In this example you will learn how to create a projection of a collection object. Using projection we can create a new collection with only a specific property from the original collection.

As an example, instead of returning a collection of Book objects we would like only to have the titles of the books. To do this we can use the Spring EL projection operator. The symbol use for this operator is ![].

Let’s begin by creating the Spring configuration file:

<?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" xmlns:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">

    <util:list id="books">
        <bean class="org.kodejava.spring.core.el.Book" p:title="Essential C# 4.0" p:author="Michaelis" p:pages="450" />
        <bean class="org.kodejava.spring.core.el.Book" p:title="User Stories Applied" p:author="Mike Cohen"
              p:pages="268" />
        <bean class="org.kodejava.spring.core.el.Book" p:title="Learning Android" p:author="Marco Gargenta"
              p:pages="245" />
        <bean class="org.kodejava.spring.core.el.Book" p:title="The Ruby Programming Language"
              p:author="David Flanagan & Yukihiro Matsumoto" p:pages="250" />
        <bean class="org.kodejava.spring.core.el.Book" p:title="Einstein" p:author="Walter Isaacson" p:pages="1000" />
    </util:list>

    <bean id="library" class="org.kodejava.spring.core.el.Library">
        <property name="bookTitles" value="#{books.![title]}" />
    </bean>

</beans>

Here are the definition of the Book and Library class. The getters and setters methods were removed for simplicity of the snippet.

package org.kodejava.spring.core.el;

public class Book {
    private Long id;
    private String title;
    private String author;
    private String type;
    private int pages;

    // Getters & Setters
}

package org.kodejava.spring.core.el;

import java.util.List;

public class Library {
    private List<Book> books;
    private List<String> bookTitles;

    // Getters & Setters
}

Now, let’s talk about the Spring configuration above. The configuration was start by creating a collection of Books using the <util:elements>. The part that use the projection operator is this part of the configuration:

<bean id="library" class="org.kodejava.spring.core.el.Library">
    <property name="bookTitles" value="#{books.![title]}"/>
</bean>

The bean element above create a library bean of type org.kodejava.spring.core.el.Library. We assign the bean’s bookTitles property with values that are a projection of the books collection where we take only the title of the books. ![projectionExpression] is the syntax of the projection operator.

The code snippet below will demonstrate and run our configuration file and print out the result:

package org.kodejava.spring.core.el;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpELProjectionExample {
    public static void main(String[] args) {
        try (ClassPathXmlApplicationContext context =
                     new ClassPathXmlApplicationContext("spel-projection.xml")) {

            Library library = context.getBean("library", Library.class);

            for (String title : library.getBookTitles()) {
                System.out.println("title = " + title);
            }
        }
    }
}

And here are the result:

title = Essential C# 4.0
title = User Stories Applied
title = Learning Android
title = The Ruby Programming Language
title = Einstein

Maven Dependencies

<dependencies>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-core</artifactId>
        <version>5.3.23</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-beans</artifactId>
        <version>5.3.23</version>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context-support</artifactId>
        <version>5.3.23</version>
    </dependency>
</dependencies>