To split a monolithic model into packages, you must first identify distinct domains within your current structure, extract related elements into new namespaces, and then systematically resolve the resulting dependencies. This process begins by isolating high-cohesion groups of classes and moves toward organizing the resulting hierarchy to map directly to your project’s physical directory structure.

The Four-Step Refactoring Process

Moving from a flat structure to a modular hierarchy requires a disciplined approach. The goal is to enhance maintainability without breaking the model’s integrity. Follow these specific actions to successfully split your model.

Step 1: Identify High-Cohesion Domains

The first action involves analyzing your existing flat list of classes and components. You are looking for natural clustering based on responsibility.

1. Group classes that share the same lifecycle or data domain together.
2. Identify entities that are frequently modified or extended together.
3. Determine the core business concept that links a specific set of elements.

For example, if you have classes like CustomerOrder, Invoice, and PaymentProcessor, they likely belong to a “Billing” domain. Separating them from “Shipping” classes like Shipment and Tracking is the foundation of your package structure. You must define the boundaries clearly before creating any new nodes in your diagram.

Step 2: Extract and Isolate Elements

Once the domains are identified, create the new package nodes. This is the physical act of splitting the model.

Drag the identified classes from the root level or the flat root node into the new package nodes. Ensure that the visual structure reflects the domain separation. Avoid overlapping responsibilities where one class might logically fit into multiple packages. If a class is shared, keep it in the most fundamental package and reference it.

This step creates a new hierarchy. The result is a flattened tree where the root is still the model, but the immediate children are now logical domains. This improves readability and allows you to focus on one specific area at a time.

Step 3: Analyze and Map Dependencies

The act of splitting the model into packages often reveals hidden coupling issues. You must now review the dependencies that cross the new package boundaries.

• Check for direct dependencies between classes in different packages.
• Identify circular dependencies that might now exist between new packages.
• Review interface definitions to ensure they are correctly exposed.

Dependencies should flow in a single direction whenever possible. If Package A depends on Package B, it should not simultaneously depend on a class in Package A that depends on Package B. This is a critical check to prevent architectural degradation.

Step 4: Organize Physical Mapping

Finally, align your UML package structure with your source code directory structure. This step ensures that the diagram remains valid as you implement the changes.

Create a package for every root namespace used in your project. Ensure that the naming conventions match your development environment. If your code structure uses src/domain/billing, your package should reflect domain.billing. This alignment allows for automated code generation and synchronization between the design and the implementation.

Common Challenges During Refactoring

Splitting a large model is rarely a smooth process. Specific obstacles arise when dealing with tight coupling and legacy structures.

Handling Interdependencies

When you split a model, you often encounter classes that interact heavily with classes in a different domain. This creates a strong coupling problem. To resolve this, you should introduce an abstraction layer.

Create an interface or a generic class that acts as a bridge between the two packages. This decouples the concrete implementations. The dependent package only needs to know about the interface, not the full implementation of the target package.

Dealing with Circular References

Circular dependencies are the most dangerous byproduct of modularization. If Package A imports Package B, and Package B imports Package A, the system cannot compile or execute correctly.

Break the circle by moving one of the shared types to a third “Common” package. This common package should contain only the definitions that are truly shared. Both original packages then depend on the common package, creating a stable star topology instead of a fragile cycle.

Mapping Logic to Code Structures

The ultimate goal of this process is to bridge the gap between your design diagram and your software architecture. A successful split of a model into packages results in a one-to-one mapping with the file system.

Directory Hierarchy Strategy

Define a clear convention for how packages map to folders. Typically, a package named com.example.module corresponds to a folder structure com/example/module.

Ensure that your tooling supports this mapping. Modern UML tools can often generate the initial skeleton code structure based on the diagram. This feature saves significant time and reduces the risk of manual file creation errors.

Version Control Alignment

When splitting the model, consider how changes will be versioned. A monolithic model often results in massive merge conflicts in version control systems.

With a modular approach, developers working on different domains modify different packages. This reduces the likelihood of conflicts. Ensure that your team agrees on package ownership. This clarity prevents accidental modifications to unrelated modules during the development cycle.

Advanced Refactoring Scenarios

Some scenarios require deeper analysis than a standard extraction.

Legacy Code Modernization

Refactoring a massive legacy model can feel overwhelming. Do not attempt to refactor the entire system at once. Use a “strangler fig” pattern.

Identify one distinct domain to extract first. Split that specific model into packages and migrate the code for that domain. Once that is stable, move to the next. This incremental approach reduces risk and allows for continuous delivery of value.

Microservices Alignment

If your project is moving toward microservices, your package structure must reflect the service boundaries. Splitting the model into packages should mirror the boundaries of your potential services.

This ensures that the code generated from the packages can be compiled and deployed independently. If a package is too large, it will result in a “monolithic microservice,” defeating the purpose of the architecture.

Key Takeaways

• Identify natural domains based on responsibility before creating packages.
• Ensure that dependencies flow in a single direction to maintain stability.
• Break circular dependencies by extracting shared types into a common package.
• Map package names directly to your source code directory structure.
• Refactor large models incrementally to reduce risk and merge conflicts.