AmrLength.java

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;

/**
 * Copy from https://blog.csdn.net/fjh658/article/details/12869073
 */
public class AmrLength {

    public synchronized static long getAmrDuration(File file) throws IOException {
        long duration = -1;
        int[] packedSize = {12, 13, 15, 17, 19, 20, 26, 31, 5, 0, 0, 0, 0, 0, 0, 0};
        try (RandomAccessFile randomAccessFile = new RandomAccessFile(file, "rw")) {
            long length = file.length();
            int pos = 6;
            int frameCount = 0;

            byte[] datas = new byte[1];
            while (pos <= length) {
                randomAccessFile.seek(pos);
                if (randomAccessFile.read(datas, 0, 1) != 1) {
                    duration = (length - 6) / 650;
                    break;
                }
                int packedPos = (datas[0] >> 3) & 0x0F;
                pos += packedSize[packedPos] + 1;
                frameCount++;
            }

            duration += frameCount * 20;
        }
        return duration;
    }
}