Spring Singleton scope bean
last modified October 18, 2023
Spring Singleton scoped bean tutorial shows how to use a Singleton scoped bean in a Spring application.
Spring is a popular Java application framework for creating enterprise applications.
Spring Singleton bean
Singleton beans are created when the Spring container is created and are destroyed when the container is destroyed. Singleton beans are shared; only one instance of a singleton bean is created per Spring container. Singleton scope is the default scope for a Spring bean.
Other bean scopes are: prototype, request, session, global session, and application.
Spring Singleton bean example
The application creates two singleton scoped beans and checks if they are identical. The application is a classic Spring 5 console application.
<?xml version="1.0" encoding="UTF-8"?>
<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>com.zetcode</groupId>
<artifactId>scopesingletonex</artifactId>
<version>1.0-SNAPSHOT</version>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<maven.compiler.source>17</maven.compiler.source>
<maven.compiler.target>17</maven.compiler.target>
<spring-version>5.3.23</spring-version>
</properties>
<dependencies>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.4.0</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>${spring-version}</version>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-core</artifactId>
<version>${spring-version}</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>exec-maven-plugin</artifactId>
<version>3.1.0</version>
<configuration>
<mainClass>com.zetcode.Application</mainClass>
</configuration>
</plugin>
</plugins>
</build>
</project>
In the pom.xml file, we have basic Spring dependencies spring-core
and spring-context and logging logback-classic dependency.
The exec-maven-plugin is used for executing Spring application from the
Maven on the command line.
myapp.name=ScopeSingleton myapp.author=Jan Bodnar
We have two basic properties in the my.properties file.
They are using in the Message bean.
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<logger name="org.springframework" level="ERROR"/>
<logger name="com.zetcode" level="INFO"/>
<appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender">
<encoder>
<Pattern>%d{HH:mm:ss.SSS} [%thread] %blue(%-5level) %magenta(%logger{36}) - %msg %n
</Pattern>
</encoder>
</appender>
<root>
<level value="INFO" />
<appender-ref ref="consoleAppender" />
</root>
</configuration>
The logback.xml is a configuration file for the Logback logging library.
package com.zetcode.bean;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.PropertySource;
import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Component;
@Component
@Scope("singleton") // change to prototype
@PropertySource("classpath:my.properties")
public class Message {
@Value("${myapp.name}")
private String name;
@Value("${myapp.author}")
private String author;
public String getMessage() {
return String.format("Application %s was created by %s", name, author);
}
}
The Message is a Spring bean managed by the Spring container.
It has singleton scope.
@Component
@Scope("singleton") // change to prototype
@PropertySource("classpath:my.properties")
public class Message {
The @Scope("singleton") annotation is not necessary; the
default scope is singleton if not specified. With the @PropertySource
annotation we specify the properties file. The properties are later read
with @Value.
package com.zetcode;
import com.zetcode.bean.Message;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
@ComponentScan(basePackages = "com.zetcode")
public class Application {
private static final Logger logger = LoggerFactory.getLogger(Application.class);
public static void main(String[] args) {
var ctx = new AnnotationConfigApplicationContext(Application.class);
var app = ctx.getBean(Application.class);
var bean1 = ctx.getBean(Message.class);
var bean2 = ctx.getBean(Message.class);
app.run(bean1, bean2);
ctx.close();
}
public void run(Message a, Message b) {
logger.info("running Application");
logger.info(a.getMessage());
if (a.equals(b)) {
logger.info("The beans are the same");
} else {
logger.info("The beans are not the same");
}
}
}
This is the main application class.
var bean1 = ctx.getBean(Message.class); var bean2 = ctx.getBean(Message.class); app.run(bean1, bean2);
We get the two beans from the application context and pass them to
the run method for comparison.
logger.info(a.getMessage());
We read the message from the bean.
if (a.equals(b)) {
logger.info("The beans are the same");
} else {
logger.info("The beans are not the same");
}
We test if the two beans are identical.
$ mvn -q exec:java 21:28:35.573 [com.zetcode.Application.main()] INFO com.zetcode.Application - running Application 21:28:35.575 [com.zetcode.Application.main()] INFO com.zetcode.Application - Application ScopeSingleton was created by Jan Bodnar 21:28:35.576 [com.zetcode.Application.main()] INFO com.zetcode.Application - The beans are the same
We run the application. Change the scope of the Message bean to prototype and
compare the results.
In this article we have worked with a Singleton Spring bean.
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