数据库在多线程方面非常灵活。我的应用程序从许多不同的线程同时击中它们的db，它做得很好。在某些情况下，我有多个进程同时访问数据库，这也很好。

你的异步任务-当你可以使用相同的连接，但如果你必须，它可以从不同的任务访问数据库。

您可以尝试应用谷歌I/O 2017上宣布的新架构方法。

它还包括新的ORM库Room

它包含三个主要组件:@Entity， @Dao和@Database

User.java

@Entity
public class User {
  @PrimaryKey
  private int uid;

  @ColumnInfo(name = "first_name")
  private String firstName;

  @ColumnInfo(name = "last_name")
  private String lastName;

  // Getters and setters are ignored for brevity,
  // but they're required for Room to work.
}

UserDao.java

@Dao
public interface UserDao {
  @Query("SELECT * FROM user")
  List<User> getAll();

  @Query("SELECT * FROM user WHERE uid IN (:userIds)")
  List<User> loadAllByIds(int[] userIds);

  @Query("SELECT * FROM user WHERE first_name LIKE :first AND "
       + "last_name LIKE :last LIMIT 1")
  User findByName(String first, String last);

  @Insert
  void insertAll(User... users);

  @Delete
  void delete(User user);
}

AppDatabase.java

@Database(entities = {User.class}, version = 1)
public abstract class AppDatabase extends RoomDatabase {
  public abstract UserDao userDao();
}

在纠结了几个小时之后，我发现每次db执行只能使用一个db helper对象。例如,

for(int x = 0; x < someMaxValue; x++)
{
    db = new DBAdapter(this);
    try
    {

        db.addRow
        (
                NamesStringArray[i].toString(), 
                StartTimeStringArray[i].toString(),
                EndTimeStringArray[i].toString()
        );

    }
    catch (Exception e)
    {
        Log.e("Add Error", e.toString());
        e.printStackTrace();
    }
    db.close();
}

相对于:

db = new DBAdapter(this);
for(int x = 0; x < someMaxValue; x++)
{

    try
    {
        // ask the database manager to add a row given the two strings
        db.addRow
        (
                NamesStringArray[i].toString(), 
                StartTimeStringArray[i].toString(),
                EndTimeStringArray[i].toString()
        );

    }
    catch (Exception e)
    {
        Log.e("Add Error", e.toString());
        e.printStackTrace();
    }

}
db.close();

每次循环迭代时创建一个新的DBAdapter是我通过助手类将字符串放入数据库的唯一方法。

对于长时间运行的操作(50ms+)，使用Thread或AsyncTask。测试你的应用，看看在哪里。大多数操作(可能)不需要线程，因为大多数操作(可能)只涉及几行。使用线程进行批量操作。 线程之间为磁盘上的每个DB共享一个SQLiteDatabase实例，并实现一个计数系统来跟踪打开的连接。

对于这些场景有什么最佳实践吗?

在所有类之间共享一个静态字段。我过去常常保持一个单例，用于那些需要共享的东西。还应该使用计数方案(通常使用AtomicInteger)来确保不会过早关闭数据库或让数据库保持打开状态。

我的解决方案:

我写的旧版本可以在https://github.com/Taeluf/dev/tree/main/archived/databasemanager上找到，并且没有维护。如果您想了解我的解决方案，请查看代码并阅读我的注释。我的笔记通常都很有用。

将代码复制/粘贴到名为DatabaseManager的新文件中。(或从github下载) 扩展DatabaseManager和实现onCreate和onUpgrade就像你通常会。您可以为一个DatabaseManager类创建多个子类，以便在磁盘上拥有不同的数据库。 实例化您的子类并调用getDb()来使用SQLiteDatabase类。 为您实例化的每个子类调用close()

复制/粘贴的代码:

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;

import java.util.concurrent.ConcurrentHashMap;

/** Extend this class and use it as an SQLiteOpenHelper class
 *
 * DO NOT distribute, sell, or present this code as your own. 
 * for any distributing/selling, or whatever, see the info at the link below
 *
 * Distribution, attribution, legal stuff,
 * See https://github.com/JakarCo/databasemanager
 * 
 * If you ever need help with this code, contact me at support@androidsqlitelibrary.com (or support@jakar.co )
 * 
 * Do not sell this. but use it as much as you want. There are no implied or express warranties with this code. 
 *
 * This is a simple database manager class which makes threading/synchronization super easy.
 *
 * Extend this class and use it like an SQLiteOpenHelper, but use it as follows:
 *  Instantiate this class once in each thread that uses the database. 
 *  Make sure to call {@link #close()} on every opened instance of this class
 *  If it is closed, then call {@link #open()} before using again.
 * 
 * Call {@link #getDb()} to get an instance of the underlying SQLiteDatabse class (which is synchronized)
 *
 * I also implement this system (well, it's very similar) in my <a href="http://androidslitelibrary.com">Android SQLite Libray</a> at http://androidslitelibrary.com
 * 
 *
 */
abstract public class DatabaseManager {
    
    /**See SQLiteOpenHelper documentation
    */
    abstract public void onCreate(SQLiteDatabase db);
    /**See SQLiteOpenHelper documentation
     */
    abstract public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion);
    /**Optional.
     * *
     */
    public void onOpen(SQLiteDatabase db){}
    /**Optional.
     * 
     */
    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {}
    /**Optional
     * 
     */
    public void onConfigure(SQLiteDatabase db){}



    /** The SQLiteOpenHelper class is not actually used by your application.
     *
     */
    static private class DBSQLiteOpenHelper extends SQLiteOpenHelper {

        DatabaseManager databaseManager;
        private AtomicInteger counter = new AtomicInteger(0);

        public DBSQLiteOpenHelper(Context context, String name, int version, DatabaseManager databaseManager) {
            super(context, name, null, version);
            this.databaseManager = databaseManager;
        }

        public void addConnection(){
            counter.incrementAndGet();
        }
        public void removeConnection(){
            counter.decrementAndGet();
        }
        public int getCounter() {
            return counter.get();
        }
        @Override
        public void onCreate(SQLiteDatabase db) {
            databaseManager.onCreate(db);
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            databaseManager.onUpgrade(db, oldVersion, newVersion);
        }

        @Override
        public void onOpen(SQLiteDatabase db) {
            databaseManager.onOpen(db);
        }

        @Override
        public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            databaseManager.onDowngrade(db, oldVersion, newVersion);
        }

        @Override
        public void onConfigure(SQLiteDatabase db) {
            databaseManager.onConfigure(db);
        }
    }

    private static final ConcurrentHashMap<String,DBSQLiteOpenHelper> dbMap = new ConcurrentHashMap<String, DBSQLiteOpenHelper>();

    private static final Object lockObject = new Object();


    private DBSQLiteOpenHelper sqLiteOpenHelper;
    private SQLiteDatabase db;
    private Context context;

    /** Instantiate a new DB Helper. 
     * <br> SQLiteOpenHelpers are statically cached so they (and their internally cached SQLiteDatabases) will be reused for concurrency
     *
     * @param context Any {@link android.content.Context} belonging to your package.
     * @param name The database name. This may be anything you like. Adding a file extension is not required and any file extension you would like to use is fine.
     * @param version the database version.
     */
    public DatabaseManager(Context context, String name, int version) {
        String dbPath = context.getApplicationContext().getDatabasePath(name).getAbsolutePath();
        synchronized (lockObject) {
            sqLiteOpenHelper = dbMap.get(dbPath);
            if (sqLiteOpenHelper==null) {
                sqLiteOpenHelper = new DBSQLiteOpenHelper(context, name, version, this);
                dbMap.put(dbPath,sqLiteOpenHelper);
            }
            //SQLiteOpenHelper class caches the SQLiteDatabase, so this will be the same SQLiteDatabase object every time
            db = sqLiteOpenHelper.getWritableDatabase();
        }
        this.context = context.getApplicationContext();
    }
    /**Get the writable SQLiteDatabase
     */
    public SQLiteDatabase getDb(){
        return db;
    }

    /** Check if the underlying SQLiteDatabase is open
     *
     * @return whether the DB is open or not
     */
    public boolean isOpen(){
        return (db!=null&&db.isOpen());
    }


    /** Lowers the DB counter by 1 for any {@link DatabaseManager}s referencing the same DB on disk
     *  <br />If the new counter is 0, then the database will be closed.
     *  <br /><br />This needs to be called before application exit.
     * <br />If the counter is 0, then the underlying SQLiteDatabase is <b>null</b> until another DatabaseManager is instantiated or you call {@link #open()}
     *
     * @return true if the underlying {@link android.database.sqlite.SQLiteDatabase} is closed (counter is 0), and false otherwise (counter > 0)
     */
    public boolean close(){
        sqLiteOpenHelper.removeConnection();
        if (sqLiteOpenHelper.getCounter()==0){
            synchronized (lockObject){
                if (db.inTransaction())db.endTransaction();
                if (db.isOpen())db.close();
                db = null;
            }
            return true;
        }
        return false;
    }
    /** Increments the internal db counter by one and opens the db if needed
    *
    */
    public void open(){
        sqLiteOpenHelper.addConnection();
        if (db==null||!db.isOpen()){
                synchronized (lockObject){
                    db = sqLiteOpenHelper.getWritableDatabase();
                }
        } 
    }
}

我对SQLiteDatabase api的理解是，如果你有一个多线程应用程序，你不能有超过1个SQLiteDatabase对象指向单个数据库。

对象当然可以创建，但是如果不同的线程/进程(也)开始使用不同的SQLiteDatabase对象(就像我们在JDBC Connection中使用的那样)，则插入/更新会失败。

这里唯一的解决方案是坚持使用1个SQLiteDatabase对象，无论何时startTransaction()在多个线程中使用，Android管理跨不同线程的锁定，并且一次只允许一个线程具有独占更新访问。

你也可以从数据库中“读取”，并在不同的线程中使用相同的SQLiteDatabase对象(而另一个线程写入)，永远不会有数据库损坏，即“读线程”不会从数据库中读取数据，直到“写线程”提交数据，尽管两者都使用相同的SQLiteDatabase对象。

这与JDBC中的连接对象不同，在JDBC中，如果在读写线程之间传递连接对象(使用相同的方法)，那么我们也可能打印未提交的数据。

在我的企业应用程序中，我尝试使用条件检查，以便UI线程永远不必等待，而BG线程持有SQLiteDatabase对象(独占)。我试图预测UI动作和推迟BG线程从运行'x'秒。还可以维护PriorityQueue来管理分发SQLiteDatabase Connection对象，以便UI线程首先获得它。

德米特罗的回答很适合我。 我认为最好将函数声明为同步的。至少在我的情况下，它会调用空指针异常，否则，例如getWritableDatabase尚未返回在一个线程和opendatabase在另一个线程同时调用。

public synchronized SQLiteDatabase openDatabase() {
    if(mOpenCounter.incrementAndGet() == 1) {
        // Opening new database
        mDatabase = mDatabaseHelper.getWritableDatabase();
    }
    return mDatabase;
}