Master Spring @Configuration and @Bean: From Java Classes to Fully Configured Beans

Introduction

Before Spring 3.0 configuration required XML files; since Spring 3.0 the @Configuration and @Bean annotations enable Java‑based configuration, and Spring Boot has made annotation‑driven setup the default approach.

Java‑based configuration requirements

Configuration classes must not be final.

They must be top‑level (cannot be defined inside another class or be private).

They must provide a public no‑arg constructor.

Basic usage

Annotate a class with @Configuration and annotate factory methods with @Bean. Spring registers the method return values as beans; the default bean name is the method name.

@Configuration
public class DataSourceConfig {
    @Bean
    public MysqlDataSource mysqlDataSource() {
        return new MysqlDataSource();
    }

    @Bean(name = "oracleDataSource")
    public OracleDataSource oracleDataSource() {
        return new OracleDataSource();
    }
}

The same configuration expressed in XML would be:

<bean id="mysqlDataSource" class="com.example.MysqlDataSource"/>
<bean id="oracleDataSource" class="com.example.OracleDataSource"/>

@Configuration annotation definition

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {
    String value() default "";
}

Because @Configuration is itself annotated with @Component, it is discovered during component scanning.

@Bean annotation definition and attributes

@Target({ElementType.METHOD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Bean {
    @AliasFor("name") String[] value() default {};
    @AliasFor("value") String[] name() default {};
    Autowire autowire() default Autowire.NO;
    String initMethod() default "";
    String destroyMethod() default "(inferred)";
}

name / value : specify one or more bean names (equivalent to XML name attribute).

initMethod : method invoked after bean initialization (equivalent to XML init-method).

destroyMethod : method invoked before bean destruction (equivalent to XML destroy-method).

autowire : bean‑property autowiring strategy; possible values are Autowire.BY_NAME, Autowire.BY_TYPE, or Autowire.NO.

Bean scope

@Bean creates a singleton by default. To obtain a prototype bean, add @Scope.

@Bean
@Scope("prototype")
public MysqlDataSource mysqlDataSource() {
    return new MysqlDataSource();
}

Component scanning

When a package is scanned, Spring detects classes annotated with @Component, @Controller, @Service, @Repository, and also @Configuration because it carries @Component.

Unit‑test example

public class ConfigBeanTest {
    @Test
    public static void testGetBean() {
        ApplicationContext ctx = new AnnotationConfigApplicationContext(DataSourceConfig.class);
        MysqlDataSource mysql = ctx.getBean(MysqlDataSource.class);
        mysql.print(); // prints "I am MysqlDataSource!"
    }
}

Practical tip

Instead of autowiring fields inside a @Configuration class, you can call other @Bean methods directly to obtain their instances.

@Bean
public CommonDataSource commonDataSource() {
    CommonDataSource common = new CommonDataSource();
    common.setMysqlDataSource(mysqlDataSource());
    common.setOracleDataSource(oracleDataSource());
    return common;
}

Note that the setter receives the result of the method call, not a field.

Conclusion

The article demonstrates how @Configuration and @Bean replace XML configuration in modern Spring applications, shows the underlying annotation definitions, bean scopes, component scanning, unit testing, and a practical technique for composing beans without field injection, enabling a smooth migration to annotation‑driven Spring development.