Android Database 并行访问

1、假设你已经有了自己的SQLiteOpenHelper实例

    public class DatabaseHelper extends SQLiteOpenHelper { ... }

2、现在你需要在不同的线程中写数据库

// Thread 1   Context context = getApplicationContext();   DatabaseHelper helper = new DatabaseHelper(context);   SQLiteDatabase database = helper.getWritableDatabase();   database.insert(…);   database.close();     // Thread 2   Context context = getApplicationContext();   DatabaseHelper helper = new DatabaseHelper(context);   SQLiteDatabase database = helper.getWritableDatabase();   database.insert(…);   database.close();

执行后，得到错误

android.database.sqlite.SQLiteDatabaseLockedException: database is locked (code 5)

错误的原因：每次new SQLiteOpenHelper的时候，实质上是创建一个Database Connection。如果在不同的Connection同时写数据库，其中一个就会Failed。

如果在多个线程中使用数据库，需要确保使用同一个Database Connection。

3、为了解决第二个问题，我们去创建一个DatabaseManager类，通过单例去获取SQLiteOpenHelper的对象。

    public class DatabaseManager {                private static DatabaseManager instance;          private static SQLiteOpenHelper mDatabaseHelper;                public static synchronized void initialize(Context context, SQLiteOpenHelper helper) {              if (instance == null) {                  instance = new DatabaseManager();                  mDatabaseHelper = helper;              }          }                public static synchronized DatabaseManager getInstance() {              if (instance == null) {                  throw new IllegalStateException(DatabaseManager.class.getSimpleName() +                          " is not initialized, call initialize(..) method first.");              }                    return instance;          }                public synchronized SQLiteDatabase getDatabase() {              return new mDatabaseHelper.getWritableDatabase();          }            }

4、那么现在第二步中的功能，应该这样去实现

    // In your application class       DatabaseManager.initializeInstance(getApplicationContext());             // Thread 1       DatabaseManager manager = DatabaseManager.getInstance();       SQLiteDatabase database = manager.getDatabase()       database.insert(…);       database.close();             // Thread 2       DatabaseManager manager = DatabaseManager.getInstance();       SQLiteDatabase database = manager.getDatabase()       database.insert(…);       database.close();

执行后，得到错误

java.lang.IllegalStateException: attempt to re-open an already-closed object: SQLiteDatabase

因为我们使用同一个Database Connection，所以Thread1和Thread2使用的是同一个实例。现在Thread1关闭了Connection，但是Thread2却仍然在用。

所以，我们在关闭Connection之前需要确保没有线程在使用这个Connection。一个解决方案是永远的不去关闭Connection，但是在Logcat里面会出现如下的消息：

    Leak found      Caused by: java.lang.IllegalStateException: SQLiteDatabase created and never closed

5、我们可以使用AtomicInteger处理这种并发操作

    public class DatabaseManager {                private AtomicInteger mOpenCounter = new AtomicInteger();                private static DatabaseManager instance;          private static SQLiteOpenHelper mDatabaseHelper;          private SQLiteDatabase mDatabase;                public static synchronized void initializeInstance(SQLiteOpenHelper helper) {              if (instance == null) {                  instance = new DatabaseManager();                  mDatabaseHelper = helper;              }          }                public static synchronized DatabaseManager getInstance() {              if (instance == null) {                  throw new IllegalStateException(DatabaseManager.class.getSimpleName() +                          " is not initialized, call initializeInstance(..) method first.");              }                    return instance;          }                public synchronized SQLiteDatabase openDatabase() {              if(mOpenCounter.incrementAndGet() == 1) {                  // Opening new database                  mDatabase = mDatabaseHelper.getWritableDatabase();              }              return mDatabase;          }                public synchronized void closeDatabase() {              if(mOpenCounter.decrementAndGet() == 0) {                  // Closing database                  mDatabase.close();                    }          }      }

最后，我们这样使用就可以实现安全的数据库并发操作

    SQLiteDatabase database = DatabaseManager.getInstance().openDatabase();      database.insert(...);      // database.close(); Don't close it directly!      DatabaseManager.getInstance().closeDatabase(); // correct way

更多相关文章

随机推荐