Servlet中带有数据库的Java Mail API,错误为NullPointerException
我正在从我的数据库向注册学生发送电子邮件,并获取用户的电子邮件地址。通过检查数据库中的电子邮件,我使用smtp服务器向用户发送一封关于其prn(即id)和密码的电子邮件。我有一个servlet、一个模型类和一个jsp。我已经附上了代码的屏幕截图。谢谢,这是个很小的问题,一定要帮忙。我是一名学生。提前感谢。:)我只得到一个错误,它是NullPointerExceptionServlet中带有数据库的Java Mail API,错误为NullPointerException,java,database,jsp,servlets,Java,Database,Jsp,Servlets,我正在从我的数据库向注册学生发送电子邮件,并获取用户的电子邮件地址。通过检查数据库中的电子邮件,我使用smtp服务器向用户发送一封关于其prn(即id)和密码的电子邮件。我有一个servlet、一个模型类和一个jsp。我已经附上了代码的屏幕截图。谢谢,这是个很小的问题,一定要帮忙。我是一名学生。提前感谢。:)我只得到一个错误,它是NullPointerException package com.student.connection; import java.sql.Connection; imp
package com.student.connection;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.*;
import javax.mail.*;
import javax.mail.internet.*;
import javax.mail.Transport;
import javax.activation.*;
public class Mailer {
private Connection conn = null;
private PreparedStatement ps = null;
private Statement stmt = null;
private ResultSet rs = null;
public void send(String emailaddress) throws SQLException, ClassNotFoundException {
int prn = 0;
final String username = "12030142084@sicsr.ac.in";
final String password = "mypassword";
String email = new String();
String pass = new String();
System.out.println("Test");
String query = "select prn,email,password from studentdetails where email=" + emailaddress;
conn = DBConnection.createDBConnection();
stmt = conn.createStatement();
rs = stmt.executeQuery(query);
while (rs.next()) {
prn = rs.getInt(1);
email = rs.getString("email");
pass = rs.getString("password");
}
Properties props = new Properties();
props.put("mail.smtp.auth", "true");
props.put("mail.smtp.starttls.enable", "true");
props.put("mail.smtp.host", "smtp.gmail.com");
props.put("mail.smtp.port", "587");
Session session = Session.getInstance(props,
new javax.mail.Authenticator() {
protected PasswordAuthentication getPasswordAuthentication() {
return new PasswordAuthentication(username, password);
}
});
try {
MimeMessage message = new MimeMessage(session);
message.setFrom(new InternetAddress("12030142084@sicsr.ac.in"));
message.setRecipients(Message.RecipientType.TO,
InternetAddress.parse(email));
message.setSubject("Password Recover");
message.setText("Your Prn is " + prn + "You Password is " + pass);
Transport.send(message);
System.out.println("Email Sent Succsessfully");
} catch (MessagingException e) {
//throw new RuntimeException(e);
} finally {
conn.close();
}
}
}
Jsp文件是正确的,它只需要一个用户输入,id是emailadd
有人请帮忙。那是因为
mailer
尚未初始化
private Mailer mailer; // you're just declaring mailer
...
mailer.send("emailaddress"); // without initialization you're trying to call a method on it.
在init()
方法中,需要执行以下操作:-
mailer = new Mailer(); // just an example. But you need to initialize your mailer.
将导致NullPointerException,因为在servlet代码中没有初始化
邮件程序的位置。mailer对象初始化为null,对其调用任何方法都将导致NPE。在调用send或任何其他方法之前,请确保先初始化它。谢谢。我现在已经初始化了。。但我发现了一些新错误:警告:StandardWrapperValve[ContactServlet]:PWC1406:Servlet.service()对于servlet ContactServlet抛出异常org.apache.derby.client.am.SqlException:“EMAILADDRESS”列不在FROM列表中的任何表中,或者出现在联接规范中,并且不在联接规范的范围内,或者出现在HAVING子句中,并且不在GROUP BY列表中。如果这是CREATE或ALTER TABLE语句,则“EMAILADDRESS”不是目标表中的列。在org.apache.derby.client.am.Statement.completeSqlca(未知源代码)上,您的查询似乎存在一些问题。不幸的是,这不属于这个问题的范围。请关闭此问题并将该问题作为单独问题发布。String query=“从studentdetails中选择prn、电子邮件、密码,其中email=“+emailaddress;只要检查一下。。如果你能建议的话,那真的很有帮助。我会马上关闭的我已经初始化了长官。。但现在我得到了这个错误:WARNING:StandardWrapperValve[ContactServlet]:PWC1406:Servlet.service()对于servlet ContactServlet抛出异常org.apache.derby.client.am.SqlException:“EMAILADDRESS”列不在FROM列表中的任何表中,或者出现在联接规范中,并且不在联接规范的范围内,或者出现在HAVING子句中,并且不在GROUP BY列表中。如果这是CREATE或ALTER TABLE语句,则“EMAILADDRESS”不是目标表中的列。在org.apache.derby.client.am.Statement.completeSqlca(未知源代码)中,在init方法中初始化mailer
。。。
private Mailer mailer; // you're just declaring mailer
...
mailer.send("emailaddress"); // without initialization you're trying to call a method on it.
mailer = new Mailer(); // just an example. But you need to initialize your mailer.
mailer.send("emailaddress"); // here is the error