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;
    }
}

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();
        }
    }

}