[JAVA] Introduction to Spring Boot ② ~ AOP ~

Learn about AOP following the previous DI. Please note that the content will be super-introductory.

AOP（Aspect-Oriented Programming）

Aspect-oriented programming is one of the concepts of development methods like object-oriented programming. When writing a method, processing that is not directly related to what you want to do, such as logging processing and authentication processing, will be mixed in. It is a development method to describe such auxiliary processing (concerns) separately. For example, suppose you have the following drinkAlcohol () method.

➀ Start log: I'll drink!
	↓
➁ Drink alcohol
	↓
➂ End log: I got drunk!

In this case, ① and ③ are processes that are not directly related to the act of "drinking alcohol", so they will be described separately in different places. Then, set ① before executing the method and ③ after execution. Now the drinkAlcohol () method doesn't care about the logging process, just focus on ② "drinking" processing.

The idea of separating and modularizing auxiliary processing (concerns) in this way is called AOP. It's old, but [here](http://netail.net/aosdwiki/index.php?%A5%A2%A5%B9%A5%DA%A5%AF%A5%C8%BB%D8%B8%FE The explanation of% A5% D7% A5% ED% A5% B0% A5% E9% A5% DF% A5% F3% A5% B0) was easy to understand.

Glossary

the term	meaning
Aspect	A summary of common processing (concerns) behavior and points to apply =A collection of Advice and Pointcut
Advice	Separate common processing behavior The process itself that is actually executed
JoinPoint	Timing to put advice Before or after executing a method, etc.
PointCut	Conditions to apply Advice Conditional expression to determine whether to execute Advice when JoinPoint is reached

JoinPoint annotation

Annotations are provided to set the timing of advice execution.

Annotation	timing
@Before	Executed before method execution
@After	Executed after method execution Execution result does not matter
@Around	Executed instead of the method. Processing before and after the method
@AfterReturning	Executed when the method ends normally
@AfterThrowing	Executed when an exception occurs in the method

PointCut directive

Describes the conditional expression that executes the advice. Let's take a look at the commonly used ʻexecution` format.

ʻExecution (method modifier return value package name.class name.method name (argument type) throws exception) `

Method qualifiers and throwing exceptions are optional.

@Before("execution(* com.sample..*(..))")

In the above example, it will be executed before the method under the com.sample package is executed. Wildcards can be used in conditional expressions.

Wildcard	meaning
*	Instead of any type, or part of a class or package name
..	Instead of any argument, or part of the class or package name
+	Specify all subclass and interface implementations by writing to the right of the class name and interface name

In addition to ʻexecution`, the following directives are available.

PointCut directive	Execution condition
within(name of the class)	Applies to methods of the specified class
target(name of the class)	Applies to methods of classes that inherit the specified class (applies to both parent and child classes)
args(Argument type)	Applies to methods with arguments that match the specified arguments
@annotation(Annotation)	指定したAnnotationが付いているメソッドに適用

Sample code

I tried to realize the above drinkAlcohol () method with AOP.

`Drink.java`


public class Drink {

	public void drinkAlcohol() {
		System.out.println("drinking...");
	}
}

Do not write the logging process, but describe only (2) the process of drinking alcohol.

`SampleAspect.java`


@Aspect
@Component
public class SampleAspect {
	
	@Before("execution(* com.sample..*(..))")
	public void beforeDrinking() {
		System.out.println("[@Before]start drinking alcohol!");
	}

	@After("execution(* com.sample..*(..))")
	public void afterDrinking() {
		System.out.println("[@After]I got drunk...zzZ");
	}
}

Annotate the class with @Aspect and @Component. @Before is (1) start log and @After is (3) end log.

The execution result is as follows.

[@Before]start drinking alcohol!
drinking...
[@After]I got drunk...zzZ

Logs are output before and after the method properly.

@Around

Now, let's realize the same process with @Around.

`SampleAspect.java`


@Aspect
@Component
public class SampleAspect {
	
	@Around("execution(* com.sample..*(..))")
	public void aroundDrinking(ProceedingJoinPoint pjp) {
		System.out.println("[@Around]start drinking alcohol!");	//All processing (➀ start log)
		try {
			pjp.proceed(); //Call the method (➁ drinkAlcohol()）
		} catch (Throwable e) {
			e.printStackTrace();
		}
		System.out.println("[@Around]I got drunk...zzZ"); //Post-processing (➂ end log)
	}
}

@Around will execute Around advice instead of the target method. You can call the method with ProceedingJoinPoint.proceed (), so it feels like putting logging processing before and after that.

Reference material

AOP in Spring Roughly interpret AOP used in Spring Spring Aspect Oriented Programming Spring AOP-Blog of Paper SE