NoClassDefFoundError 仅在通过命令行运行代码时发生

Question

当我通过命令行运行此代码时出现异常。当我通过 Eclipse IDE 运行它时，它运行良好，没有例外。我不知道发生了什么，但肯定会在 Session 语句中出现异常。

public static void sendEmail(String sendFile) {
    // Send the log file via email.
    final String username = "firstname.lastname@mycompany.com";
    final String password = "myPassword";

    // Strings that contain from, to, subject, body and file path to the attachment.
    String from = username;
    String to = username;
    String subject = "Baggage URL failures - Please check";
    String body = "FareDB Bad URL(s) Report. The following URL(s) could not be loaded...";
    String filename = sendFile;
    System.out.println("Mail header info set!");

    // Set smtp properties
    Properties properties = new Properties();
    properties.put("mail.smtp.starttls.enable", "true");
    properties.put("mail.smtp.auth", "true");
    properties.put("mail.smtp.host", "smtp.gmail.com");
    properties.put("mail.smtp.port", "587");
    System.out.println("SMTP properties set!");

    Session session = Session.getInstance(properties, null);
    System.out.println("Mail session declared!");

    try {
        MimeMessage message = new MimeMessage(session);
        message.setFrom(new InternetAddress(from));
        // Just me for testing
        message.setRecipient(Message.RecipientType.TO, new InternetAddress(username));
        message.setSubject(subject);
        message.setSentDate(new Date());
        System.out.println("Mail body info set!");

        // Set the email body
        MimeBodyPart messagePart = new MimeBodyPart();
        messagePart.setText(body);
        System.out.println("Mime message body set!");

        // Set the email attachment file
        MimeBodyPart attachmentPart = new MimeBodyPart();
        FileDataSource fileDataSource = new FileDataSource(filename) {
            @Override
            public String getContentType() {
                return "application/octet-stream";
            }
        };
        attachmentPart.setDataHandler(new DataHandler(fileDataSource));
        attachmentPart.setFileName(fileDataSource.getName());
        System.out.println("Mail attachment info set!");

        // Add all parts of the email to Multipart object
        Multipart multipart = new MimeMultipart();
        multipart.addBodyPart(messagePart);
        multipart.addBodyPart(attachmentPart);
        message.setContent(multipart);
        System.out.println("Mail parts added!");

        // Send email
        Transport.send(message);
        System.out.println("Mail sent!");
    } catch (MessagingException e) {
        e.printStackTrace();
    }
}

我得到的错误只是在命令行运行时：

 Exception in thread "main" java.lang.NoClassDefFoundError: com/sun/mail/util/MailLogger
    at javax.mail.Session.initLogger(Session.java:221)
    at javax.mail.Session.<init>(Session.java:206)
    at javax.mail.Session.getInstance(Session.java:242)
    at VerifyUrl.sendEmail(VerifyUrl.java:186)
    at VerifyUrl.main(VerifyUrl.java:50)
 Caused by: java.lang.ClassNotFoundException: com.sun.mail.util.MailLogger
    at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
    ... 5 more

有什么可能导致这种情况的想法吗？

Answer 1

这是 Java 开发人员面临的最常见问题。

• 其中java有包结构机制。
• 要运行任何使用其他包类/接口/枚举的 java 程序，您需要将其导入类。
• 如果您使用的是外部库或其他包，那么您需要在您的情况下为该类设置类路径com.sun.mail.util.MailLogger。
• Eclipse 和 IntellijIdea 等 IDE 为您打包和设置类路径。

• 如果您尝试运行上述程序或任何其他程序，您可以遵循类似

  的语法

  java -cp /relative_path_to_your_dependency_jars ClassName

  在你的情况下是这样的。

  java -cp /downloadpath/javax.mail-1.5.0.jar YouMainClass

  类似的编译类

  java -cp /downloadpath/javax.mail-1.5.0.jar YouMainClass.java

Answer 2

这原来是一个类路径问题，并且还使用了 javax.mail 而不是 javax.mail-api。

问题已经解决了。