Java编码Base64，MimeBodyPart附件

要通过电子邮件发送Base64编码的PDF文件，并确保收件人能够直接从邮件中打开它，您需要将Base64字符串解码回字节流，然后将其作为附件添加到MimeMessage中。以下是一个使用JavaMail API实现此操作的示例代码片段：

import javax.mail.*;
import javax.mail.internet.*;
import java.io.*;
import java.util.Base64;

public class EmailWithBase64PDFAttachment {
    public static void main(String[] args) throws MessagingException, IOException {
        String from = "your_email@example.com";
        String sendTo = "recipient_email@example.com";
        String host = "smtp.example.com"; // SMTP服务器地址
        String password = "your_password";

        Properties properties = System.getProperties();
        properties.put("mail.smtp.host", host);
        properties.put("mail.smtp.port", "587"); // 或者是其他端口，如465
        properties.put("mail.smtp.auth", "true");
        properties.put("mail.smtp.starttls.enable", "true");

        Session session = Session.getInstance(properties, new javax.mail.Authenticator() {
            protected PasswordAuthentication getPasswordAuthentication() {
                return new PasswordAuthentication(from, password);
            }
        });

        Message message = new MimeMessage(session);
        message.setFrom(new InternetAddress(from));
        message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(sendTo));
        message.setSubject("Subject - " + new Date());

        Multipart multipart = new MimeMultipart();

        // 假设你有一个Base64编码的PDF字符串
        String base64PDF = "your_base64_encoded_string_here";
        byte[] pdfBytes = Base64.getDecoder().decode(base64PDF);

        MimeBodyPart attachment = new MimeBodyPart();
        DataSource dataSource = new ByteArrayDataSource(pdfBytes, "application/pdf");
        attachment.setDataHandler(new DataHandler(dataSource));
        attachment.setFileName("myFileName.pdf");
        multipart.addBodyPart(attachment);

        message.setContent(multipart);
        
        Transport.send(message);
        System.out.println("Email Sent Successfully.");
    }
}

// 注意：ByteArrayDataSource 类可能需要导入额外的库或自行定义。
class ByteArrayDataSource implements DataSource {
    private byte[] data;
    private String type;

    public ByteArrayDataSource(byte[] data, String type) {
        super();
        this.data = data;
        this.type = type;
    }

    @Override
    public InputStream getInputStream() throws IOException {
        return new ByteArrayInputStream(data);
    }

    @Override
    public OutputStream getOutputStream() throws IOException {
        throw new IOException("Not Supported");
    }

    @Override
    public String getContentType() {
        return type;
    }

    @Override
    public String getName() {
        return "Generated PDF";
    }
}

这段代码首先设置邮件的基本信息和SMTP服务器配置，然后创建一个MimeMessage实例。关键在于，我们不再直接处理Base64编码的文件路径，而是将Base64字符串解码为字节数组，然后使用ByteArrayDataSource包装这个字节数组，最后设置到MimeBodyPart中作为邮件的附件。

请确保替换your_email@example.com、recipient_email@example.com、smtp.example.com以及your_password等占位符为实际的发件人邮箱地址、收件人邮箱地址、SMTP服务器地址和密码。