Builder and public all arg constructor exposes aggregate internals. What is advantage of static factory method and not to use constructor?

Of course. This is one of the most subtle but powerful concepts in DDD, and it's fantastic that you're digging into it. The difference isn't just technical; it's about intent, semantics, and protecting your business rules.

Let's use an analogy.

• A public static create(...) method is like a Car Factory Assembly Line.
• A public static load(...) method is like a Mechanic's Repair Bay.

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The Car Factory (public static create(...))

When you use a method like Asset.onAssetCreated(...), you are telling the system: "I am manufacturing a brand new asset that has never existed before."

The assembly line has very strict rules for this process:

1. It assigns a new Vehicle Identification Number (VIN). You can't bring your own. The factory generates it (new AssetId(UUID.randomUUID())).
2. It starts in a specific, initial state. A new car always comes off the line with 0 miles on the odometer and a "new" status. Your asset is created with the status STORED_IN_TEMP_LOCATION.
3. It triggers other business processes. When a new car is built, the factory notifies the sales, inventory, and marketing departments. Your onAssetCreated method fires a NewAssetEvent. This is a critical business fact that other parts of your system need to know about.
4. It validates the raw materials. The factory checks if the parts are valid for building this model. Your method validates the inputs to ensure a valid asset can be created.

The Mechanic's Repair Bay (public static load(...))

When your repository fetches data from the database and uses Asset.load(...), you are telling the system: "I am re-assembling an existing asset that we already know about."

The mechanic in the repair bay has a different set of tasks:

1. It works with an existing VIN. The car already has an ID. The load method receives an existing AssetId from the database. It does not create a new one.
2. It restores the car to its last known state. If the car had 50,000 miles and a "Used" status when it was brought in, the mechanic restores it to that state. Your load method takes the status, type, and other details directly from the database and trusts they are correct because they were valid when last saved.
3. It does not announce a new car has been built. The mechanic simply finishes the job. Your load method must not fire a NewAssetEvent. If it did, every time you loaded an asset from the database, you would trigger the "new asset" workflow again (e.g., send a welcome notification, start a processing job, etc.), which would be a catastrophic bug.
4. It trusts the car was built correctly the first time. The mechanic isn't re-validating the original factory specifications; they are just putting the pieces back together.

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The Real Benefit: Preventing Misuse and Bugs

Now, let's see what happens if you just use a single public constructor instead of these two distinct factory methods.

The "Bad" Scenario: A Single Public Constructor

// In your Asset aggregate
public Asset(AssetId assetId, AssetType assetType, AssetStatus assetStatus, AssetDetails details) {
    // What logic goes here?!
    // Do I fire an event? Only if it's new? How do I know?
    // Do I assign a default status? Or trust the one passed in?
    // This constructor is now confused about its purpose.
    this.assetId = assetId;
    this.assetType = assetType;
    this.assetStatus = assetStatus;
    this.assetDetails = details;
    // Maybe I fire an event here? BAD IDEA!
    // registerEvent(new NewAssetEvent(...));
}

Now, a developer in the application service wants to create a new asset. They might do this:

// In Application Service (creating a NEW asset)
// PROBLEM 1: The developer has to know to generate a new ID themselves.
AssetId newId = new AssetId(UUID.randomUUID());

// PROBLEM 2: The developer can create an asset in an invalid initial state.
// This should be forbidden! An asset can't be created as "DELETED".
AssetStatus invalidInitialStatus = AssetStatus.DELETED;

// PROBLEM 3: No NewAssetEvent is fired! The rest of the system is blind to this new asset.
Asset newAsset = new Asset(newId, AssetType.IMAGE, invalidInitialStatus, details);

assetRepository.save(newAsset); // The system now contains a broken, silent asset.

The public constructor completely destroyed the aggregate's ability to protect itself. It puts the responsibility of enforcing rules onto the developer, which is the exact opposite of what an Aggregate Root is for.

The "Good" Scenario: create() and load()

public class Asset {
    // Constructor is PRIVATE! No one can call it directly except the class itself.
    private Asset(...) { ... }

    // INTENT: Make a new one.
    public static Asset onAssetCreated(...) {
        // Enforces ALL rules for creation.
        // Generates new ID.
        // Sets initial status.
        // Fires NewAssetEvent.
        return new Asset(...);
    }

    // INTENT: Rebuild an existing one.
    public static Asset load(AssetId id, AssetStatus status, ...) {
        // Bypasses initial creation rules.
        // Does NOT fire an event.
        return new Asset(id, status, ...);
    }
}

Now, the developer has no choice but to do the right thing:

• To create an asset: They must call Asset.onAssetCreated(...). It's impossible to create an invalid asset or forget the event.
• To load an asset: The repository/mapper is the only part of the code that should call Asset.load(...). It's clear that this is a technical reconstitution, not a business operation.

Summary of Benefits

Feature	public static create() (The Factory)	public static load() (The Mechanic)	public Constructor (The Problem)
Intent	Create a new business object	Rebuild an existing object from storage	Ambiguous; can be used for both, poorly
ID Generation	Creates a new ID	Receives an existing ID	Developer must remember to create one
Initial State	Enforces a valid, default state	Trusts the state from the database	Developer can set any state, even invalid ones
Domain Events	Must fire ...CreatedEvent	Must NOT fire ...CreatedEvent	Easy to forget to fire the event
Encapsulation	Excellent. The aggregate protects its rules.	Excellent. The aggregate controls its lifecycle.	Terrible. All rules can be bypassed.

The real benefit is making your domain model robust, expressive, and bug-resistant. You are using the type system to enforce a clear separation between the business act of creation and the technical act of reconstitution.