从jdbc转换到打开jpa

Question

我是新来打开jpa环境的，我需要转换jdbc代码才能打开jpa，请帮我解决这个问题。我发布这个帖子的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

This是他们的文档，它似乎是一个体面的描述如何使用它。 Here，是一些样本的链接。

有关使用JPA查询的示例，请参阅上述文档链接的第10节。

一个简单的例子（从他们的文档）可能是这样的：

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