11

我的应用程序使用 Java 类RandomAccessFile通过实现SeekableByteChannel接口随机读取/写入字节到 SD 卡上的文件。现在我需要用新的 Lollipop API 为 Android 5.0 重写它。

我找到了阅读的唯一方法：

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

和写：

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

从/到新 API 中的文件。

我希望能够在某个随机位置设置通道并将字节读/写到该位置。是否可以在新的 SDK 21 中做到这一点？新的SDK是否暗示以这种方式获取渠道：

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

或其他方法？

4

1 回答 1

11

似乎对 SDK 21 (Lollipop) 的 SD 卡上的文件进行随机读/写访问的唯一方法如下：

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 15/03/2017： 一点点优化版本看起来像

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();
    }
}
于 2015-03-02T08:35:00.070 回答