Information AboutJdbc |
| CATEGORIES ABOUT JAVA DATABASE CONNECTIVITY | |
| java platform | |
| java specification requests | |
| application programming interfaces | |
| sql data access | |
| databases | |
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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 . {Link without Title} OVERVIEW 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 . 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:
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 package that allows for scrollable result sets and cursor support among other things. EXAMPLE The method 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 .) Class.forName( "com.somejdbcvendor.TheirJdbcDriver" ); In JDBC 4.0, it's no longer necessary to explicitly load JDBC drivers using Class.forName(). See JDBC 4.0 Enhancements in Java SE 6 When a class is loaded, it creates an instance of itself and registers it with the . 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 should 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 {
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.
+ "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 is raised. There is typically very little one can 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. For an example of a CallableStatement (to call stored procedures in the database), see the .JDBC DRIVERS JDBC Drivers are client-side adaptors (they are installed on the client machine, not on the server) that convert requests from Java programs to a protocol that the DBMS can understand. Types There are commercial and free drivers available for most relational database servers. These drivers fall into one of the following types:
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