22-AOP

Question 1

What are cross-cutting concerns? Give some examples

Answer 1

Generic functionality that is needed in many places in your application
Examples:
1. Logging and tracing
2. Security
3. Custom Business Rules

Question 2

What does Spring-AOP enable?

Answer 2

Modulization of cross-cutting concerns
i.e. The code for a cross-cutting concern is in a single place, in a module - in Java, a class represents a module

Question 3

What’re the problems if failing to modularize cross-cutting concerns?

Answer 3

1. Code tangling i.e. coupling of concerns

2. Code scattering i.e. the same concern spread across modules

Question 4

What is aspect weaving?

Answer 4

Technique by which aspects are combined with main code

Question 5

What library does Spring AOP use?

Answer 5

AspectJ

Question 6

What is a join point?

Answer 6

A point in the execution of a program such as a method call or exception thrown

Question 7

What is a pointcut?

Answer 7

An expression that selects one or more Join Points

Question 8

What is an Advice?

Answer 8

Code to be executed at each selected JoinPoint

Question 9

What is an Aspect?

Answer 9

A module that encapsulates pointcuts and advice

Question 10

What is AOP proxy?

Answer 10

An “enhanced” class that stands in place of your original.
With extra behavior (Aspect) added (woven) into it

Question 11

Write an aspect component to advise all functions start with set and return void

Answer 11

@Aspect @Component
public class Tracker {
   @Before("execution(void set*(*))")
   public void method() {}
}

Question 12

List out all @EnableXXX annotations

Answer 12

@EnableAspectJAutoProxy

• @EntityScan w/SpringBootApplication
• @EnableJpaRepository(basePackage=”…”) with @Configuration
• @EnableWebSecurity with @Configuration
• @EnableGlobalMethodSecurity with
• @AutoConfigureMockMvc with @SpringBootTest
• @AutoConfigureTestDatabase
• @EnableHypermediaSupport(type=HypermediaType.HAL) with @Configuration
• @EnableWebMvc with @Configuration
• @EnableAutoConfiguration
• @EnableConfigurationProperties
• @EnableTransactionManager

Question 13

What is @EnableAspectJAutoProxy equivalent to?

Answer 13

XML: aop:aspectj-autoproxy

Question 14

What aop:aspectj-autoproxy supports but @EnableAspectJAutoProxy doesn’t?

Answer 14

it has aop:include to sepecify ids if beans to use in aspect –> doesn’t need to scan all aspect components. But Spring AOP doesn’t have this

Question 15

How to enable AOP in Spring app?

Answer 15

no need for Spring Boot app. Need to add @Configuration @EnableAspectJAutoProxy for non-Spring boot app

Question 16

When is the point-cut expression used?

Answer 16

1. “weaving” aspect and target

2. on every invocation, to decide if the advice should actually apply

Question 17

List out pointcut designators

Answer 17

within: package

execution

Question 18

How to chain pointcuts together?

Answer 18

Using: && or || or !

Question 19

What is the method pattern in execution(method-pattern)?

Answer 19

[Modifiers] ReturnType [ClassType]

MethodName (Arguments) [throws ExceptionType]

Question 20

What are mandatory in pointcut method pattern?

Answer 20

[Modifiers] ReturnType [ClassType]
MethodName (Arguments) [throws ExceptionType]

Question 21

Can the pointcut work with the private/protected method in class?

Answer 21

yes if using CGLIB proxies

Question 22

Using pointcut, how to match method with 0, 1, 2 and any arguments

Answer 22

0: set()
1: set()
2: set(*, )
any: set(..)

Question 23

The powerful match-all pointcut expression

Answer 23

• *(..)

Question 24

Pointcut: match method starts with send, having an argument of type rewards.Dining and return void

Answer 24

execution(void send*(rewards.Dining))

Question 25

Pointcut: match method send, doesn’t care about return type but the first argument must be int. Can have 1+ argument

Answer 25

execution(* send(int, ..))

Question 26

Will this match MessageServiceImpl.send?

execution(void example.MessageService.send(*))

Answer 26

Yes. Matching including sub-class

Question 27

Pointcut to match all methods with @Logger

Answer 27

execution( @Logger * *(..) )

Question 28

Pointcut: match zero or more packages

Answer 28

*..

should not be .. because an expression with a leading .. does not work - hence *..

Question 29

Pointcut: match 1 directory between rewards and restaurant

Answer 29

execution(* rewards..restaurant..*(..))

Question 30

Pointcut: match several directories between rewards

and restaurant

Answer 30

execution(* rewards..restaurant..(..))

Question 31

Pointcut: Any sub-package called restaurant

Answer 31

execution(* ..restaurant..*(..))

Question 32

Write and advice function to print out the name of the joinpoint, first args, and name of the bean (having that joinpoint)

Answer 32

void adviceFunction(JoinPoint jp) {}

1. Name of joinpoint: jp.getSignature().getName()
2. arg: jp.getArgs()[0]
3. bean name: jp.getTarget().toString()

Question 33

Meaning of * in pointcut

Answer 33

matches once (return type, package, class, method name, argument)

Question 34

Meaning of .. in pointcut

Answer 34

matches zero or more (arguments or packages)

Question 35

Define an advice & pointcut to re-thrown all IOException to MyIOException and wrap the original exception inside

Answer 35

@AfterThrowing(value="execution(* *(..) throws IOException", throwing="e")
void fun(JoinPoint jp, IOException e) {
    throw new MyIOException(e);
}

Note: The @AfterThrowing advice will not stop the exception from propagating

Question 36

Write advice function for @Around pointcut

Answer 36

Object adviceFn(ProceedingJoinPoint jp) throws Throwable {
    return jp.proceed();
}

Note: ProceedingJoinPoint not JointPoint
must throws Throwable because of jp.proceed()

Question 37

Limitation of Spring AOP

Answer 37

1. can only advise non-private methods
2. apply for Spring Beans only
3. Limitation of weaving proxies
   When using proxies, suppose method a() calls method b() on the same class/interface
   advice will never be executed for method b()

Question 38

How to force CGLib proxy?

Answer 38

XML:

aop: config proxy-target-class=”true”
aop: aspectj-autoproxy proxy-target-class=”true”

Question 39

Pointcut to bind target bean and argument to arguments in the advice method

Answer 39

@Before("execution(something) && target(server) && args(host, port)")
void listen(Server server, String host, int port) {}

Question 40

Pointcut to bind annotation to argument in the advice method

Answer 40

@Before("execution(something) && @annotation(txn)")
void listen(Transactional txn) {}

Question 41

In Spring AOP, ________ can be of multiple types, such as “around”, “before” and “after”.

Answer 41

Advice