Java tutorial
// Copyright 2016 The Chromium Authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. package org.chromium.chrome.browser.util; import android.content.Context; import android.database.Cursor; import android.database.MatrixCursor; import android.net.Uri; import android.os.ParcelFileDescriptor; import android.provider.MediaStore; import android.support.v4.content.FileProvider; import java.io.FileNotFoundException; import java.util.Arrays; /** * A file provider class that can share a potentially non-existent file and blocks the client * application from accessing the file till it is written. * * This class serves as the default file provider, but also lets us share blocked files. * For blocked files, it generates an unique identifier for the file to be shared and the embedder * must write the file and notify that the file is ready using the unique uri generated. The client * application is blocked from accessing the file till the file is ready. This provider allows only * one blocked file to be shared at a given time. */ public class ChromeFileProvider extends FileProvider { private static final String AUTHORITY_SUFFIX = ".FileProvider"; private static final String BLOCKED_FILE_PREFIX = "BlockedFile_"; // All these static objects must be accesseed in a synchronized block: private static Object sLock = new Object(); private static boolean sIsFileReady; private static Uri sCurrentBlockingUri; private static Uri sFileUri; /** * Returns an unique uri to identify the file to be shared and block access to it till * notifyFileReady is called. * * This function clobbers any uri that was previously created and the client application * accessing those uri will get a null file descriptor. * @param context Activity context that is used to access package manager. */ public static Uri generateUriAndBlockAccess(final Context context) { String authority = context.getPackageName() + AUTHORITY_SUFFIX; String fileName = BLOCKED_FILE_PREFIX + String.valueOf(System.nanoTime()); Uri blockingUri = new Uri.Builder().scheme("content").authority(authority).path(fileName).build(); synchronized (sLock) { sCurrentBlockingUri = blockingUri; sFileUri = null; sIsFileReady = false; // In case the previous file never got ready. sLock.notify(); } return blockingUri; } /** * Notify that the file is ready to be accessed by the client application. * * @param blockingUri The unique uri that was generated by generateUriAndBlockAccess. * @param fileUri The Uri for actual file given by FileProvider. */ public static void notifyFileReady(Uri blockingUri, Uri fileUri) { synchronized (sLock) { sFileUri = fileUri; // Ready is set only if the current file is ready. sIsFileReady = doesMatchCurrentBlockingUri(blockingUri); sLock.notify(); } } @Override public ParcelFileDescriptor openFile(Uri uri, String mode) throws FileNotFoundException { Uri fileUri = getFileUriWhenReady(uri); return fileUri != null ? super.openFile(fileUri, mode) : null; } @Override public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) { Uri fileUri = getFileUriWhenReady(uri); if (fileUri == null) return null; // Workaround for a bad assumption that particular MediaStore columns exist by certain third // party applications. // http://crbug.com/467423. Cursor source = super.query(fileUri, projection, selection, selectionArgs, sortOrder); String[] columnNames = source.getColumnNames(); String[] newColumnNames = columnNamesWithData(columnNames); if (columnNames == newColumnNames) return source; MatrixCursor cursor = new MatrixCursor(newColumnNames, source.getCount()); source.moveToPosition(-1); while (source.moveToNext()) { MatrixCursor.RowBuilder row = cursor.newRow(); for (int i = 0; i < columnNames.length; i++) { switch (source.getType(i)) { case Cursor.FIELD_TYPE_INTEGER: row.add(source.getInt(i)); break; case Cursor.FIELD_TYPE_FLOAT: row.add(source.getFloat(i)); break; case Cursor.FIELD_TYPE_STRING: row.add(source.getString(i)); break; case Cursor.FIELD_TYPE_BLOB: row.add(source.getBlob(i)); break; case Cursor.FIELD_TYPE_NULL: default: row.add(null); break; } } } source.close(); return cursor; } @Override public int delete(Uri uri, String selection, String[] selectionArgs) { if (uri != null && uri.getPath().contains(BLOCKED_FILE_PREFIX)) { synchronized (sLock) { if (!doesMatchCurrentBlockingUri(uri)) return 0; sFileUri = null; sIsFileReady = false; sCurrentBlockingUri = null; } } return super.delete(uri, selection, selectionArgs); } /** * Waits and returns file uri iff the file is ready to be accessed, or returns null if file is * replaced. */ protected static Uri getFileUriWhenReady(Uri uri) { // If the uri passed is not a blocked file, then the given uri can be directly used. if (uri == null || !uri.getPath().contains(BLOCKED_FILE_PREFIX)) return uri; synchronized (sLock) { // Wait only if the file is not ready and the current file has not changed. while (!sIsFileReady && doesMatchCurrentBlockingUri(uri)) { try { sLock.wait(); } catch (InterruptedException e) { break; } } // If the current file has changed while waiting, return null. if (doesMatchCurrentBlockingUri(uri)) return sFileUri; } return null; } private static boolean doesMatchCurrentBlockingUri(Uri uri) { return uri != null && sCurrentBlockingUri != null && sCurrentBlockingUri.equals(uri); } private String[] columnNamesWithData(String[] columnNames) { for (String columnName : columnNames) { if (MediaStore.MediaColumns.DATA.equals(columnName)) return columnNames; } String[] newColumnNames = Arrays.copyOf(columnNames, columnNames.length + 1); newColumnNames[columnNames.length] = MediaStore.MediaColumns.DATA; return newColumnNames; } }