深入JAVA注解之方法注解

以获取数据库连接为例，建立maven项目

 <project xmlns="http://maven.apache.org/POM/4.0.0"
     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
     <groupId>org.guangsoft</groupId>
     <artifactId>annotation</artifactId>
     <version>1.0</version>
     <packaging>war</packaging>
     <name>annotation</name>
     <dependencies>
         <dependency>
             <groupId>com.mchange</groupId>
             <artifactId>mchange-commons-java</artifactId>
             <version>0.2.9</version>
         </dependency>
         <dependency>
             <groupId>com.mchange</groupId>
             <artifactId>c3p0</artifactId>
             <version>0.9.5</version>
         </dependency>
         <dependency>
             <groupId>mysql</groupId>
             <artifactId>mysql-connector-java</artifactId>
             <version>8.0.11</version>
         </dependency>
     </dependencies>
 </project>

建立注解类

 package org.guangsoft.annotation.entity;

 import java.lang.annotation.ElementType;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;

 @Target(ElementType.METHOD)
 @Retention(RetentionPolicy.RUNTIME)
 public @interface DBInfo {
     //属性名参考com.mchange.v2.c3p0.DriverManagerDataSource
     String driverClass() default "com.mysql.jdbc.Driver";
     String jdbcUrl() default "jdbc:mysql://192.168.50.30:3306/guanghe";
     String user() default "root";
     String password() default "root";
 }

建立dao层

 package org.guangsoft.annotation.dao;

 import org.guangsoft.annotation.entity.DBInfo;

 import com.mchange.v2.c3p0.ComboPooledDataSource;

 public class CommDAO {

     private ComboPooledDataSource dataSource;

     @DBInfo
     public void setDataSource(ComboPooledDataSource dataSource) {
         this.dataSource = dataSource;
     }

     public ComboPooledDataSource getDataSource() {
         return dataSource;
     }

 }

建立工厂类

 package org.guangsoft.annotation.utils;

 import java.beans.BeanInfo;
 import java.beans.Introspector;
 import java.beans.PropertyDescriptor;
 import java.lang.reflect.Method;

 import javax.sql.DataSource;

 import org.guangsoft.annotation.dao.CommDAO;
 import org.guangsoft.annotation.entity.DBInfo;

 public class JDBCUtil2 {

     //将注解注入到数据源类
     private static DataSource getDataSourceByDBInfo (DBInfo dbInfo, DataSource dataSource) {
         Method[] methods = DBInfo.class.getMethods();
         for(Method method : methods) {
             String name = method.getName();
             try {
                 //注解类没有属性，反射注解类的方法名，内省出数据源类设置参数的方法, 找不到会抛异常，进入下次循环
                 PropertyDescriptor propertyDescriptor = new PropertyDescriptor(name, dataSource.getClass());
                 //注解类的方法能得到实际注解的值
                 Object value = method.invoke(dbInfo, null);
                 //用数据源的方法将注解类的值注入
                 propertyDescriptor.getWriteMethod().invoke(dataSource, value);
             } catch(Exception e) {
                 continue;
             }
         }
         return dataSource;
     }

     //工厂模式下的创建DAO
     public static CommDAO createCommDAO() {
         CommDAO commDAO = new CommDAO();
         try {
             BeanInfo beanInfo = Introspector.getBeanInfo(commDAO.getClass(), Object.class);
             PropertyDescriptor[] propertyDescriptors = beanInfo.getPropertyDescriptors();
             if(propertyDescriptors != null) {
                 for(PropertyDescriptor propertyDescriptor : propertyDescriptors) {
                     Method setMethod = propertyDescriptor.getWriteMethod();
                     DBInfo dbInfo = setMethod.getAnnotation(DBInfo.class);
                     if(dbInfo != null) {
                         //获取dao中dataSource的实体类ComboPooledDataSource
                         Class dataSourceClass = propertyDescriptor.getPropertyType();
                         DataSource dataSource = (DataSource) dataSourceClass.newInstance();
                         dataSource = getDataSourceByDBInfo(dbInfo, dataSource);
                         setMethod.invoke(commDAO, dataSource);
                     }
                 }
             }
         }catch(Exception e) {
             e.printStackTrace();
         }
         return commDAO;
     }

 }

建立测试类

 package org.guangsoft.annotation.service;

 import java.sql.Connection;

 import javax.sql.DataSource;

 import org.guangsoft.annotation.dao.CommDAO;
 import org.guangsoft.annotation.utils.JDBCUtil2;

 public class CommService {

     public static void main(String args[]) throws Exception {
         CommDAO commDAO = JDBCUtil2.createCommDAO();
         DataSource dataSource = commDAO.getDataSource();
         Connection connection = dataSource.getConnection();
         System.out.println(connection);
     }

 }

测试结果：成功