Type instance analysis of java annotations

1. Custom annotation

Use keywords to define annotation: @interface

// #1 定义注解
public @interface MyAnno1{
}

2. Meta annotation

Annotations used to modify annotations.

Five kinds of meta-annotations provided by JDK:

(1) @Target: used to determine the location of modified custom annotations

(2) @Retention: used To determine the life cycle of the modified custom annotation

(3) @Inherited: Indicates that the annotation has inheritance (understanding)

(4) @Documented: When using javadoc to generate api documentation, Whether to include this annotation (understand)

(5) @Repeatable: The annotation is in the same position and can only appear once. With @Repeatable, you can use it multiple times in the same place.

package util;
 
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
 
import anno.JDBCConfig;
 
@JDBCConfig(ip = "127.0.0.1", database = "test", encoding = "UTF-8", loginName = "root", password = "admin")
@JDBCConfig(ip = "127.0.0.1", database = "test", encoding = "UTF-8", loginName = "root", password = "admin")
public class DBUtil {
    static {
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
 
    public static Connection getConnection() throws SQLException, NoSuchMethodException, SecurityException {
        JDBCConfig config = DBUtil.class.getAnnotation(JDBCConfig.class);
        System.out.println(config);
 
        String ip = config.ip();
        int port = config.port();
        String database = config.database();
        String encoding = config.encoding();
        String loginName = config.loginName();
        String password = config.password();
 
        String url = String.format("jdbc:mysql://%s:%d/%s?characterEncoding=%s", ip, port, database, encoding);
        return DriverManager.getConnection(url, loginName, password);
    }
 
    public static void main(String[] args) throws NoSuchMethodException, SecurityException, SQLException {
        Connection c = getConnection();
        System.out.println(c);
    }
}

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