Why am I getting this exception when I instantiate a SimpleEmail class in Java?

问题

All I want to do is send an email in Java. Here is the example I found:

import org.apache.commons.mail.SimpleEmail;

public class Email {

    public static void sendMessage(String emailaddress, String subject, String body) {
        try {
            SimpleEmail email = new SimpleEmail();
            email.setHostName("valid ip address here");
            email.addTo(emailaddress);
            email.setFrom("noreply@example.com", "No reply");
            email.setSubject(subject);
            email.setMsg(body);
            email.send();
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

I immediately get the following exception on the SimpleEmail email = new SimpleEmail(); line:

java.lang.ClassNotFoundException: org.apache.commons.mail.SimpleEmail

I have the following JAR in my project (using Netbeans): commons-email-1.2.jar

What am I doing wrong?

Thanks.

回答1:

I tried to run your code and got this:

Exception in thread "main" java.lang.NoClassDefFoundError: javax/mail/Message
    at javaapplication3.Email.sendMessage(Email.java:9)
    at javaapplication3.JavaApplication3.main(JavaApplication3.java:7)
Caused by: java.lang.ClassNotFoundException: javax.mail.Message
    at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
        at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
    at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:248)
... 2 more
Java Result: 1`

Appears that some import is missing. You need the javax.mail package to run your code.

回答2:

Not sure why SimpleEmail didn't work. But this did:

import javax.mail.*;
  import javax.mail.internet.*;

  import java.util.Properties;

  public class Email {

     public static void sendMessage(String emailaddress, String subject, String body) {
        try {
           Properties props = new Properties();
           props.setProperty("mail.transport.protocol", "smtp");
           props.setProperty("mail.host", "myhost");

           Session mailSession = Session.getDefaultInstance(props, null);
           Transport transport = mailSession.getTransport();

           MimeMessage message = new MimeMessage(mailSession);
           message.setSubject("Testing javamail plain");
           message.setContent("This is a test", "text/plain");
           message.addRecipient(Message.RecipientType.TO, new InternetAddress("test@example.com"));

           transport.connect();
           transport.sendMessage(message,
                   message.getRecipients(Message.RecipientType.TO));
           transport.close();
        } catch (Exception ex) {
           ex.printStackTrace();
        }
     }
  }

Thanks for the suggestions.

回答3:

The SimpleEmail class is in there, and you have the correct package, so I'd have to say this is a problem in NetBeans configuration for your project.

Since this is a simple example, you could try compiling from the command-line and adding the JAR to your classpath that way. It should work. Then you can figure out why it's not being picked up in NetBeans.

回答4:

You could also add the source code of the project to your src folder. But most of the times a clean & build helps with these problems.

If nothing helps, close netbeans, delete manually the build & dist folder of your project, start netbeans up again and perform a clean & build. Helped me once out of a mysterious problem.

来源：https://stackoverflow.com/questions/6847178/why-am-i-getting-this-exception-when-i-instantiate-a-simpleemail-class-in-java