如何通过为 Lollipop 提供的 API 随机访问 SD 卡上的文件？

Question

我的应用程序使用 Java class RandomAccessFile to read/write bytes to a file on SD card randomly by means of realization of SeekableByteChannel 接口。现在我需要用新的 Lollipop API 为 Android 5.0 重写它。

我找到the only way阅读：

InputStream inputStream = getContentResolver().openInputStream(uri);

然后写：

ParcelFileDescriptor pfd = getActivity().getContentResolver().openFileDescriptor(uri, "w");
FileOutputStream fileOutputStream = new FileOutputStream(pfd.getFileDescriptor());

from/to 新建一个文件 API.

我希望能够在某些随机位置和 read/write 字节的位置设置频道。可以在新的 SDK 21 中做到这一点吗？新的SDK是否暗示以这种方式获取渠道：

FieInputChannel fieInputChannel = fileInputStream.getChannel();
FieOutputChannel fieOutputChannel = fileOutputStream.getChannel();

或其他一些方法？

Answer 1

对于 SDK 21 (Lollipop)，随机 read/write 访问 SD 卡上的文件的唯一方法似乎如下：

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;
    }
}

编辑 2017 年 3 月 15 日： 稍微优化一下的版本看起来像

import android.content.Context;
import android.net.Uri;
import android.os.ParcelFileDescriptor;

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 {
    private ParcelFileDescriptor pfdInput, pfdOutput;
    private FileInputStream fis;
    private FileOutputStream fos;
    private long position;

    public SecondaryCardChannel(Uri treeUri, Context context) {
        try {
            pfdInput = context.getContentResolver().openFileDescriptor(treeUri, "r");
            pfdOutput = context.getContentResolver().openFileDescriptor(treeUri, "rw");
            fis = new FileInputStream(pfdInput.getFileDescriptor());
            fos = new FileOutputStream(pfdOutput.getFileDescriptor());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }

    public int read(ByteBuffer buffer) {
        try {
            FileChannel fch = fis.getChannel();
            fch.position(position);
            int bytesRead = fch.read(buffer);
            position = fch.position();
            return bytesRead;
        } catch (IOException e) {
            e.printStackTrace();
            return -1;
        }
    }

    public int write(ByteBuffer buffer) {
        try {
            FileChannel fch = fos.getChannel();
            fch.position(position);
            int bytesWrite = fch.write(buffer);
            position = fch.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 fis.getChannel().size();
    }

    public void force(boolean metadata) throws IOException {
        fos.getChannel().force(metadata);
        pfdOutput.getFileDescriptor().sync();
    }

    public long truncate(long size) throws Exception {
        FileChannel fch = fos.getChannel();
        try {
            fch.truncate(size);
            return fch.size();
        } catch (Exception e){ // Attention! Truncate is broken on removable SD card of Android 5.0
            e.printStackTrace();
            return -1;
        }
    }

    public void close() throws IOException {
        FileChannel fch = fos.getChannel();
        fch.close();

        fos.close();
        fis.close();
        pfdInput.close();
        pfdOutput.close();
    }
}

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