使用Java语言，写出便利邮件Content内容的代码

以下是使用Java语言，便利邮件Content内容的代码示例：

import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.util.Base64;

import javax.activation.DataHandler;
import javax.activation.DataSource;
import javax.activation.FileDataSource;
import javax.mail.BodyPart;
import javax.mail.MessagingException;
import javax.mail.Multipart;
import javax.mail.internet.MimeBodyPart;
import javax.mail.internet.MimeMessage;
import javax.mail.internet.MimeMultipart;

public class EmailContentBuilder {
    
    private MimeMessage message;
    private Multipart multipart;

    public EmailContentBuilder(MimeMessage message) throws MessagingException {
        this.message = message;
        this.multipart = new MimeMultipart();
        this.message.setContent(this.multipart);
    }

    public void setText(String text) throws MessagingException {
        BodyPart bodyPart = new MimeBodyPart();
        bodyPart.setText(text);
        this.multipart.addBodyPart(bodyPart);
    }

    public void setHtml(String html) throws MessagingException {
        BodyPart bodyPart = new MimeBodyPart();
        bodyPart.setContent(html, "text/html");
        this.multipart.addBodyPart(bodyPart);
    }

    public void addAttachment(String filePath) throws IOException, MessagingException {
        File file = new File(filePath);
        DataSource dataSource = new FileDataSource(file);
        BodyPart bodyPart = new MimeBodyPart();
        bodyPart.setDataHandler(new DataHandler(dataSource));
        bodyPart.setFileName(file.getName());
        this.multipart.addBodyPart(bodyPart);
    }

    public void addInlineImage(String filePath, String contentId) throws IOException, MessagingException {
        File file = new File(filePath);
        DataSource dataSource = new FileDataSource(file);
        BodyPart bodyPart = new MimeBodyPart();
        bodyPart.setDataHandler(new DataHandler(dataSource));
        bodyPart.setHeader("Content-ID", "<" + contentId + ">");
        this.multipart.addBodyPart(bodyPart);
    }

    public void addBase64EncodedImage(String base64Content, String contentId) throws MessagingException {
        byte[] bytes = Base64.getDecoder().decode(base64Content);
        DataSource dataSource = new ByteArrayDataSource(bytes, "image/png");
        BodyPart bodyPart = new MimeBodyPart();
        bodyPart.setDataHandler(new DataHandler(dataSource));
        bodyPart.setHeader("Content-ID", "<" + contentId + ">");
        this.multipart.addBodyPart(bodyPart);
    }

    private static class ByteArrayDataSource implements DataSource {

        private byte[] bytes;
        private String contentType;

        public ByteArrayDataSource(byte[] bytes, String contentType) {
            this.bytes = bytes;
            this.contentType = contentType;
        }

        @Override
        public InputStream getInputStream() {
            return new ByteArrayInputStream(this.bytes);
        }

        @Override
        public OutputStream getOutputStream() {
            throw new UnsupportedOperationException("Not supported");
        }

        @Override
        public String getContentType() {
            return this.contentType;
        }

        @Override
        public String getName() {
            return null;
        }
    }

}

上述代码实现了一个邮件内容构造器，支持添加文本、HTML、附件、内联图片和Base64编码图片。使用示例：

MimeMessage message = new MimeMessage(session);
message.setFrom(new InternetAddress("sender@example.com"));
message.setRecipients(Message.RecipientType.TO, InternetAddress.parse("recipient@example.com"));
message.setSubject("My Email Subject");

EmailContentBuilder contentBuilder = new EmailContentBuilder(message);
contentBuilder.setText("Hello world!");
contentBuilder.setHtml("<h1>Hello world!</h1>");
contentBuilder.addAttachment("/path/to/myfile.pdf");
contentBuilder.addInlineImage("/path/to/myimage.png", "myimage");
contentBuilder.addBase64EncodedImage("iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAYAAABzenr0AA...", "mybase64image");

Transport.send(message);

上述代码会发送一封邮件，包含文本、HTML、附件、内联图片和Base64编码图片。