How do I combine filter and projection operation in Spring EL?

Using Spring Expression Language (SpEL) we can filter a collection based on some criteria. We can also create a projection of a collection by collecting only a particular property from the collection objects.

Now you know that you have two good features of SpEL that are really powerful to use when working with collection objects manipulation. But you are wondering how to combine both of these filter and projection in one expression. Can you do this in Spring EL? The answer is yes! You can combine them both in one expression. Let’s see an example below.

We are going to use the same configuration used in the previous example:

<?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.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="library" class="org.kodejava.example.spring.model.Library">
        <property name="bookTitles" value="#{books.?[pages gt 250].![title]}"/>
    </bean>
</beans>

In the configuration above, when we define the library bean we set its bookTitles property using the filtering and projection operator. First we take only books that have more that 250 pages and then we create the projection that contains only the book title. So this expression give us all the book’s title of a book that has more than 250 pages.

To make the example complete here again the definition of the Book and the Library class.

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
    //
}
package org.kodejava.example.spring.model;

import java.util.List;

public class Library {
    private List<Book> books;
    private List<String> bookTitles;
    
    //
    // Getters & Setters
    //
}

The main class the run the configuration file:

package org.kodejava.example.spring;

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

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

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

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

The result of the code snippet:

INFO: Loading XML bean definitions from class path resource [spel-filter-projection.xml]
title = Essential C# 4.0
title = User Stories Applied
title = Einstein

How do I create a projection of a collection using Spring EL?

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.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="library" class="org.kodejava.example.spring.model.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.example.spring.model;

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

    //
    // Getters & Setters
    //
}
package org.kodejava.example.spring.model;

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.example.spring.model.Library">
    <property name="bookTitles" value="#{books.![title]}"/>
</bean>

The bean element above create a library bean of type org.kodejava.example.spring.model.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.example.spring;

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

public class SpELProjectionExample {
    public static void main(String[] args) {
        ApplicationContext 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:

INFO: Loading XML bean definitions from class path resource [spel-projection.xml]
title = Essential C# 4.0
title = User Stories Applied
title = Learning Android
title = The Ruby Programming Language
title = Einstein

How do I filter collection members based on some criteria using Spring EL?

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}

How do I read system environment variables in Spring EL?

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

How do I read a value from properties file using Spring EL?

In the previous two examples you have seen how to access member of a collection and access a map element using the square-braces [] operator in Spring EL. In this example you will see how to use the [] operator to read a value from a properties file or java.util.Properties. Let’s say we have a database properties file called database.properties with the following entries in it:

jdbc.driverClassName=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost/mydb
jdbc.username=root
jdbc.password=r00t

First, let’s create the spring configuration file. In this configuration we will use the <util:properties> to load the properties configuration into Spring. And then we will use Spring EL to access the value of this properties and assign it to some bean’s properties.

<?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.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd">

    <util:properties id="database" location="classpath:database.properties"/>

    <bean id="dataSource" class="org.kodejava.example.spring.model.MyDataSource">
        <property name="driverClassName" value="#{database['jdbc.driverClassName']}"/>
        <property name="url" value="#{database['jdbc.url']}"/>
        <property name="username" value="#{database['jdbc.username']}"/>
        <property name="password" value="#{database['jdbc.password']}"/>
    </bean>
</beans>

To read a value from properties file what you do is the same as how we access an element of a map object. We pass the name of the properties as the key in the Spring EL.

<property name="driverClassName" value="#{database['jdbc.driverClassName']}"/>

The MyDataSource class is an imaginary data source object. It has some properties such as the driverClassName, url, username and password. It’s a common parameter you use to connect to a database using a JDBC driver. For simplicity the getters and setters we removed from the class.

package org.kodejava.example.spring.model;

public class MyDataSource {
    private String driverClassName;
    private String url;
    private String username;
    private String password;

    //
    // Getters & Setters
    //
}

As always, to run the Spring configuration above we will need to create a main class that load and execute the application context. This class will obtain the dataSource bean from the application context and print out its properties whose values are read from a properties file called database.properties.

package org.kodejava.example.spring;

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

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

        MyDataSource dataSource = (MyDataSource) context.getBean("dataSource");
        System.out.println("driverClassName = " + dataSource.getDriverClassName());
        System.out.println("url             = " + dataSource.getUrl());
        System.out.println("username        = " + dataSource.getUsername());
        System.out.println("password        = " + dataSource.getPassword());
    }
}

Here are the result you get when running the code snippet:

driverClassName = com.mysql.jdbc.Driver
url             = jdbc:mysql://localhost/mydb
username        = root
password        = r00t