Spring Core - Learn Java by Examples

How do I inject into bean properties?

By I Wayan Saryada 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>

How do I create default init-method and destroy method in Spring?

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

When many beans in a context definition will have the same initialization and destroy method name, you can define a default-init-method and default-destroy-method in the attributes of the beans element.

This way you don’t have to define individual init-method and destroy-method for each of your beans. When the beans do not supply the method that match the name of defined default-init-method or default-destroy-method nothing will happen to those beans.

Let’s see an example code below. Firs here is our simple bean, the AutoEngine bean.

package org.kodejava.spring.core;

public class AutoEngine {
    public void initialize() {
        System.out.println("AutoEngine.initialize");
    }

    public void destroy() {
        System.out.println("AutoEngine.destroy");
    }
}

And then we define our Spring configuration file. You can see that there are default-init-method and default-destroy-method in the attribute of the beans element.

<?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"
       default-init-method="initialize" default-destroy-method="destroy">

    <bean id="engine" class="org.kodejava.spring.core.AutoEngine" />

</beans>

And finally a small program to run our demo.

package org.kodejava.spring.core;

import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class DefaultInitDestroyDemo {
    public static void main(String[] args) {
        ConfigurableApplicationContext context =
                new ClassPathXmlApplicationContext("default-init-destroy.xml");

        AutoEngine engine = (AutoEngine) context.getBean("engine");

        // context.close will remove the bean from the container.
        // This will call our bean destroy method.
        context.close();
    }
}

When you run this example you’ll get the following output printed on your screen:

AutoEngine.initialize
AutoEngine.destroy

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 use InitializingBean and DisposableBean interfaces in Spring?

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

In the other example, How do I initialize and destroy beans in Spring? you can see how to initialize and destroy a bean using the Spring configuration init-method and destroy-method.

In the following example you’ll see how to achieve the same thing using the Spring API. In this case our class need to implements the InitializingBean and DisposableBean. These interfaces are located under the org.springframework.beans.factory package.

The InitializingBean interface requires us to implements the afterPropertiesSet() method. This method will be the init method of our bean. The destroy() method which is a contract defined in the DisposableBean interface is where we’ll place the clean-up logic of our bean.

The drawback of using this approach is that our classes will be required to use the Spring API. If you want to use the class outside the Spring container using the other approach mentioned above is a better way to go.

Now, let’s see some code in action.

package org.kodejava.spring.core;

import org.springframework.beans.factory.DisposableBean;
import org.springframework.beans.factory.InitializingBean;

public class InitDisposeService implements InitializingBean, DisposableBean {

    /**
     * Do some processes.
     */
    public void doSomething() {
        System.out.println("InitDisposeService.doSomething");
    }

    /**
     * Initialize bean after property set.
     */
    @Override
    public void afterPropertiesSet() {
        System.out.println("InitDisposeService.afterPropertiesSet");
    }

    /**
     * Clean-up bean when the context is closed.
     */
    @Override
    public void destroy() {
        System.out.println("InitDisposeService.destroy");
    }
}

As usual, we define our Spring configuration (init-dispose.xml) to register our bean. In this case we will create a bean with id called service and will be using the InitDisposeService class.

<?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="service" class="org.kodejava.spring.core.InitDisposeService" />

</beans>

Below is a small Java program that we can use to run our application. This will load the Spring configuration, get the bean from the container and execute the bean. We’ll see that the afterPropertiesSet method is called. And when the context is closed the destroy method is also executed.

package org.kodejava.spring.core;

import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class InitDisposeDemo {
    public static void main(String[] args) {
        ConfigurableApplicationContext context =
                new ClassPathXmlApplicationContext("init-dispose.xml");

        InitDisposeService service =
                (InitDisposeService) context.getBean("service");
        service.doSomething();

        context.close();
    }
}

Here are the output printed on the screen:

InitDisposeService.afterPropertiesSet
InitDisposeService.doSomething
InitDisposeService.destroy

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 initialize and destroy beans in Spring?

By I Wayan Saryada in Spring Core Last modified: June 4, 2023 1 Comment

When creating an instance of a bean you might need to do some initialization to the bean. Likewise, when the bean is no longer needed and removed from the Spring container you might want to do some cleanup routine or destroy the bean.

To do this initialization and destroy routine you can use the init-method and destroy-method attribute when declaring a bean in spring configuration using the <bean> element.

By defining the init-method and destroy-method it will allow the Spring Container to call the initialization method right after the bean created. And just before the bean removed and discarded from the container, the defined destroy method will be called. Let’s see some code snippet as an example.

package org.kodejava.spring.core;

public class AutoEngine {
    public void initialize() {
        System.out.println("AutoEngine.initialize");
    }

    public void destroy() {
        System.out.println("AutoEngine.destroy");
    }
}

Below is the Spring configuration file that where we register the bean. You’ll see in the configuration there are additional attributes that we add to the bean.

<?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="engine" class="org.kodejava.spring.core.AutoEngine" init-method="initialize" destroy-method="destroy" />

</beans>

Create a small program to execute our demo:

package org.kodejava.spring.core;

import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class InitDestroyDemo {
    public static void main(String[] args) {
        ConfigurableApplicationContext context =
                new ClassPathXmlApplicationContext("init-destroy.xml");

        AutoEngine engine = (AutoEngine) context.getBean("engine");

        // context.close will remove the bean from the container. 
        // This will call our bean destroy method.
        context.close();
    }
}

When you run the program it will print the following output:

AutoEngine.initialize
AutoEngine.destroy

The advantage of using this method to initialize or clean up the bean is that it does not mandate our bean to implement or extend any Spring API which will make our bean reusable on other container beside Spring.

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 bean scoping in Spring?

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

Bean scoping is how the bean is created and returned from the spring container to the bean requester. By default, the scope of all bean is singleton. The spring container will always return the same bean whenever this bean is required, for instance when in injected or call using the getBean() method from the application context.

There are five types of bean scope define in the Spring Container:

Scope	Description
`singleton`	Scope the bean definition to a single bean instance per Spring Container. This is the default scope when no scope is defined when creating a bean.
`prototype`	Scope the bean to allow multiple times bean creation. This will create a new bean for each time the bean is required.
`request`	Scope the bean definition to a single HTTP request. ^*
`session`	Scope the bean definition to a single HTTP session. ^*
`global-session`	Scope the bean definition to a global HTTP session. ^*

*) This is only valid when using the web-capable Spring context.

Let’s see the difference between Singleton and Prototype scope. First we’ll create our DummyService class.

package org.kodejava.spring.core;

public class DummyService {
    private String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

Singleton Scope

By default, when no scope defined it will be a singleton.

<?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="service" class="org.kodejava.spring.core.DummyService" />

</beans>

Now create a program to run our example. First will run it using the singleton.xml as the configuration.

package org.kodejava.spring.core;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class BeanScopeDemo {
    public static void main(String[] args) {
        try (var context = new ClassPathXmlApplicationContext("singleton.xml")) {
            DummyService serviceA = (DummyService) context.getBean("service");
            serviceA.setMessage("Hello From A");
            System.out.println("Message A = " + serviceA.getMessage());

            DummyService serviceB = (DummyService) context.getBean("service");
            System.out.println("Message B = " + serviceB.getMessage());
        }
    }
}

The output of the singleton configuration is:

Message A = Hello From A
Message B = Hello From A

The above output show that the message printed by the serviceB is the same as the serviceA. We don’t even set the message in the serviceB explicitly. It prints the same message because the getBean() method actually return the same bean both for serviceA and serviceB. This is the singleton scope.

Prototype Scope

To change the scope to prototype use the scope attribute in the bean element as shown below.

<?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="service" class="org.kodejava.spring.core.DummyService" scope="prototype" />

</beans>

package org.kodejava.spring.core;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class BeanScopePrototypeDemo {
    public static void main(String[] args) {
        try (var context = new ClassPathXmlApplicationContext("prototype.xml")) {
            DummyService serviceA = (DummyService) context.getBean("service");
            serviceA.setMessage("Hello From A");
            System.out.println("Message A = " + serviceA.getMessage());

            DummyService serviceB = (DummyService) context.getBean("service");
            System.out.println("Message B = " + serviceB.getMessage());
        }
    }
}

Now if you try to run the same program again but changing the configuration to prototype.xml you’ll get the following output printed:

Message A = Hello From A
Message B = null

The serviceB now print a different message. This is the effect of using the prototype scope. When we call the getBean() a new bean will be created per request.

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>