Tuesday, April 3, 2007

Introduction to JDBC

JDBC is an API for the Java programming language that defines how a client may access a database. It provides methods for querying and updating data in a database. JDBC is oriented towards relational databases.

The Java Platform, Standard Edition includes the JDBC API together with an ODBC implementation of the API enabling connections to any relational database that supports ODBC. This driver is native code and not Java, and is closed source।

JDBC has been part of the Java Standard Edition since the release of JDK 1.1. The JDBC classes are contained in the Java package java.sql. Starting with version 3.0, JDBC has been developed under the Java Community Process. JSR 54 specifies JDBC 3.0 (included in J2SE 1.4), JSR 114 specifies the JDBC Rowset additions, and JSR 221 is the specification of JDBC 4.0 (included in Java SE 6).

JDBC allows multiple implementations to exist and be used by the same application. The API provides a mechanism for dynamically loading the correct Java packages and registering them with the JDBC Driver Manager. The Driver Manager is used as a connection factory for creating JDBC connections.

JDBC connections support creating and executing statements. These statements may be update statements such as SQL CREATE, INSERT, UPDATE and DELETE or they may be query statements using the SELECT statement. Additionally, stored procedures may be invoked through a statement. Statements are one of the following types:

  • Statement – the statement is sent to the database server each and every time.
  • PreparedStatement – the statement is cached and then the execution path is pre determined on the database server allowing it to be executed multiple times in an efficient manner.
  • CallableStatement – used for executing stored procedures on the database.

Update statements such as INSERT, UPDATE and DELETE return an update count that indicates how many rows were affected in the database. These statements do not return any other information.

Query statements return a JDBC row result set. The row result set is used to walk over the result set. Individual columns in a row are retrieved either by name or by column number. There may be any number of rows in the result set. The row result set has metadata that describes the names of the columns and their types.

There is an extension to the basic JDBC API in the javax.sql package that allows for scrollable result sets and cursor support among other things.

The method Class.forName(String) is used to load the JDBC driver class. The line below causes the JDBC driver from some jdbc vendor to be loaded into the application. (Some JVMs also require the class to be instantiated with .newInstance().)

Class.forName( "com.somejdbcvendor.TheirJdbcDriver" );

When a Driver class is loaded, it creates an instance of itself and registers it with the DriverManager. This can be done by including the needed code in the driver class's static block. e.g. DriverManager.registerDriver(Driver driver)

Now when a connection is needed, one of the DriverManager.getConnection() methods is used to create a JDBC connection.

Connection conn = DriverManager.getConnection(
    "jdbc:somejdbcvendor:other data needed by some jdbc vendor",
    "myLogin",
    "myPassword" );

The URL used is dependent upon the particular JDBC driver. It will always begin with the "jdbc:" protocol, but the rest is up to the particular vendor. Once a connection is established, a statement must be created.

Statement stmt = conn.createStatement();
try {
   stmt.executeUpdate( "INSERT INTO MyTable( name ) VALUES ( 'my name' ) " );
} finally {
   //It's important to close the statement when you are done with it
   stmt.close();
}

Note that connections, statements, and resultsets often tie up operating system resources such as sockets or file descriptors. In the case of connections to remote database servers, further resources are tied up on the server, eg. cursors for currently open resultsets. It is vital to close() any JDBC object as soon as it has played its part; garbage collection shall not be relied upon. Forgetting to close() things properly results in spurious errors and misbehaviour. The above try-finally construct is a recommended code pattern to use with JDBC objects.

Data is retrieved from the database using a database query mechanism. The example below shows creating a statement and executing a query.

Statement stmt = conn.createStatement();
try {
   ResultSet rs = stmt.executeQuery( "SELECT * FROM MyTable" );
   try {
       while ( rs.next() ) {
           int numColumns = rs.getMetaData().getColumnCount();
           for ( int i = 1 ; i <= numColumns ; i++ ) {
              //Column numbers start at 1.
              //Also there are many methods on the result set to return
              // the column as a particular type. Refer to the Sun documentation
              // for the list of valid conversions.
              System.out.println( "COLUMN " + i + " = " + rs.getObject(i) );
           }
       }
   } finally {
       rs.close();
   }
} finally {
   stmt.close();
}

Typically, however, it would be rare for a seasoned Java programmer to code in such a fashion. The usual practice would be to abstract the database logic into an entirely different class and to pass preprocessed strings (perhaps derived themselves from a further abstracted class) containing SQL statements and the connection to the required methods. Abstracting the data model from the application code makes it more likely that changes to the application and data model can be made independently.

An example of a PreparedStatement query. Using conn and class from first example.

   PreparedStatement ps = conn.prepareStatement( "SELECT i.*, j.* FROM Omega i, Zappa j"
    + "WHERE i = ? AND j = ?" );
  try {
    // In the prepared statement ps, the question mark denotes variable input,
    // which can be passed through a parameter list, for example.
    // The following replaces the question marks,
    // with the string or int, before sending it to SQL.
    // The first parameter corresponds to the first occurrence of the ?,
    // the second parameter tells Java to replace it with
    // the second item.
    // The nth parameter corresponds to the nth ?
    ps.setString(1, "Poor Yorick");
    ps.setInt(2, 8008);
    // The ResultSet rs, receives the SQL Query response.
    ResultSet rs = ps.executeQuery();
    try {
      while ( rs.next() ) {
        int numColumns = rs.getMetaData().getColumnCount();
        for ( int i = 1 ; i <= numColumns ; i++ ) {
          //Column numbers start at 1.
          //Also there are many methods on the result set to return
          // the column as a particular type. Refer to the Sun documentation
          // for the list of valid conversions.
          System.out.println( "COLUMN " + i + " = " + rs.getObject(i) );
        } // for
      } // while
    } finally {
      rs.close();
    }
  } finally {
    ps.close;
  } // try

When a database operation fails, an SQLException is raised. There is typically very little one could do to recover from such an error, apart from logging it with as much detail as possible. It is recommended that the SQLException be translated into an application domain exception (an unchecked one) that eventually results in a transaction rollback and a notification to the user.

Here are examples of host database types, Java can convert to with a function.

setXXX()Methods

Oracle Datatype setXXX()
CHAR
setString()
VARCHAR2
setString()
NUMBER
setBigDecimal()
setBoolean()
setByte()
setShort()
setInt()
setLong()
setFloat()
setDouble()
INTEGER setInt()
FLOAT setDouble()
CLOB setClob()
BLOB setBlob()
RAW
setBytes()
LONGRAW
setBytes()
DATE
setDate()
setTime()
setTimestamp()

For an example of a CallableStatement (to call stored procedures in the database), see the JDBC API Guide.

Drivers are client-side adaptors (they are installed in the client machine, not in the server) that convert requests from Java programs to a protocol that the DBMS can understand.

There are commercial and free drivers available for most relational database servers. These drivers fall into one of the following types:

  • Type 1, the JDBC-ODBC bridge
  • Type 2, the Native-API driver
  • Type 3, the network-protocol driver
  • Type 4, the native-protocol drivers
  • Internal JDBC driver, driver embedded with JRE in Java-enabled SQL databases. Used for Java stored procedures.
  • JDBC URL, all Database Connection String

  • SQLSummit.com publishes list of drivers, including JDBC drivers and vendors
  • Sun Microsystems provides a list of some JDBC drivers and vendors
  • Simba Technologies ships an SDK for building custom JDBC Drivers for any custom/proprietary relational data source
  • DataDirect Technologies provides a comprehensive suite of fast Type 4 JDBC drivers for all major database
  • IDS Software provides a Type 3 JDBC driver for concurrent access to all major databases. Supported features include resultset caching, SSL encryption, custom data source, dbShield.
  • i-net software provides fast Type 4 JDBC drivers for all major databases
  • OpenLink Software ships JDBC Drivers for a number of target databases, including Bridges to other data access mechanisms (e.g., ODBC, JDBC) which can provide more functionality than the targeted mechanism
  • JDBaccess is a Java persistence library for MySQL and Oracle which defines major database access operations in an easy usable API above JDBC

Posted by themaster at 1:13 AM

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