让class只有一个实例的例子

最近在学习android 的webkit,看到一些自己认为是好的代码或者方式就把它记录下来，方便以后复习与借鉴。
WebViewDatabase是一个单实例对象，通过getInstance方法获取WebViewDatabase的实例。WebViewDatabase是WebKit模块中的内部对象，仅供WebKit框架内部使用。

private static WebViewDatabase mInstance = null;private WebViewDatabase() {// Singleton only, use getInstance()}public static synchronized WebViewDatabase getInstance(Context context) {if (mInstance == null) {mInstance = new WebViewDatabase();mDatabase = context.openOrCreateDatabase(DATABASE_FILE, 0, null);// mDatabase should not be null,// the only case is RequestAPI test has problem to create dbif (mDatabase != null && mDatabase.getVersion() != DATABASE_VERSION) {mDatabase.beginTransaction();try {upgradeDatabase();mDatabase.setTransactionSuccessful();} finally {mDatabase.endTransaction();}}if (mDatabase != null) {// use per table Mutex lock, turn off database lock, this// improves performance as database's ReentrantLock is expansivemDatabase.setLockingEnabled(false);}mCacheDatabase = context.openOrCreateDatabase(CACHE_DATABASE_FILE,0, null);// mCacheDatabase should not be null,// the only case is RequestAPI test has problem to create dbif (mCacheDatabase != null&& mCacheDatabase.getVersion() != CACHE_DATABASE_VERSION) {mCacheDatabase.beginTransaction();try {upgradeCacheDatabase();bootstrapCacheDatabase();mCacheDatabase.setTransactionSuccessful();} finally {mCacheDatabase.endTransaction();}// Erase the files from the file system in the// case that the database was updated and the// there were existing cache contentCacheManager.removeAllCacheFiles();}if (mCacheDatabase != null) {// use InsertHelper for faster insertionmCacheInserter = new DatabaseUtils.InsertHelper(mCacheDatabase,"cache");mCacheUrlColIndex = mCacheInserter.getColumnIndex(CACHE_URL_COL);mCacheFilePathColIndex = mCacheInserter.getColumnIndex(CACHE_FILE_PATH_COL);mCacheLastModifyColIndex = mCacheInserter.getColumnIndex(CACHE_LAST_MODIFY_COL);mCacheETagColIndex = mCacheInserter.getColumnIndex(CACHE_ETAG_COL);mCacheExpiresColIndex = mCacheInserter.getColumnIndex(CACHE_EXPIRES_COL);mCacheMimeTypeColIndex = mCacheInserter.getColumnIndex(CACHE_MIMETYPE_COL);mCacheEncodingColIndex = mCacheInserter.getColumnIndex(CACHE_ENCODING_COL);mCacheHttpStatusColIndex = mCacheInserter.getColumnIndex(CACHE_HTTP_STATUS_COL);mCacheLocationColIndex = mCacheInserter.getColumnIndex(CACHE_LOCATION_COL);mCacheContentLengthColIndex = mCacheInserter.getColumnIndex(CACHE_CONTENTLENGTH_COL);}}return mInstance;}

WebViewDatabase db = WebViewDatabase.getInstance(this);

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