what is java reflection

##What is reflection?

Since reflection has "reverse", it means that it is a reverse process. So let’s first take a look at what the forward process is like.

Design a class, instantiate the class into an object, and the object calls methods or properties to implement some functions.

What is reflection?

I don’t know if you still remember the picture above. This is the knowledge I learned when I first came into contact with java. My own understanding is that now I know 1, infer to 2, and then get some other information through 2. (This is just my speculation. As for whether it is true or not, we need to delve into it in the future)

The analogy to the real-life scenario is:

你：妈妈，你知道剪刀在哪吗？
妈妈：我那天放针线的时候好像在针线盒里看到了，你去找找有没有。针线盒在客厅的茶几上。
你：好的。

Then you derived the "sewing box" from "sewing" ", and then found "Scissors" from the "Sewing Box".

Our reflection is to deduce the Class class from the class or object, and then obtain the class information from the Class class.

More confused? Don't worry, please look down.

Find the sewing box - get the Class class

First of all, what is the Class class?

The Class class is the class of the People class, just like the People class is the class of p. The Class class is the "grandfather" of p.

We can understand it this way: the instance object of java.lang.Class is People, and the instance object of People is p. Our idea is to obtain the Class class through the People class or p object, and then obtain the People class information, such as attributes, methods, constructors, etc., through the Class class.

Now that we know what the Class class is, let’s talk about how to get it.

There are three ways to find the Class class:

It is worth mentioning that c1, c2, and c3 are called the "class types" of the People class. Okay, now let’s summarize the known conditions:

People: Class

p: Object

c: Class Type

Find the scissors - get the information of the People class

Application 1: Get all the methods of the class

In the method of obtaining the People class, ms is an array that stores the People class method. Through a loop, its name, parameter type, and return value are obtained layer by layer.

Finally, just call it directly in the main function

The partial results obtained are as follows:

Application 2: Get all the attributes of the class

##The call is similar to the calling method:

Run result:

Application 3: Obtain the constructor of the class

The constructor is similar, but not Again.

Application 4: Obtain a specific method of the class

The running results are as follows. The results of the two calling methods are the same:

The above is just a list of individual functions. For more specific information, you can find it in the API documentation: http:/ /tool.oschina.net/apidocs/apidoc?api=jdk-zh

##The concept of reflection

At the end of the article, we summarize the concept of reflection and let our thoughts sublime.

What is the definition of reflection?

The JAVA reflection mechanism is that in the running state, for any class, you can know all the properties and methods of this class; for any object, you can call any of its methods. This The function of dynamically obtaining and calling object methods is called the reflection mechanism of the Java language.

Is reflection necessary?

"The reflection mechanism is the cornerstone of many java frameworks".

(1) Write the configuration in the xml file or properties, then parse the content in the xml or properties in the Java class, get a string, and then use the reflection mechanism to obtain a certain string based on this string Class instance of the class, so that you can dynamically configure something. You don't have to add new or do other things in the code every time. If you want to change it in the future, you can directly change the configuration file, which makes the code maintenance very convenient.

(2) Sometimes to meet certain needs, it may not be possible to directly call other methods in the Java class. At this time, it can also be achieved through the reflection mechanism.

Disadvantages of reflection?

We can also see in the code that the reflected code is more code than the normal call code, and the performance is slower, so the use of reflection should be avoided. This is equivalent to if you already know where the scissors are, there is no need to go through the needle and thread-sewing box-scissors path.

What are the criteria for whether to use reflection?

If a function can be completed without reflection, then it is best not to use it.

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