What are the common Java annotations?

Annotation

(Recommended tutorial: java introductory tutorial)

• From JDK Starting from 5.0, Java has added support for metadata (MetaData), that is, Annotation

• Annotation is actually a special mark in the code. These marks can be used during compilation and class loading. , is read at runtime, and corresponding processing is performed. By using Annotation, we can embed some supplementary information in the source file without changing the original logic. Code analysis tools, development tools, and deployment tools can use this supplementary information for verification or deployment.

• Annotation can be used like a modifier and can be used to modify the declaration of packages, classes, constructors, methods, member variables, parameters, and local variables. This information is saved in Annotation. "name=value" pairs.

• In JavaSE, the use of annotations is relatively simple, such as marking obsolete functions, ignoring warnings, etc. Annotations play a more important role in JavaEE/Android, for example, they are used to configure any aspect of the application, replacing the cumbersome code and XML configuration left in the old version of JavaEE.

• Future development models are all based on annotations, JPA is based on annotations, Spring 2.5 and above are all based on annotations, Hibernate3.x and later are also based on annotations, and now Struts2 Part of it is also based on annotations, and annotations are a trend. To a certain extent it can be said: framework = annotation reflection design pattern.

Common annotations

When using an Annotation, add the @ symbol in front of it and use the Annotation as a modifier. Used to decorate the program elements it supports.

Generate documentation-related annotations

• @author indicates the author who developed this type of module, used to separate multiple authors

• @version indicates the version of this type of module

• @see reference direction, that is, related topics

• @since which version to start from The added

• @param is a description of a certain parameter in the method. If there are no parameters, it cannot be written.

• @return A description of the method return value , if the return value type of the method is void, you cannot write

• @exception. Explain the exceptions that the method may throw. If the method does not explicitly throw exceptions with throws, you cannot write it.

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Note:

• @param @return These three tags, @exception, are only used for methods.

• @param format requirements: @param formal parameter name formal parameter type formal parameter description

• @return format requirements: @return return Value type return value description

• @exception format requirements: @exception Exception type exception description

• @param and @exception can be multiple in parallel

Code example:

/**
 * @author wushanghui
 * @date 2020/7/16 17:51
 * @see Math
 * @version	 1.0
 *
 */
public class JavadocAnnotation {

    /**
     * 程序的主方法，程序的入口
     *
     * @param args String[] 命令行参数
     */
    public static void main(String[] args) {
        System.out.println(getArea(1.0d)); // 3.141592653589793
    }


    /**
     * 求圆面积的方法
     * @since 1.0
     * @param radius double 半径值
     * @return double 圆的面积
     */
    public static double getArea(double radius) {
        return Math.PI * radius * radius;
    }
}

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