JDBC

Business Transactions

• The process of retrieving a value from database, changing it and pushing it back to database is termed as a business transaction.
• Business transaction encompass one or more database transactions.
• Following things must be taken care while handling business transactions.
• Inconsistent data must be handled before writing to database.
• If inconsistent data cannot be handled then the database should be rolled back to previous persistent state. 

Transaction Management

• Transactions is a very important topic in various Database related persistent technologies like hibernate and  JPA there transaction management is a separate topic.
• After learning hibernate we will learn Spring in Spring we have a separate topic called as transactions.
• Transactions is a set of related operations which should be executed all together or none should be executed.
• The various properties of a transaction are as follows
• Atomicity
• Consistency
• Isolation
• Durability
• There are 2 types of transactions
• Local Transactions
• All operations are performed on same database
• JDBC only supports local transactions
• Global Transactions
• Operations are performed on multiple databases.
• If we want to go for global transactions we need support from EJB or Spring Framework.
• To implement transactions in JDBC we use 3 methods of Connection Interface.
• con.setAutoCommit(false);
• We need to set autoCommit to false initially.
• con.Commit();
• If all the operations complete successfully we use con.commit() to finalize changes to database.
• con.rollback();
• is used to rollback if any operations fails.
• We also need to catch SQLException in case of any problems while performing operations and call rollback in that block.

Connection Pooling

• It is a method to keep database connection open so that they can be reused.
• Opening a database connection is an expensive process so we keep the existing one opened and ready to use when a database connection is requested.
• Server first checks the pool for a suitable connection if one is available it is given to the client else a new connection is created.
• Rather than getting connection object through DriverManager class,we get them through an instance of the DataSource class.
• DataSource object maintains a pool of persistent database connections in order to eliminate the time consuming step of establishing a database connection each time the application interacts with the database.

Configuring the Data Source

• There are 2 ways to configure a datasource
• First way is to include the library for a DataSource implementation and create DataSource by interacting with the library.
• Second way is to tell the web application container(like Tomcat etc) to create DataSource for us.
• This approach is referred to as container managed connection pooling because the web application container that hosts the web application creates and destroys the data source that implements connection pooling.
• A DataSource is a static property in a servlet
• This helps in DataSource pooling as only one instance is passed to the multiple instances of the servlet.
• Secondly DataSource is already created even before Servlet is created so we do not have to check the creation of Servlet Instance First and then the DataSource.
• When using DataSource Connection pooling, it is important to close connections and their associated objects when finished with them.
• If this is not done the pool of available connections inside the DataSource will become exhausted and application threads will block and wait indefinitely for an available connection.

Object Persistence

• Code devoted to manage the persistent state of objects compromises a large percentage of total code.
• Because this type of code can be generalized libraries have been provided which act as a framework to implement object persistence in their applications.
• These frameworks are referred to as object-relational mapping(ORM) frameworks because they provide the means to move data represented as objects in code into and out of relational databases.
• Two well known open Source ORM frameworks include Hibernate and iBatis