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;
}
}
