jpa - 从 jdbc 到 open jpa 的转换

Question

我是打开jpa环境的新手，我需要转换jdbc代码才能打开jpa，请帮我解决这个问题。我正在用这篇文章发布 jdbc 代码。

//error.java//
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.Statement;


public class ErrorDAO
{

DataConnection jdbcConnection = null;

public ErrorDAO()
{
    jdbcConnection = new DataConnection();  

}
 public static void main(String[] args) {
    ErrorDAO errorDAO = new ErrorDAO();
    long errid = errorDAO.getErrorId("POTENTIAL VIOLATION");
    String[] details = errorDAO.getErrorDetails(errid);

    System.out.println(details[0]);
    System.out.println(details[1]);

}


public String[] getErrorDetails(long errid)
{
    String[] details = new String[2];
    System.out.println("The input error id = " + errid);

    try {
        Connection connection = jdbcConnection.getConnectionInstance();
        Statement stmt = connection.createStatement();
        String query = "SELECT * FROM VARBALAS.ERROR_MASTER WHERE ERRORID = "+errid;
        ResultSet rs = stmt.executeQuery(query);

        while (rs.next())
        {
            details[0] = rs.getString("ERRDESC");
            details[1] = rs.getString("SEVERITY");


        }
       rs.close();

}
catch(Exception e)
{
    e.printStackTrace();
}

    return details;
}



public long getErrorId(String errDesc)
{
    long errid = 0 ;
    try {
        Connection connection = jdbcConnection.getConnectionInstance();
        Statement stmt = connection.createStatement();
        String query = "SELECT ERRORID FROM VARBALAS.ERROR_MASTER WHERE ERRDESC = '"+errDesc+"'";
        ResultSet rs = stmt.executeQuery(query);

        while (rs.next())
        {
            errid = rs.getLong("ERRORID");

        }
        System.out.println("Printing the queried ERROR id "+errid);

        System.out.println("End of Data");
        rs.close();

}
catch(Exception e)
{
    e.printStackTrace();
}

    return errid;
}

}

Answer 1

这是他们的文档，它似乎对如何使用它提供了一个不错的描述。这里，是他们的一些样本的链接。

有关使用 JPA 查询的示例，请查看上述文档链接中的第 10 节。

一个简单的例子（来自他们的文档）可能是这样的：

EntityManager em=...
Query q = em.createQuery("SELECT x FROM Magazine x");
List<Magazine> results = (List<Magazine>) q.getResultList();