---

# (DD24) Business Logic — OwnerController.showOwner() [9 LOC]

| Field | Value |
|-------|-------|
| Fully Qualified Name | `org.springframework.samples.petclinic.owner.OwnerController` |
| Layer | Controller |
| Module | `owner` (Package: `org.springframework.samples.petclinic.owner`) |

## 1. Role

### OwnerController.showOwner()

This method acts as the request-handling entry point for the owner detail page in the Petclinic owner module. Its business responsibility is to retrieve a single owner record by the incoming path variable, prepare the web model, and route the user to the owner details view. The method implements a simple routing and delegation pattern: it converts an HTTP path parameter into a repository lookup, then either renders the owner details page or fails fast when the requested owner cannot be found.

From a business perspective, this is the screen-level “view owner profile” operation used when a user navigates to an owner’s detail page. It handles only one service type: owner master data display. The success branch populates the MVC model with the owner aggregate, while the failure branch raises an `IllegalArgumentException` to signal that the requested business entity does not exist or the identifier is invalid. The method therefore serves as a thin controller boundary that bridges the web layer and persistence layer without adding transformation rules or business calculations.

## 2. Processing Pattern (Detailed Business Logic)

```mermaid
flowchart TD
    START(["showOwner(ownerId)"])
    CREATE_MAV(["Create ModelAndView with view owners/ownerDetails"])
    FIND_OWNER(["Call owners.findById(ownerId)"])
    HAS_OWNER{"Optional contains Owner?"}
    THROW_ERROR(["Throw IllegalArgumentException - Owner not found with id"])
    ADD_MODEL(["Add Owner object to ModelAndView model"])
    RETURN_OK(["Return ModelAndView"])

    START --> CREATE_MAV
    CREATE_MAV --> FIND_OWNER
    FIND_OWNER --> HAS_OWNER
    HAS_OWNER -- "Yes" --> ADD_MODEL
    HAS_OWNER -- "No" --> THROW_ERROR
    ADD_MODEL --> RETURN_OK
```

## 3. Parameter Analysis

| No | Parameter Name | Type | Business Description |
|----|---------------|------|---------------------|
| 1 | `ownerId` | `@PathVariable("ownerId") int` | Business identifier for the owner whose profile is being displayed. It is supplied from the URL path and is used as the lookup key for retrieving a single owner record. Any integer value may be received; valid existing IDs lead to the detail screen, while missing or incorrect IDs trigger the not-found exception path. |

Instance fields / external state read by this method:
- `this.owners` — repository dependency used to locate the owner record.
- `"owners/ownerDetails"` — MVC view name used for the detail screen.

## 4. CRUD Operations / Called Services

### Pre-computed evidence from code analysis graph:

| CRUD | SC / CBS | SC Code | Entity / DB | Operation Description |
|------|----------|---------|-------------|----------------------|
| R | `OwnerRepository.findById` | OwnerRepository | Owner | Calls `findById` in `OwnerRepository` |

Analyze all method calls within this method and classify each as a CRUD operation.

| CRUD | SC / CBS | SC Code | Entity / DB | Operation Description |
|------|----------|---------|-------------|----------------------|
| R | `OwnerRepository.findById` | OwnerRepository | Owner | Reads a single owner aggregate by primary key for detail display. |
| R | `Optional.orElseThrow` | JDK Optional | Owner | Converts the read result into a guaranteed owner instance or raises a not-found exception. |
| C | `ModelAndView.addObject` | Spring MVC ModelAndView | MVC Model | Adds the retrieved owner to the model for rendering the details page. |
| C | `new ModelAndView("owners/ownerDetails")` | Spring MVC ModelAndView | MVC View | Creates the view model container for the owner details screen. |

## 5. Dependency Trace

Trace who calls this method and what this method ultimately calls.

| # | Caller (Screen/Batch) | Call Chain (Full Path to this Method) | Terminal (SC / CRUD / Entity) |
|---|----------------------|--------------------------------------|-------------------------------|
| 1 | Test: `OwnerControllerTests` | `OwnerControllerTests.showOwner` -> `OwnerController.showOwner` | `OwnerRepository.findById [R] Owner` |

## 6. Per-Branch Detail Blocks

**Block 1** — [TRY / SEQUENCE] `(method entry)` (L167)

> Initializes the MVC response and starts owner lookup for the requested detail screen.

| # | Type | Code |
|---|------|------|
| 1 | SET | `ModelAndView mav = new ModelAndView("owners/ownerDetails");` // prepare owner detail view |
| 2 | CALL | `this.owners.findById(ownerId);` // retrieve owner by business identifier |

**Block 2** — [IF] `(optionalOwner contains an Owner)` (L169)

> Success branch for an existing owner record.

| # | Type | Code |
|---|------|------|
| 1 | CALL | `optionalOwner.orElseThrow(...);` // resolve the owner or fail fast |
| 2 | SET | `Owner owner = ...;` // bind retrieved owner for model population |
| 3 | CALL | `mav.addObject(owner);` // expose owner data to the view model |
| 4 | RETURN | `return mav;` // return populated owner details view |

**Block 3** — [ELSE] `(optionalOwner is empty)` (L169)

> Failure branch for a missing owner record.

| # | Type | Code |
|---|------|------|
| 1 | CALL | `optionalOwner.orElseThrow(() -> new IllegalArgumentException("Owner not found with id: " + ownerId + ". Please ensure the ID is correct "));` // raise not-found error |
| 2 | RETURN | `throw IllegalArgumentException;` // terminate request with an exception |

## 7. Glossary

| Term | Type | Business Meaning |
|------|------|------------------|
| `ownerId` | Field | Unique identifier of the owner record displayed on the detail page. |
| `Owner` | Entity | Pet owner master data displayed in the UI. |
| `owners` | Repository | Persistence gateway used to retrieve owner records from storage. |
| `ModelAndView` | Technical term | Spring MVC container that carries both the selected view and the model attributes. |
| `owners/ownerDetails` | View name | Owner detail screen rendered after a successful lookup. |
| `Optional` | Technical term | Container used to represent a possibly missing owner record. |
| `IllegalArgumentException` | Exception | Indicates an invalid or missing owner identifier in the request. |
| MVC | Acronym | Model-View-Controller architecture used by the web layer. |
| CRUD | Acronym | Create, Read, Update, Delete — standard data operation categories. |
| repository | Technical term | Data access abstraction that hides the underlying persistence mechanism. |
| Petclinic | Business term | Sample pet management application domain used by the module. |