Week 4 Review — Exceptions

Goal

This review checks whether I can name a failure, keep the cause, close what I opened, and map it once at the edge.

Week 4 topics:

1. Throwable / Error / checked / unchecked
2. throw, throws, catch order, cause
3. Try-with-resources, suppressed, finally
4. Domain exceptions and translation
5. REST mapping and @Transactional rollback

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1. Week 4 big picture

OrderService.place
   throws OrderNotFoundException / DuplicateOrderException   (unchecked)
        │
        │  @Transactional: rollback on those runtime types
        ▼
RestExceptionHandler (@ControllerAdvice)
   maps type → HTTP 404 / 409 / ProblemDetail
   logs once

Persistence adapter
   SQLException → DataAccessException → DuplicateOrderException (cause kept)

Import job
   try (InputStream in = ...) { ... }   // I opened it, I close it

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2. Core memory sentences

Error is the JVM. RuntimeException is unchecked. Other Exception types are checked.

throw unwinds. throws declares a checked contract.

Catch specific before general. Empty catch is a production bug.

Wrap with the cause. Do not keep only the message.

Prefer try-with-resources; finally is for non-closeable cleanup.

Close what I opened. Do not close the container’s EntityManager.

Name the business failure. Translate once at the adapter edge.

Services throw domain types. The web layer chooses the status code.

Default rollback is unchecked. Checked commits unless rollbackFor.

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3. Speak these without notes

1. Checked vs unchecked, and what a REST service throws.
2. Why Spring invented DataAccessException.
3. Body vs close() both throw — which one does the caller see?
4. Why return in finally is a bug.
5. @Transactional plus a checked exception: commit or rollback?

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4. Tiny code proofs

Proof A — catch order

Throw FileNotFoundException into catch (IOException) vs a prior catch (FileNotFoundException). Confirm first-fit.

Proof B — suppressed

An AutoCloseable whose close() throws, body also throws. Print getSuppressed().

Proof C — wrap with cause

Catch IOException, throw UncheckedIOException("read failed", e), then printStackTrace() and find both.

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5. Common mix-ups from this week

Mix-up	Clear line
Checked is morally better	Checked is a compiler contract; REST uses unchecked + mapping
Catch everything in the service	Catch at adapter or advice
finally return is cleanup	It swallows the exception
I close EntityManager	Spring opened it
Checked exception undoes the insert	Default @Transactional commits

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6. Interview drill

Open Java fundamentals:

• Checked vs unchecked — what should a REST service throw?
• try-with-resources vs finally

Then peek at REST and MVC when you want the HTTP side, and Spring book Week 4 Day 4 when you study MVC.

Use the four-part answer.

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7. Ready for Week 5?

I am ready if I can draw the type tree, wrap with a cause, and say the transactional rollback default without hedging.

Week 5 is generics: erasure, wildcards, Optional as a return type.

Next: Week 5 Day 1 — Why Generics and Type Parameters