Android SQLiteDatabaseLockedException: database is locked

问题场景描述：

在页面中用到了ViewPager控件，ViewPager中的内容分别是两个ListView，两个ListView的数据都来自本地数据库（先从网络下载数据，然后更新本地数据库），在实际的使用过程中发现会出现SQLiteDatabaseLockedException: database is locked的问题。

经网上搜索资料，发现是读写数据库时存在的同步问题，所以采用单例+同步锁的方法，并且在每次数据库操作后都关闭数据库，经测试后发现没有在出现上述问题。

以下是两个主类

DBHelper.java（这个类用来管理数据库）

public class DBHelper extends SQLiteOpenHelper {

	private final String TAG = this.getClass().getSimpleName();
	
	public final static String DATABASE_NAME = "test.db";
	public final static String TABLE = "table";
	public final static int DATABASE_VERSION = 2;

	public DBHelper(Context context) {
		super(context, DATABASE_NAME, null, DATABASE_VERSION);
	}

	private static DBHelper mInstance;

	public synchronized static DBHelper getInstance(Context context) {
		if (mInstance == null) {
			mInstance = new DBHelper(context);
		}
		return mInstance;
	};

	@Override
	public void onCreate(SQLiteDatabase db) {
		try {
			db.execSQL("CREATE TABLE IF NOT EXISTS " + TABLE
					+ "(id INTEGER PRIMARY KEY ,data BLOB)");
		} catch (SQLiteException e) {
			e.printStackTrace();
		}
	}

	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {	
		
		// 删除原来的数据表
		db.execSQL("DROP TABLE IF EXISTS " + TABLE);
		// 重新创建
		onCreate(db);
	}

	public static byte[] objectToBytes(Object obj) throws Exception {

		ByteArrayOutputStream out = new ByteArrayOutputStream();
		ObjectOutputStream sOut = new ObjectOutputStream(out);
		sOut.writeObject(obj);
		sOut.flush();
		byte[] bytes = out.toByteArray();
		return bytes;
	}

	public static Object bytesToObject(byte[] bytes) throws Exception {

		ByteArrayInputStream in = new ByteArrayInputStream(bytes);
		ObjectInputStream sIn = new ObjectInputStream(in);
		return sIn.readObject();
	}
}

DBStudentManager类（这里可以定义自己的管理类）

public class DBStudentManager {

	private DBHelper helper;
	private SQLiteDatabase db;

	public DBStudentManager(Context context) {
		helper = DBHelper.getInstance(context);
		db = helper.getWritableDatabase();
	}

	// 插入
	private void insert(Student student) {

		synchronized (helper) {
			// 看数据库是否关闭
			if (!db.isOpen()) {
				db = helper.getWritableDatabase();
			}
			// 开始事务
			db.beginTransaction();
			try {
				db.execSQL(
						"INSERT INTO " + DBHelper.TABLE + " VALUES(?,?)",
						new Object[] { student.mID,
								DBHelper.objectToBytes(student) });
				db.setTransactionSuccessful(); // 设置事务成功完成
			} catch (SQLException e) {
				e.printStackTrace();
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				db.endTransaction();
				db.close();
			}
		}
	}

	// 更新
	private void update(Student student) {

		synchronized (helper) {
			if (!db.isOpen()) {
				db = helper.getWritableDatabase();
			}
			db.beginTransaction();
			try {
				db.execSQL("UPDATE " + DBHelper.TABLE
						+ "SET data = ? WHERE id = ?",
						new Object[] { DBHelper.objectToBytes(student),
								student.mID });
				db.setTransactionSuccessful(); // 设置事务成功完成
			} catch (SQLException e) {
				e.printStackTrace();
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				db.endTransaction();
				db.close();
			}
		}
	}

	// 同步
	public void synchronous(List<Student> students) {
		if (students == null) {
			return;
		}
		for (Student student : students) {
			if (query(student.mID) == null) {
				insert(student);
			} else {
				update(student);
			}
		}
	}

	// 删除指定数据
	public void delete(String id) {

		synchronized (helper) {
			if (!db.isOpen()) {
				db = helper.getWritableDatabase();
			}
			db.beginTransaction();
			try {
				db.execSQL("DELETE FROM " + DBHelper.TABLE + " WHERE id = ? ",
						new String[] { id });
				db.setTransactionSuccessful();
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				db.endTransaction();
				db.close();
			}
		}
	}

	// 删除所有数据
	public void delete() {

		synchronized (helper) {
			if (!db.isOpen()) {
				db = helper.getWritableDatabase();
			}
			db.beginTransaction();
			try {
				db.execSQL("DELETE * FROM " + DBHelper.TABLE);
				db.setTransactionSuccessful();
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				db.endTransaction();
				db.close();
			}
		}
	}

	// 查找所有的Students
	public List<Student> query() {

		List<Student> students = new ArrayList<Student>();
		synchronized (helper) {
			if (!db.isOpen()) {
				db = helper.getWritableDatabase();
			}
			Cursor c = queryTheCursor();
			Student student = null;
			try {
				while (c.moveToNext()) {

					byte[] bytes = c.getBlob((c.getColumnIndex("data")));
					student = (Student) DBHelper.bytesToObject(bytes);
					students.add(student);
				}
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				c.close();
			}
		}
		return students;
	}

	// 查找指定ID的Student
	public Student query(String id) {

		Student student = null;
		synchronized (helper) {
			if (!db.isOpen()) {
				helper.getWritableDatabase();
			}
			Cursor c = queryTheCursor(id);
			try {
				while (c.moveToNext()) {

					byte[] bytes = c.getBlob((c.getColumnIndex("data")));
					student = (Student) DBHelper.bytesToObject(bytes);
					break;
				}
			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				c.close();
			}
		}
		return student;
	}

	// 获取游标
	public Cursor queryTheCursor(String id) {
		Cursor c = db.rawQuery("SELECT FROM " + DBHelper.TABLE
				+ " WHERE id = ?", new String[] { id });
		return c;
	}

	// 获取游标
	public Cursor queryTheCursor() {
		Cursor c = db.rawQuery("SELECT * FROM " + DBHelper.TABLE);
		return c;
	}

	class Student {

		String mID;
		String mName;
		int mAge;
	}
}

使用继承ContentProvider的方式，也可以解决该异常。

但是这种方式有2个弊端：

1.使用继承ContentProvider方式，会将项目的数据库暴露给第三方。一般系统级应用，如存储联系人， 存储短信的数据库才有必要暴露给第三方。

2.使用方式不如直接继承SQLiteOpenHelper对象简单。