Module java.sql

# Package javax.sql

package javax.sql
Provides the API for server side data source access and processing from the Java programming language. This package supplements the `java.sql` package and, as of the version 1.4 release, is included in the Java Platform, Standard Edition (Java SE). It remains an essential part of the Java Platform, Enterprise Edition (Java EE).

The `javax.sql` package provides for the following:

1. The `DataSource` interface as an alternative to the `DriverManager` for establishing a connection with a data source
2. Connection pooling and Statement pooling
3. Distributed transactions
4. Rowsets

Applications use the `DataSource` and `RowSet` APIs directly, but the connection pooling and distributed transaction APIs are used internally by the middle-tier infrastructure.

## Using a `DataSource` Object to Make a Connection

The `javax.sql` package provides the preferred way to make a connection with a data source. The `DriverManager` class, the original mechanism, is still valid, and code using it will continue to run. However, the newer `DataSource` mechanism is preferred because it offers many advantages over the `DriverManager` mechanism.

These are the main advantages of using a `DataSource` object to make a connection:

- Changes can be made to a data source's properties, which means that it is not necessary to make changes in application code when something about the data source or driver changes.
- Connection and Statement pooling and distributed transactions are available through a `DataSource` object that is implemented to work with the middle-tier infrastructure. Connections made through the `DriverManager` do not have connection and statement pooling or distributed transaction capabilities.

Driver vendors provide `DataSource` implementations. A particular `DataSource` object represents a particular physical data source, and each connection the `DataSource` object creates is a connection to that physical data source.

A logical name for the data source is registered with a naming service that uses the Java Naming and Directory Interface (JNDI) API, usually by a system administrator or someone performing the duties of a system administrator. An application can retrieve the `DataSource` object it wants by doing a lookup on the logical name that has been registered for it. The application can then use the `DataSource` object to create a connection to the physical data source it represents.

A `DataSource` object can be implemented to work with the middle tier infrastructure so that the connections it produces will be pooled for reuse. An application that uses such a `DataSource` implementation will automatically get a connection that participates in connection pooling. A `DataSource` object can also be implemented to work with the middle tier infrastructure so that the connections it produces can be used for distributed transactions without any special coding.

## Connection Pooling and Statement Pooling

Connections made via a `DataSource` object that is implemented to work with a middle tier connection pool manager will participate in connection pooling. This can improve performance dramatically because creating new connections is very expensive. Connection pooling allows a connection to be used and reused, thus cutting down substantially on the number of new connections that need to be created.

Connection pooling is totally transparent. It is done automatically in the middle tier of a Java EE configuration, so from an application's viewpoint, no change in code is required. An application simply uses the `DataSource.getConnection` method to get the pooled connection and uses it the same way it uses any `Connection` object.

The classes and interfaces used for connection pooling are:

- `ConnectionPoolDataSource`
- `PooledConnection`
- `ConnectionEvent`
- `ConnectionEventListener`
- `StatementEvent`
- `StatementEventListener`

The connection pool manager, a facility in the middle tier of a three-tier architecture, uses these classes and interfaces behind the scenes. When a `ConnectionPoolDataSource` object is called on to create a `PooledConnection` object, the connection pool manager will register as a `ConnectionEventListener` object with the new `PooledConnection` object. When the connection is closed or there is an error, the connection pool manager (being a listener) gets a notification that includes a `ConnectionEvent` object.

If the connection pool manager supports `Statement` pooling, for `PreparedStatements`, which can be determined by invoking the method `DatabaseMetaData.supportsStatementPooling`, the connection pool manager will register as a `StatementEventListener` object with the new `PooledConnection` object. When the `PreparedStatement` is closed or there is an error, the connection pool manager (being a listener) gets a notification that includes a `StatementEvent` object.

## Distributed Transactions

As with pooled connections, connections made via a `DataSource` object that is implemented to work with the middle tier infrastructure may participate in distributed transactions. This gives an application the ability to involve data sources on multiple servers in a single transaction.

The classes and interfaces used for distributed transactions are:

- `XADataSource`
- `XAConnection`

These interfaces are used by the transaction manager; an application does not use them directly.

The `XAConnection` interface is derived from the `PooledConnection` interface, so what applies to a pooled connection also applies to a connection that is part of a distributed transaction. A transaction manager in the middle tier handles everything transparently. The only change in application code is that an application cannot do anything that would interfere with the transaction manager's handling of the transaction. Specifically, an application cannot call the methods `Connection.commit` or `Connection.rollback`, and it cannot set the connection to be in auto-commit mode (that is, it cannot call `Connection.setAutoCommit(true)`).

An application does not need to do anything special to participate in a distributed transaction. It simply creates connections to the data sources it wants to use via the `DataSource.getConnection` method, just as it normally does. The transaction manager manages the transaction behind the scenes. The `XADataSource` interface creates `XAConnection` objects, and each `XAConnection` object creates an `XAResource` object that the transaction manager uses to manage the connection.

## Rowsets

The `RowSet` interface works with various other classes and interfaces behind the scenes. These can be grouped into three categories.

1. Event Notification
   - `RowSetListener`
     A `RowSet` object is a JavaBeans component because it has properties and participates in the JavaBeans event notification mechanism. The `RowSetListener` interface is implemented by a component that wants to be notified about events that occur to a particular `RowSet` object. Such a component registers itself as a listener with a rowset via the `RowSet.addRowSetListener` method.

     When the `RowSet` object changes one of its rows, changes all of it rows, or moves its cursor, it also notifies each listener that is registered with it. The listener reacts by carrying out its implementation of the notification method called on it.

   - `RowSetEvent`
     As part of its internal notification process, a `RowSet` object creates an instance of `RowSetEvent` and passes it to the listener. The listener can use this `RowSetEvent` object to find out which rowset had the event.
2. Metadata
   - `RowSetMetaData`
     This interface, derived from the `ResultSetMetaData` interface, provides information about the columns in a `RowSet` object. An application can use `RowSetMetaData` methods to find out how many columns the rowset contains and what kind of data each column can contain.

     The `RowSetMetaData` interface provides methods for setting the information about columns, but an application would not normally use these methods. When an application calls the `RowSet` method `execute`, the `RowSet` object will contain a new set of rows, and its `RowSetMetaData` object will have been internally updated to contain information about the new columns.

3. The Reader/Writer Facility
   A `RowSet` object that implements the `RowSetInternal` interface can call on the `RowSetReader` object associated with it to populate itself with data. It can also call on the `RowSetWriter` object associated with it to write any changes to its rows back to the data source from which it originally got the rows. A rowset that remains connected to its data source does not need to use a reader and writer because it can simply operate on the data source directly.
   - `RowSetInternal`
     By implementing the `RowSetInternal` interface, a `RowSet` object gets access to its internal state and is able to call on its reader and writer. A rowset keeps track of the values in its current rows and of the values that immediately preceded the current ones, referred to as the original values. A rowset also keeps track of (1) the parameters that have been set for its command and (2) the connection that was passed to it, if any. A rowset uses the `RowSetInternal` methods behind the scenes to get access to this information. An application does not normally invoke these methods directly.
   - `RowSetReader`
     A disconnected `RowSet` object that has implemented the `RowSetInternal` interface can call on its reader (the `RowSetReader` object associated with it) to populate it with data. When an application calls the `RowSet.execute` method, that method calls on the rowset's reader to do much of the work. Implementations can vary widely, but generally a reader makes a connection to the data source, reads data from the data source and populates the rowset with it, and closes the connection. A reader may also update the `RowSetMetaData` object for its rowset. The rowset's internal state is also updated, either by the reader or directly by the method `RowSet.execute`.
   - `RowSetWriter`
     A disconnected `RowSet` object that has implemented the `RowSetInternal` interface can call on its writer (the `RowSetWriter` object associated with it) to write changes back to the underlying data source. Implementations may vary widely, but generally, a writer will do the following:
     - Make a connection to the data source
     - Check to see whether there is a conflict, that is, whether a value that has been changed in the rowset has also been changed in the data source
     - Write the new values to the data source if there is no conflict
     - Close the connection

The `RowSet` interface may be implemented in any number of ways, and anyone may write an implementation. Developers are encouraged to use their imaginations in coming up with new ways to use rowsets.

## Package Specification

- JDBC 4.3 Specification

## Related Documentation

The Java Series book published by Addison-Wesley Longman provides detailed information about the classes and interfaces in the `javax.sql` package:

- “JDBC™API Tutorial and Reference, Third Edition”

- Related Packages
  Module
  Package
  Description
  java.sql.rowset
  javax.sql.rowset
  Standard interfaces and base classes for JDBC `RowSet` implementations.
- Class
  Description
  CommonDataSource
  Interface that defines the methods which are common between `DataSource`, `XADataSource` and `ConnectionPoolDataSource`.
  ConnectionEvent
  An `Event` object that provides information about the source of a connection-related event.
  ConnectionEventListener
  An object that registers to be notified of events generated by a `PooledConnection` object.
  ConnectionPoolDataSource
  A factory for `PooledConnection` objects.
  DataSource
  A factory for connections to the physical data source that this `DataSource` object represents.
  PooledConnection
  An object that provides hooks for connection pool management.
  PooledConnectionBuilder
  A builder created from a `ConnectionPoolDataSource` object, used to establish a connection to the database that the `data source` object represents.
  RowSet
  The interface that adds support to the JDBC API for the JavaBeans component model.
  RowSetEvent
  An `Event` object generated when an event occurs to a `RowSet` object.
  RowSetInternal
  The interface that a `RowSet` object implements in order to present itself to a `RowSetReader` or `RowSetWriter` object.
  RowSetListener
  An interface that must be implemented by a component that wants to be notified when a significant event happens in the life of a `RowSet` object.
  RowSetMetaData
  An object that contains information about the columns in a `RowSet` object.
  RowSetReader
  The facility that a disconnected `RowSet` object calls on to populate itself with rows of data.
  RowSetWriter
  An object that implements the `RowSetWriter` interface, called a writer.
  StatementEvent
  A `StatementEvent` is sent to all `StatementEventListener`s which were registered with a `PooledConnection`.
  StatementEventListener
  An object that registers to be notified of events that occur on PreparedStatements that are in the Statement pool.
  XAConnection
  An object that provides support for distributed transactions.
  XAConnectionBuilder
  A builder created from a `XADataSource` object, used to establish a connection to the database that the `data source` object represents.
  XADataSource
  A factory for `XAConnection` objects that is used internally.