如何正确编码MimeMessage为字符串以适配Microsoft Graph？

问题根源 #

• 错误使用了URL安全的Base64编码：Base64.encodeBase64URLSafeString会将+替换为-、/替换为_，不符合Graph要求的标准Base64格式
• 请求体格式不正确：Graph的sendMail接口要求传入JSON结构，而非直接传递编码后的字符串，且Content-Type需设为application/json

修正后的代码 #

1. 正确编码MIME消息 #

将URL安全编码替换为标准Base64编码，同时移除编码字符串中的换行符：

private void doMyTest(EmailDetails emailDetails, OAuthAccessToken accessToken) {
    MimeMessage mimeMessage = createMimeMessageEmail(emailDetails, true);
    String encodedEmail = null;
    try {
        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        mimeMessage.writeTo(buffer);
        byte[] bytes = buffer.toByteArray();
        // 标准Base64编码，移除自动生成的换行符
        encodedEmail = Base64.encodeBase64String(bytes).replaceAll("\\r?\\n", "");
    } catch (Exception e) {
        throw new RuntimeException(e);
    }

    // 后续请求代码见下文
}

2. 修正请求体与请求头 #

构建Graph要求的JSON结构请求体，设置正确的Content-Type：

try {
    // 构造符合Graph接口要求的JSON请求体
    String jsonBody = String.format(
        "{\"message\":{\"mimeContent\":{\"contentType\":\"text/html\",\"content\":\"%s\"}},\"saveToSentItems\":true}",
        encodedEmail
    );
    
    HttpPost post = new HttpPost("https://graph.microsoft.com/v1.0/me/sendMail");
    post.setEntity(new StringEntity(jsonBody, ContentType.APPLICATION_JSON));
    post.setHeader("Authorization", "Bearer " + accessToken.getTokenValue());
    
    try (CloseableHttpClient httpclient = HttpClients.createDefault()) {
        Integer responseCode = httpclient.execute(post, httpResponse -> {
            int code = httpResponse.getCode();
            String responseBody = EntityUtils.toString(httpResponse.getEntity());
            if (code != 202) {
                System.err.println("请求失败：" + responseBody);
            }
            return code;
        });
    }
} catch (Exception e) {
    e.printStackTrace();
}

额外注意事项 #

• 确认createMimeMessageEmail方法使用的是jakarta.mail包的类，版本需与依赖适配
• Graph的sendMail接口成功响应为202 Accepted，而非200 OK，需调整判断逻辑
• 若邮件包含附件或多部分内容，需确保MimeMessage构建完整，编码后内容无损坏

内容的提问来源于stack exchange，提问作者HeronAlgoSearch