Spring - Injecting Collection

You have seen how to configure primitive data type using value attribute and object references using ref attribute of the <property> tag in your Bean configuration file. Both the cases deal with passing singular value to a bean.

Now what if you want to pass plural values like Java Collection types such as List, Set, Map, and Properties. To handle the situation, Spring offers four types of collection configuration elements which are as follows −

You can use either <list> or <set> to wire any implementation of java.util.Collection or an array.

You will come across two situations (a) Passing direct values of the collection and (b) Passing a reference of a bean as one of the collection elements.

Example

Let us have a working Eclipse IDE in place and take the following steps to create a Spring application −

Here is the content of JavaCollection.java file −

package com.howcodex;
import java.util.*;

public class JavaCollection {
   List addressList;
   Set  addressSet;
   Map  addressMap;
   Properties addressProp;

   // a setter method to set List
   public void setAddressList(List addressList) {
      this.addressList = addressList;
   }
   
   // prints and returns all the elements of the list.
   public List getAddressList() {
      System.out.println("List Elements :"  + addressList);
      return addressList;
   }
   
   // a setter method to set Set
   public void setAddressSet(Set addressSet) {
      this.addressSet = addressSet;
   }
   
   // prints and returns all the elements of the Set.
   public Set getAddressSet() {
      System.out.println("Set Elements :"  + addressSet);
      return addressSet;
   }
   
   // a setter method to set Map
   public void setAddressMap(Map addressMap) {
      this.addressMap = addressMap;
   }
   
   // prints and returns all the elements of the Map.
   public Map getAddressMap() {
      System.out.println("Map Elements :"  + addressMap);
      return addressMap;
   }
   
   // a setter method to set Property
   public void setAddressProp(Properties addressProp) {
      this.addressProp = addressProp;
   }
   
   // prints and returns all the elements of the Property.
   public Properties getAddressProp() {
      System.out.println("Property Elements :"  + addressProp);
      return addressProp;
   }
}

Following is the content of the MainApp.java file −

package com.howcodex;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");
      JavaCollection jc=(JavaCollection)context.getBean("javaCollection");

      jc.getAddressList();
      jc.getAddressSet();
      jc.getAddressMap();
      jc.getAddressProp();
   }
}

Following is the configuration file Beans.xml which has configuration for all the type of collections −

<?xml version = "1.0" encoding = "UTF-8"?>

<beans xmlns = "http://www.springframework.org/schema/beans"
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation = "http://www.springframework.org/schema/beans
   http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

   <!-- Definition for javaCollection -->
   <bean id = "javaCollection" class = "com.howcodex.JavaCollection">
      
      <!-- results in a setAddressList(java.util.List) call -->
      <property name = "addressList">
         <list>
            <value>INDIA</value>
            <value>Pakistan</value>
            <value>USA</value>
            <value>USA</value>
         </list>
      </property>

      <!-- results in a setAddressSet(java.util.Set) call -->
      <property name = "addressSet">
         <set>
            <value>INDIA</value>
            <value>Pakistan</value>
            <value>USA</value>
            <value>USA</value>
         </set>
      </property>

      <!-- results in a setAddressMap(java.util.Map) call -->
      <property name = "addressMap">
         <map>
            <entry key = "1" value = "INDIA"/>
            <entry key = "2" value = "Pakistan"/>
            <entry key = "3" value = "USA"/>
            <entry key = "4" value = "USA"/>
         </map>
      </property>
      
      <!-- results in a setAddressProp(java.util.Properties) call -->
      <property name = "addressProp">
         <props>
            <prop key = "one">INDIA</prop>
            <prop key = "one">INDIA</prop>
            <prop key = "two">Pakistan</prop>
            <prop key = "three">USA</prop>
            <prop key = "four">USA</prop>
         </props>
      </property>
   </bean>

</beans>

Once you are done creating the source and bean configuration files, let us run the application. If everything is fine with your application, it will print the following message −

List Elements :[INDIA, Pakistan, USA, USA] 
Set Elements :[INDIA, Pakistan, USA] 
ap Elements :{1 = INDIA, 2 = Pakistan, 3 = USA, 4 = USA} 
Property Elements :{two = Pakistan, one = INDIA, three = USA, four = USA} 

Injecting Bean References

The following Bean definition will help you understand how to inject bean references as one of the collection's element. Even you can mix references and values all together as shown in the following code snippet −

<?xml version = "1.0" encoding = "UTF-8"?>

<beans xmlns = "http://www.springframework.org/schema/beans"
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation = "http://www.springframework.org/schema/beans
   http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

   <!-- Bean Definition to handle references and values -->
   <bean id = "..." class = "...">

      <!-- Passing bean reference  for java.util.List -->
      <property name = "addressList">
         <list>
            <ref bean = "address1"/>
            <ref bean = "address2"/>
            <value>Pakistan</value>
         </list>
      </property>
      
      <!-- Passing bean reference  for java.util.Set -->
      <property name = "addressSet">
         <set>
            <ref bean = "address1"/>
            <ref bean = "address2"/>
            <value>Pakistan</value>
         </set>
      </property>
      
      <!-- Passing bean reference  for java.util.Map -->
      <property name = "addressMap">
         <map>
            <entry key = "one" value = "INDIA"/>
            <entry key = "two" value-ref = "address1"/>
            <entry key = "three" value-ref = "address2"/>
         </map>
      </property>
   </bean>

</beans>

To use the above bean definition, you need to define your setter methods in such a way that they should be able to handle references as well.

Injecting null and empty string values

If you need to pass an empty string as a value, then you can pass it as follows −

<bean id = "..." class = "exampleBean">
   <property name = "email" value = ""/>
</bean>

The preceding example is equivalent to the Java code: exampleBean.setEmail("")

If you need to pass a NULL value, then you can pass it as follows −

<bean id = "..." class = "exampleBean">
   <property name = "email"><null/></property>
</bean>

The preceding example is equivalent to the Java code: exampleBean.setEmail(null)