Introduction to SpEL (Spring Expression Language)

When writing bean wiring configuration in Spring it happens from time to time that we need something more than just primitive values and references to other beans to setup our bean. For example we may want to set bean property to value of some class static field or maybe we need to initialize that property with result of some method call. Of course we could use bean init-method and initialize these properties from Java code but doing this from configuration file would be more convenient and would keep our business logic separated from configuration. Fortunately Spring allows us to use simple expressions while wiring beans via SpEL - Spring Expression Language.

SpEL expression in configuration files must be put inside #{ } delimiters.
As every self respecting language SpEL supports literals:

<bean id="spelDemo" class="SpELDemo">
  <property name="boolValue" value="#{true}" />
  <property name="intValue" value="#{22}" />
  <property name="doubleValue" value="#{1.23e4}" />
  <property name="stringValue" value="#{'foo'}" />
</bean>
<!-- this produces bean with values:
  boolValue=true, intValue=22, 
  doubleValue=12300.0, stringValue=foo -->

SpEL also supports simple arithmetic (+,-,*,/,% and ^ - exponentiation), string concatenation (via +) and string interpolation:

<bean id="spelDemo" class="SpELDemo">
  <property name="intExpr" value="#{1+2+3}" />
  <property name="stringExpr" value="#{'foo'+'bar'}" />
  <property name="stringInterpolation" 
      value="[#{'foo'}: #{3*4}]" />
</bean>
<!-- this produces bean with values:
  intExpr=6, stringExpr=foobar,
  stringInterpolation=[foo: 12] -->

SpEL supports full range of boolean (!, not, and, or) and comparison operators (eq, ne, lt, le, gt, ge). We can refer to boolean negation using either ! or not. Comparison operators can be written using mnemonics e.g. lt or symbols e.g. <, since in XML files we must escape < and > characters it’s better to stick to mnemonics. Again example will be handy here:

<property name="bool1" value="#{4 lt 8}" />
<property name="bool2" value="#{true or false}" />
<property name="bool3" value="#{(3 gt 5) or not (5 gt 3)}" />
<property name="bool4" value="#{8 ne 8}" />

<!-- property values:
  bool1=true, bool2=true, 
  bool3=false, bool4=false -->

Now it’s time to move to more advanced SpEL features, let’s start with method calls and reading JavaBeans properties:

<bean id="myJavaBean" class="MyJavaBean">
  <property name="myProperty" value="SpEL is awesome!" />
</bean>

<bean id="spelDemo" class="SpELDemo">
  <!-- when reading properties we may write obj.foo
        instead of obj.getFoo() -->
  <property name="stringExpr" value="#{myJavaBean.myProperty}" />

  <!-- method calls work just as in Java, we may also
        chain them e.g. 'foo'.substring(1).toUpperCase() -->
  <property name="boolExpr" value="#{'foo'.contains('o')}" />
</bean>

<!-- this produces bean with values:
  stringExpr=SpEL is awesome!, boolExpr=true -->

The last example showed important feature of SpEL - inside SpEL expressions we may refer to other beans by their id. This allows us to create complex wirings. For example let’s say that we have two consumers and a single producer beans that must share the same instance of queue. To complicate things queue is a part of the producer bean (imagine that we use some complex legacy library that we cannot change). With SpEL we can produce such wiring with ease:

<bean id="producer" class="...">...</bean>

<bean id="consumer1" class="...">
  <property name="inQueue" value="#{producer.outQueue}" />
</bean>
<bean id="consumer2" class="...">
  <property name="inQueue" value="#{producer.outQueue}" />
</bean>

The other scenario when SpEL shines is injection of beans created by factory method. For example let’s say we have emailService bean that takes collection of MessageFilters. All message filters are created via filterFactory bean, and we want to keep configuration of what filters to use in XML. We can code such scenario using SpEL:

<bean id="filterFactory" class="FilterFactory"></bean>
<bean id="emailService" class="EmailService">
  <constructor-arg>
    <list>
      <value>#{filterFactory.createFilter("SPAM")}</value>
      <value>#{filterFactory.createFilter("ANTIVIRUS")}</value>
      <value>#{filterFactory.createFilter("SIZE")}</value>
    </list>
  </constructor-arg>
</bean>

As mentioned previously SpEL allows us to access static class members using syntax T(full.class.name).member, for example:

<property name="doubleValue" 
  value="#{T(java.lang.Math).random()}" />

<property name="doubleValue2" 
  value="#{T(java.lang.Math).PI}" />

Sometimes when writing SpEL expressions we want to inject different beans depending on some condition. For example we may want to inject TextMessageNotificationService only when text messages are enabled in configuration. Fortunately for us SpEL supports good old ?: ternary operator:

<bean id="configuration" class="Configuration"
  init-method="loadConfiguration">
  <!-- this bean loads configuration from database -->
</bean>

<bean id="textMessageNotificationService"
   class="TextMessageNotificationService" />
<bean id="emailNotificationService"
   class="EmailNotificationService" />
 
<bean id="orderingService" class="OrderingService">
   <property name="notificationService"
      value="#{configuration.useTextMessages ? 
         textMessageNotificationService : 
         emailNotificationService}" />
</bean>

SpEL also supports so known Elvis operator ?:. Writing foo ?: bar is equivalent to writing (foo != null) ? foo : bar, in other words Elvis operator is mostly used to provide default values:

<property name="myProperty" 
  value="#{someBean.stringProperty ?: 'default'}" />

The last thing that I want to show is how to use SpEL to get values from .properties file. First we must load properties using Spring util:properties element which will create instance of java.util.Properties class and register it as a bean with appProperties name. Then inside SpEL expression we may use appProperties.propertyName or appProperties['propertyName'] to get actual property value:

<util:properties id="appProperties"
    location="classpath:app.properties" />

<bean id="someBean" class="MyJavaBean">
   <property name="foo" value="#{appProperties.foo}" />
   <property name="bar" value="#{appProperties['bar']}" />
</bean>
# app.properties
foo=1
bar=2

That’s all for today! We only scratched the surface of what SpEL can do so if you get interested don’t forget to check friendly Spring docs.

marcin-chwedczuk

A Programmer, A Geek, A Human