安卓:如何从InputStream中随机访问？

Question

我有一个InputStream，以及相对的文件名和大小。

我需要访问/阅读InputStream中的一些随机(增加)职位。此位置存储在整数数组(命名偏移量)中。

InputStream inputStream = ...

String fileName = ...
int fileSize = (int) ...

int[] offsets = new int[]{...};  // the random (increasing) offsets array

现在，给定一个InputStream，我发现只有两个可能的解决方案可以跳到文件的任意(增加)位置。

第一个方法是使用InputStream的InputStream方法(请注意，我实际上使用了BufferedInputStream，因为我需要马克()和重置()文件指针)。

//Open a BufferInputStream:
BufferedInputStream bufferedInputStream = new BufferedInputStream(inputStream);

byte[] bytes = new byte[1];

int curFilePointer = 0;
long numBytesSkipped = 0;
long numBytesToSkip = 0;
int numBytesRead = 0;

//Check the file size:
if ( fileSize < offsets[offsets.length-1] ) {  // the last (bigger) offset is bigger then the file size... 
    //Debug:
    Log.d(TAG, "The file is too small!\n");

    return;
}

for (int i=0, k=0; i < offsets.length; i++, k=0) {  // for each offset I have to jump...
    try {
        //Jump to the offset [i]:
        while( (curFilePointer < offsets[i]) && (k < 10) ) {  // until the correct offset is reached (at most 10 tries)
            numBytesToSkip = offsets[i] - curFilePointer;
            numBytesSkipped = bufferedInputStream.skip(numBytesToSkip);

            curFilePointer += numBytesSkipped;  // move the file pointer forward

            //Debug:
            Log.d(TAG, "FP: " + curFilePointer + "\n");

            k++;
        }

        if ( curFilePointer != offsets[i] ) {  // it did NOT jump properly... (what's going on?!)
            //Debug:
            Log.d(TAG, "InputStream.skip() DID NOT JUMP PROPERLY!!!\n");

            break;
        }

        //Read the content of the file at the offset [i]:
        numBytesRead = bufferedInputStream.read(bytes, 0, bytes.length);
        curFilePointer += numBytesRead;  // move the file pointer forward

        //Debug:
        Log.d(TAG, "READ [" + curFilePointer + "]: " + bytes[0] + "\n");
    }
    catch ( IOException e ) {
        e.printStackTrace();

        break;
    }
    catch ( IndexOutOfBoundsException e ) {
        e.printStackTrace();

        break;
    }
}

//Close the BufferInputStream:
bufferedInputStream.close() 

问题是，在我的测试中，对于一些(通常是大的)偏移，在跳过正确的字节数之前，它已经循环了5次或更多次。正常吗？而且，最重要的是，我能/应该推跳()吗？(也就是说:10个周期是否足以确保它始终到达正确的偏移量？)

我发现的唯一替代方法是通过RandomAccessFile和下面的函数从InputStream创建一个File.createTempFile(前缀、后缀、目录)。

public static RandomAccessFile toRandomAccessFile(InputStream inputStream, File tempFile, int fileSize) throws IOException {
    RandomAccessFile randomAccessFile = new RandomAccessFile(tempFile, "rw");

    byte[] buffer = new byte[fileSize];
    int numBytesRead = 0;

    while ( (numBytesRead = inputStream.read(buffer)) != -1 ) {
        randomAccessFile.write(buffer, 0, numBytesRead);
    }

    randomAccessFile.seek(0);

    return randomAccessFile;
}

拥有一个RandomAccessFile实际上是一个更好的解决方案，但性能却是指数级的(最重要的是因为我将拥有不止一个文件)。

编辑：使用byte[]缓冲器=新的bytefileSize加速(和很多) RandomAccessFile的创建！

//Create a temporary RandomAccessFile:
File tempFile = File.createTempFile(fileName, null, context.getCacheDir());
RandomAccessFile randomAccessFile = toRandomAccessFile(inputStream, tempFile, fileSize);

byte[] bytes = new byte[1];
int numBytesRead = 0;

//Check the file size:
if ( fileSize < offsets[offsets.length-1] ) {  // the last (bigger) offset is bigger then the file size...
    //Debug:
    Log.d(TAG, "The file is too small!\n");

    return;
}

for (int i=0, k=0; i < offsets.length; i++, k=0) {  // for each offset I have to jump...
    try {
        //Jump to the offset [i]:
        randomAccessFile.seek(offsets[i]);

        //Read the content of the file at the offset [i]:
        numBytesRead = randomAccessFile.read(bytes, 0, bytes.length);

        //Debug:
        Log.d(TAG, "READ [" + (randomAccessFile.getFilePointer()-4) + "]: " + bytes[0] + "\n");
    }
    catch ( IOException e ) {
        e.printStackTrace();

        break;
    }
    catch ( IndexOutOfBoundsException e ) {
        e.printStackTrace();

        break;
    }
}

//Delete the temporary RandomAccessFile:
randomAccessFile.close();
tempFile.delete();

现在，是否有更好(或更优雅)的解决方案可以从InputStream获得“随机”访问？

Answer 1

您一开始就有一个InputStream，这有点不幸，但是在这种情况下，在文件中缓冲流是没有用的，如果您总是跳过前一步。但是，您不必计算调用skip的次数，这并不是真正的兴趣所在。

您必须检查流是否已经结束，以防止无限循环。检查默认skip实现的skip，我认为您必须一直调用skip，直到它返回0为止。这将指示已经到达流的结束。就我的口味而言，JavaDoc对此有点不清楚。

Answer 2

不能，InputStream是一个流，也就是说是一个顺序的构造。你的问题在措辞上有矛盾之处。