PERSİSTENCE OBJECT RELATİON MAPPİNG

Transcription

1 PERSİSTENCE Most of the applications require storing and retrieving objects in a persistent storage mechanism. This chapter introduces how to store and retrieve information in a persistent storage with JDBC. In fact, there are many free open source persistence frameworks that provides an abstraction of underlying storage mechanisms. For example, Hibernate ( is the widely used framework which provides such functionality. Therefore, instead of providing persistence related functionality by yourself, one of these frameworks could be used easily. Persistent objects are the objects which can be stored and retrieved to/from a persistent storage. There are three broad categories for persistent storage: Object Databases: The storage mechanism allows storing and retrieving information as objects. Therefore, there is no need for additional persistence related functionality. Relational Databases: This is the widely used choice. Such persistent storages are record-oriented. Therefore, an object-relation mapping service is required to store and retrieve objects in such storage. That is, individual records should be translated to objects and vice versa before using them. Other: Flat files or XML structures, and other storage structure can be used to store and retrieve objects. This chapter introduces basic mechanisms to store and retrieve objects in a relational database. A fully-fledged object relation mapping mechanism is not covered in this chapter. Instead, how to access information stored and how to store information in a relational database by using JDBC API is explained. OBJECT RELATİON MAPPİNG Relational databases are usually maintained by a database server (e.g., Oracle, Microsoft SQL Server) or a specially build API. If a database maintained by a database server is concerned, the server maintains a set of databases, each of which contains many Tables. Each table contains a set of records with the same set of fields. Therefore, a table could be seen as a matrix of records and fields. In order to store and retrieve instances of a class to the database elements, a mapping is required. Usually instances of the same class are mapped to a single table in a database. Then each row in a table corresponds to an instance of the related class. Attributes of primitive data types in each object are also mapped to fields of the table. In the following a simple mapping is illustrated.

2 In order to have a consistent of relating objects to records, an object identifier is attached to each record and object. The identifier is usually an alphanumeric value which is unique within the table. This identifier will also play primary key for the records within the table. Primary keys are used to refer to a unique record in a table. Note: In order to map the attributes of complex types (i.e., reference to other objects) to fields in a table, the referred object s object identifier could be used. This reference will play a foreign key role in the table (the field in the record points to another record in another table). DATABASE ACCESS WİTH JDBC JDBC API which is included in the Java Development Kit (JDK) allows access to various database systems simple and uniform way. In order to use JDBC, the JDBC API should have been installed in the development platform. In order to access a database management system (DBMS), a suitable driver should first be loaded into the program. There are drivers available for several DMBSs. If a specific DBMS is to be used, a JDBC driver written for that DBMS could be used. Most

3 driver installations consist of copying the driver onto the development platform without any configuration. It is also possible to access databases configured by ODBC using sun.jdbc.odbc.jdbcodbcdriver bridge driver. Drivers provide an interface to open a connection with the DBMS. By using the provided Connection interface, SQL commands could be sent to the DBMS by creating and executing Statements. The results of these commands will be returned after the statement execution. If the command returns a set of records, the result will be a Resultset object which allows navigation within the records returned. ESTABLİSHİNG A CONNECTİON Before sending commands a connection should be established with the database. In order to establish a connection DriverManager and Connection classes in java.sql package could be used. To connect to a database with DriverManager class, the driver should first be registered. The registration could be made simply by issuing: Class.forName(className); The classname could be learned from the driver s documentation. For example, JDBC-ODBC bridge driver could be loaded as: Class.forName( sun.jdbc.odbc.jdbcodbcdriver ); After loading the driver, a connection could be established by specifying the url of the database to connect to, a user name, and a password as DriverManager.getConnection(url, username, userpassword); For example, a connection to ODBC data source mydb could be established as: Connection con = DriverManager.getConnection("jdbc:odbc:myDB", user, password); How to specify driver url and additional properties usually explained in driver documentations. The additional properties may include the datasource (or database name within a DBMS), user name, password, and other DBMS specific configuration. CREATİNG AND EXECUTİNG STATEMENTS Through a connection SQL statements could be sent to a database. By using these commands database schema could be modified, or data in the tables could be manipulated. The commands to modify database schema include CREATE, ALTER, and DROP statements. The commands to modify data in tables include SELECT, INSERT, UPDATE, and DELETE statements. Information about these commands could be obtained from SQL specification and DBMS documentation. In order to execute a command, a Statement instance should first be obtained from the Connection instance as: Statement statement = con.getstatement();

4 Then a command could be supplied to the Statement instance and the Statement could be instructed to execute the command. Two commonly used execute methods are executequery(commandstring) and executeupdate(commandstring). If the command to be execute will not return records from the database, executeupdate could be called. executeupdate returns the number of records affected from the executed command. executequery should be used when the command to be executed will produce a set of records. executequery command will return a Resultset object which contains the records and fields returned by the command executed. In the following, three example commands to create a table and insert a record to the database connected and retrieve the record inserted from the database are given. statement.executeupdate( CREATE TABLE Persons ( "person_id INT, person_name VARCHAR(20)," + " person_surname VARCHAR(20))"); statement.executeupdate("insert INTO Persons(" + " person_id, person_name, person_surname)" + " VALUES(203, 'Bob', 'Brown')"); Resultset res = statement.executequery("select * FROM Persons"); WORKİNG WİTH RESULTSETS If the command to be executed will return a set of records from the database, executequery method of the Statement should be used. A call to executequery returns a Resultset object which allows navigation in the records returned. In order to retrieve individual field s values in a record returned, the cursor within the Resultset should be positioned and the value of each field should be retrieved by an appropriate getter method. In order to position the cursor, first(), next(), previous(), last(), absolute(recordno), etc. methods of the Recordset class could be used. Method first() next() previous() last() absolute(int) Explanation Moves cursor to the first record in the Resultset Moves cursor to the next record (relative to current cursor position) in the Resultset Moves cursor to the previous record (relative to current cursor position) in the Resultset Moves cursor to the last record in the Resultset Moves cursor to the record at the specified position In order to retrieve values of the fields in the current record, the getter method corresponding to the type of data in the field should be used. Some of these getter methods are given in the following table.

5 JDBC Data Type of the field Appropriate Getter Method TINYINT getbyte SMALLINT getshort INTEGER getint BIGINT getlong REAL getfloat FLOAT getdouble DOUBLE getdouble DECIMAL getbigdecimal NUMERIC getbigdecimal BIT getboolean CHAR getstring VARCHAR getstring BINARY getbytes VARBINARY getbytes DATE getdate TIME gettime TIMESTAMP gettimestamp