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Spring

The container and dependency injection, AOP and transactions, the path a web request takes, and what Spring Boot does for you.

IoC container and DI

• The Spring container and beans — You never wrote new, so who creates the object and passes it in?
• Bean creation and injection — In what order does Spring create objects and wire them together?
• Singleton beans and state — My order shows someone else's name
• Bean scopes — It is a prototype bean, so why is there only one object?
• Bean lifecycle callbacks — Why is an injected field null when you use it in the constructor?

AOP and proxies

• Proxies and self-invocation — Why does the annotation have no effect?
• AOP: advice and pointcuts — Which methods does advice apply to, and in what order does it run?

Data access and transactions

• Transaction basics — What if the withdrawal went through but the deposit did not?
• Transaction propagation — How far does a rollback reach when an exception is thrown?
• JdbcTemplate and exception translation — What does JdbcTemplate do for you, and what does it translate exceptions to?
• Connection pool and OSIV — Why connections run short even though the queries finish quickly

Web MVC

• Servlets — How an HTTP request reaches Java code
• Spring MVC request flow — How one request reaches the controller, and how it goes back
• Request mapping and parameter binding — How does a value sent in a request become a method parameter?
• Validation — The field has @NotBlank, so why does an empty value still get in?
• REST APIs and message converters — Where does the object returned by a controller become JSON?

Spring Boot

• Spring Boot auto-configuration — Why do beans appear when all you did was add a dependency?
• Configuration precedence — When the same property is set in several places, which value wins?
• Actuator and health checks — What does the health check answer when the connection to the payment provider is lost?

Other features

• Testing — When tests run, what goes into the Spring context, and how many times is it created?
• Calling external APIs and timeouts — When a call to another server gets no answer, how long does it wait?
• Cache abstraction — With @Cacheable on a method, when does the method body not run?
• Security filter chain — Where is it decided whether the response is 401 or 403?