Mastering Spring’s @Import: Three Powerful Ways to Load Configurations

1. Introduction

In the Spring framework, the @Import annotation allows developers to import configuration classes, dependencies, or custom configurations into the Spring container, improving maintainability and readability.

2. Use Cases of @Import

Import configuration classes that define beans.

Import dependencies by importing a class into another.

Import custom configurations using @ImportResource .

3. Examples

Directly importing a regular class array

<code>static class B {}
static class A {
    @Bean
    public B b() {
        return new B();
    }
}
@Import({A.class})
@Configuration
static class ImportConfig {}
</code>

Both A and B are managed by the Spring IoC container. Multiple classes can be imported. Note: before Spring 4.2 the imported class had to be annotated with @Configuration ; from 4.2 onward a plain class is sufficient.

Importing via ImportSelector

<code>static class B {}
static class A {}
@Import({C.class})
@Configuration
static class ImportConfig {}
static class C implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        return new String[] {"com.pack.main.import_class.Import_02.B", "com.pack.main.import_class.Import_02.A"};
    }
}
</code>

The returned strings must be fully qualified class names. The ImportSelector itself is not registered as a bean. An empty array is allowed, but null is not.

Importing via ImportBeanDefinitionRegistrar

<code>static class A {
    private String name;
    public void setName(String name) { this.name = name; }
    @Override
    public String toString() { return "A [name=" + this.name + "]"; }
}
static class B implements ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
                                        BeanDefinitionRegistry registry) {
        GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
        beanDefinition.setBeanClass(A.class);
        beanDefinition.getPropertyValues().addPropertyValue("name", "张三");
        registry.registerBeanDefinition("a", beanDefinition);
    }
}
@Import({B.class})
@Configuration
static class ImportConfig {}
static class D {
    @Resource private A a;
    @Override public String toString() { return "D [a=" + a + "]"; }
}
</code>

The ImportBeanDefinitionRegistrar implementation itself is not registered as a bean.

Conclusion: The @Import annotation offers flexible ways to load configuration classes, dependencies, and custom beans, helping developers organize and manage application configuration more effectively.