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SQLiteSession


android.database.sqlite

Class SQLiteSession



  • public final class SQLiteSession
    extends Object
    Provides a single client the ability to use a database.

    About database sessions

    Database access is always performed using a session. The session manages the lifecycle of transactions and database connections.

    Sessions can be used to perform both read-only and read-write operations. There is some advantage to knowing when a session is being used for read-only purposes because the connection pool can optimize the use of the available connections to permit multiple read-only operations to execute in parallel whereas read-write operations may need to be serialized.

    When Write Ahead Logging (WAL) is enabled, the database can execute simultaneous read-only and read-write transactions, provided that at most one read-write transaction is performed at a time. When WAL is not enabled, read-only transactions can execute in parallel but read-write transactions are mutually exclusive.

    Ownership and concurrency guarantees

    Session objects are not thread-safe. In fact, session objects are thread-bound. The SQLiteDatabase uses a thread-local variable to associate a session with each thread for the use of that thread alone. Consequently, each thread has its own session object and therefore its own transaction state independent of other threads.

    A thread has at most one session per database. This constraint ensures that a thread can never use more than one database connection at a time for a given database. As the number of available database connections is limited, if a single thread tried to acquire multiple connections for the same database at the same time, it might deadlock. Therefore we allow there to be only one session (so, at most one connection) per thread per database.

    Transactions

    There are two kinds of transaction: implicit transactions and explicit transactions.

    An implicit transaction is created whenever a database operation is requested and there is no explicit transaction currently in progress. An implicit transaction only lasts for the duration of the database operation in question and then it is ended. If the database operation was successful, then its changes are committed.

    An explicit transaction is started by calling beginTransaction(int, android.database.sqlite.SQLiteTransactionListener, int, android.os.CancellationSignal) and specifying the desired transaction mode. Once an explicit transaction has begun, all subsequent database operations will be performed as part of that transaction. To end an explicit transaction, first call setTransactionSuccessful() if the transaction was successful, then call #end. If the transaction was marked successful, its changes will be committed, otherwise they will be rolled back.

    Explicit transactions can also be nested. A nested explicit transaction is started with beginTransaction(int, android.database.sqlite.SQLiteTransactionListener, int, android.os.CancellationSignal), marked successful with setTransactionSuccessful()and ended with endTransaction(android.os.CancellationSignal). If any nested transaction is not marked successful, then the entire transaction including all of its nested transactions will be rolled back when the outermost transaction is ended.

    To improve concurrency, an explicit transaction can be yielded by calling yieldTransaction(long, boolean, android.os.CancellationSignal). If there is contention for use of the database, then yielding ends the current transaction, commits its changes, releases the database connection for use by another session for a little while, and starts a new transaction with the same properties as the original one. Changes committed by yieldTransaction(long, boolean, android.os.CancellationSignal) cannot be rolled back.

    When a transaction is started, the client can provide a SQLiteTransactionListener to listen for notifications of transaction-related events.

    Recommended usage:

     // First, begin the transaction.
     session.beginTransaction(SQLiteSession.TRANSACTION_MODE_DEFERRED, 0);
     try {
         // Then do stuff...
         session.execute("INSERT INTO ...", null, 0);
    
         // As the very last step before ending the transaction, mark it successful.
         session.setTransactionSuccessful();
     } finally {
         // Finally, end the transaction.
         // This statement will commit the transaction if it was marked successful or
         // roll it back otherwise.
         session.endTransaction();
     }
     

    Database connections

    A SQLiteDatabase can have multiple active sessions at the same time. Each session acquires and releases connections to the database as needed to perform each requested database transaction. If all connections are in use, then database transactions on some sessions will block until a connection becomes available.

    The session acquires a single database connection only for the duration of a single (implicit or explicit) database transaction, then releases it. This characteristic allows a small pool of database connections to be shared efficiently by multiple sessions as long as they are not all trying to perform database transactions at the same time.

    Responsiveness

    Because there are a limited number of database connections and the session holds a database connection for the entire duration of a database transaction, it is important to keep transactions short. This is especially important for read-write transactions since they may block other transactions from executing. Consider calling yieldTransaction(long, boolean, android.os.CancellationSignal) periodically during long-running transactions.

    Another important consideration is that transactions that take too long to run may cause the application UI to become unresponsive. Even if the transaction is executed in a background thread, the user will get bored and frustrated if the application shows no data for several seconds while a transaction runs.

    Guidelines:

    • Do not perform database transactions on the UI thread.
    • Keep database transactions as short as possible.
    • Simple queries often run faster than complex queries.
    • Measure the performance of your database transactions.
    • Consider what will happen when the size of the data set grows. A query that works well on 100 rows may struggle with 10,000.

    Reentrance

    This class must tolerate reentrant execution of SQLite operations because triggers may call custom SQLite functions that perform additional queries.

    • Field Detail

      • TRANSACTION_MODE_DEFERRED

        public static final int TRANSACTION_MODE_DEFERRED
        Transaction mode: Deferred.

        In a deferred transaction, no locks are acquired on the database until the first operation is performed. If the first operation is read-only, then a SHARED lock is acquired, otherwise a RESERVED lock is acquired.

        While holding a SHARED lock, this session is only allowed to read but other sessions are allowed to read or write. While holding a RESERVED lock, this session is allowed to read or write but other sessions are only allowed to read.

        Because the lock is only acquired when needed in a deferred transaction, it is possible for another session to write to the database first before this session has a chance to do anything.

        Corresponds to the SQLite BEGIN DEFERRED transaction mode.

        See Also:
        Constant Field Values
      • TRANSACTION_MODE_IMMEDIATE

        public static final int TRANSACTION_MODE_IMMEDIATE
        Transaction mode: Immediate.

        When an immediate transaction begins, the session acquires a RESERVED lock.

        While holding a RESERVED lock, this session is allowed to read or write but other sessions are only allowed to read.

        Corresponds to the SQLite BEGIN IMMEDIATE transaction mode.

        See Also:
        Constant Field Values
      • TRANSACTION_MODE_EXCLUSIVE

        public static final int TRANSACTION_MODE_EXCLUSIVE
        Transaction mode: Exclusive.

        When an exclusive transaction begins, the session acquires an EXCLUSIVE lock.

        While holding an EXCLUSIVE lock, this session is allowed to read or write but no other sessions are allowed to access the database.

        Corresponds to the SQLite BEGIN EXCLUSIVE transaction mode.

        See Also:
        Constant Field Values
    • Constructor Detail

      • SQLiteSession

        public SQLiteSession(SQLiteConnectionPool connectionPool)
        Creates a session bound to the specified connection pool.
        Parameters:
        connectionPool - The connection pool.
    • Method Detail

      • hasTransaction

        public boolean hasTransaction()
        Returns true if the session has a transaction in progress.
        Returns:
        True if the session has a transaction in progress.
      • hasNestedTransaction

        public boolean hasNestedTransaction()
        Returns true if the session has a nested transaction in progress.
        Returns:
        True if the session has a nested transaction in progress.
      • hasConnection

        public boolean hasConnection()
        Returns true if the session has an active database connection.
        Returns:
        True if the session has an active database connection.
      • yieldTransaction

        public boolean yieldTransaction(long sleepAfterYieldDelayMillis,
                               boolean throwIfUnsafe,
                               CancellationSignal cancellationSignal)
        Temporarily ends a transaction to let other threads have use of the database. Begins a new transaction after a specified delay.

        If there are other threads waiting to acquire connections, then the current transaction is committed and the database connection is released. After a short delay, a new transaction is started.

        The transaction is assumed to be successful so far. Do not call setTransactionSuccessful() before calling this method. This method will fail if the transaction has already been marked successful.

        The changes that were committed by a yield cannot be rolled back later.

        Before this method was called, there must already have been a transaction in progress. When this method returns, there will still be a transaction in progress, either the same one as before or a new one if the transaction was actually yielded.

        This method should not be called when there is a nested transaction in progress because it is not possible to yield a nested transaction. If throwIfNested is true, then attempting to yield a nested transaction will throw IllegalStateException, otherwise the method will return false in that case.

        If there is no nested transaction in progress but a previous nested transaction failed, then the transaction is not yielded (because it must be rolled back) and this method returns false.

        Parameters:
        sleepAfterYieldDelayMillis - A delay time to wait after yielding the database connection to allow other threads some time to run. If the value is less than or equal to zero, there will be no additional delay beyond the time it will take to begin a new transaction.
        throwIfUnsafe - If true, then instead of returning false when no transaction is in progress, a nested transaction is in progress, or when the transaction has already been marked successful, throws IllegalStateException.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        True if the transaction was actually yielded.
        Throws:
        IllegalStateException - if throwIfNested is true and there is no current transaction, there is a nested transaction in progress or if setTransactionSuccessful() has already been called for the current transaction.
        SQLiteException - if an error occurs.
        OperationCanceledException - if the operation was canceled.
        See Also:
        beginTransaction(int, android.database.sqlite.SQLiteTransactionListener, int, android.os.CancellationSignal), endTransaction(android.os.CancellationSignal)
      • prepare

        public void prepare(String sql,
                   int connectionFlags,
                   CancellationSignal cancellationSignal,
                   SQLiteStatementInfo outStatementInfo)
        Prepares a statement for execution but does not bind its parameters or execute it.

        This method can be used to check for syntax errors during compilation prior to execution of the statement. If the outStatementInfo argument is not null, the provided SQLiteStatementInfo object is populated with information about the statement.

        A prepared statement makes no reference to the arguments that may eventually be bound to it, consequently it it possible to cache certain prepared statements such as SELECT or INSERT/UPDATE statements. If the statement is cacheable, then it will be stored in the cache for later and reused if possible.

        Parameters:
        sql - The SQL statement to prepare.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        outStatementInfo - The SQLiteStatementInfo object to populate with information about the statement, or null if none.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error.
        OperationCanceledException - if the operation was canceled.
      • execute

        public void execute(String sql,
                   Object[] bindArgs,
                   int connectionFlags,
                   CancellationSignal cancellationSignal)
        Executes a statement that does not return a result.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.
      • executeForLong

        public long executeForLong(String sql,
                          Object[] bindArgs,
                          int connectionFlags,
                          CancellationSignal cancellationSignal)
        Executes a statement that returns a single long result.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        The value of the first column in the first row of the result set as a long, or zero if none.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.
      • executeForString

        public String executeForString(String sql,
                              Object[] bindArgs,
                              int connectionFlags,
                              CancellationSignal cancellationSignal)
        Executes a statement that returns a single String result.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        The value of the first column in the first row of the result set as a String, or null if none.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.
      • executeForBlobFileDescriptor

        public ParcelFileDescriptor executeForBlobFileDescriptor(String sql,
                                                        Object[] bindArgs,
                                                        int connectionFlags,
                                                        CancellationSignal cancellationSignal)
        Executes a statement that returns a single BLOB result as a file descriptor to a shared memory region.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        The file descriptor for a shared memory region that contains the value of the first column in the first row of the result set as a BLOB, or null if none.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.
      • executeForChangedRowCount

        public int executeForChangedRowCount(String sql,
                                    Object[] bindArgs,
                                    int connectionFlags,
                                    CancellationSignal cancellationSignal)
        Executes a statement that returns a count of the number of rows that were changed. Use for UPDATE or DELETE SQL statements.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        The number of rows that were changed.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.
      • executeForLastInsertedRowId

        public long executeForLastInsertedRowId(String sql,
                                       Object[] bindArgs,
                                       int connectionFlags,
                                       CancellationSignal cancellationSignal)
        Executes a statement that returns the row id of the last row inserted by the statement. Use for INSERT SQL statements.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        The row id of the last row that was inserted, or 0 if none.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.
      • executeForCursorWindow

        public int executeForCursorWindow(String sql,
                                 Object[] bindArgs,
                                 CursorWindow window,
                                 int startPos,
                                 int requiredPos,
                                 boolean countAllRows,
                                 int connectionFlags,
                                 CancellationSignal cancellationSignal)
        Executes a statement and populates the specified CursorWindow with a range of results. Returns the number of rows that were counted during query execution.
        Parameters:
        sql - The SQL statement to execute.
        bindArgs - The arguments to bind, or null if none.
        window - The cursor window to clear and fill.
        startPos - The start position for filling the window.
        requiredPos - The position of a row that MUST be in the window. If it won't fit, then the query should discard part of what it filled so that it does. Must be greater than or equal to startPos.
        countAllRows - True to count all rows that the query would return regagless of whether they fit in the window.
        connectionFlags - The connection flags to use if a connection must be acquired by this operation. Refer to SQLiteConnectionPool.
        cancellationSignal - A signal to cancel the operation in progress, or null if none.
        Returns:
        The number of rows that were counted during query execution. Might not be all rows in the result set unless countAllRows is true.
        Throws:
        SQLiteException - if an error occurs, such as a syntax error or invalid number of bind arguments.
        OperationCanceledException - if the operation was canceled.


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