Java 注解实例

package com.annotation;

import java.lang.annotation.Retention;

import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.*;

import static java.lang.annotation.RetentionPolicy.*;

/**

 * 

 * @author cong

 * 数据库连接注解

 *

 */

@Target(value = {FIELD})//Target:此注解用到的地方有TYPE、FIELD、METHOD

@Retention(value = RUNTIME)//注解的运行环境

public @interface CustomConnection {

    String url() default "";

    String driverClass() default "";

    String username() default "";

    String password() default "";

}

 

注解处理
package com.annotation.processor;



import java.lang.reflect.Constructor;

import java.lang.reflect.Field;

import java.sql.Connection;

import java.sql.DriverManager;



import com.annotation.CustomConnection;



@SuppressWarnings("unchecked")//不显示警告提示

public class CustomAnnotationProcessor {

    public static <T> void processor(Class<T> clazz) {

        try {

            //T t = clazz.newInstance();

            T t = null;

            Field[] fields = clazz.getDeclaredFields();//获取T对象声明变量

            

            Constructor<?>[] constructors = clazz.getDeclaredConstructors();//获取T对象的构造函数，因为构造函数的访问级别问题

            for(Constructor<?> constructor : constructors) {

                constructor.setAccessible(true);//无论T对象的构造函数是什么访问级别都可以访问其方法

                t = (T)constructor.newInstance(new Object[]{});

            }

            

            for(Field field : fields) {

                if(field.isAnnotationPresent(CustomConnection.class)) {

                    CustomConnection customConnection = field.getAnnotation(CustomConnection.class);

                    String url = customConnection.url();

                    String driverClass = customConnection.driverClass();

                    String username = customConnection.username();

                    String password = customConnection.password();

                    Class.forName(driverClass);

                    Connection connection = DriverManager.getConnection(url, username, password);

                    field.setAccessible(true);

                    field.set(t, connection);

                    continue;

                }

            } 

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}

package com.util;



import java.sql.Connection;



import com.annotation.CustomConnection;

/**

 * 

 * @author Administrator

 * 调用注解连接数据库

 *

 */

public class DBUtil {



    private DBUtil() {//构造方法

        

    }

    

    @CustomConnection(url = "jdbc:oracle:thin:@//192.168.40.29:1521/PDBMYORACLE",

            driverClass = "oracle.jdbc.driver.OracleDriver",

            username = "victop",

            password = "victop")

    private static Connection connection;

    

    public static Connection getConnection() {

        return connection;

    }

}

测试

package com.victop;



import java.sql.Connection;

import java.sql.PreparedStatement;

import java.sql.ResultSet;

import java.sql.SQLException;



import com.annotation.processor.CustomAnnotationProcessor;

import com.util.DBUtil;



public class Test {

    

    public static void main(String[] args) {

        try {

            CustomAnnotationProcessor.processor(DBUtil.class);

            Connection connection = DBUtil.getConnection();

            PreparedStatement preparedStatement = connection.prepareStatement("select * from Users");

            ResultSet resultSet = preparedStatement.executeQuery();

            while(resultSet.next()) {

                System.out.println(resultSet.getString("username"));

            }

        } catch (SQLException e) {

            e.printStackTrace();

        }

    }



}