spring mvc 的jpa JpaRepository数据层

1. Spring Data 应用场景

Spring Data包含多个子项目：
Spring Data JPA
Spring Data Mongo DB
Spring Data Redis
Spring Data Solr

2. JDBC和JPA的区别

Java数据库连接，（Java Database Connectivity，简称JDBC）是Java语言中用来规范客户端程序如何来访问数据库的应用程序接口，提供了诸如查询和更新数据库中数据的方法

JDBC: is a standard for Database Access (是Java的底层技术，包括也是很多database JPA的底层技术)
JPA: is a standard for ORM. This can "hide" the SQL from the developer so that all they deal with are Java classes, and the provider allows you to save them and load them magically. The most famous JPA provider is Hibernate

JDBC的一个例子：

    public String getAllEnglishWords() throws ClassNotFoundException, SQLException {
        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
        Connection con = DriverManager.getConnection("jdbc:sqlserver://127.0.0.1:1434;databaseName=DailyEnglish", "username", "password");
         Statement stmt = con.createStatement(); 
         ResultSet rs = stmt.executeQuery("select * from Words"); 
         //while(rs.next()) {
            // System.out.println(rs.getString("english"));
             //System.out.println(rs.getString("chinese"));
         //}
         String result = resultSetToJson(rs);
         System.out.println(result);
         rs.close();
         stmt.close();
         con.close();
        return result;
    }

3. Spring Data JPA

JPA (Java Persistence API)
ORM:Object-Relational Mapping: 让你map entity class到sql table
Class <-> Table
当你用JAVA连接关系型数据库事，你需要JDBC，并且运行queries，然后得到结果后，把它转为对象；
References:
https://juejin.im/post/5aa9fb2a518825557e781bbf