bytedeco/javacv

MP4 Files Unplayable After Crash Despite Using frag_keyframe+empty_moov with FFmpegFrameRecorder

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#2358 opened on Sep 26, 2025

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help wantedquestion

Description

I’m currently using the following code to record video as an MP4 file:

String outputFile = outputDir + File.separator +
        "video_" + new SimpleDateFormat("yyyyMMdd_HHmmss").format(new Date()) + ".mp4";

recorder = new FFmpegFrameRecorder(outputFile, width, height, audioChannels);
recorder.setFormat("mp4");
recorder.setVideoCodec(avcodec.AV_CODEC_ID_H264);
recorder.setAudioCodec(avcodec.AV_CODEC_ID_AAC);
recorder.setSampleRate(grabber.getSampleRate());
recorder.setFrameRate(grabber.getFrameRate() <= 0 || grabber.getFrameRate() > 120 ? 25 : grabber.getFrameRate());
recorder.setAudioChannels(audioChannels);
recorder.start();

The problem is, if the service crashes during recording, the generated MP4 file becomes unplayable. From my research, this is because the MP4 moov atom is written at the end of the file. If the process crashes, the moov atom never gets written, which makes the file corrupt.

I also tried setting the following option to fix it:

recorder.setOption("movflags", "frag_keyframe+empty_moov+faststart");

However, it doesn’t seem to work. What could be causing this, and how can I reliably generate MP4 files that remain playable even if the recording is interrupted?

Contributor guide

Tech stack
java
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
JavaUnderstanding of MP4 container format
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
65
Research direction
Investigate why the 'frag keyframe+empty moov+faststart' movflags are not preventing corruption after a crash. Examine FFmpegFrameRecorder's implementation of setOption to ensure flags are passed correctly to FFmpeg. Consider alternative approaches such as writing to a temporary file and moving on success (atomic write), or using a format that writes moov atom at the beginning (e.g., fragmented MP4). Look for existing discussions in the JavaCV repository (issue #2358) for community insights.