This post presents a look at the overall application components and architecture of the Spring framework

Application Domains

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Our application components break down into two fundamental categories, the System and Problem Domains.

• System Domain – infrastructure components, the plumbing, this is Spring’s sweet spot!
• Problem Domain – business components, typically use-case driven, this is what most of developers are paid to solve

Application Layers

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Application components (beans) should be separated into distinct layers, and categories.

1. Controllers (for MVC, System Domain)
2. Services (Problem Domain)
3. Repository (System Domain)

• Data Transfer Objects (Problem Domain)
• System Functions (System Domain)

Controller Beans

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More on Controllers in an upcoming blog on Enterprise Spring Best Practices MVC blog – TBD

Service Beans

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Service Beans are Problem Domain components. These are the MOST significant in the application. Service beans are the Fundamental component of SOA.

• These are POJOs
• Always defined from interfaces
• NEVER include infrastructure components
• NO import of Spring or utility libraries
• NO infrastructure annotations
• Always declare transaction boundaries by public functions
• Create implementation/concrete classes in a sub-package named internal

Spring annotations can be very useful for services, useful annotations are @Service and @Transactional. To abstract away the infrastructure from the business services, create a project specific meta annotation.

package com.gordondickens.service.annotation;

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Service
@Transactional
public @interface AppService {
    String value() default "";
}

...
@AppService
public class MyClass() {
  ...
}

Repository Beans

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Repository beans are in the System Domain. More on this in Enterprise Spring Best Practices ORM blog – TBD

• DO NOT contain business logic
• Do use Spring and JPA annotations
• Should be considered disposable

Data Transfer Beans

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Data Transfer Objects (DTO) are the fundamental objects in and out of our system. DTOs are simple public POJOs that receive and send data as a logical set.

• Always Public Beans
• Annotate with JAXB2 Annotations

Conversion Beans

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Spring provides a rich conversion registry at it’s core. The conversion service in Spring is based on the original Bean Specification PropertyEditor.

PropertyEditors are focussed on String data into and out of our application.

The Spring class org.springframework.beans.PropertyEditorRegistrySupport shows the built in String <–> object classes. Which we use, usually without knowledge, in our applications. When we configure our applications with XML and send in property values, Spring uses reflection to determine the argument type, if that type is not a String, Spring looks for a PropertyEditor that can convert from String to the target type. We also can create our own PropertyEditor’s and register them for types such as US Social Security Number or Telephone Number. See: Craig Wall’s Spring in Action, 3rd Ed for examples.

• Primitive wrapper types: Long, Integer, etc
• Collection types: List, Property, Set, Map, etc.
• Arrays
• Utility types: URL, TimeZone, Locale, etc.

Spring 3.0 introduced the Conversion service providing us the ability to register a conversion service for object <–> other conversion. To use the conversion registry, we can register a conversion class that will automatically convert to/from MyObject <–> MyOtherObject. See Using Spring Customer Type Converter Blog.

Further Reading

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• Enterprise Spring Best Practices – Part 1
• Enterprise Spring Best Practices – Source Code
• FREE Spring Framework PDF (848 pages)
• Craig Wall’s Spring in Action, 3rd Ed