What exactly do the following SpEL expression?

Question

I have some doubt about what exactly do these 3 Spring SpEL example:

1) FIRST EXAMPLE:

<bean id="rewardsDb" class="com.acme.RewardsTestDatabase">
    <property name="keyGenerator" value="#{strategyBean.databaseKeyGenerator}" />
</bean>

It seems to me that this code snippet injet an inner property named databaseKeyGenerator (that is inside the strategyBean bean). So in this case SpEL is used to access to a specific bean property in the classica OO logic. Is it true?

2) SECOND EXAMPLE:

<bean id="strategyBean" class="com.acme.DefaultStrategies">
    <property name="databaseKeyGenerator" ref="myKeyGenerator"/>
</bean>

It seems to me that SpEL is not used, or am I missing something?

3) THIRD EXAMPLE:

<bean id="taxCalculator" class="com.acme.TaxCalculator">
    <property name="defaultLocale" value="#{ systemProperties['user.region'] }"/>
</bean> Equivalent

It is used to inject a property value taken from a property file

Is it correct or I a missing something or am I misinterpreting the SpEL logic?

Answer 1

The first and second examples come together. The second actually uses no SpEL at all. Its sole purpose is to help understand the first one. So you are not missing something regarding the first two.

As for the third one, systemProperties is a predefined variable and you use it to access system properties. Except from the standard VM system properties you can also access those that you pass with -D when starting the application.

You can access a property file the same way, after creating a bean to reference them, by using the bean id instead of systemProperties. For example

<util:properties id="appProps" location="classpath:application.properties" />

and then

<property name="propOne" value="#{appProps['some.property'] }"/>