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This article will explain in detail how to use Java annotations, Xiaobian thinks it is quite practical, so share it for everyone to make a reference, I hope you can gain something after reading this article.

defined

Annotations, also known as raw data, are a feature introduced in JDK 1.5 and later, which can be declared in front of classes, methods, variables, etc. to describe these elements.

role

Generating documentation: generating doc documentation from comments identified in code

Code analysis: analysis of code by comments identified in the code

Compilation checking: comments identified in the code enable the compiler to perform basic compilation checking

The difference between comments and comments

Comments are for compilers, comments are for programmers.

Standard annotations built into JDK

@Override

Indicates that the method overrides the parent class. If there is no parent class in a method and the method uses @Override, an error will be reported.

@Deprecated

This means that the method is outdated, but it can still be used.

@SuppressWarnings

Suppress warning

Usually used in front of a class, passing the parameter "all" means to suppress all warnings of the class

This is the parent code.

public class Fu { public void eat(){ System.out.println("Fu Eat...... "); }}

This is subclass code.

@SuppressWarnings("all")//Suppress all warnings under this class public class Zi extends Fu{ @Override //indicates that the method overrides the parent class public void eat(){ System.out.println("Zi eat...... "); } @Deprecated //indicates that the method is obsolete public void work(){ }}

Here are the main classes

custom annotation

Format:

meta-annotation

public @interface annotation name {

attribute list

}

An annotation is essentially an interface that inherits the Annotation interface by default.

public interface annotation name extends java.lang.annotation.Annotation {}

meta-annotation

A comment is a meta-comment.

Below is the source code of @Override annotation. We can see that there are some annotations in front of @Override. Let's introduce these annotations.

We need to master five meta-annotations:

@Target

Indicates where annotations can be used

Parameter ElementType Value

TYPE: Indicates that annotations can be used on classes

METHOD: Indicates that comments can be used on methods

FIELD: Indicates that annotations can be used on member variables

@Target({ElementType.TYPE, ElementType.FIELD, ElementType.METHOD}) //When {} has only one content,{} can omit public @interface PersonWrite { int age(); String name() default "22";}

@Retention（）

Indicate the stage at which comments are retained:

Annotations are retained until the source code stage

@Retention(RetentionPolicy.SOURCE)

Comments are retained until compilation

@Retention(RetentionPolicy.CLASS)

Note 1 remains in operation

@Retention(RetentionPolicy.RUNTIME)

@Documented

Identification comments are extracted into doc documents

@Inherited

Identification annotation inherited by subclass

attribute

Properties in annotations are abstract methods

The return type of an attribute can only be of the following types:

basic data types

String

enumeration

annotation

Arrays of the above types

If attributes (interfaces) are defined in the annotation, attribute assignment is required when used.

If default is used to assign an attribute when defining an attribute, then you can use an annotation without assigning the attribute (of course, you can also assign it again). If there is only one annotation, and the name of the annotation is value, you can directly write the parameter by using the time parameter. If you do not need to write the attribute name array assignment, you can use {} to wrap it. If there is only one value in the array,{} can be omitted.

Here are the custom comments:

@Target({ElementType.TYPE, ElementType.FIELD, ElementType.METHOD})@Retention(RetentionPolicy.RUNTIME)public @interface PersonWrite { int age(); String name() default "Li Si";}

The following is the use of this annotation:

package Java advanced features. comments;@PersonWrite(age = 222)public class Main { public static void main(String[] args) { Zi zi = new Zi(); zi.work(); }} About "Java annotations how to use" this article is shared here, I hope the above content can be of some help to everyone, so that you can learn more knowledge, if you think the article is good, please share it to let more people see.

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