import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshottable.SnapshotDiffInfo; //导入依赖的package包/类
/**
 * Get the difference between two snapshots (or between a snapshot and the
 * current status) of a snapshottable directory.
 * 
 * @param path The full path of the snapshottable directory.
 * @param fromSnapshot Name of the snapshot to calculate the diff from. Null
 *          or empty string indicates the current tree.
 * @param toSnapshot Name of the snapshot to calculated the diff to. Null or
 *          empty string indicates the current tree.
 * @return A report about the difference between {@code fromSnapshot} and 
 *         {@code toSnapshot}. Modified/deleted/created/renamed files and 
 *         directories belonging to the snapshottable directories are listed 
 *         and labeled as M/-/+/R respectively. 
 * @throws IOException
 */
SnapshotDiffReport getSnapshotDiffReport(String path,
    String fromSnapshot, String toSnapshot) throws IOException {
  SnapshotDiffInfo diffs = null;
  final FSPermissionChecker pc = getPermissionChecker();
  readLock();
  try {
    checkOperation(OperationCategory.READ);
    if (isPermissionEnabled) {
      checkSubtreeReadPermission(pc, path, fromSnapshot);
      checkSubtreeReadPermission(pc, path, toSnapshot);
    }
    diffs = snapshotManager.diff(path, fromSnapshot, toSnapshot);
  } finally {
    readUnlock();
  }
  
  if (auditLog.isInfoEnabled() && isExternalInvocation()) {
    logAuditEvent(true, "computeSnapshotDiff", null, null, null);
  }
  return diffs != null ? diffs.generateReport() : new SnapshotDiffReport(
      path, fromSnapshot, toSnapshot,
      Collections.<DiffReportEntry> emptyList());
}
 

import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshottable.SnapshotDiffInfo; //导入依赖的package包/类
/**
 * Compute the difference between two snapshots of a directory, or between a
 * snapshot of the directory and its current tree.
 */
public SnapshotDiffInfo diff(final String path, final String from,
    final String to) throws IOException {
  if ((from == null || from.isEmpty())
      && (to == null || to.isEmpty())) {
    // both fromSnapshot and toSnapshot indicate the current tree
    return null;
  }

  // Find the source root directory path where the snapshots were taken.
  // All the check for path has been included in the valueOf method.
  INodesInPath inodesInPath = fsdir.getINodesInPath4Write(path.toString());
  final INodeDirectorySnapshottable snapshotRoot = INodeDirectorySnapshottable
      .valueOf(inodesInPath.getLastINode(), path);
  
  return snapshotRoot.computeDiff(from, to);
}
 

import org.apache.hadoop.hdfs.server.namenode.snapshot.INodeDirectorySnapshottable.SnapshotDiffInfo; //导入依赖的package包/类
/**
 * Get the difference between two snapshots (or between a snapshot and the
 * current status) of a snapshottable directory.
 * 
 * @param path The full path of the snapshottable directory.
 * @param fromSnapshot Name of the snapshot to calculate the diff from. Null
 *          or empty string indicates the current tree.
 * @param toSnapshot Name of the snapshot to calculated the diff to. Null or
 *          empty string indicates the current tree.
 * @return A report about the difference between {@code fromSnapshot} and 
 *         {@code toSnapshot}. Modified/deleted/created/renamed files and 
 *         directories belonging to the snapshottable directories are listed 
 *         and labeled as M/-/+/R respectively. 
 * @throws IOException
 */
SnapshotDiffReport getSnapshotDiffReport(String path,
    String fromSnapshot, String toSnapshot) throws IOException {
  SnapshotDiffInfo diffs = null;
  checkOperation(OperationCategory.READ);
  final FSPermissionChecker pc = getPermissionChecker();
  readLock();
  try {
    checkOperation(OperationCategory.READ);
    if (isPermissionEnabled) {
      checkSubtreeReadPermission(pc, path, fromSnapshot);
      checkSubtreeReadPermission(pc, path, toSnapshot);
    }
    diffs = snapshotManager.diff(path, fromSnapshot, toSnapshot);
  } finally {
    readUnlock();
  }
  
  if (auditLog.isInfoEnabled() && isExternalInvocation()) {
    logAuditEvent(true, "computeSnapshotDiff", null, null, null);
  }
  return diffs != null ? diffs.generateReport() : new SnapshotDiffReport(
      path, fromSnapshot, toSnapshot,
      Collections.<DiffReportEntry> emptyList());
}