In this article, we will discuss how to create a bean, scoped as a singleton, using the @Scope annotation.
We use @Scope to define the scope of a @Component class or a @Bean definition. It can be either singleton, prototype, request, session, globalSession or some custom scope. In this article, we will discuss a singleton scope with an example.
When a Spring bean is scoped as a singleton, the Spring IoC container creates exactly one instance of the object defined by that bean definition.
By default, the Spring IoC container creates and initializes all beans as a singleton. But we can define the scope of a bean as a singleton using the scope="singleton" attribute of the element or using the @Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON) annotation.
We will demonstrate this example using Annotation based (@Component) as well as Java-based configuration(@Bean).
Spring @Scope Annotation + Singleton Scope + @Component Example
Let's create an example to demonstrates usage of use @Scope annotation with a singleton scope in a spring application.
Tools and technologies used
- Spring Framework - 5.1.0.RELEASE
- JDK - 8 or later
- Maven - 3.2+
- IDE - Eclipse Mars/STS
Create a Simple Maven Project
Create a simple maven project using your favorite IDE and refer below section for packaging structure. If you are new to maven then read this article How to Create a Simple Maven Project.
Project Structure
Below diagram shows a project structure for your reference -
The pom.xml File
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>net.javaguides.spring</groupId>
<artifactId>spring-dependson-annotation</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>spring-scope-example</name>
<url>http://maven.apache.org</url>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>5.1.0.RELEASE</version>
</dependency>
</dependencies>
<build>
<sourceDirectory>src/main/java</sourceDirectory>
<plugins>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.5.1</version>
<configuration>
<source>1.8</source>
<target>1.8</target>
</configuration>
</plugin>
</plugins>
</build>
</project>
Create MessageService interface as follows.
MessageService.java
package net.javaguides.spring.scope;
public interface MessageService {
String getMessage();
void setMessage(String message);
}
Let's create TwitterMessageService class which implements MessageService interface.
TwitterMessageService.java
package net.javaguides.spring.scope;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Component;
@Component
@Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
public class TwitterMessageService implements MessageService {
private String message;
@Override
public String getMessage() {
return message;
}
@Override
public void setMessage(String message) {
this.message = message;
}
}
Annotation Based Configuration - AppConfig.java
Declare the above beans in java based configuration class.
package net.javaguides.spring.scope;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
@Configuration
@ComponentScan(basePackages = "net.javaguides.spring")
public class AppConfig {
}
@ComponentScan annotation scans all beans whose class is annotated by the @Component annotation in a package specified by basePackages attribute.
Running Spring Application - Application.java
Let's create a main class and run an application.
package net.javaguides.spring.scope;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
public class Application {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
MessageService messageService = context.getBean(MessageService.class);
messageService.setMessage("TwitterMessageService Implementation");
System.out.println(messageService.getMessage());
MessageService messageService1 = context.getBean(MessageService.class);
System.out.println(messageService1.getMessage());
context.close();
}
}
Output
TwitterMessageService Implementation
TwitterMessageService Implementation
Let's develop the same example using Java-based configuration with @Bean annotation.
Spring @Scope Annotation + Singleton Scope + @Bean Example
Create MessageService interface as follows.
MessageService.java
package net.javaguides.spring.scope;
public interface MessageService {
String getMessage();
void setMessage(String message);
}
Let's create TwitterMessageService class which implements MessageService interface.
TwitterMessageService.java
package net.javaguides.spring.scope;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Component;
public class TwitterMessageService implements MessageService {
private String message;
@Override
public String getMessage() {
return message;
}
@Override
public void setMessage(String message) {
this.message = message;
}
}
Annotation Based Configuration - AppConfig.java
Declare the above beans in java based configuration class.
package net.javaguides.spring.scope;
import org.springframework.beans.factory.config.ConfigurableBeanFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Scope;
@Configuration
public class AppConfig {
@Bean
@Scope(value = ConfigurableBeanFactory.SCOPE_SINGLETON)
public MessageService messageService() {
return new TwitterMessageService();
}
}
@ComponentScan annotation scans all beans whose class is annotated by the @Component annotation in a package specified by basePackages attribute.
Running Spring Application - Application.java
Let's create a main class and run an application.
package net.javaguides.spring.scope;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
public class Application {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
MessageService messageService = context.getBean(MessageService.class);
messageService.setMessage("TwitterMessageService Implementation");
System.out.println(messageService.getMessage());
MessageService messageService1 = context.getBean(MessageService.class);
System.out.println(messageService1.getMessage());
context.close();
}
}
Output
TwitterMessageService Implementation
TwitterMessageService ImplementationThe source code of this article is available on my GitHub repository https://github.com/RameshMF/spring-core-tutorial
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