Using JPA/Hibernate

JPA or Java Persistence API provides fundamental frameworks over relational data. The JPA follows Hibernate footsteps.

Hibernate was one of the first and the most popular library, which provided Object-Relational Mapping (ORM) to simplify code of handling relational data in Java.

Hibernate is currently part of JBoss offering, which in its turn is under the RedHat company umbrella.

Over the years most of the Hibernate features appeared in the mainstream Java as JPA.

Hibernate is especially beneficial when a programmer deals with One-To-One relationships.
A data table can be described as a class called entity.
This is the simplest Object-Relational Mapping (ORM), which allows a programmer to manipulate by Java classes, while JPA/Hibernate deals with the data tables behind the scene.

Here is the sample of such a class Account, describing a data table Accounts with the ID, Email, and Password fields.


@Entity

@Table(name="accounts")

public class Account implements Serializable {

   private int id;

   private String email;

   private String password;

}

It is a bit more complicated to deal with One-To-Many relationships, for example, describing multiple record types, resulting from different combinations of WHERE clauses with many JOIN in SELECT statements.

JPA/Hibernate frameworks save tremendous time, which used to be consumed by describing the flavors of different database types.

This is done behind the scene by powerful classes like javax.persistence.EntityManager, org.hibernate.Session, javax.persistence.Query and more.

Here is the extract from the constructor of the JpaDataService class:


		  emf = Persistence.createEntityManagerFactory(jpaUnitName, properties); // using persistence.xml 

		  em = emf.createEntityManager();

		  session = em.unwrap(Session.class);

Just a reminder: the JpaDataService class is a parent class of the DataBaseService, which is a parent class of the DataService.
The properties for the EntityManagerFactory are derived from the persistence.xml (default) and can be replaced by the DB parameters, if any, in the web.xml.

The JpaDataService class keeps a table of EntityManager instances mapped to their Data Source names. Each EntityManager instance is a Singletone, created once for all users.

Here is the method that returns the JpaDataService object with the EntityManager and the Hibernate Session instances for a specific data source name.


	public static JpaDataService getJds(String appName, String jpaUnitName) {

		if(tableOfJpaServices == null) {

			tableOfJpaServices = new Hashtable();

		}

		JpaDataService jds = tableOfJpaServices.get(jpaUnitName);

		if(jds == null) {

			jds = new JpaDataService(appName, jpaUnitName);

			jds.setDsName(appName+"/"+jpaUnitName);

			tableOfJpaServices.put(jpaUnitName, jds);

		}

		return jds;

	}

Was it clear so far?

The methods of the JpaDataService class support the main functions of the DataService APIs, such as:


int nRecords = DataService.setData(sql, dsName);



List records = DataService.getData(sql, dsName);



// using sqlName to get SQL 

String sqlName = "updatePassword"; // update password set psw=? where userId=?

// create run-time parameters to update password

String[] runTimeParams = new String[] {psw, userId};

// update

int nRecords = DataService.setPrepDataBySqlName(sqlName, runTimeParams, dsName);



// retrieve data records with sqlName and run-time parameters in where clause

List records = DataService.getPrepDataBySqlName(sqlName, runTimeParams, dsName);

The DataService framework uses a powerful Hibernate Query object to retrieve and change data.
While changing data, any method calls a Transaction object providing thread safety.

There are many convenience functions implemented by the DataService framework.
For example:


// get a string with all table names, separated by comma, for that data source

String allDsTableNames = DataService.getAllTables(dsName); 



// get the lists of column names and column types for a specific table in a specific data source

List[] tableColumnNamesAndTypes = DataService.getColumnNamesAndTypes(dsName, tableName);



// get a page of data with a specific SQL from any data source 

int start = 3000; // starting with the record #3000 of the selection result

int pageSize = 40; // deliver 40 records (starting from the record #3000 of the selection)

List pageOfRecords = DataService.getPageData(sql, start, pageSize, dsName);



And many more...

Assignments:
1. Create a project 4.4.5.1.Hibernate in Eclipse (similar to 4.4.5.Hibernate) and copy/paste several examples from the web.
2. Provide the links to your examples on the web in the email to dean@ituniversity.us.

public static JpaDataService getJds(String appName, String jpaUnitName) { if(tableOfJpaServices == null) { tableOfJpaServices = new Hashtable<String, JpaDataService>(); } JpaDataService jds = tableOfJpaServices.get(jpaUnitName); if(jds == null) { jds = new JpaDataService(appName, jpaUnitName); jds.setDsName(appName+"/"+jpaUnitName); tableOfJpaServices.put(jpaUnitName, jds); } return jds; }

int nRecords = DataService.setData(sql, dsName); List<Object[]> records = DataService.getData(sql, dsName); // using sqlName to get SQL String sqlName = "updatePassword"; // update password set psw=? where userId=? // create run-time parameters to update password String[] runTimeParams = new String[] {psw, userId}; // update int nRecords = DataService.setPrepDataBySqlName(sqlName, runTimeParams, dsName); // retrieve data records with sqlName and run-time parameters in where clause List<Object[]> records = DataService.getPrepDataBySqlName(sqlName, runTimeParams, dsName);

// get a string with all table names, separated by comma, for that data source String allDsTableNames = DataService.getAllTables(dsName); // get the lists of column names and column types for a specific table in a specific data source List<String>[] tableColumnNamesAndTypes = DataService.getColumnNamesAndTypes(dsName, tableName); // get a page of data with a specific SQL from any data source int start = 3000; // starting with the record #3000 of the selection result int pageSize = 40; // deliver 40 records (starting from the record #3000 of the selection) List<Object[]> pageOfRecords = DataService.getPageData(sql, start, pageSize, dsName); And many more...