Designing a Persistence Framework

Transcription

1 Designing a Persistence Framework Working directly with code that uses JDBC is low-level data access; As application developers, one is more interested in the business problem that requires this data access. What we d really like is a framework relieving us of this low-level drudgery. However, how is easy is to build such a framework? There are excellent free, robust, industrial-strength open source persistence frameworks (based on JDBC). For example, Hibernate is very widely used in the Java domain ( It solves most or all problems in object-relational mapping, performance, transaction support, and so forth. The persistence framework is presented here to introduce key concepts in designing such a framework.

2 Persistence Framework First Ideas Mapping There must be some mapping between a class and its persistent store (for example, a table in a database), and between object attributes and the fields in a record. Database mapper A Pure Fabrication database mapper is responsible for retrieving and storing objects of a class.

3 Pattern: Representing Objects as Tables Define a table in an RDB for each persistent object class. Object attributes containing primitive data types (number, string, boolean, and so on) map to columns. name city Manufacturer : Manufacturer name = Now&Zen city = Mumbai MANUFACTURER TABLE name Now&Zen Celestial Shortening city Mumbai San Ramon

4 The problem of identity Java objects define two different notions of sameness : Object identity (roughly equivalent to memory location, checked with a==b) Equality as determined by the implementation of the equals()method (also called equality by value) On the other hand, the identity of a database row is expressed as the primary key value. Neither equals()nor == is naturally equivalent to the primary key value. It s common for several (nonidentical) objects to simultaneously represent the same row of the database, for example, in concurrently running application threads. We might change in a thread the value of attributes with the result the equals() to not work properly.

5 Patterns: Object Identifier It s recommended we use surrogate keys whenever we can t find a good natural key A surrogate key column is a primary key column with no meaning to the user In other words, a key that is not presented to the user and only used for identification of data inside the software system. Object Identifier pattern proposes assigning an object identifier (OID) to each record and object. An OID is usually an alphanumeric value; each is unique to a specific object. Every table will have an OID as primary key, and each object will also have an OID.

6 Manufacturer city name oid : OID : Manufacturer city = Mumbai name = Now&Zen oid = xyz123 MANUFACTURER TABLE OID xyz123 abc345 name Now&Zen Celestial Shortening city Mumbai San Ramon primary key aggregate signifies a referential constraint: a ProductDescription row can't exist without a related Manufacturer row «Table» Manufacturer «PK» OID : char(16) Name : varchar(100) City : varchar(50) 1 PK - primary key FK - foreign key * «Table» ProductDescription «PK» OID : char(16) Description : varchar(100) «FK» Manu_OID : char(16)

7 Accessing a Persistence Service with a Facade PersistenceFacade 1 : SomeClass 1 : PersistenceFacade getinstance() : PersistenceFacade pd = get() get( OID, Class ) : Object put( OID, Object ) // example use of the facade OID oid = new OID("XYZ123"); ProductDescription pd = (ProductDescription) PersistenceFacade.getInstance().get( oid, ProductDescription.class );

8 Mapping Objects: Database Mapper Pattern It proposes making a class that is responsible for materialization, dematerialization, and object caching. A different mapper class is defined for each persistent object class.

9 In UML UML notation: This is a qualified assocation. It means: 1. There is a 1-M association from PersistenceFacade to IMapper objects. 2. With a key of type Class, an IMapper is found (e.g., via a HashMap lookup) PersistenceFacade getinstance() : PersistenceFacade get( OID, Class ) : Object put( OID, Object ) 1 Class 1 «interface» IMapper get(oid) : Object put( OID, Object ) note that the Class as a parameter is no longer needed in this version of get, as the class is "hardwired" for a particular persistent type ProductSpecification Mapper get( OID) : Object put( OID, Object ) Manufacturer Mapper get( OID) : Object put( OID, Object ) each mapper gets and puts objects in its own unique way, depending on the kind of data store and format

10 PersistenceFacade public class PersistenceFacade { public Object get(int oid, Class class) { IMapper mapper = (IMapper) themapperfactory.getmapper(class); // delegate return mapper.get( oid ); public void put(object o) { IMapper mapper = (IMapper) themapperfactory.getmapper(o.getclass()); // delegate mapper.put(o);

11 PersistenceFacade (Cont ed) public class PersistenceFacade { //Finds all the objects maching the template. public List find(object template) { IMapper mapper = (IMapper) themapperfactory.getmapper(template.getclass()); // delegate return mapper.find(template); public void delete (Object o) { //add code

12 MapperFactory public class MapperFactory { public IMapper getmapper (Class class) { if (class.getname().equals("bookstore.bookdescription")) return BookDescriptionMapper.getInstance(); if (class.getname().equals("bookstore.review")) return ReviewMapper.getInstance(); return null; //Better to be done with a hashmap

13 IMapper public interface IMapper { public Object get(int oid); public void put(object o); public int getmaxid(); public java.util.list find(object template);

14 BookDescriptionMapper public class BookDescriptionMapper implements IMapper { private static BookDescriptionMapper instance = null; private ConnectionManager theconnectionmanager; private BookDescriptionMapper() { this.theconnectionmanager = ConnectionManager.getInstance(); public static BookDescriptionMapper getinstance() { if (instance == null) instance = new BookDescriptionMapper(); return instance;

15 public Object get(int oid) { Connection conn = theconnectionmanager.getconnection(); try { Statement stmt = conn.createstatement(); ResultSet rset = stmt.executequery( "SELECT * " + "FROM BookDescription " + "WHERE id = " + oid); rset.next(); BookDescription bd = new BookDescription( rset.getstring("title"), rset.getstring("author"), rset.getstring("isbn"), rset.getstring("publisher"), rset.getint("price") ); stmt.close(); theconnectionmanager.returnconnection(conn); return bd; catch (SQLException ex) { theconnectionmanager.returnconnection(conn); return null;

16 synchronized public void put(object o) { if (! (o instanceof BookDescription) ) return; BookDescription bd = (BookDescription) o; bd.setid (getmaxid()+1); Connection conn = theconnectionmanager.getconnection(); try { PreparedStatement stmt = conn.preparestatement( "INSERT INTO BookDescription (id, title, author, ISBN, publisher, price) " + "VALUES(?,?,?,?,?,?)" ); stmt.setint(1, bd.getid()); stmt.setstring(2, bd.gettitle()); stmt.setstring(3, bd.getauthor()); stmt.setstring(4, bd.getisbn()); stmt.setstring(5, bd.getpublisher()); stmt.setint(6, bd.getprice()); stmt.execute();

17 stmt.close(); theconnectionmanager.returnconnection(conn); catch (SQLException ex) { System.out.println(ex); theconnectionmanager.returnconnection(conn);

18 public int getmaxid() { Connection conn = theconnectionmanager.getconnection(); try { Statement stmt = conn.createstatement(); ResultSet rset = stmt.executequery( "SELECT MAX(id) FROM BookDescription"); rset.next(); int maxid = rset.getint(1); stmt.close(); theconnectionmanager.returnconnection(conn); return maxid; catch (SQLException ex) { theconnectionmanager.returnconnection(conn); return 0;

19 public java.util.list find(object template) { if (! (template instanceof BookDescription) ) return null; BookDescription bd = (BookDescription) template; java.util.list result = new java.util.list(); Connection conn = theconnectionmanager.getconnection(); try { PreparedStatement stmt = conn.preparestatement( "SELECT * " + "FROM BookDescription " + "WHERE " + "title LIKE? AND " + "author LIKE? AND " + "isbn LIKE? AND " + "publisher LIKE? " );

20 String title = (bd.gettitle() == null)? "" : bd.gettitle(); String author = (bd.getauthor() == null)? "" : bd.getauthor(); String isbn = (bd.getisbn() == null)? "" : bd.getisbn(); String publisher = (bd.getpublisher() == null)? "" : bd.getpublisher(); stmt.setstring(1, "%"+title+"%"); stmt.setstring(2, "%"+author+"%"); stmt.setstring(3, "%"+isbn+"%"); stmt.setstring(4, "%"+publisher+"%"); ResultSet rset = stmt.executequery(); while (rset.next()) { BookDescription bd_res = new BookDescription( rset.getstring("title"), rset.getstring("author"), rset.getstring("isbn"), rset.getstring("publisher"), rset.getint("price") ); bd_res.setid(rset.getint("id"));

21 result.add(bd_res); catch (SQLException ex) { System.out.println(ex); theconnectionmanager.returnconnection(conn); theconnectionmanager.returnconnection(conn); if(result.size() > 0) return result; else return null;

22 ConnectionManager public class ConnectionManager { private static ConnectionManager instance = null; private Stack connections; private ConnectionManager () { connections = new Stack(); try { DriverManager.registerDriver (new oracle.jdbc.driver.oracledriver()); catch (Exception ex) { System.out.println(ex); public static ConnectionManager getinstance() { if (instance == null) instance = new ConnectionManager(); return instance;

23 public Connection getconnection() { Connection conn = null; if (!connections.empty()) conn = (Connection) connections.pop(); else { //No one left in the stack, create a new one try { conn = DriverManager.getConnection ("jdbc:oracle:thin:@oracle.csc.uvic.ca:1521:teach", "thomo", password"); catch (SQLException ex) { System.out.println("SQLException: " + ex); return conn; public void returnconnection(connection conn) { if (conn!= null) connections.push(conn);