Getting ClassCastException in java

message.getContent() returns javax.mail.util.SharedByteArrayInputStream here, but the SharedByteArrayInputStream is not cast-able to a Multipart instance, because you may not necessarily have a multipart message.

You could check if its mimetype is a multipart anything:

if (message.isMimeType("multipart/*") {
    Multipart mp = (Multipart)message.getContent();
    // more stuff
}

Or you can do instance of...

if (message.getContent() instanceof Multipart) {
    Multipart mp = (Multipart)message.getContent();
    // more
}

You're getting an exception because the return value of getContent is a reference to a javax.mail.util.SharedByteArrayInputStream and that class doesn't implement Multipart. Presumably this isn't a multipart mail message.

As the documentation for Part.getContent states:

Return the content as a Java object. The type of the returned object is of course dependent on the content itself. For example, the object returned for "text/plain" content is usually a String object. The object returned for a "multipart" content is always a Multipart subclass. For content-types that are unknown to the DataHandler system, an input stream is returned as the content

So basically if you want to handle multipart messages in a particular way, you'll need to use:

Object content = message.getContent();
if (content instanceof Multipart)
{
    Multipart multipart = (Multipart) content;
    // ...
}
else
{
    // Handle non-multipart content
}

Ok, so here is what is happening. It looks like you're trying to get the content from an object which implements javax.mail.Part, but the format is unknown, in which case MimeMessage will return an input stream. In this case, it is returning a javax.mail.util.SharedByteArrayInputStream. Regardless, an input stream cannot be converted to the Multipart interface.

You can test to see if it is an implementer of multipart using isMimeType (birryree's suggestion):

if (message.isMimeType("multipart/*") 
{
    Multipart multipart = (Multipart) content;
    // what you have above.
}
else
{
    // it is not multi-part
}

Or you can test for a direct match (my original suggestion):

// other string comparisons will work here too.
if(message.getContentType().equals("multipart"))
{
     Multipart multipart = (Multipart) message.getContent();
    // what you have above.
}
else
{
    // it is not multi-part
}

getContentType is also on the Part interface. Its documentation can be found here.
You can see a list of all possible content types here.

Or you can test based on the result of instanceof (Jon Skeet's answer):

Object content = message.getContent();
if (content instanceof Multipart)
{
    Multipart multipart = (Multipart) content;
    // what you have above.
}
else
{
    // it is not multi-part
}

For unknown mime-types the MimeMessage class returns with ShraedByteArrayInputstream as the documentation says.

Check the returning type with instanceof then cast.

Update:

If you're using the same source as you did in Eclipse and the response of getContent() method is still different, then you can try modifying the file.encoding property.

Example:

java -Dfile.encoding=UTF8 -jar something.jar

Update2:

Maybe an older version of the loaded class is used in your jar. Please check your classpath for the loaded classes.

When you export the Runnable jar file, choose package required libraries into generated JAR, which solves the problem.

This might because some charset can't be found properly, so the returned multipart object is not parsed.