Arkitektur Embedded SQL Tema Persistens Databaser Arkitektur Embedded SQL Tema Persistens

Databasearkitektur Basis valg (constraints): En objekt orienteret tilgangsvinkel Data gemmes i en relationel RDBMS Spørgsmålet er i forbindelse med database arkitekturen: Hvordan kan vi på den ene side adskille implementations detaljer vedr. databasen (sql’en) fra forretningskoden OG stadigvæk bruge faciliteterne I databasen

Problemet Employee String fname; String lname; String ssn; String adress; ……. Employee superssn; Department dno; Objekt model / relationel model Fname Lname Ssn ………. superssn dno Dname Dnumber Mgrssn mgrstartdate

Architecture When mapping between an object model and a relational model – it has to be implemented in the application. There are 3 fundamental strategies: Brute Force Data Access Objects Persistence Framework

Brute Force The fundamental strategy is that the business objects access directly to the data source. It is used when no database layer is available. It is simple an presumes that the programmer has full control of how the business objects interact with the database. This access is the right when the demand for access are straight forward. Will give code with low cohesion and high coupling (difficult to maintain)

Data access objects Data access objects encloses the required database logic from the business objects. One data access class for each business object The class consist of the code, which is necessary for accessing the database (sql code). The advantage is that all sql code is in the data access objects (high cohesion, low coupling) You have to write a DAO for each business class

Persistence framework Seal the database access completely from you business object. In stead of writing the logic of the database access, you define the Meta data which represents the mapping. PF generate the necessary code which enables the business objects to become persistent. You will have to buy a framework

Architecture Layered architecture, Data access objects GUI - Layer Control Layer Model Layer DB Layer

DB layer Consist of : A connection class (implemented as a singelton) An interface for each business class (Interface) One class for each business class

Connection class Implemented using the singleton pattern The singleton pattern ensures that only one instance of a class is created. All objects that uses an instance of that class use the same instance. The purpose is to get a connection to the database

private DbConnection() // private implemented as a Singleton { String url = driver + databaseName; try{ //load af driver Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); } catch(Exception e){ System.out.println("Can not find the driver"); System.out.println(e.getMessage()); }//end catch //connection to the database con = DriverManager.getConnection(url, userName,password); //set autocommit con.setAutoCommit(true); dma = con.getMetaData(); // get meta data System.out.println("Connection to " + dma.getURL()); System.out.println("Driver " + dma.getDriverName()); System.out.println("Database product name " + dma.getDatabaseProductName()); }//end try System.out.println("Problems with the connection to the database"); }//end constructor

//DBConnection cont.. //this method is used to get the instance of the connection public static DbConnection getInstance() { if (instance == null) { instance = new DbConnection(); } return instance;

Interface ……… public interface IFDBEmp { // get all employees public ArrayList getAllEmployees(boolean retriveAssociation); //get one employee having the ssn public Employee findEmployee(String empssn, boolean retriveAssociation); //find one employee having the name public Employee searchEmployeeFname( String fname, boolean retriveAssociation); public Employee searchEmployeeLname( String fname, boolean retriveAssociation); public Employee searchEmployeeSsn( String fname, boolean retriveAssociation); //insert a new employee public void inserEmployee(Employee emp); //update information about an employee public int updateEmployee(Employee emp); }

Company Example DB classes – Private methods singleWhere (is used to retrieve a singel tuple from the database) michWhere ( is used to retrieve multiple tuples from the database) buildquery (build the sql select-statement) build<class> (build the object, with information from the database)

Tilgang til databasen fra Java Databaser Tilgang til databasen fra Java

Database tilgang fra java Database tilgang tilbydes af pakken sql - Vi anvender følgende klasser DriverManager Connection Statement ResultSet DatabaseMetaData ResultSetMetaData

Fremgangsmåde Åben forbindelsen til databasen Udfører SQL for at opdaterer/læse fra databasen Opbyg et string objekt der indeholder sql statment der skal udføres Lav et Statement objekt Udfør statement Behandling af resultatet Luk forbindelsen

Åben forbindelsen Load af driver Skaf connection object // mySQL Class.forName("org.gjt.mm.mysql.Driver"); //SQL Server Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); Skaf connection object con = DriverManager.getConnection(url,bruger,password); url = "jdbc:odbc:company” bruger og password som oprettet under installering Eksempel: klassen DbConnection Databasenavnet

Udfør SQL Opbyg SQL-statement i et String objekt: String query="SELECT * FROM Employee WHERE ssn = " + id; Lav Statement objekt: Statement stmt = con.createStatement(); stmt.setQueryTimeout(5); Udfør ResultSet results = stmt.executeQuery(query); executeUpdate, ved delete, update og insert

Behandling af resultatet Objektet opbygges på baggrund af resultatet af søgningen if( results.next() ){ emp.setFname(results.getString(2)); emp.setAdresse(results.getString(4)); } stmt.close(); results er af typen ResultSet

Luk forbindelsen con.close(); Eksempel: Company

Opsætning af Driver Inden vi kan anvende en database fra Java skal driveren være tilgængelig