Saturday, July 5, 2008

JDBC Questions 1

1.What is the JDBC?

Java Database Connectivity (JDBC) is a standard Java API to interact with relational databases form Java. JDBC has set of classes and interfaces which can use from Java application and talk to database without learning RDBMS details and using Database Specific JDBC Drivers.

2.What are the new features added to JDBC 4.0?

The major features added in JDBC 4.0 include :

  • Auto-loading of JDBC driver class
  • Connection management enhancements
  • Support for RowId SQL type
  • DataSet implementation of SQL using Annotations
  • SQL exception handling enhancements
  • SQL XML support
3.Explain Basic Steps in writing a Java program using JDBC?

JDBC makes the interaction with RDBMS simple and intuitive. When a Java application needs to access database :

  • Load the RDBMS specific JDBC driver because this driver actually communicates with the database (Incase of JDBC 4.0 this is automatically loaded).
  • Open the connection to database which is then used to send SQL statements and get results back.
  • Create JDBC Statement object. This object contains SQL query.
  • Execute statement which returns resultset(s). ResultSet contains the tuples of database table as a result of SQL query.
  • Process the result set.
  • Close the connection.

4.Exaplain the JDBC Architecture.

The JDBC Architecture consists of two layers:

  • The JDBC API, which provides the application-to-JDBC Manager connection.
  • The JDBC Driver API, which supports the JDBC Manager-to-Driver Connection.

The JDBC API uses a driver manager and database-specific drivers to provide transparent connectivity to heterogeneous databases. The JDBC driver manager ensures that the correct driver is used to access each data source. The driver manager is capable of supporting multiple concurrent drivers connected to multiple heterogeneous databases. The location of the driver manager with respect to the JDBC drivers and the Java application is shown in Figure 1.

JDBC Architecture

Figure 1: JDBC Architecture

5.What are the main components of JDBC ?

The life cycle of a servlet consists of the following phases:

  • DriverManager: Manages a list of database drivers. Matches connection requests from the java application with the proper database driver using communication subprotocol. The first driver that recognizes a certain subprotocol under JDBC will be used to establish a database Connection.

  • Driver: The database communications link, handling all communication with the database. Normally, once the driver is loaded, the developer need not call it explicitly.

  • Connection : Interface with all methods for contacting a database.The connection object represents communication context, i.e., all communication with database is through connection object only.

  • Statement : Encapsulates an SQL statement which is passed to the database to be parsed, compiled, planned and executed.

  • ResultSet: The ResultSet represents set of rows retrieved due to query execution.

6.How the JDBC application works?

A JDBC application can be logically divided into two layers:

1. Driver layer

2. Application layer

  • Driver layer consists of DriverManager class and the available JDBC drivers.
  • The application begins with requesting the DriverManager for the connection.
  • An appropriate driver is choosen and is used for establishing the connection. This connection is given to the application which falls under the application layer.
  • The application uses this connection to create Statement kind of objects, through which SQL commands are sent to backend and obtain the results.
JDBC Application

Figure 2: JDBC Application

7.How do I load a database driver with JDBC 4.0 / Java 6?

Provided the JAR file containing the driver is properly configured, just place the JAR file in the classpath. Java developers NO longer need to explicitly load JDBC drivers using code like Class.forName() to register a JDBC driver.The DriverManager class takes care of this by automatically locating a suitable driver when the DriverManager.getConnection() method is called. This feature is backward-compatible, so no changes are needed to the existing JDBC code.

8.What is JDBC Driver interface?

The JDBC Driver interface provides vendor-specific implementations of the abstract classes provided by the JDBC API. Each vendor driver must provide implementations of the java.sql.Connection,Statement,PreparedStatement, CallableStatement, ResultSet and Driver.

9.What does the connection object represents?

The connection object represents communication context, i.e., all communication with database is through connection object only.

10.What is Statement ?

Statement acts like a vehicle through which SQL commands can be sent. Through the connection object we create statement kind of objects.
Through the connection object we create statement kind of objects.

Statement stmt = conn.createStatement();

This method returns object which implements statement interface.

11.What is PreparedStatement?

A prepared statement is an SQL statement that is precompiled by the database. Through precompilation, prepared statements improve the performance of SQL commands that are executed multiple times (given that the database supports prepared statements). Once compiled, prepared statements can be customized prior to each execution by altering predefined SQL parameters.

 PreparedStatement pstmt = conn.prepareStatement("UPDATE EMPLOYEES SET SALARY = ? WHERE ID = ?");
pstmt.setBigDecimal(1, 153833.00);
pstmt.setInt(2, 110592);

Here: conn is an instance of the Connection class and "?" represents parameters.These parameters must be specified before execution.

12.What is the difference between a Statement and a PreparedStatement?

Statement PreparedStatement
A standard Statement is used to create a Java representation of a literal SQL statement and execute it on the database. A PreparedStatement is a precompiled statement. This means that when the PreparedStatement is executed, the RDBMS can just run the PreparedStatement SQL statement without having to compile it first.
Statement has to verify its metadata against the database every time. While a prepared statement has to verify its metadata against the database only once.
If you want to execute the SQL statement once go for STATEMENT If you want to execute a single SQL statement multiple number of times, then go for PREPAREDSTATEMENT. PreparedStatement objects can be reused with passing different values to the queries

13.What are callable statements ?

Callable statements are used from JDBC application to invoke stored procedures and functions.

14.How to call a stored procedure from JDBC ?

PL/SQL stored procedures are called from within JDBC programs by means of the prepareCall() method of the Connection object created. A call to this method takes variable bind parameters as input parameters as well as output variables and creates an object instance of the CallableStatement class.

The following line of code illustrates this:

   CallableStatement stproc_stmt = conn.prepareCall("{call procname(?,?,?)}");

Here conn is an instance of the Connection class.

15.What are types of JDBC drivers?

There are four types of drivers defined by JDBC as follows:

  • Type 1: JDBC/ODBC—These require an ODBC (Open Database Connectivity) driver for the database to be installed. This type of driver works by translating the submitted queries into equivalent ODBC queries and forwards them via native API calls directly to the ODBC driver. It provides no host redirection capability.

  • Type2: Native API (partly-Java driver)—This type of driver uses a vendor-specific driver or database API to interact with the database. An example of such an API is Oracle OCI (Oracle Call Interface). It also provides no host redirection.

  • Type 3: Open Protocol-Net—This is not vendor specific and works by forwarding database requests to a remote database source using a net server component. How the net server component accesses the database is transparent to the client. The client driver communicates with the net server using a database-independent protocol and the net server translates this protocol into database calls. This type of driver can access any database.

  • Type 4: Proprietary Protocol-Net(pure Java driver)—This has a same configuration as a type 3 driver but uses a wire protocol specific to a particular vendor and hence can access only that vendor's database. Again this is all transparent to the client.

Note: Type 4 JDBC driver is most preferred kind of approach in JDBC.

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