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This article shows you how to interpret Java Spring AOP, the content is concise and easy to understand, can definitely brighten your eyes, through the detailed introduction of this article, I hope you can gain something.

First impression of AOP

First of all, give a more professional term (from Baidu):

In the software industry, AOP is the abbreviation of Aspect Oriented Programming, which means: a technology for aspect-oriented programming, which realizes the unified maintenance of program functions through precompilation and runtime dynamic agents. AOP is the continuation of OOP, is a hot spot in software development, is also an important content of Spring framework, and is a derivative paradigm of functional programming. Each part of the business logic can be isolated by using AOP, which reduces the coupling between the various parts of the business logic, improves the reusability of the program, and improves the efficiency of development at the same time.

Then let's give an example that is easier to understand:

To understand aspect programming, you need to understand what aspect is. Divide a watermelon into two slices with a knife, and the cut is the noodles; stir-fry, pan and stove together to complete the frying, the pot and stove is the noodles. In web level design, web layer-> gateway layer-> service layer-> data layer, there is also an aspect between each layer. In programming, there are aspects between objects, between methods, and between modules.

When we do activities, we usually check the validity of each interface (whether to start, end, etc.), and whether this interface requires users to log in.

According to normal logic, we can do this.

The problem here is that how many interfaces there are, how many times the code copy. For a "lazy person", this is intolerable. Okay, come up with a public method, and each interface calls this interface. It smells a little bit of noodles here.

There is also a problem, although I do not have to copy code every time, but every interface has to call this method. So I have the concept of aspect, and I inject the method somewhere in the interface call (pointcut).

In this way, the interface only needs to care about the specific business, and does not need to pay attention to other logic or processing that is not concerned with the interface.

At the red box, it is aspect-oriented programming.

II. Related concepts in AOP

After reading the above example, I think you already have a rough prototype of AOP in your mind, but there are some ambiguities in terms such as the section mentioned above. Next, let's explain the relevant concepts in AOP and understand the concepts in AOP. Only by understanding the concepts in AOP can we really grasp the essence of AOP.

Here, let's give a more professional concept definition:

Aspect: the Aspect declaration is similar to the class declaration in Java and contains some Pointcut and the corresponding Advice in the Aspect.

Joint point (join point): represents a point clearly defined in a program, typically including method calls, access to class members, execution of exception handler blocks, and so on. It can also nest other joint point itself.

Pointcut (pointcut): represents a set of joint point that are combined either by logical relationships or by wildcards, regular expressions, etc., and define where the corresponding Advice is going to occur.

Advice (enhancements): Advice defines the specific actions to be done by the program points defined in the Pointcut, distinguishing between before, after, and around before, after, or instead of executing code for each joint point.

Target (target object): the target object woven into Advice.

Weaving: the process of connecting Aspect to other objects and creating an Adviced object

Then give an easy-to-understand example:

After reading the above theoretical knowledge, I believe there are still many friends who feel that the concept of AOP is still very vague and do not have a thorough understanding of the various concepts in AOP. In fact, this is normal, because there are too many concepts in AOP, and it took me a long time to sort it out.

Let me use a simple example to compare the relationship between Aspect, Joint point, Pointcut and Advice in AOP.

Let's suppose that there was a murder in a small county called Java on a dark and windy night. The murderer was so cunning that there were no valuable clues left at the scene. Fortunately, Lao Wang, who had just returned from next door, accidentally discovered the process of the murderer at this time, but because it was getting late and the murderer was masked, Lao Wang did not see the face of the murderer clearly, only that the murderer was a male and was about seven feet five inches tall. According to Lao Wang's description, the county magistrate of Java County ordered the gatekeeper to arrest and interrogate any man who was found to be seven feet five inches tall. Of course, the soldiers dared not disobey the county magistrate's orders, so they had to arrest all qualified people entering and leaving the city.

Let's take a look at how the above short story corresponds to AOP.

First of all, we know that in Spring AOP, Joint point refers to the execution point of all methods, while point cut is a description information, which modifies Joint point. Through point cut, we can determine which Joint point can be woven into Advice. Corresponding to the example we have given above, we can make a simple analogy. Joint point is equivalent to the people in a small county in Java. Pointcut is equivalent to the accusation made by Lao Wang, that is, the killer is a male, about 7 feet 5 inches tall, while Advice is applied to match the suspect described by Lao Wang: arrest and interrogation.

Why can we make such an analogy?

Joint point: people in a small county town in Java: because by definition, Joint point is all the candidate points that may be woven into Advice, in Spring AOP, it can be considered that all method execution points are Joint point. In our above example, the murder took place in a small county, where everyone is supposed to be a suspect.

Pointcut: male, about seven feet five inches tall: we know that all methods (joint point) can be woven into Advice, but we do not want to weave Advice in all methods, and the function of Pointcut is to provide a set of rules to match joinpoint and add Advice to the joinpoint that meets the rules. By the same token, for the county magistrate, no matter how stupid he is, he knows that he cannot arrest and interrogate all the people in the county seat, but arrest those who meet the requirements according to the fact that the killer is a male, about seven feet five inches tall. Here the killer is a male, about seven feet five inches tall is a modifier predicate, it limits the scope of the murderer, the people who meet the modification rules are suspects and need to be arrested and interrogated.

Advice: grab and interrogate. Advice is an action, that is, a piece of Java code that acts on those Joint point defined by point cut. By the same token, in our example, the act of arrest and interrogation works on people in Java, a small town that satisfies men and is about seven feet five inches tall.

Aspect::Aspect is a combination of point cut and Advice, so here we can make an analogy: "according to Lao Wang's clue, any man who is seven feet five inches tall should be arrested for questioning." the whole action can be regarded as an Aspect.

Finally, there is a diagram that describes the relationship between these concepts:

Third, some other contents

There are many types of Joinpoint in AOP: constructor call, field setting and acquisition, method call, method execution, exception handling execution, class initialization. That is to say, in the concept of AOP, we can weave our custom Advice on the above Joinpoint, but we don't implement all the above joinpoint in Spring. To be exact, Spring only supports Joinpoint of method execution type.

Type of Advice

Before advice, the advice that was executed before join point. Although before advice is executed before join point, it does not prevent the execution of join point unless an exception occurs (that is, we cannot artificially decide whether to continue to execute the code in join point in the before advice code)

After return advice, the advice executed after a join point returns normally

After throwing advice, the advice that executes when an join point throws an exception

After (final) advice, whether a join point exits normally or has an exception, will be executed by advice.

Around advice, the advice that executes before join point and after joint point exits. This is the most commonly used advice.

Introduction,introduction can add new properties and methods to existing objects.

In Spring, AOP is realized by dynamic proxy and dynamic bytecode technology.

The above is how to interpret Java Spring AOP. Have you learned any knowledge or skills? If you want to learn more skills or enrich your knowledge reserve, you are welcome to follow the industry information channel.

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