16-Annotation-Driven Configuration

Question 1

What happens if auto-wiring a non-existing bean?

Answer 1

NoSuchDefinitionException thrown

Question 2

What injection modes are supported via @Autowired?

Answer 2

1. Field
2. Constructor
3. ## Setter/method methodNote: although you can auto-wire methods other than setters, this is probably not a good idea, because you are starting to run code during start-up

Question 3

With constructor, in what case we can drop @Autowired?

Answer 3

If it’s the only constructor in the component

Question 4

How to inject an optional bean?

Answer 4

Using @Autowired(required = false)

Question 5

What is the recommended place to check if an optional bean has been set or not?

Answer 5

In @PostConstruct void method() {}

Question 6

When should use setter DI?

Answer 6

1. Circulate dependencies

2. Want subclass to inherited parent’s setters automatically

Question 7

When should use constructor DI?

Answer 7

1. Mandatory dependencies
2. Immutable
   Constructor injection is generally preferred

Question 8

Given 2 beans that implement the same service interface e.g. UserService.
Inject bean with @Autowired void method(UserService)
What happens? How to fix it?

Answer 8

1. NoSuchDefinitionException because of 2 beans with the same type
2. Using void method(@Qualifier(“name”) UserService)

Question 9

What are the resolution rules for @Autowired?

Answer 9

1. First, search by type
2. If there are multiple beans of the same type, it looks for @Qualifier
3. Match by bean-name and field-name

Question 10

What is the equivalent @Autowired in JSR-330?

Answer 10

@Resource

Question 11

What are the resolution rules for @Resource in JSR-250?

Answer 11

Name first, falling back to type if required.

Question 12

Can @Resource use in constructor DI?

Answer 12

@Resource cannot be used on a constructor.

Question 13

What is the bean name for the following cases?

1. @Service class UserServiceImpl implements UserService {}
2. @Bean UserService myUserService() {}

Answer 13

1. userServiceImpl

2. myUserService

Question 14

How to auto-wire a property to the constructor parameter?

Answer 14

@Autowired

MyConstructor(@Value(“${property}”) property) {}

Question 15

When is a @Lazy bean created?

Answer 15

1. When dependency injected

2. By ApplicationContext.getBean() method

Question 16

What is the default value of @Lazy.value?

Answer 16

false

Beans are normally created on startup when application context created

Question 17

If there’re multiple constructors, which one will be invoked?

Answer 17

1. The one with @Autowired. If there’s no @Autowired constructor
2. Invoke the default constructor (no arguments)

Question 18

Ways to add a method that runs after the construction done?

Answer 18

1. create a method with no params and only return void
2. mark with @PostConstruct or init-method in XML
   or add @Bean(initMethod=”methodName”) if it’s with @Bean. Only available in @Bean

Note: @PostConstruct is in javax.annotation, JSR-250

Question 19

Ways to add a method that runs before destruction?

Answer 19

1. create a method with no params and only return void
2. mark with @PreDestroy or destroy-method in XML
   or add @Bean(destroyMethod=”methodName”) if it’s with @Bean. Only available in @Bean

Note: @PreDestroy is in javax.annotation, JSR-250

Question 20

What is the result of SpringApplication.run?

Answer 20

ConfigurableApplicationContext (not ApplicationContext)

Question 21

Ways to shut down a Spring application?

Answer 21

1. ConfigurableApplicationContext.close()

2. SpringApplication.exit(ConfigurableApplicationContext, ExitCodeGenerator…)

Question 22

With @Bean, which methods in the bean does Spring infer to call before deconstruction?

Answer 22

If exist in bean: public void close(), public void shutdown()

Question 23

What ConfigurableApplicationContext.registerShutdownHook() does?

Answer 23

Register a shutdown hook with the JVM runtime, closing this context on JVM shutdown unless it has already been closed at that time.

Question 24

When does JVM shut down?

Answer 24

1. The last non-daemon thread ends
2. The JVM is interrupted (by using ctrlC or sending SIGINT).
3. The JVM is terminated (by sending SIGTERM)
4. One of the threads calls System.exit() or Runtime.exit().

Question 25

List out 6 stereotype annotations

Answer 25

1. @Component
2. @Controller
3. @RestController
4. @Service
5. @Repository
6. @Configuration

Note that other Spring projects provide their own stereotype annotations
@Bean is not a stereotype annotation

Question 26

Is @Bean a stereotype annotation?

Answer 26

No

Question 27

What is meta-annotation?

Answer 27

Annotation which can be used to annotate other annotations

Question 28

When creating a new annotation, what RetentionPolicy should be set so that Spring can scan it?

Answer 28

Note:
CLASS
Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time.
RUNTIME
Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.
SOURCE
Annotations are to be discarded by the compiler.

Question 29

What is the execution order of post-construction methods?

Answer 29

@PostConstruct –> afterPropertiesSet (if impl InitializingBean) –> init-method

Question 30

What is the execution order of pre-destruction methods?

Answer 30

@PreDestroy –> detroy (if impl DisposableBean) –> destroy-method
Note that if it’s an @Bean object, public void close() and public void shutdown() can be considered as default destroy-method

Question 31

In JSR-330, what does @Named do?

Answer 31

Equivalent to @Component + @Qualifier