DDD is a business-centric design idea in Java enterprise applications and is suitable for complex business scenarios. 1. Core concepts include entities (with unique identification), value objects (without identification, only value view), aggregation (object boundary), and warehousing (aggregation-oriented access method). 2. The hierarchical structure should be clear: the user interface layer handles requests, the domain layer implements core logic, the infrastructure layer provides technical support, and domain events are used to decouple. 3. When combined with Spring Boot, Entity and Value Object are placed in the domain package as POJO. Repository defines the interface and implements it by infrastructure. Domain Service and Application Service are marked with @Service respectively. 4. DDD is suitable for projects with complex business logic, requiring multiple teams to collaborate, and long-term maintenance. It is not recommended to use it in a simple CRUD system. Design ideas can be gradually introduced instead of forcing standard DDD from the beginning. The key is to understand business logic and clearly divide responsibilities, rather than piling up terms.

DDD is indeed a common topic in Java enterprise applications, but it is not used well. It is not a set of frameworks or tools, but a design idea, and the core lies in driving design with business as the core . If your project business logic is complex and has many rules, then DDD may be the direction you are worth trying.

The following is based on several common questions and usage scenarios, and talk about how to implement DDD in Java enterprise applications.

1. What is the core concept? Don't be scared by the terms

DDD has several core concepts, such as Domain, Entity, Value Object, Aggregate, Repository , etc. These terms sound academic, but they are actually very plain in nature.

• Entity : An object with a unique identification, such as a user. Even if the attribute changes, as long as the ID remains unchanged, it will still be it.
• Value object : An object that does not have a unique identifier, only looks at the value, such as the address (Address). As long as the address content is the same, it is considered to be the same.
• Aggregation : A set of boundaries of related objects, such as an order (Order) and line item (OrderItem), form an aggregate, and line items can only be operated by orders outside.
• Warehousing : It is not a database access layer, but provides a "collection" access method for aggregation, such as OrderRepository.

Understanding these concepts is not to write more "standards", but to express business logic more clearly when designing.

2. How to design a hierarchical structure? Don't mess up the code

In DDD, common stratification is like this:

• User interface layer (Application Layer) : Process requests, such as Controller.
• Domain Layer : Core logic, including entities, aggregations, services, etc.
• Infrastructure Layer : Provides technical support, such as database access, message queues, etc.
• Domain Events : used to decouple business logic, such as triggering emails after placing an order.

For example, when you are placing an order, the Controller calls the Application Service, the Application Service calls the Domain Service to handle order creation, and finally save it through Repository, or publish an event that "order creation is successful".

Common misunderstandings: Many people write Repository as DAO, and even directly call DAO in the Controller, which loses the layered meaning of DDD.

3. How to combine it with Spring Boot? Don't force it, be flexible

Although Spring Boot is not designed for DDD, its component management mechanisms (such as @Component, @Service, @Repository) can be used to organize the structure of DDD.

• Entity and Value Object : Put it in the domain package, without annotation, pure POJO.
• Aggregate Root : Usually Entity, you can add @Aggregate annotation (if you use Axon and other frameworks), but generally not.
• Repository : defined by interface, implementation can be placed in infrastructure package and implemented using JPA or MyBatis.
• Domain Service : You can use @Service annotation and put it in the domain package.
• Application Service : Use @Service the same, but put it in the application package.

Structure example:

 com.example.order
├── application
│ └── OrderApplicationService.java
├── domain
│ ├── Order.java (Entity)
│ ├── OrderService.java (Domain Service)
│ └── OrderRepository.java (Interface)
└── infrastructure
    └── JpaOrderRepository.java (implementation)

Note: Spring scans subpackages by default, so make sure your package structure is reasonable and don't let Spring find the beans.

4. When is DDD suitable? Don't make a big deal

DDD is not omnipotent, it is more suitable for:

• A system with complex business logic and changing rules
• Projects that require multiple teamwork and unified language
• Want a system with long-term maintenance and clear structure

If you just build a CRUD system, such as user management and logging, using DDD will increase the complexity.

Suggestion: You can start with a simple "anemia model" and gradually introduce the concept of DDD after the business complexity increases, such as first introducing aggregation and warehousing, and then introducing field services, events, etc.

DDD is not achieved overnight, it is more like a gradual evolution of design. The key is to understand the business, not the terminology. Clear structure and clear responsibilities are more important than "looking like DDD".

Basically that's it.

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