我的应用程序使用Java类RandomAccessFile通过SeekableByteChannel接口(interface)的实现来将字节随机读取/写入SD卡上的文件中。现在,我需要使用新的Lollipop API将其重写为Android 5.0。
我发现the only way读取:
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中设置 channel ,并在该位置读取/写入字节。在新的SDK 21中可以做到这一点吗?新SDK是否暗示以这种方式获取 channel :
FieInputChannel fieInputChannel = fileInputStream.getChannel();
FieOutputChannel fieOutputChannel = fileOutputStream.getChannel();
或其他方法?
最佳答案
似乎唯一可以对SD卡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();
}
}
关于android-5.0-lollipop - 如何通过为Lollipop提供的API随机访问SD卡上的文件?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/28698199/