在AS2消息中，MIME头部通常不被加密。MIME（Multipurpose Internet Mail Extensions）是一种用于扩展电子邮件格式的标准，用于在AS2消息中指示消息的类型、编码和其他相关信息。

以下是一个示例代码，说明如何在AS2消息中添加MIME头部：

import org.apache.commons.codec.binary.Base64;
import org.apache.commons.io.FileUtils;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpVersion;
import org.apache.http.entity.mime.MultipartEntityBuilder;
import org.apache.http.entity.mime.content.ByteArrayBody;
import org.apache.http.entity.mime.content.StringBody;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.message.BasicHeader;
import org.apache.http.message.BasicHttpResponse;
import org.apache.http.message.BasicStatusLine;

import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.security.PrivateKey;
import java.security.cert.X509Certificate;

public class AS2MessageEncryptionExample {

    public static void main(String[] args) throws Exception {
        // Load private key and certificate
        PrivateKey privateKey = loadPrivateKeyFromFile("private.key");
        X509Certificate certificate = loadCertificateFromFile("certificate.crt");

        // Load message content
        String messageContent = "This is the content of the AS2 message";
        
        // Create MIME headers
        String contentType = "text/plain";
        String contentDisposition = "attachment; filename=message.txt";

        // Encrypt the message
        byte[] encryptedMessage = encryptMessage(messageContent, certificate);

        // Create AS2 message
        HttpResponse as2Response = createAS2Message(encryptedMessage, contentType, contentDisposition, privateKey, certificate);

        // Process AS2 response
        if (as2Response.getStatusLine().getStatusCode() == 200) {
            System.out.println("AS2 message sent successfully.");
        } else {
            System.out.println("Failed to send AS2 message. Error: " + as2Response.getStatusLine().getReasonPhrase());
        }
    }

    private static PrivateKey loadPrivateKeyFromFile(String privateKeyFilePath) throws Exception {
        // Load private key from file
        File privateKeyFile = new File(privateKeyFilePath);
        byte[] privateKeyBytes = FileUtils.readFileToByteArray(privateKeyFile);

        // TODO: Implement private key loading logic

        return null; // Replace with actual private key
    }

    private static X509Certificate loadCertificateFromFile(String certificateFilePath) throws Exception {
        // Load certificate from file
        File certificateFile = new File(certificateFilePath);
        byte[] certificateBytes = FileUtils.readFileToByteArray(certificateFile);

        // TODO: Implement certificate loading logic

        return null; // Replace with actual certificate
    }

    private static byte[] encryptMessage(String messageContent, X509Certificate certificate) throws Exception {
        // TODO: Implement message encryption logic

        return null; // Replace with actual encrypted message
    }

    private static HttpResponse createAS2Message(byte[] encryptedMessage, String contentType, String contentDisposition,
                                                 PrivateKey privateKey, X509Certificate certificate) throws IOException {
        // Create AS2 message
        CloseableHttpClient httpClient = HttpClients.createDefault();
        BasicHttpResponse as2Response = new BasicHttpResponse(new BasicStatusLine(HttpVersion.HTTP_1_1, 200, "OK"));

        // Create MIME entity
        HttpEntity entity = MultipartEntityBuilder.create()
                .addPart("Content-Disposition", new StringBody(contentDisposition, StandardCharsets.US_ASCII))
                .addPart("Content-Type", new StringBody(contentType, StandardCharsets.US_ASCII))
                .addPart("Content-Transfer-Encoding", new StringBody("base64", StandardCharsets.US_ASCII))
                .addPart("AS2-From", new StringBody("sender@example.com", StandardCharsets.US_ASCII))
                .addPart("AS2-To", new StringBody("receiver@example.com", StandardCharsets.US_ASCII))
                .addPart("AS2-Version", new StringBody("1.0", StandardCharsets.US_ASCII))
                .addPart("AS2-Message-Id", new StringBody("AS2-Message-ID-12345", StandardCharsets.US_ASCII))
                .addPart("AS2-Date", new StringBody("Tue, 01 Jan 2022 00:00:00 GMT", StandardCharsets.US_ASCII))
                .addPart("AS2-Subject", new StringBody("AS2 Message Subject", StandardCharsets.US_ASCII))
                .addPart("AS2-From-Name", new StringBody("Sender Name", StandardCharsets.US_ASCII))
                .addPart("AS2-To-Name", new StringBody("Receiver Name", StandardCharsets.US_ASCII))
                .