) - which syntactic constructs are accepted at parse time.
- Options (`JexlOptions`) - how the interpreter behaves at runtime (strict, silent, safe, cancellable, lexical, math).
- Bindings (namespaces, imports, arithmetic, class loader, caches) - the environment scripts execute against.

For a declarative alternative, `JexlConfigLoader` builds a pre-configured `JexlBuilder` from a YAML document covering permissions, features, arithmetic, namespaces and imports - convenient for externalizing configuration or restoring pre-3.7 defaults.

### Security: permissions, sandbox and `@NoJexl`

`JexlPermissions` is the primary security gate; it controls which packages, classes and members the introspection layer will expose. Four constants are provided: `JexlPermissions.UNRESTRICTED` (everything), `JexlPermissions.RESTRICTED` (a broad but curated allow-list), `JexlPermissions.SECURE` (the minimal safe allow-list and the default since 3.7) and `JexlPermissions.NONE` (deny everything). A permission set is configured with `JexlBuilder.permissions(org.apache.commons.jexl3.introspection.JexlPermissions)` and can be composed from a textual DSL through `JexlPermissions.parse(java.lang.String...)` or `RESTRICTED.compose("com.example.api +{}")`. To build a closed-world, deny-by-default policy from scratch, start from `NONE` (or `JexlPermissions.create(java.lang.String...)`) and compose only what scripts need, e.g. `JexlPermissions.create("com.example.api +{}")`. To diagnose what a permission set allows or denies under your workload, wrap it with `JexlPermissions.logging()`.

Two finer-grained mechanisms complement permissions: the `NoJexl` annotation completely shields annotated classes and members from introspection at the source level, while a `JexlSandbox` - set through `JexlBuilder.sandbox(org.apache.commons.jexl3.introspection.JexlSandbox)` - gives per-class allow/deny control over properties and methods at configuration time.

### Language features (parse time)

`JexlFeatures` restricts the grammar a script may use; violations are reported as `JexlException.Feature` when the script is compiled, before any evaluation. Among the toggles are `newInstance` (`new(...)`), `loops`, `sideEffect`/`sideEffectGlobal`, `lambda`, `methodCall`, `structuredLiteral`, `pragma`, `annotation` and lexical scoping. A feature set is applied with `JexlBuilder.features(org.apache.commons.jexl3.JexlFeatures)`; convenient bases are `JexlFeatures.createDefault()`, `JexlFeatures.createScript()`, `JexlFeatures.createAll()` and `JexlFeatures.createNone()`.

Since expressions are parsed as a single expression, statement-level features (loops, multiple statements, ...) never apply to them regardless of the feature set; they are relevant to scripts only.

### Runtime options

`JexlOptions` carries the behavioral flags evaluated at runtime. They can be set as engine defaults either through the dedicated builder shortcuts or by mutating the builder's option set returned by `JexlBuilder.options()`:

- `JexlBuilder.strict(boolean)` - whether `null` operands, unknown variables or failed calls are errors.
- `JexlBuilder.silent(boolean)` - whether errors throw `JexlException` (unchecked) or are logged and yield `null`.
- `JexlBuilder.safe(boolean)` - whether safe-navigation (`x?.y`) tolerates `null` on dereference.
- `JexlBuilder.cancellable(boolean)` - whether interrupting the evaluating thread raises `JexlException.Cancel`.
- `JexlBuilder.lexical(boolean)` / `JexlBuilder.lexicalShade(boolean)` - lexical scoping of local variables.
- arithmetic strictness and math context, carried by the `JexlArithmetic` instance set with `JexlBuilder.arithmetic(org.apache.commons.jexl3.JexlArithmetic)`.

These engine defaults can be overridden per evaluation: a `JexlContext` that also implements `JexlContext.OptionsHandle` supplies a fresh `JexlOptions` for each evaluation. (The legacy `JexlEngine.Options` interface is deprecated in favor of this mechanism.)

### Namespaces, imports and arithmetic

`JexlBuilder.namespaces(java.util.Map)` registers your own classes or instances as namespaces, exposing their methods as `prefix:function(...)` calls.

     public static class MyMath {
       public double cos(double x) {
         return Math.cos(x);
       }
     }
     Map<String, Object> funcs = new HashMap<String, Object>();
     funcs.put("math", new MyMath());
     // MyMath is in your own package; compose it into the permissions.
     JexlEngine jexl = new JexlBuilder()
         .namespaces(funcs)
         .permissions(JexlPermissions.SECURE.compose("com.example +{}"))
         .create();
     JexlContext jc = new MapContext();
     jc.set("pi", Math.PI);
     JexlExpression e = jexl.createExpression("math:cos(pi)");
     Object 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.

`JexlBuilder.imports(java.util.Collection)` declares packages whose classes can be referenced by their unqualified name in `new(...)` and type references, mimicking Java imports. `JexlBuilder.arithmetic(org.apache.commons.jexl3.JexlArithmetic)` substitutes a custom `JexlArithmetic` - see customization.

### 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).

The library-wide defaults applied by a bare `new JexlBuilder()` are themselves configurable through static setters: `JexlBuilder.setDefaultPermissions(org.apache.commons.jexl3.introspection.JexlPermissions)`, `JexlBuilder.setDefaultFeatures(org.apache.commons.jexl3.JexlFeatures)`, and `JexlBuilder.setDefaultOptions(String...)` (runtime evaluation flags such as `strict`, `safe`, and `cancellable`; see `JexlOptions.setDefaultFlags(String...)`). The pre-3.7 permissive feature set is exposed as `JexlBuilder.FULL` so an application can restore the legacy behavior process-wide in one place.

Of particular importance is `JexlBuilder.loader(java.lang.ClassLoader)` 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 `JexlEngine.setClassLoader(java.lang.ClassLoader)` and all the scripts created through this engine instance will automatically point to the newly loaded classes.

### Caches & debugging

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.

`JexlBuilder.cache(int)` sets how many expressions can be simultaneously cached by the JEXL engine, `JexlBuilder.cacheThreshold(int)` caps the size of cached expressions, and `JexlBuilder.cacheFactory(java.util.function.IntFunction)` lets you supply an alternative `JexlCache` implementation. JxltEngine allows defining the cache size through its constructor.

`JexlBuilder.debug(boolean)` makes stack traces carried by JexlException more meaningful; in particular, these traces will carry the exact caller location the Expression was created from.

### Dynamic Configuration

Beyond options, a `JexlContext` can implement a number of handles to override engine behavior on a per-evaluation basis. The `MapContext`/`ObjectContext` implementations cover the common cases; for finer control, implement one or more of:

- `JexlContext.OptionsHandle` - supply per-evaluation `JexlOptions`.
- `JexlContext.NamespaceResolver` - override namespace resolution and the default namespace map defined through `JexlBuilder.namespaces(java.util.Map)`.
- `JexlContext.AnnotationProcessor` - handle `@annotation` statements.
- `JexlContext.PragmaProcessor` - react to `#pragma` directives.
- `JexlContext.ModuleProcessor` - resolve `#pragma jexl.module` imports.
- `JexlContext.ClassNameResolver` - resolve unqualified class names.
- `JexlContext.CancellationHandle` - expose a cancellation flag to interrupt evaluation.
- `JexlContext.ThreadLocal` - bind context data to the evaluating thread.

## JEXL Customization

The `JexlContext`, `JexlBuilder` and `JexlOptions` 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 `ObjectContext` which may already cover some of your needs.

`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: `JexlArithmetic.arrayBuilder(int)`
- map literals: `JexlArithmetic.mapBuilder(int)`
- set literals: `JexlArithmetic.setBuilder(int)`
- range objects: `JexlArithmetic.createRange(java.lang.Object, java.lang.Object)`

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 a `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 described in `JexlOperator`:

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).

Property Accessors
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.

`Engine` can be extended to let you capture your own configuration defaults regarding cache sizes and various flags. Implementing your own `JexlCache` - instead of the default soft-reference based one - would be another possible extension.

`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.

`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.

- Related Packages
  Package
  Description
  org.apache.commons.jexl3.annotations
  Provides annotation for introspection services.
  org.apache.commons.jexl3.introspection
  Provides high-level introspective services.
  org.apache.commons.jexl3.scripting
  Contains the JSR-223 Scripting Engine for JEXL script.
- Class
  Description
  JexlArithmetic
  Perform arithmetic, implements JexlOperator methods.
  JexlArithmetic.ArrayBuilder
  Helper interface used when creating an array literal.
  JexlArithmetic.CoercionException
  Marker class for coercion operand exceptions.
  JexlArithmetic.MapBuilder
  Helper interface used when creating a map literal.
  JexlArithmetic.NullOperand
  Marker class for null operand exceptions.
  JexlArithmetic.SetBuilder
  Helper interface used when creating a set literal.
  JexlArithmetic.Uberspect
  The interface that uberspects JexlArithmetic classes.
  JexlBuilder
  Configures and builds a JexlEngine.
  JexlCache<K,V>
  Caching script or template interface.
  JexlCache.Reference
  A cached reference.
  JexlConfigLoader
  Loads a YAML configuration file and applies it to a `JexlBuilder`.
  JexlContext
  Manages variables which can be referenced in a JEXL expression.
  JexlContext.AnnotationProcessor
  A marker interface of the JexlContext that processes annotations.
  JexlContext.CancellationHandle
  A marker interface of the JexlContext sharing a cancelling flag.
  JexlContext.ClassNameResolver
  A marker interface that solves a simple class name into a fully-qualified one.
  JexlContext.ModuleProcessor
  A marker interface of the JexlContext that processes module definitions.
  JexlContext.NamespaceFunctor
  A marker interface of the JexlContext, NamespaceFunctor allows creating an instance to delegate namespace methods calls to.
  JexlContext.NamespaceResolver
  A marker interface of the JexlContext that declares how to resolve a namespace from its name; it is used by the interpreter during evaluation.
  JexlContext.OptionsHandle
  A marker interface of the JexlContext that exposes runtime evaluation options.
  JexlContext.PragmaProcessor
  A marker interface of the JexlContext that processes pragmas.
  JexlContext.ThreadLocal
  A marker interface of the `JexlContext` that indicates the interpreter to put this context in the `JexlEngine` thread local context instance during evaluation.
  JexlEngine
  Creates and evaluates JexlExpression and JexlScript objects.
  JexlEngine.EmptyContext
  The empty context class, public for instrospection.
  JexlEngine.EmptyNamespaceResolver
  The empty/static/non-mutable JexlNamespace class, public for instrospection.
  JexlEngine.Options
  Deprecated.
  3.2
  JexlException
  Wraps any error that might occur during interpretation of a script or expression.
  JexlException.Ambiguous
  Thrown when parsing fails due to an ambiguous statement.
  JexlException.Annotation
  Thrown when an annotation handler throws an exception.
  JexlException.Assignment
  Thrown when parsing fails due to an invalid assignment.
  JexlException.Break
  Thrown to break a loop.
  JexlException.Cancel
  Thrown to cancel a script execution.
  JexlException.Continue
  Thrown to continue a loop.
  JexlException.Feature
  Thrown when parsing fails due to a disallowed feature.
  JexlException.Method
  Thrown when a method or ctor is unknown, ambiguous, or inaccessible.
  JexlException.Operator
  Thrown when an operator fails.
  JexlException.Parsing
  Thrown when parsing fails.
  JexlException.Property
  Thrown when a property is unknown.
  JexlException.Return
  Thrown to return a value.
  JexlException.StackOverflow
  Thrown when reaching stack-overflow.
  JexlException.Throw
  Thrown to throw a value.
  JexlException.Tokenization
  Thrown when tokenization fails.
  JexlException.TryFailed
  Thrown when method/ctor invocation fails.
  JexlException.Variable
  Thrown when a variable is unknown.
  JexlException.VariableIssue
  Enumerates types of variable issues.
  JexlExpression
  Represents a single JEXL expression.
  JexlFeatures
  A set of language feature options.
  JexlInfo
  Helper class to carry information such as a url/file name, line and column for debugging information reporting.
  JexlInfo.Detail
  Describes errors more precisely.
  JexlOperator
  Enumerates JEXL operators.
  JexlOperator.Uberspect
  Uberspect that solves and evaluates JexlOperator overloads.
  JexlOptions
  Flags and properties that can alter the evaluation behavior.
  JexlScript
  A JEXL Script.
  JxltEngine
  A simple "JeXL Template" engine.
  JxltEngine.Exception
  The sole type of (runtime) exception the JxltEngine can throw.
  JxltEngine.Expression
  A unified expression that can mix immediate, deferred and nested sub-expressions as well as string constants; The "immediate" syntax is of the form `"...`
  JxltEngine.Template
  A template is a JEXL script that evaluates by writing its content through a Writer.
  MapContext
  Wraps a map in a context.
  ObjectContext<T>
  Wraps an Object as a JEXL context and NamespaceResolver.