code: package com.lody.virtual.helper.utils;
  
  import android.util.Log;
  
  import java.io.File;
  import java.io.FileInputStream;
  import java.io.FileNotFoundException;
  import java.io.FileOutputStream;
  import java.io.IOException;
  
  /**
   * Static library support version of the framework's {@link android.util.AtomicFile},
   * a helper class for performing atomic operations on a file by creating a
   * backup file until a write has successfully completed.
   * <p>
   * Atomic file guarantees file integrity by ensuring that a file has
   * been completely written and sync'd to disk before removing its backup.
   * As long as the backup file exists, the original file is considered
   * to be invalid (left over from a previous attempt to write the file).
   * </p><p>
   * Atomic file does not confer any file locking semantics.
   * Do not use this class when the file may be accessed or modified concurrently
   * by multiple threads or processes.  The caller is responsible for ensuring
   * appropriate mutual exclusion invariants whenever it accesses the file.
   * </p>
   */
  public class AtomicFile {
      private final File mBaseName;
      private final File mBackupName;
  
      /**
       * Create a new AtomicFile for a file located at the given File path.
       * The secondary backup file will be the same file path with ".bak" appended.
       */
      public AtomicFile(File baseName) {
          mBaseName = baseName;
          mBackupName = new File(baseName.getPath() + ".bak");
      }
  
      /**
       * Return the path to the base file.  You should not generally use this,
       * as the data at that path may not be valid.
       */
      public File getBaseFile() {
          return mBaseName;
      }
  
      /**
       * Delete the atomic file.  This deletes both the base and backup files.
       */
      public void delete() {
          mBaseName.delete();
          mBackupName.delete();
      }
  
      /**
       * Start a new write operation on the file.  This returns a FileOutputStream
       * to which you can write the new file data.  The existing file is replaced
       * with the new data.  You <em>must not</em> directly close the given
       * FileOutputStream; instead call either {@link #finishWrite(FileOutputStream)}
       * or {@link #failWrite(FileOutputStream)}.
       * <p>
       * <p>Note that if another thread is currently performing
       * a write, this will simply replace whatever that thread is writing
       * with the new file being written by this thread, and when the other
       * thread finishes the write the new write operation will no longer be
       * safe (or will be lost).  You must do your own threading protection for
       * access to AtomicFile.
       */
      public FileOutputStream startWrite() throws IOException {
          // Rename the current file so it may be used as a backup during the next read
          if (mBaseName.exists()) {
              if (!mBackupName.exists()) {
                  if (!mBaseName.renameTo(mBackupName)) {
                      Log.w("AtomicFile", "Couldn't rename file " + mBaseName
                              + " to backup file " + mBackupName);
                  }
              } else {
                  mBaseName.delete();
              }
          }
          FileOutputStream str = null;
          try {
              str = new FileOutputStream(mBaseName);
          } catch (FileNotFoundException e) {
              File parent = mBaseName.getParentFile();
              if (!parent.mkdir()) {
                  throw new IOException("Couldn't create directory " + mBaseName);
              }
              try {
                  str = new FileOutputStream(mBaseName);
              } catch (FileNotFoundException e2) {
                  throw new IOException("Couldn't create " + mBaseName);
              }
          }
          return str;
      }
  
      /**
       * Call when you have successfully finished writing to the stream
       * returned by {@link #startWrite()}.  This will close, sync, and
       * commit the new data.  The next attempt to read the atomic file
       * will return the new file stream.
       */
      public void finishWrite(FileOutputStream str) {
          if (str != null) {
              sync(str);
              try {
                  str.close();
                  mBackupName.delete();
              } catch (IOException e) {
                  Log.w("AtomicFile", "finishWrite: Got exception:", e);
              }
          }
      }
  
      /**
       * Call when you have failed for some reason at writing to the stream
       * returned by {@link #startWrite()}.  This will close the current
       * write stream, and roll back to the previous state of the file.
       */
      public void failWrite(FileOutputStream str) {
          if (str != null) {
              sync(str);
              try {
                  str.close();
                  mBaseName.delete();
                  mBackupName.renameTo(mBaseName);
              } catch (IOException e) {
                  Log.w("AtomicFile", "failWrite: Got exception:", e);
              }
          }
      }
  
      /**
       * Open the atomic file for reading.  If there previously was an
       * incomplete write, this will roll back to the last good data before
       * opening for read.  You should call close() on the FileInputStream when
       * you are done reading from it.
       * <p>
       * <p>Note that if another thread is currently performing
       * a write, this will incorrectly consider it to be in the state of a bad
       * write and roll back, causing the new data currently being written to
       * be dropped.  You must do your own threading protection for access to
       * AtomicFile.
       */
      public FileInputStream openRead() throws FileNotFoundException {
          if (mBackupName.exists()) {
              mBaseName.delete();
              mBackupName.renameTo(mBaseName);
          }
          return new FileInputStream(mBaseName);
      }
  
      /**
       * A convenience for {@link #openRead()} that also reads all of the
       * file contents into a byte array which is returned.
       */
      public byte[] readFully() throws IOException {
          FileInputStream stream = openRead();
          try {
              int pos = 0;
              int avail = stream.available();
              byte[] data = new byte[avail];
              while (true) {
                  int amt = stream.read(data, pos, data.length - pos);
                  //Log.i("foo", "Read " + amt + " bytes at " + pos
                  //        + " of avail " + data.length);
                  if (amt <= 0) {
                      //Log.i("foo", "**** FINISHED READING: pos=" + pos
                      //        + " len=" + data.length);
                      return data;
                  }
                  pos += amt;
                  avail = stream.available();
                  if (avail > data.length - pos) {
                      byte[] newData = new byte[pos + avail];
                      System.arraycopy(data, 0, newData, 0, pos);
                      data = newData;
                  }
              }
          } finally {
              stream.close();
          }
      }
  
      /**
       * @deprecated This is not safe.
       */
      public void truncate() throws IOException {
          try {
              FileOutputStream fos = new FileOutputStream(mBaseName);
              fos.getFD().sync();
              fos.close();
          } catch (FileNotFoundException e) {
              throw new IOException("Couldn't append " + mBaseName);
          } catch (IOException e) {
          }
      }
  
      /**
       * @deprecated This is not safe.
       */
      @Deprecated public FileOutputStream openAppend() throws IOException {
          try {
              return new FileOutputStream(mBaseName, true);
          } catch (FileNotFoundException e) {
              throw new IOException("Couldn't append " + mBaseName);
          }
      }
  
      static boolean sync(FileOutputStream stream) {
          try {
              if (stream != null) {
                  stream.getFD().sync();
              }
              return true;
          } catch (IOException e) {
          }
          return false;
      }
  }