Spring Framework Constructor Injection with Collections Example

In this tutorial, we will modify our previous example and add a few collections so we can illustrate how we use constructor injection with differing collection types. Specifically, we will show how to perform constructor injection with List, Map and Set objects.

Client Class (Client.java)

package com.avaldes;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;

public class Client {
  private int id;
  private String name;
  private String email;
  private boolean active;
  private ArrayList<String> children;
  private HashMap<String, String> phones;
  private HashSet<Integer> favoriteNumbers; 
  
  public Client(int id, String name, String email, 
                boolean active, 
                ArrayList<String> children, 
                HashMap<String, String> phones, 
                HashSet<Integer> favoriteNumbers) {

    this.id = id;
    this.name = name;
    this.email = email;
    this.active = active;
    this.children = children;
    this.phones = phones;
    this.favoriteNumbers = favoriteNumbers;
  }

  @Override
  public String toString() {
    return "Client [id=" + id + 
                ", name=" + name + 
                ", email=" + email + 
                ", active=" + active + 
                ", children=" + children + 
                ", phones=" + phones + 
                ", favoriteNumbers=" + favoriteNumbers + "]";
  }
}

I have taken the liberty of removing all the getter and setter methods from the Client class and modified the constructor to allow us to pass in children, phones and favoriteNumbers (I know, its a stretch — humor me)…

Constructor Injection – Populating List Collection Elements

For the List collection, we use the constructor argument with name attribute of “children“, create the list node and add value nodes with each of Albert Einstein’s children.

<constructor-arg name="children">
  <list>
    <value>Hans Albert Einstein</value>
    <value>Eduardo Einstein</value>
  </list>
</constructor-arg>

Constructor Injection – Populating Map Collection Elements

Next, we will work on Map collection, which has key/value pairs. In this example, the collection is aptly named “phones“. We create the Map node, but will need to add entry key nodes with each key correctly labeled and the value for each phone in the corresponding value node.

<constructor-arg name="phones">
  <map>
    <entry key="home">
      <value>212-555-1212</value>
    </entry>
    <entry key="mobile">
      <value>212-444-1313</value>
    </entry>
    <entry key="work">
      <value>212-333-3456</value>
    </entry>
  </map>
</constructor-arg>

Constructor Injection – Populating Set Collection Elements

In this last example, we are populating a Set which looks very similar to List, except that the node will correspond to set instead of list. The main differing quality between a Set and List is that sets do not contain duplicate elements.

<constructor-arg name="favoriteNumbers">
  <set>
    <value>11</value>
    <value>17</value>
    <value>25</value>
    <value>39</value>
  </set>
</constructor-arg>

Constructor Injection – Full (spring-config.xml)

<bean id="client6" class="com.avaldes.Client">
  <constructor-arg name="email" value="albert.einstein@princeton.edu">
  </constructor-arg>
  <constructor-arg name="active" value="false"></constructor-arg>
  <constructor-arg name="name" value="Albert Einstein"></constructor-arg>  
  <constructor-arg name="id" type="int" value="789"></constructor-arg>
  <constructor-arg name="children">
    <list>
      <value>Hans Albert Einstein</value>
      <value>Eduardo Einstein</value>
    </list>
  </constructor-arg>
  <constructor-arg name="phones">
    <map>
      <entry key="home">
        <value>212-555-1212</value>
      </entry>
      <entry key="mobile">
        <value>212-444-1313</value>
      </entry>
      <entry key="work">
        <value>212-333-3456</value>
      </entry>
    </map>
  </constructor-arg>
  <constructor-arg name="favoriteNumbers">
    <set>
      <value>11</value>
      <value>17</value>
      <value>25</value>
      <value>39</value>
    </set>
  </constructor-arg>
</bean>

Output – Working with different collections

Take a look at the completed object, and notice you the children, phones and favoriteNumbers are constructed and how the output looks.

Client [id=789, name=Albert Einstein, email=albert.einstein@princeton.edu, active=false, children=[Hans Albert Einstein, Eduardo Einstein], phones={home=212-555-1212, mobile=212-444-1313, work=212-333-3456}, favoriteNumbers=[11, 17, 25, 39]]

That’s It

As you can see from the above examples, adding collections as constructor arguments is not that difficult a task to perform. Enjoy Spring!

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