vb.net generate qr barcode Overview of the JDBC Process in Java

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Overview of the JDBC Process
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Although each J2ME application is different, J2ME applications use a similar process for interacting with a DBMS This process is divided into five routines: loading the JDBC driver, connecting to the DBMS, creating and executing a statement, processing data returned by the DBMS, and terminating the connection with the DBMS It is sometimes better to get a general understanding of how the process works before delving into the details of each routine of the process Therefore, the next few sections provide an overview of the process and each routine A more detailed discussion of each routine is provided later in this chapter
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JDBC Objects
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Load the JDBC Driver
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The JDBC driver must be loaded before the J2ME application can connect to the DBMS The ClassforName() method is used to load the JDBC driver Suppose a developer wants to work offline and write a J2ME application that interacts with Microsoft Access on the developer s PC The developer must write a routine that loads the JDBC/ODBC Bridge driver called sunjdbcodbcJdbcOdbcDriver The driver is loaded by calling the ClassforName() method and passing it the name of the driver, as shown in the following code segment:
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ClassforName( "sunjdbcodbcJdbcOdbcDriver");
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Connect to the DBMS
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Once the driver is loaded, the J2ME application must connect to the DBMS using the DriverManagergetConnection() method The javasqlDriverManager class is the highest class in the javasql hierarchy and is responsible for managing driver information The DriverManagergetConnection() method is passed the URL of the database, along with the user ID and password if required by the DBMS The URL is a String object that contains the driver name and the name of the database that is being accessed by the J2ME application The DriverManagergetConnection() returns a Connection interface that is used throughout the process to reference the database The javasqlConnection interface is another member of the javasql package that manages communications between the driver and the J2ME application It is the javasqlConnection interface that sends statements to the DBMS for processing Listing 10-1 illustrates the use of the DriverManagergetConnection() method to load the JDBC/ODBC Bridge and connect to the CustomerInformation database
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Listing 10-1 Open a connection with a database
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J2ME DATA MANAGEMENT
String url = "jdbc:odbc:CustomerInformation"; String userID = "jim"; String password = "keogh"; Statement DataRequest; private Connection Db; try { ClassforName( "sunjdbcodbcJdbcOdbcDriver"); Db = DriverManagergetConnection(url,userID,password); }
Create and Execute an SQL Statement
The next step after the JDBC driver is loaded and a connection is successfully made with a particular database managed by the DBMS is to send an SQL query to the DBMS for
J2ME: The Complete Reference
processing An SQL query consists of a series of SQL commands that direct the DBMS to do something, such as return rows of data to the J2ME application You ll learn how to write queries in the next chapter The ConnectcreateStatement() is used to create a Statement object The Statement object is then used to execute a query and return a ResultSet object that contains the response from the DBMS, which is usually one or more rows of information requested by the J2ME application Typically, the query is assigned to a String object, which is passed to the Statement object s executeQuery() method, as illustrated in the next code segment Once the ResultSet is received from the DBMS, the close() method is called to terminate the statement Listing 10-2 retrieves all the rows and columns from the Customers table
Listing 10-2 Retrieve all the rows from the Customers table
Statement DataRequest; ResultSet Results; try { String query = "SELECT * FROM Customers"; DataRequest = DatabasecreateStatement(); DataRequest = DbcreateStatement(); Results = DataRequestexecuteQuery (query); DataRequestclose(); }
Process Data Returned by the DBMS
The javasqlResultSet object is assigned the results received from the DBMS after the query is processed The javasqlResultSet object consists of methods used to interact with data that is returned by the DBMS to the J2ME application Later in this chapter you ll learn the details of using the javasqlResultSet object However, the following code is an abbreviated example that gives you a preview of a commonly used routine for extracting data returned by the DBMS Error-catching code is purposely removed from this example in order to minimize code clutter You ll find the completed version of this routine later in this chapter and throughout 11 Assume for Listing 10-3 that a J2ME application requested a customer s first name and last name from a table The result returned by the DBMS is already assigned to the ResultSet object called Results The first time that the next() method of the ResultSet is called, the ResultSet pointer is positioned at the first row in the ResultSet and returns a boolean value If false, this indicates that no rows are present in the ResultSet The if statement in Listing 10-3 traps this condition and displays the End of data message on the screen A true value returned by the next() method means at least one row of data is present in the ResultSet, which causes the code to enter the do while loop The getString() method of the ResultSet object is used to copy the value of a specified column in the current row of the ResultSet to a String object The getString() method is passed the name of the column in the ResultSet whose content needs to be copied, and the getString() method returns the value from the specified column
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