Java邮件 : get size of a MimeMessage

Question

我正在尝试获取 MimeMessage 的大小。getSize() 方法总是简单地返回 -1。

这是我的代码:

MimeMessage m = new MimeMessage(session);
m.setFrom(new InternetAddress(fromAddress, true));
m.setRecipient(RecipientType.TO, new InternetAddress(toAddress, true));
m.setSubject(subject);

MimeBodyPart bodyPart = new MimeBodyPart();
bodyPart.setContent(body, "text/html");
Multipart mp = new MimeMultipart();
mp.addBodyPart(bodyPart);
m.setContent(mp);

m.getSize(); // -1 is returned

这是对我的问题的回答:

ByteArrayOutputStream os = new ByteArrayOutputStream();
m.writeTo(os);
int bytes = os.size();

Answer 1

下面是一个更有效的解决方案，但需要外部库:

public static long getReliableSize(MimeMessage m) throws IOException, MessagingException {
    try (CountingOutputStream out = new CountingOutputStream(new NullOutputStream())) {
        m.writeTo(out);
        return out.getByteCount();
    }
}

CountingOutputStream 和 NullOutputStream 都在 Apache Common IO 中可用。该解决方案不需要使用临时字节缓冲区(写入、分配、重新分配等)