Hibernate Criteria Sample Implementation

The Criteria interface in Hibernate is used to create and execute object-oriented queries.
The following are some commonly used criterion.
(i) To query based on simple search criteria like Employee Type Permanent and Confirmation Date less than Request Date

Criteria criteria = session.createCriteria( Emp.class );
Criterion[] criterions = new Criterion[] {
Restrictions.eq( "empType", 'P' ),
Restrictions.and( Restrictions.isNotNull( "confirmationDate" ),
Restrictions.le( "confirmationDate",
requestDate ) )
};
for ( Criterion criterion : criterions ) {
criteria.add( criterion );
}
List result=criteria.list( );

(ii) To Retrieve values between

Criteria criteria = session.createCriteria( Emp.class );
Criterion[] criterions = new Criterion[] {
Restrictions.between( "joiningDate", startDate,endDate )};
for ( Criterion criterion : criterions ) {
criteria.add( criterion );
}
List result=criteria.list( );

(iii) Using In Clause
Criteria criteria = session.createCriteria( Emp.class );
Criterion[] criterions = new Criterion[] {
Restrictions.in( "empType",new Object[]{'P','C','R' )};
for ( Criterion criterion : criterions ) {
criteria.add( criterion );
}
List result=criteria.list( );

(iv) To find Record Count.
Criteria criteria = session.createCriteria( Emp.class );
criteria.setProjection(Projections.rowCount());
result=criteria.list( );
if ((null != result) && (!result.isEmpty())) {
Integer rowCount = (Integer) result.get(0);
}

(v) Returning 'n' columns from the Entity List based on Search Criteria
Criteria criteria = session.createCriteria( Emp.class );
Criterion[] criterions = new Criterion[] {
Restrictions.in( "empType",new Object[]{'P','C','R' )};
for ( Criterion criterion : criterions ) {
criteria.add( criterion );
}
criteria.setProjection( Projections.projectionList( )
.add( Projections.property( fieldName1 )) .add( Projections.property( fieldName2 ) ) );
List result=criteria.list( );

(vi) Return Result based on descending order
Criteria criteria = session.createCriteria( Emp.class );
Criterion[] criterions = new Criterion[] {
Restrictions.in( "empType",new Object[]{'P','C','R' )};
for ( Criterion criterion : criterions ) {
criteria.add( criterion );
}
crietrai.addOrder( Order.desc("empName") )
List result=criteria.list( );

Named Queries

NamedQuery feature in Hibernate allows Queries to added in Entity or Mapping Documents.The Queries can be of HQL or SQL type.NamedQuery structure has a name and query part.The name identifies the Query and should be unique in the context.If there are multiple named queries for an Entity,it can be specified within a NamedQueries construct.
@NamedQueries({

@NamedQuery(
name = "findEmpByAge",
query = "from emp e where e.age>?"
),

@NamedQuery(
name = "findEmpByRole",
query = "from emp e where e.role=?"
)
})

Adding additional fields in @Entity Class

In the Entity class, the persistent fields are mapped with @Column construct.But there could be some computed or other fields which are not in the table but are required in the Entity class.Adding these additional fields in the entity would give rise to Invalid Identifier Exception.To avoid this,these additional fields can be used with @Transient construct.When the fields are marked as Transient ,they are not associated with the Session Factory and their instances would be destroyed by the GC.

Cloud Computing

Cloud computing is a technology that uses the Internet and central remote servers to deliver hosted services over the Internet. It allows consumers and businesses to use applications without any installation on the computer .This technology enhances the efficiency of computing by centralizing storage, memory, processing and bandwidth. Typical examples of Cloud Services is Email Services like Yahoo,G Mail etc.

The distinct characteristics that differentiate Cloud Services from traditional hosting are
(1) It is sold on demand, the consumer pays based on the usage similar to any other utilities like water ,electricity.
(2) It is fully managed by the provider (the consumer needs nothing but a personal computer and Internet access).

The Cloud are of 3 types
-A public cloud that sells services to anyone on the Internet
-A private cloud is a proprietary network or data center that has hosted services to limited audience.
-A virtual private cloud that uses public cloud resources but is available for limited audience.

Cloud Computing Services are broadly divided into three categories:
Infrastructure-as-a-Service ,in which the Cloud Service provides virtual server instances with unique IP addresses and blocks of storage on demand. Customers use the provider's application program interface (API) to start, stop, access and configure their virtual servers and storage.

Platform-as-a-service ,is a set of software and product development tools hosted on the provider's infrastructure. Developers create applications on the provider's platform over the Internet.

Software-as-a-service ,where the vendor supplies the hardware infrastructure, the software product and interacts with the user through a front-end portal. Services can be anything from Web-based email to inventory control and database processing. Because the service provider hosts both the application and the data, the end user can avail the services from any computer.

Step [null] has neither a element nor a 'ref' attribute referencing a Tasklet

In the course of developing an Application with Spring Batch was once stumbled upon 'Step [null] has neither a element nor a 'ref' attribute referencing a Tasklet' exception.There was no documentation on the cause or work around for this Exception.Later identified the cause of the problem was the error in configuring the Job.In the Job Context the value for Job name was given by ref but it should have been by value.

Steps Execution in Spring Batch

In the real world scenario a batch task may need more than one step to complete the processing. The steps may or may not be independent of each other. Spring batch provides provision to define multiple steps for a Job.Each Step has a StepExecutionContext which provides the Information about the status of the step.The order of the steps execution can be made conditional

When a job consists of multiple steps ,it may be necessary to pass information between the steps. Each Step has a StepExecutionContext but this exists only for the span of the step. To make the data available to future Steps, it will have to be passed to the Job ExecutionContext after the step has finished. Spring Batch provides the ExecutionContextPromotionListener for this purpose. The listener must be configured with the keys related to the data in the ExecutionContext that must be promoted and must be registered with the Step.

The sample of Job with multiple steps

Spring Batch

Batch applications are used to process high volume business critical transactional data.

Spring Batch is the first Java based framework catered for batch process .For more information refer to http://static.springsource.org/spring-batch/reference/html-single/index.html#springBatchBackground

Building a batch application using Spring Batch Framework is relatively simple. The operation to be accomplished as a Batch task should be defined as a Job Bean. A typical batch program comprises of 3 logical steps which

1. reads a large number of records from a database, file, or queue
2. processes the data in some fashion, and
3. then writes back data in a modified form

The Spring batch framework provides interfaces to read(ItemReader),process(ItemProcessor) and write(ItemWriter) to achieve the same.

It provides better performance by using a chunk-oriented approach. For each Job a commit size can be provided,the Job would read and process the data and the actual writing into Database or File system is done only when the commit size is reached.

The following is sample of a simple Job Config with one step.