DocumentFile RandomAccessFile

Question

有没有办法从给定的DocumentFile中获取RandomAccessFile

我知道通过getUri获取InputStream是可能的

InputStream inputStream = getContentResolver().openInputStream(DocumentFile.getUri());

但是我需要一个RandomAccessFile

谢谢你的帮忙,

延斯

Answer 1

对于SDK 21 (棒棒糖)SD卡上的文件进行随机读/写访问的唯一方法似乎如下所示：

import android.content.Context;
import android.net.Uri;
import android.os.ParcelFileDescriptor;
import com.jetico.bestcrypt.FileManagerApplication;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class SecondaryCardChannel {//By analogy with the java.nio.channels.SeekableByteChannel
    private FileChannel fileChannel;
    private ParcelFileDescriptor pfd;
    private boolean isInputChannel;
    private long position;

    public SecondaryCardChannel(Uri treeUri, Context context) {
        try {
            pfd = context.getContentResolver().openFileDescriptor(treeUri, "rw");
            FileInputStream fis = new FileInputStream(pfd.getFileDescriptor());
            fileChannel = fis.getChannel();
            isInputChannel = true;
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }

    public int read(ByteBuffer buffer) {
        if (!isInputChannel) {
            try {
                fileChannel.close();
                FileInputStream fis = new FileInputStream(pfd.getFileDescriptor());
                fileChannel = fis.getChannel();
                isInputChannel = true;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        try {
            fileChannel.position(position);
            int bytesRead = fileChannel.read(buffer);
            position = fileChannel.position();
            return bytesRead;
        } catch (IOException e) {
            e.printStackTrace();
            return -1;
        }
    }

    public int write(ByteBuffer buffer) {
        if (isInputChannel) {
            try {
                fileChannel.close();
                FileOutputStream fos = new FileOutputStream(pfd.getFileDescriptor());
                fileChannel = fos.getChannel();
                isInputChannel = false;
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        try {
            fileChannel.position(position);
            int bytesWrite = fileChannel.write(buffer);
            position = fileChannel.position();
            return bytesWrite;
        } catch (IOException e) {
            e.printStackTrace();
            return -1;
        }
    }

    public long position() throws IOException {
        return position;
    }

    public SecondaryCardChannel position(long newPosition) throws IOException {
        position = newPosition;
        return this;
    }

    public long size() throws IOException {
        return fileChannel.size();
    }

    public SecondaryCardChannel truncate(long size) throws IOException {
        fileChannel.truncate(size);
        return this;
    }
}

Answer 2

从API级别21 (棒棒糖)开始，这可能是一个低级别的替换：

ParcelFileDescriptor pfd = context.getContentResolver().openFileDescriptor(uri, "rw");
FileDescriptor fd = pfd.getFileDescriptor();
// seek to offset 10 from beginning of file
android.system.Os.lseek(fd, 10, OsConstants.SEEK_SET);

其他低级方法，如read(fd, ...)? write(fd, ...) fstat(fd)，也可以在android.system.OS中找到。

确保您具有具有读/写访问权限的uri。

Answer 3

创建文本文件，其中包含您要随机打开的所有文件的路径

in = new BufferedReader(new FileReader("randomfilepaths.txt"));

创建一个函数来获取一个随机文件的路径，这可以由readLine()来完成。

代码：

protected String getRandomPath() {
     String returnval = null;
 try{
   if((returnval = in.readLine()) == null){
    in.close();
    moreFiles = false;
   }
 }catch(Exception e){}
 return returnval;
}

这里的returnval将包含一个随机文件的字符串路径。