Java Unified Expression Language

Java Unified Expression Language is an implementation of the unified expression language (EL) standard specified by JSP 2.1 (JSR-245). This software has a javax.el.ExpressionFactory implementation, specifically de.odysseus.el.ExpressionFactoryImpl.

The package de.odysseus.el.util provides a variety of useful subclasses, including javax.el.ELContext and javax.el.ELResolver. For example, utilizing this package would allow you to create a simple EL context:

```
de.odysseus.el.util.SimpleContext context = new de.odysseus.el.util.SimpleContext(new de.odysseus.el.util.SimpleResolver());
```

You can also define functions and variables within the context. For instance, to define the function math:max(int, int):

```
context.setFunction("math", "max", Math.class.getMethod("max", new Class[]{int.class, int.class}));
```

To set the variable “foo” with a value of 0:

```
context.setVariable("foo", factory.createValueExpression(0, null));
```

After defining the context, you can create an expression using the ExpressionFactory:

```
javax.el.ValueExpression e = factory.createValueExpression(context, "${math:max(foo,bar)}", null);
```

By setting the value of the top-level property ”bar” to 1, you can then evaluate the expression and return the value using:

```
System.out.println(e.getValue(context)); // --> 1
```

Additionally, JUEL jar may be run from the command line to dump the parse tree of an EL expression:

```
$ java -jar juel-2.1.x.jar "#{unified(expression[language])}"
+- #{...}
|
+- unified(...)
|
+- [...]
|
+- expression
|
+- language.
```

Overall, Java Unified Expression Language is a useful implementation of the unified expression language standard, providing several essential features for software developers seeking efficient expression evaluation.