Spring - Reading values from file and injecting to bean property using SPEL

Sometimes you would need to inject value for spring bean property by reading values from a file. Good use case for this would be injecting passwords into bean property by reading value from a file rather than hard coding password into application context xml.

This can be done by creating property file with values we want to inject and use spring util properties to read values into application context as properties. Then, this property value can be injected using Spring SpEL.

To demonstrate this, lets pick example of initializing Jetty SslContextFactory with keystore file and key store password.

First, Create a property file say keyStorePwd.properties with values

passwd=mypass1234

Then, we can use following code in Spring Application Context to read that property file and inject value into keyStorePassword property of org.eclipse.jetty.util.ssl.SslContextFactory class.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xmlns:util="http://www.springframework.org/schema/util"
             xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
           http://www.springframework.org/schema/util
           http://www.springframework.org/schema/util/spring-util.xsd">

    <util:properties id="keyStorePwdProperties" location="keyStorePwd.properties" />
    
    <bean id="sslContextFactory" class="org.eclipse.jetty.util.ssl.SslContextFactory">
        <property name="keyManagerPassword" value="#{keyStorePwdProperties.passwd}"/> 
        <constructor-arg value="file:keyStoreFile.jks"/>
    </bean>

</beans>

The Spring SpEL expression #{keyStorePwdProperties.passwd} takes value of passwd property read from keyStorePwd.properties and injects into spring bean property.