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