Detailed explanation of basic concepts of Java language annotation

1、RetentionPolicy. Source: the annotation is only kept in the source file. When the java file is compiled into a class file, the annotation is abandoned;

2、RetentionPolicy. Class: the annotation is kept in the class file, but it is abandoned when the JVM loads the class file. This is the default life cycle;

3、RetentionPolicy. Runtime: the annotation is not only saved in the class file, but still exists after the JVM loads the class file;

These three life cycles correspond to: Java source file (. Java file) -- > Class file -- > bytecode in memory.

How to choose the appropriate annotation life cycle?

First of all, the life cycle length source < class < runtime should be clear, so where the former can work, the latter must also work. Generally, if you need to dynamically obtain annotation information at runtime, you can only use runtime annotation; If you want to perform some preprocessing operations during compilation, such as generating some auxiliary code (such as butterknife), use the class annotation; if you only do some checking operations, such as @ override and @ suppresswarnings, you can use the source annotation.

Here is a brief introduction to the use of runtime annotations.

Get annotation

You need to get runtime annotations through reflection, which can be obtained from package, class, field, method The basic methods are the same. Several common methods are as follows:

To use these functions, you must first obtain the corresponding elements through reflection: class, field, method, etc.

Custom annotation

Let's take a look at the simple usage of custom annotations. Here, we first define three runtime annotations:

These three annotations are applicable to different elements and have different attributes. You need to set these attribute values when using annotations.

Define a test class to use these annotations:

It is still very simple to use. Finally, let's look at how to obtain annotation information in the code:

All operations are to obtain the corresponding element through reflection, then obtain the annotation on the element, and finally obtain the attribute value of the annotation.

Let's take a look at the output. Here I directly display it on my mobile phone:

summary

The above is all about the detailed explanation of the basic concepts of Java language annotation in this paper. I hope it will be helpful to you. Interested friends can continue to refer to other related topics on this site. If there are deficiencies, please leave a message to point out. Thank you for your support!