I just released the Starter Class of "Learn Spring Security":
1. Introduction
Expression Language (EL), is a scripting language that’s seen adoption within many Java frameworks, such as Spring with SpEL and JBoss with JBoss EL.
In this article, we’ll focus at the JSF’s implementation of this scripting language – Unified EL.
EL is currently in version 3.0, a major upgrade that allows the processing engine to be used in standalone mode – for example, on the Java SE platform. Prior versions were dependent on a JavaEE-compliant application server or web container. This article discusses EL version 2.2.
2. Immediate and Deferred Evaluation
The primary function of EL in JSF is to connect the JSF view (usually XHTML markup) and the java-based back-end. The back-end can be user-created managed beans, or container-managed objects like the HTTP session.
We will be looking at EL 2.2. EL in JSF comes in two general forms, immediate syntax EL and deferred syntax EL.
2.1. Immediate Syntax EL
Otherwise known as JSP EL, this is a scripting format that’s a holdover from the JSP days of java web application development.
The JSP EL expressions start with the dollar sign ($), then followed by the left curly bracket ({), then followed by the actual expression, and finally closed with the right curly bracket (}):
${ELBean.value > 0}
This syntax:
- Is evaluated only once (at the beginning) in the lifecycle of a page. What this means is that the value that is. being read by the expression in the example above must be set before the page is loaded.
- Provides read-only access to bean values.
- And as a result, requires adherence to the JavaBean naming convention.
For most uses, this form of EL is not very versatile.
2.2. Deferred Execution EL
Deferred Execution EL is the EL designed for JSF proper. It’s major syntactical difference with JSP EL is that it’s marked with a “#” instead of a “$“.
#{ELBean.value > 0}
Deferred EL:
- Is in sync with the JSF lifecycle. This means that an EL expression in deferred EL is evaluated at different points in the rendering of a JSF page (at the beginning and the end).
- Provides read and write access to bean values. This allows one to set a value in a JSF backing-bean (or anywhere else) using EL.
- Allows a programmer to invoke arbitrary methods on an object and depending on the version of EL, pass arguments to such methods.
Unified EL is the specification that unifies both deferred EL and JSP EL, allowing both syntax in the same page.
3. Unified EL
Unified EL allows two general flavors of expressions, value expressions and method expressions.
And a quick note – the following sections will show some examples, which are all available in the app (see the Github link at the end) by navigating to:
http://localhost:8080/jsf/el_intro.jsf
3.1. Value Expressions
A value expression allows us to either read or set a managed bean property, depending on where it’s placed.
The following expression reads a managed bean property onto the page:
Hello, #{ELBean.firstName}
The following expression however, allows us to set a value on the user object:
<h:inputText id="firstName" value="#{ELBean.firstName}" required="true"/>
The variable must follow JavaBean naming convention to be eligible for this kind of treatment. For the value of the bean to be committed, the enclosing form just needs to be saved.
3.2. Method Expressions
Unified EL provides method expressions to execute public, non-static methods from within a JSF page. The methods may or may not have return values.
Here’s a quick example:
<h:commandButton value="Save" action="#{ELBean.save}"/>
The save() method being referred to is defined on a backing bean named ELBean.
Starting from EL 2.2, you can also pass arguments to the method that’s accessed using EL. This can allow us to rewrite our example thus:
<h:inputText id="firstName" binding="#{firstName}" required="true"/>
<h:commandButton value="Save"
action="#{ELBean.saveFirstName(firstName.value.toString().concat('(passed)'))}"/>
What we’ve done here, is to create a page-scoped binding expression for the inputText component and directly pass the value attribute to the method expression.
Note that the variable is passed to the method without any special notation, curly braces or escape characters.
3.3. Implicit EL Objects
The JSF EL engine provides access to several container-managed objects. Some of them are:
- #{Application}: Also available as the #{servletContext}, this is the object representing the web application instance
- #{applicationScope}: a map of variables accessible web application-wide
- #{Cookie}: a map of the HTTP Cookie variables
- #{facesContext}: the current instance of FacesContext
- #{flash}: the JSF Flash scoped-object
- #{header}: a map of the HTTP headers in the current request
- #{initParam}: a map of the context initialization variables of the web application
- #{param}: a map of the HTTP request query parameters
- #{request}: the HTTPServletRequest object
- #{requestScope}: a request-scoped map of variables
- #{sessionScope}: a session-scoped map of variables
- #{session}: the HTTPSession object
- #{viewScope}: a view (page-) scoped map of variables
The following simple example lists all the request headers and values by accessing the headers implicit object:
<c:forEach items="#{header}" var="header">
<tr>
<td>#{header.key}</td>
<td>#{header.value}</td>
</tr>
</c:forEach>
4. What You Can Do in EL
In it’s versatility, EL can be featured in Java code, XHTML markup, Javascript and even in JSF configuration files like the faces-config.xml file. Let’s examine some concrete use-cases.
4.1. Use EL in Page Markup
EL can be featured in standard HTML tags:
<meta name="description" content="#{ELBean.pageDescription}"/>
4.2. Use EL in JavaScript
EL will be interpreted when encountered in Javascript or <script> tags:
<script type="text/javascript"> var theVar = #{ELBean.firstName};</script>
A backing bean variable will be set as a javascript variable here.
4.3. Evaluate Boolean Logic in EL Using Operators
EL supports fairly advanced comparison operators:
- eq equality operator, equivalent to “==.”
- lt less than operator, equivalent to “<.”
- le less than or equal to operator, equivalent to “<=.”
- gt greater than operator, equivalent to “>.”
- ge greater than or equal, equivalent to “>=.“
4.4. Evaluate EL in a Backing Bean
From within the backing bean code, one can evaluate an EL expression using the JSF Application. This opens up a world of possibilities, in connecting the JSF page with the backing bean. You could retrieve implicit EL objects, or retrieve actual HTML page components or their value easily from the backing bean:
FacesContext ctx = FacesContext.getCurrentInstance();
Application app = ctx.getApplication();
String firstName = app.evaluateExpressionGet(ctx, "#{firstName.value}", String.class);
HtmlInputText firstNameTextBox = app.evaluateExpressionGet(ctx, "#{firstName}", HtmlInputText.class);
This allows the developer a great deal of flexibility in interacting with a JSF page.
5. What You Can Not Do in EL
EL < 3.0 does have some limitations. The following sections discuss some of them.
5.1. No Overloading
EL doesn’t support the use of overloading. So in a backing bean with the following methods:
public void save(User theUser); public void save(String username); public void save(Integer uid);
JSF EL will not be able to properly evaluate the following expression
<h:commandButton value="Save" action="#{ELBean.save(firstName.value)}"/>
The JSF ELResolver will introspect the class definition of bean, and pick the first method returned by java.lang.Class#getMethods (a method that returns the methods available in a class). The order of the methods returned is not guaranteed and this will inevitably result in undefined behaviour.
5.2. No Enums or Constant Values
JSF EL < 3.0, doesn’t support the use of constant values or Enums in the script. So, having any of the following
public static final String USER_ERROR_MESS = "No, you can’t do that";
enum Days { Sat, Sun, Mon, Tue, Wed, Thu, Fri };
means that you won’t be able to do the following
<h:outputText id="message" value="#{ELBean.USER_ERROR_MESS}"/>
<h:commandButton id="saveButton" value="save" rendered="bean.offDay==Days.Sun"/>
5.3. No Built-in Null Safety
JSF EL < v3.0 doesn’t provide implicit null safe access, which some may find odd about a modern scripting engine.
So if person in the expression below is null, the entire expression fails with an unsightly NPE
Hello Mr, #{ELBean.person.surname}"
6. Conclusion
We’ve examined some of the fundamentals of JSF EL, strengths and limitations.
This is largely a versatile scripting language with some room for improvement; it’s also the glue that binds the JSF view, to the JSF model and controller.
The source code that accompanies this article is available at GitHub.