java - System.currentTimeMillis() 行为错误

Question

我正在创建一个文本文件，其中的时间戳源自 System.currentTimeMillis() 的结果。我的算法是这样的:

1. 创建文件并记录创建时间戳
2. 每次按下按钮时保存时间戳
3. 从按钮按下时间戳中减去文件创建时间戳
4. 将结果写入文件

出于某种奇怪的原因，文件创建的时间戳通常比按钮按下的时间戳更大(更年轻、更近)，而按钮按下的时间戳总是在文件创建后发生。这会导致第 3 步返回负值。

这是什么原因造成的？

FileCreationMenu.java

    public class FileCreationMenu extends Fragment {
            public Button toggleRecordingButton;
            @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup    container,Bundle savedInstanceState) {
                View v = inflater.inflate(R.layout.menu_fragment_calibrator, container, false); 
                toggleRecordingButton = (Button) v.findViewById(R.id.recordAudioToggle);
                toggleRecordingButton.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {
                        recordAudio = !recordAudio; //toggle audio recording on/off
                        if (recordAudio==true){
                            AudioRecorder.createFile(System.currentTimeMillis());  //generate file for new track if record is togged ON
                        }
                    }
                });

AudioRecorder.java

 public static void createFile(long time) {  //create file and record creation timeStamp
        recordingStartTime = time;
        myFile = new File(Environment.getExternalStorageDirectory(), "file.txt");  //save to external storage
        try {
            myFile.createNewFile();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void recordNote(long timeStamp){  //record timeStamps of notes
        String playedNote = (timeStamp+ "\n" );
        try {
            fos = new FileOutputStream(myFile, true);
            fos.write(playedNote.getBytes());
            fos.flush();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

   public static void playTrack(String fileName){   //playback Notes
    try {
        FileInputStream fis = new FileInputStream(myFile.getPath());
        BufferedReader reader = new BufferedReader(new InputStreamReader(fis));
        String line;
        try {
            int i =0;
            while ((line = reader.readLine()) != null) {   //read each line of input file
                historicalTime[i] = Long.parseLong(line);  //store time current line's note was played 
                if (i==0){
                    timeStream[i] = (Long.parseLong(line) - recordingStartTime); //if first note, calculate wait based on file creation time
                }
                else{
                    timeStream[i] = (Long.parseLong(line) - historicalTime[i-1]); //otherwise, calculate wait as amount of time note was played after most recent preceding note 
                }
                i++;
            }
        } catch (IOException e){
            e.printStackTrace();
        }
        for (int i=0; i<noteStream.length; i++){
            try {
                AudioRecorder.class.wait(timeStream[i]); //wait 
                //Play Note
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    }
} 

MainActivity.java

...
public static void noteDetected(){
        if (FileCreationMenu.recordAudio == true){
            AudioRecorder.recordNote(System.currentTimeMillis());
        }

同样，问题是 AudioRecorder.java 中的 RecordStartTime 通常大于 MainActivity 中 recordNote 的输入参数“timeStamp”。例如，在最近的调试 session 中，recordingStartTime =1,450,573,093,044，而 timeStamp=1,450,565,187,318。

什么可能导致这种看似不可能的行为？

Answer 1

问题出在这一行:

myFile.createNewFile();

此方法的 JavaDoc 说

Atomically creates a new, empty file named by this abstract pathname if and only if a file with this name does not yet exist

因此，每次您尝试开始新的记录时，您实际上都在重复使用同一个文件(带有很久以前的时间戳)，但记录了新的 recordStartTime。

如果您想先删除旧文件，您必须编写

myFile.delete();
myFile.createNewFile();