1. Annotation Overview
Brief introduction: Annotations are comments or metadata that are inserted into your code. These annotations can be processed using pre-compilation tools during compilation, or can be processed using the java reflection mechanism during runtime.
Annotation function : 1. Compiler check 2. Define annotation
Annotation advantages: improve development efficiency and low cost
Note shortcomings: large coupling, not conducive to later maintenance
2. Definition of annotations
The annotation is defined by the @interface keyword
Examples:
The use of annotations: the simplest annotation format @Test
Commonly used annotation: @override (indicating the coverage of the parent class method, it should be no stranger!)
3. Meta annotations
Meta annotations are annotations that can be annotated onto annotations. (A basic annotation can be applied to other annotations)
Its role and purpose is to explain other common notes
There are 5 types of meta tags: @Retention , @Documented , @Target , @Inherited , @Repeatable . (Two of them are more important)
1. @Retention (indicating the survival time of the annotation)
Value: RetentionPolicy.SOURCE The annotation is only reserved in the source code stage, it will be discarded and ignored when the compiler compiles.
The RetentionPolicy.CLASS annotation is only retained until the compilation is in progress, it will not be loaded into the JVM.
RetentionPolicy.RUNTIME annotation can be retained until the program runs, it will be loaded into the JVM,
format:
2. @ Target (specify the application of annotation)
Value: ElementType.ANNOTATION_TYPE can annotate an annotation
ElementType.CONSTRUCTOR can annotate the constructor
ElementType.FIELD can annotate attributes
ElementType.LOCAL_VARIABLE can annotate local variables
ElementType.METHOD can annotate methods
ElementType.PACKAGE can annotate a package
ElementType.PARAMETER can annotate parameters within a method
ElementType.TYPE can annotate a type, such as class, interface, enumeration
format:
4. Annotated attributes
Note that there are only member variables and no methods. '
Examples:
use:
V. Annotation extraction
Annotations are obtained through reflection. First, you can determine whether a certain annotation is applied by the isAnnotationPresent () method of the Class object.
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