如何让Flutter的just_audio兼容Amazon Polly驱动的Spring Boot服务？

我正在开发一款Flutter应用，使用just_audio库播放短语音提示。手动通过AWS控制台用Amazon Polly生成MP3并上传到S3后，just_audio可以正常播放，代码如下：

final AudioPlayer audioPlayer = AudioPlayer();
audioPlayer.setUrl('https://bucketname.s3.eu-west-2.amazonaws.com/say_someting.mp3');
audioPlayer.play();

但需要根据运行时动态文本实时生成语音，因此搭建了Java Spring Boot微服务调用Polly并暴露接口。音频时长3秒内、大小约15KB，服务代码如下：

@GetMapping(path = "/mp3", produces = "audio/mpeg")
public byte[] getAudio()
{
    // 运行时动态文本
    final String whatToSay = "The time is now " + System.currentTimeMillis();

    // 构造Polly语音合成请求
    final SynthesizeSpeechRequest synthesizeSpeechRequest = new SynthesizeSpeechRequest()
            .withOutputFormat(OutputFormat.Mp3)
            .withVoiceId(VoiceId.Amy)
            .withText(whatToSay);

    // 可启用SSML控制语调
    // synthesizeSpeechRequest.setTextType(TextType.Ssml);

    final SynthesizeSpeechResult synthesizeSpeechResult = amazonPolly.synthesizeSpeech(synthesizeSpeechRequest);

    // 将音频流转为字节数组返回
    final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    IOUtils.copy(synthesizeSpeechResult.getAudioStream(), outputStream);
    return outputStream.toByteArray();
}

服务部署到Elastic Beanstalk后，浏览器访问接口能正常播放，下载的MP3也能在Quicktime播放，但just_audio调用时抛出source error异常，代码如下：

final AudioPlayer audioPlayer = AudioPlayer();
audioPlayer.setUrl('https://my-domain/mp3');
audioPlayer.play();

已尝试的排查动作：

• 调试just_audio源码，但异步Future的错误无法追踪
• 尝试设置空请求头调用接口
• 试用flutter_tts但无法控制语调
• 改为流式输出Polly音频，问题仍存在

测试接口响应头：

HTTP/1.1 200
Server: nginx
Date: Thu, 08 Feb 2024 07:59:14 GMT
Content-Type: audio/mpeg
Content-Length: 50043
Connection: keep-alive

1. 直接转发Polly原始音频流 #

just_audio对音频格式校验严格，字节流转换可能丢失关键文件头信息。修改Spring Boot接口，直接将Polly的原始音频流写入响应输出流：

@GetMapping(path = "/mp3", produces = "audio/mpeg")
public void getAudio(HttpServletResponse response) throws IOException {
    String whatToSay = "The time is now " + System.currentTimeMillis();
    SynthesizeSpeechRequest request = new SynthesizeSpeechRequest()
            .withOutputFormat(OutputFormat.Mp3)
            .withVoiceId(VoiceId.Amy)
            .withText(whatToSay);

    SynthesizeSpeechResult result = amazonPolly.synthesizeSpeech(request);
    InputStream audioStream = result.getAudioStream();

    response.setContentType("audio/mpeg");
    response.setContentLength((int) result.getContentLength());
    IOUtils.copy(audioStream, response.getOutputStream());
    response.getOutputStream().flush();
}

2. 为just_audio添加请求头并等待异步操作 #

部分服务器会限制缺少特定头的请求，且需确保音频加载完成后再播放：

final AudioPlayer audioPlayer = AudioPlayer();
await audioPlayer.setUrl(
  'https://my-domain/mp3',
  headers: {
    'Accept': 'audio/mpeg',
  },
);
await audioPlayer.play();

3. 配置跨域(CORS) #

Flutter应用可能存在跨域限制，在Spring Boot中添加CORS配置：

@Configuration
public class CorsConfig implements WebMvcConfigurer {
    @Override
    public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/**")
                .allowedOrigins("*")
                .allowedMethods("GET", "POST")
                .allowedHeaders("*");
    }
}

也可直接在Controller方法上添加@CrossOrigin(origins = "*")注解。

4. 验证音频格式标准性 #

下载Spring Boot接口返回的音频，用ffmpeg检查格式是否符合just_audio兼容的MP3标准：

ffmpeg -i downloaded.mp3 -v info

若存在格式问题，可在服务端添加简单转码步骤确保格式合规。

内容的提问来源于stack exchange，提问作者Paul Wells