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How do I inject collections using list element in Spring?

By Wayan in Spring Core Last modified: June 4, 2023 0 Comment

The following example show you how to use the <list> element to wire collection property. We can use it to wire property of either arrays or some implementation of java.util.Collection such as java.util.ArrayList.

For this example we will create a bean called Album that have a collection of Song beans in it. So here is our bean classes.

package org.kodejava.spring.core;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Properties;

public class Album {
    private String title;
    private int year;
    private List<Song> songs = new ArrayList<>();
    private Map<String, Publisher> publisher = new HashMap<>();
    private Properties props = new Properties();

    public Album() {
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public void setSongs(List<Song> songs) {
        this.songs = songs;
    }

    public void setPublisher(Map<String, Publisher> publisher) {
        this.publisher = publisher;
    }

    public void setProps(Properties props) {
        this.props = props;
    }

    @Override
    public String toString() {
        return "Album{" +
                "title='" + title + '\'' +
                ", year=" + year +
                ", songs=" + songs +
                ", publisher=" + publisher +
                ", props=" + props +
                '}';
    }
}

package org.kodejava.spring.core;

public class Song {
    private String title;
    private String writer;

    public Song() {
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public void setWriter(String writer) {
        this.writer = writer;
    }

    @Override
    public String toString() {
        return "Song{" +
                "title='" + title + '\'' +
                ", writer='" + writer + '\'' +
                '}';
    }
}

package org.kodejava.spring.core;

public class Publisher {
    private String name;

    public Publisher() {
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Publisher{" +
                "name=" + name +
                '}';
    }
}

Here is the Spring configuration file, CollectionList.xml.

<?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="song1" class="org.kodejava.spring.core.Song">
        <property name="title" value="I Saw Her Standing There" />
        <property name="writer" value="Beatles" />
    </bean>

    <bean id="song2" class="org.kodejava.spring.core.Song">
        <property name="title" value="Misery" />
        <property name="writer" value="Beatles" />
    </bean>

    <bean id="song3" class="org.kodejava.spring.core.Song">
        <property name="title" value="Anna (Go to Him)" />
        <property name="writer" value="Beatles" />
    </bean>


    <bean id="album" class="org.kodejava.spring.core.Album">
        <property name="title" value="Please Please Me" />
        <property name="year" value="1963" />
        <property name="songs">
            <list>
                <ref bean="song1" />
                <ref bean="song2" />
                <ref bean="song3" />
            </list>
        </property>
    </bean>

</beans>

The part of the configuration that wire the song collection is inside the album bean. You can see that we have a property name songs. This property has a <list> element that contains a couple <ref> elements referring to some Song type beans.

Now let’s run it with the following code:

package org.kodejava.spring.core;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class DemoList {
    public static void main(String[] args) {
        var context = new ClassPathXmlApplicationContext("collection-list.xml");

        Album album = (Album) context.getBean("album");
        System.out.println("Album = " + album);
        context.close();
    }
}

You’ll see the following output when you run the program:

Album = Album{title='Please Please Me', year=1963, songs=[Song{title='I Saw Her Standing There', writer='Beatles'}, Song{title='Misery', writer='Beatles'}, Song{title='Anna (Go to Him)', writer='Beatles'}], publisher={}, props={}}

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>

How do I inject collections properties in Spring?

By Wayan in Spring Core Last modified: June 4, 2023 0 Comment

Beside injecting a simple value (using the value attribute) and a reference to other bean (using the ref attribute). We can also inject collections properties into a bean.

Spring provides four ways to inject collections. There are <list> and <set> that can be used to inject arrays or any implementation of java.util.Collection. The <map> that can be used to inject property of type java.util.Map. And the <props> can be used to inject property of type java.util.Properties.

Here is a table that summarize the collection configuration support in Spring.

Collection Element	Description
`<list>`	Wiring a list of values, where the values might have duplicates.
`<set>`	Wiring a set of values, where the values can not have duplicates.
`<map>`	Wiring a key-value pairs collection, where the key and value can be any type
`<props>`	Wiring a key-value pairs property, where the key and value are both type of String

You can see the following example on how to use each type of this collection configuration:

How do I wire properties with Spring’s p namespace?

By Wayan in Spring Core Last modified: June 4, 2023 0 Comment

Beside using the <property> element, Spring framework also provide us another way to wiring value or reference into the bean. We can use the Spring’s p namespaces. The p namespace has a schema URI of `http://www.springframework.org/schema/p`.

With this namespace declared in the Spring configuration file you can use the p: prefixed attribute of the <bean> element to wire bean’s property.

We update the configuration used in the previous example How do I inject into bean properties? to use the p namespace. Here is the new Spring’s 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:p="http://www.springframework.org/schema/p"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="rectangle" class="org.kodejava.spring.core.Rectangle" />

    <bean id="drawingBean" class="org.kodejava.spring.core.DrawingBean" p:colour="Red" p:shape-ref="rectangle" />

</beans>

The p:color attribute set the color property to Red, this is a simple value. To set the shape property we use the p:shape-ref attribute. The -ref suffix tell Spring that we are injecting a reference.

You can use the <property> element or using the p namespace to inject bean’s property. Both of them have the same functionality.

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>

How do I define inner bean in Spring?

By Wayan in Spring Core Last modified: June 4, 2023 0 Comment

Inner bean is a bean defined inside another bean, it can be seen as an inner class. In another word, the inner bean is a bean defined within the scope of another bean. In this case the inner bean can only be use by the outer bean. No other bean in the Spring context can refer to that bean.

So, if you sure that a bean is only use within a single bean it is a good idea to use an inner bean. Inner bean can be injected through setter injection or constructor injection.

Here is an example of Spring configuration for an inner bean injection:

<?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="racer" class="org.kodejava.spring.core.Racer">
        <property name="car">
            <bean class="org.kodejava.spring.core.Car">
                <property name="maker" value="Ferrari" />
                <property name="year" value="2021" />
            </bean>
        </property>
    </bean>

</beans>

In this configuration we use a setter injection. So we use the property element. Instead of using a ref attribute for referring to another bean we define the bean using the bean element inside the property element. And then we create the Car bean and sets its properties.

If you want to use a constructor injection you can inject the Car bean into the Racer bean by defining a bean inside the constructor-arg element in the Racer bean.

Below is our Racer and Car classes.

package org.kodejava.spring.core;

public class Racer {
    private Car car;

    public Racer() {
    }

    public Racer(Car car) {
        this.car = car;
    }

    public void setCar(Car car) {
        this.car = car;
    }

    @Override
    public String toString() {
        return "Racer{" +
                "car=" + car +
                '}';
    }
}

package org.kodejava.spring.core;

public class Car {
    private String maker;
    private int year;

    public void setMaker(String maker) {
        this.maker = maker;
    }

    public void setYear(int year) {
        this.year = year;
    }

    @Override
    public String toString() {
        return "Car{" +
                "maker='" + maker + "'" +
                ", year=" + year +
                '}';
    }
}

Let’s create our Demo class to run the program:

package org.kodejava.spring.core;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Demo {
    public static void main(String[] args) {
        var context = new ClassPathXmlApplicationContext("inner-bean.xml");

        Racer racer = (Racer) context.getBean("racer");
        System.out.println("Racer = " + racer);
        context.close();
    }
}

Here is the output of our program:

Racer = Racer{car=Car{maker='Ferrari', year=2021}}

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>

How do I inject into bean properties?

By Wayan in Spring Core Last modified: June 4, 2023 0 Comment

A bean usually have some private properties that can be accessed through a pair of accessor methods, the setters and getters. These setters, the setXXX() method can be used by Spring Framework to configure the beans.

This method of injecting beans property through their setter methods is called the setter injection. The following example will show you how to do it.

Below is our DrawingBean that have colour and shape properties. In the example we will inject both of the properties using their respective setter method. The configuration is done in the Spring application configuration file.

package org.kodejava.spring.core;

public class DrawingBean {
    private String colour;
    private Shape shape;

    public DrawingBean() {
    }

    public void drawShape() {
        getShape().draw();
        System.out.printf("The colour is %s.", getColour());
    }

    public String getColour() {
        return colour;
    }

    public void setColour(String colour) {
        this.colour = colour;
    }

    public Shape getShape() {
        return shape;
    }

    public void setShape(Shape shape) {
        this.shape = shape;
    }
}

We can inject a simple value into a bean, such as string, number, etc. We can also inject a reference to another bean. Here we define an example of other bean, the Rectangle bean that we will inject into the DrawingBean.

package org.kodejava.spring.core;

public interface Shape {
    /**
     * Draw a shape.
     */
    void draw();
}

package org.kodejava.spring.core;

public class Rectangle implements Shape {
    @Override
    public void draw() {
        System.out.println("Drawing a rectangle.");
    }
}

Let’s create the Spring application configuration file that will register our beans into the Spring context. After that we just create a simple program to execute 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"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="rectangle" class="org.kodejava.spring.core.Rectangle" />

    <bean id="drawingBean" class="org.kodejava.spring.core.DrawingBean">
        <property name="colour" value="Red" />
        <property name="shape" ref="rectangle" />
    </bean>

</beans>

In the configuration file above you can see that we use the property element to set a bean’s property. The name attribute is referring to the bean’s setter methods name, exclude the set prefix.

The value attribute of the property element is used to inject a simple value, such as string, int, etc. For injecting a reference to another bean we use the ref attribute instead.

package org.kodejava.spring.core;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class DrawingBeanDemo {
    public static void main(String[] args) {
        try (var context = new ClassPathXmlApplicationContext("drawing-bean.xml")) {
            DrawingBean bean = (DrawingBean) context.getBean("drawingBean");
            bean.drawShape();
        }
    }
}

Here are the output of our example:

Drawing a rectangle.
The colour is Red.

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>