Detailed explanation of java knowledge points: annotations and meta-annotations

This article brings you relevant knowledge about java, which mainly introduces issues related to annotations and meta-annotations, including a basic introduction to annotations, a basic introduction to Annotation, etc., Let’s take a look at it together, I hope it will be helpful to everyone.

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1. Basic introduction to annotations

      Annotation (annotation), also known as metadata (Metadata), is introduced in JDK1.5 and later versions and is used to modify and interpret data information such as packages, classes, methods, properties, constructors, and local variables. It can be used to create documentation, track dependencies in your code, and even perform basic compile-time checks.

Annotations exist in the code with '@annotation name'. According to the number of annotation parameters, we can divide annotations into three categories: mark annotations, single-value annotations, and complete annotations. kind. Like comments, annotations do not affect program logic, but they can be compiled or run and are equivalent to supplementary information embedded in the code.

In addition, you can choose at compile time whether the annotations in the code only exist at the source code level, or it can also appear in class files or runtime (SOURCE/CLASS/ RUNTIME).

In JavaSE, the purpose of using annotations is relatively simple, such as marking obsolete functions, ignoring warnings, etc. Annotations play a more important role in Java EE, such as being used to configure any aspect of the application, replacing the cumbersome code and XML configuration left in the old version of Java EE.

2. Basic introduction to Annotation

1. Three basic Annotations

@Override : To limit a method is to override the parent class method , This annotation can only be used for methods

@Deprecated : Used to represent a certain program element ( kind , Methods, etc. ) Outdated

@SuppressWarnings : Suppress compiler warnings

##2. Code examples

@Override

class Son extends Father{

    @Override
    public void play(){}

}

Note:

@Override indicates that the play method of the subclass overrides the play method of the parent class

If there is no written here @Override It will still override the parent class ply

If you write the @Override annotation, the compiler will check whether the method really overrides the parent class Method, if it is indeed rewritten, the compilation will pass. If it is not rewritten, the compilation error will occur.

@Override can only modify methods, not other classes, packages, properties, etc.

@Deprecated

@Deprecated
class A{
    @Deprecated
    public static int name = 1;

    @Deprecated
    public void play(){ }
}

Note:

    It’s obsolete, it doesn’t mean it can’t be used, it’s just not recommended, but it can still be used

      Can modify methods, classes, fields, Packages, parameters, etc.

Its function is to achieve compatibility and transition between old and new versions

@SuppressWarnings

@SuppressWarnings ("all")
public class word{ }

注意：

         关于 SuppressWarnings 作用范围是和你放置的位置相关。比如@SuppressWarnings 放置在 main 方法，那么抑制警告的范围就是 main

         通过 @SuppressWarnings 的源码可知，其注解目标为类、字段、函数、函数入参、构造函数和函数的局部变量。

三、@SuppressWarnings的指定警告类型

4. Meta-annotation

1. Basic introduction

##▶ What is meta-annotation:

Meta-annotations are annotations that explain annotations. The annotations they annotate are the annotations we introduced earlier, such as: @Override, @Deprecated, @SuppressWarnings  

▶ Four meta-annotations:

Retention: Specify the scope of the annotation, three types of SOURCE, CLASS, and RUNTIME

Target: Specify where the annotation can be used

▶ Notes on annotations:

                                                                                                                                                                                    , the annotation will exist in the class bytecode file, but cannot be obtained at runtime,

The annotation will exist in the class bytecode file, and can be obtained through reflection at runtime                              

        ①: Generally, if you need to dynamically obtain annotation information at runtime, you can only use RUNTIME annotations; ## For some pre -processing operations, such as generating some auxiliary code (such as Butterknife), use class notes;
③: If you just do some
check -in SuppressWarnings, you can use the SOURCE annotation.

2. Retention annotation

Explanation:

can only be used to modify an Annotation definition,

Annotation

How long it can be retained, @Rentention contains a member variable of type

RetentionPolicy, which must be the

value when using @Rentention

##Member variables specify values ​​(there are three types of values).                                       RetentionPolicy.SOURCE: After the compiler uses it, it directly discards the comments of this policy.

       RetentionPolicy.CLASS: The compiler will record the annotations in class In the file . When running Java Program Time ​ , JVM Annotations will not be retained. It's the default value.

RetentionPolicy.RUNTIME: The compiler will record the annotation in class ​ In the file ​ . ​ When running ​ Java ​ Program Time ​ , JVM ​ Annotations will be retained ​ . ​ The program can obtain this annotation through reflection.

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3. Target annotation

Explanation:

is used to modify the Annotation definition and specify which program elements the modified Annotation can be used to modify. @Target also contains a member variable named value.

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4, Documented annotation

Explanation:

      @Documented: Used to specify that the Annotation class modified by this meta-annotation will be extracted into a document by the javadoc tool, that is, the annotation can be seen when the document is generated.

Notice:

Annotations defined as @Documented must set the Retention value to RUNTIME.

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5. Inherited annotation

Explanation:

Annotations modified by @Inherited will have inheritance. If a class uses annotations modified by @Inherited, its subclasses will automatically have inheritance This annotation

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