In some of the previous example you have seen that we use the square-braces [] operator to select items from collection. In this blog post you will learn how to filter some members of a collection with a certain criteria while using the Spring EL. To do this Spring EL give you another special operator, the filter operator which can be typed like .?[]. Where you can define the filter criteria inside the braces.

Beside the filter operator .?[]> there are also operator that can select the first and the last matching items from collection. You can use the .^[] for selecting the first match and the .$[] operator to select the last match items from collection respectively.

As an example we will demonstrate here that we want to find a collection of books that have pages more than 250 and assign it to Library bean’s books properties. So let’s begin by defining a class called Book and Library which will have the following properties:

package org.kodejava.example.spring.model;

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

    //
    // Getters & Setters
    //

    @Override
    public String toString() {
        return "Book{" +
                "title='" + title + ''' +
                ", author='" + author + ''' +
                ", pages=" + pages +
                '}';
    }
}

package org.kodejava.example.spring.model;

import java.util.List;

public class Library {
    private List books;

    // 
    // Getter & Setter
    //
}

After creating the Book and the Library class let’s now create the Spring configuration file for our demo. We will create a file and call it as spel-select-collection.xml with the following lines of configuration in it.

<?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:p="http://www.springframework.org/schema/p"
       xmlns:util="http://www.springframework.org/schema/util"
       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.example.spring.model.Book"
              p:title="Essential C# 4.0" p:author="Michaelis" p:pages="450"/>
        <bean class="org.kodejava.example.spring.model.Book"
              p:title="User Stories Applied" p:author="Mike Cohen" p:pages="268"/>
        <bean class="org.kodejava.example.spring.model.Book"
              p:title="Learning Android" p:author="Marco Gargenta" p:pages="245"/>
        <bean class="org.kodejava.example.spring.model.Book"
              p:title="The Ruby Programming Language"
              p:author="David Flanagan &amp; Yukihiro Matsumoto" p:pages="250"/>
        <bean class="org.kodejava.example.spring.model.Book"
              p:title="Einstein" p:author="Walter Isaacson" p:pages="1000"/>
    </util:list>

    <bean id="lib1" class="org.kodejava.example.spring.model.Library">
        <property name="books" value="#{books.?[pages gt 250]}"/>
    </bean>

    <bean id="lib2" class="org.kodejava.example.spring.model.Library">
        <property name="books" value="#{books.^[pages gt 250]}"/>
    </bean>

    <bean id="lib3" class="org.kodejava.example.spring.model.Library">
        <property name="books" value="#{books.$[pages gt 250]}"/>
    </bean>
</beans>

The first thing that you can see in the configuration above is the <util-list> where we create a list of Book beans. Next we have three beans definition of the type Library where the books property was assigned with a collection of beans selected from the books list.

The lib1 will contains books that have pages more than 250. The lib2 will contains the first book found to have more than 250 pages while the lib3 will contains the last book found to have more than 250 pages.

Below is the code that will run our configuration file and the example output it will produce.

package org.kodejava.example.spring;

import org.kodejava.example.spring.model.Book;
import org.kodejava.example.spring.model.Library;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpELFilterCollection {
    public static void main(String[] args) {
        ApplicationContext context =
                new ClassPathXmlApplicationContext("spel-filter-collection.xml");

        System.out.println("Library 1");
        Library lib1 = (Library) context.getBean("lib1");
        for (Book book : lib1.getBooks()) {
            System.out.println("    Book = " + book);
        }

        System.out.println("Library 2");
        Library lib2 = (Library) context.getBean("lib2");
        for (Book book : lib2.getBooks()) {
            System.out.println("    Book = " + book);
        }

        System.out.println("Library 3");
        Library lib3 = (Library) context.getBean("lib3");
        for (Book book : lib3.getBooks()) {
            System.out.println("    Book = " + book);
        }
    }
}

INFO: Loading XML bean definitions from class path resource [spel-filter-collection.xml]
Library 1
    Book = Book{title='Essential C# 4.0', author='Michaelis', pages=450}
    Book = Book{title='User Stories Applied', author='Mike Cohen', pages=268}
    Book = Book{title='Einstein', author='Walter Isaacson', pages=1000}
Library 2
    Book = Book{title='Essential C# 4.0', author='Michaelis', pages=450}
Library 3
    Book = Book{title='Einstein', author='Walter Isaacson', pages=1000}

Previously you have seen that we can load properties file and read a value from it in this example: How do I read a value from properties file using Spring EL?. In this example you will learn how to read a special properties available to Spring EL. These properties includes the systemEnvironment and systemProperties.

The systemEnvironment property contains all the environment variables on the machine where the program is running. Meanwhile, the systemProperties contains all the properties that we set in Java when the application started, using the -D argument. Let’s see how to access both of these properties in the following 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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="program1"
          class="org.kodejava.example.spring.model.Program">
        <property name="logPath"
                  value="#{systemProperties['APP.LOG_PATH']}"/>
    </bean>
    <bean id="program2"
          class="org.kodejava.example.spring.model.Program">
        <property name="logPath"
                  value="#{systemEnvironment['HOME']}"/>
    </bean>
</beans>

In the configuration above we have two beans of Program. We set the logPath properties using a different property source. In the program1 bean we use systemProperties['APP.LOG_PATH']. Using this method the value will be pass to our program using the -DAPP.LOG_PATH=/Users/wsaryada/tmp when we are executing the program. While the program2 bean’s logPath is read from user’s home directory property available through the systemEnvironment variables.

To make the Spring configuration works you’ll need the Program class. So here is the class definition.

package org.kodejava.example.spring.model;

public class Program {
    private String logPath;

    public Program() {
    }

    public String getLogPath() {
        return logPath;
    }

    public void setLogPath(String logPath) {
        this.logPath = logPath;
    }
}

Finally, let’s create a simple class to execute the Spring configuration file above and see the result of the code.

package org.kodejava.example.spring;

import org.kodejava.example.spring.model.MyDataSource;
import org.kodejava.example.spring.model.Program;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpELEnvironment {
    public static void main(String[] args) {
        ApplicationContext context =
                new ClassPathXmlApplicationContext("spel-environment.xml");

        Program program1 = (Program) context.getBean("program1");
        System.out.println("program.getLogPath() = " + program1.getLogPath());

        Program program2 = (Program) context.getBean("program2");
        System.out.println("program.getLogPath() = " + program2.getLogPath());
    }
}

The code will print the following result:

INFO: Loading XML bean definitions from class path resource [spel-environment.xml]
program.getLogPath() = /Users/wsaryada/tmp
program.getLogPath() = /Users/wsaryada