java - 如何读取进程 getInputstream？

Question

我使用进程构建器和进程(使用 ffmpeg)，并且我必须将 ffmpeg 输出重定向到 java 输入。我写了这段简短的代码，但输入流无论如何都是空的......

public class Ffmpegtest {
public static void main(String[] args) {
    String[] cmd = {"ffmpeg ","-i ","output2.mp4 ","-f ","mp4 ", "-"};
    InputStream stream;
    try {
        stream = new ProcessBuilder(Arrays.asList(cmd)).start().getInputStream();
        System.out.println(stream.available()); // still 0
        byte[] kilo = new byte[1024];
        int i = stream.read(kilo,0,1024);
        while(i != -1){
            System.out.println("read "+1024);
            i = stream.read(kilo,0,1024);
        }

    } catch (IOException ex) {
        System.out.println(ex.getMessage());
    }
}

Answer 1

当我运行时:

ffmpeg -i file -f mp4 -

标准输出为空。然而，stderr 说:

ffmpeg version N-74516-g99b9f01 Copyright (c) 2000-2015 the FFmpeg developers
  built with Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
  configuration: --enable-gpl --samples=/Users/ronaldbultje/Movies/fate-suite-ff --enable-libvpx
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 58.100 / 56. 58.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 34.100 /  5. 34.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Input #0, ivf, from 'file.ivf':
  Duration: 00:00:00.01, start: 0.002000, bitrate: 178292 kb/s
    Stream #0:0: Video: vp9 (Profile 0) (VP90 / 0x30395056), yuv420p(pc, bt470bg/unknown/unknown), 640x480, 1k tbr, 1k tbn, 1k tbc
[mp4 @ 0x7f95aa800000] muxer does not support non seekable output
Output #0, mp4, to 'pipe:':
  Metadata:
    encoder         : Lavf56.40.101
    Stream #0:0: Video: mpeg4 ( [0][0][0] / 0x0020), yuv420p, 640x480, q=2-31, 200 kb/s, 1k fps, 1k tbn, 1k tbc
    Metadata:
      encoder         : Lavc56.58.100 mpeg4
Stream mapping:
  Stream #0:0 -> #0:0 (vp9 (native) -> mpeg4 (native))
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument

这里的关键是“复用器不支持不可查找的输出”。选择支持流式传输的不同输出格式(例如 mpegts)，或选择文件输出而不是 stdout。作为更通用的 java 规则，在使用 Process 时始终检查 stdout 和 stderr。