Transforming a result

As a developer, I love this feature, as it helps the developers to transform the returned rows to List, Map, or user-defined Bean.

How to do it...

Now we will take a look at three scenarios of the demonstration code that will convert the records returned by hibernate to List, Map, and Bean.

Here, we use the Transformers class to provide the transforming mechanism to criteria.

Scenario 1: Converting a result to List

All the demos up to this point show that if we use the criteria.list() method, the resultant data is always returned in List. However, you can still use Transformers.TO_LIST in criteria, as follows:

criteria.setResultTransformer(Transformers.TO_LIST);

This means that every row in the result will be represented as a List.

Scenario 2: Converting a result to Map

Now, let's see how to convert the resultant data in Map:

criteria.setResultTransformer(Transformers.ALIAS_TO_ENTITY_MAP);

This means every object from List represents Map.

Code

For example, the following code shows how to transform the resultant data into Map:

Criteria criteria = session.createCriteria(Employee.class);
criteria.setResultTransformer(Transformers.ALIAS_TO_ENTITY_MAP);
List list = criteria.list();
System.out.println("List: " + list);
Map map = (Map) list.get(0);
Employee employeeMap = (Employee) map.get(Criteria.ROOT_ALIAS);
System.out.println(employeeMap.toString());

Output

The output will be as follows:

List: [{this=Employee@3235025a}, {this=Employee@4e84c320}, {this=Employee@2644f3a2}, {this=Employee@7c7d8dfe}]
Employee
 id: 2
 first name: aarush
 salary: 35000.0
 department: developement

When we print out a list object in the console, we can see that it's represented as key and value format in the Map structure. We will try to get the value from map at the zero position using the following code:

Map map = (Map) list.get(0);

The key of Map is this, and the value is the object of the Employee class.

So, the standard way to access a map's value is by its key. Here, the key is this, which is the equivalent of the Root alias of criteria in hibernate. So, you can access an object of Employee with the help of the following code:

Employee employeeMap = (Employee) map.get(Criteria.ROOT_ALIAS);

Scenario 3: Converting a result to user-defined Bean

This feature is useful when we select the columns in the resultant data and want to form that data in an already defined Bean.

For example, if we select the empId, empFirstName, empSalary, and the empDeptName, we can easily form this data into an EmployeeDetail bean, as we already have a bean defined with the name EmployeeDetail.java with these four fields:

public class EmployeeDetail {
    private long empId;
    private String empFirstName;
    private double empSalary;
    private String empDeptName;

    public long getEmpId() {
      return empId;
    }

    public void setEmpId(long empId) {
      this.empId = empId;
    }

    public String getEmpFirstName() {
      return empFirstName;
    }

    public void setEmpFirstName(String empFirstName) {
      this.empFirstName = empFirstName;
    }

    public double getEmpSalary() {
      return empSalary;
    }

    public void setEmpSalary(double empSalary) {
      this.empSalary = empSalary;
    }

    public String getEmpDeptName() {
      return empDeptName;
    }

    public void setEmpDeptName(String empDeptName) {
      this.empDeptName = empDeptName;
    }
    
    @Override
    public String toString() {
      return "
EmployeeDetail "
          + "
 Employee id: " + this.empId
          + "
 Employee FirstName: " + this.empFirstName
          + "
 Employee Salary: " + this.empSalary
          + "
 Employee DepartmentName : " + this.empDeptName;
    }

}

Code

Now, the following code shows how to convert the resultant data into an EmployeeDetail bean:

Criteria criteria = session.createCriteria(Employee.class);
criteria.createAlias("department", "_department");

ProjectionList projectionList = Projections.projectionList();
projectionList.add(Projections.alias(Projections.property("id"), "empId"));
projectionList.add(Projections.alias(Projections.property("firstName"), "empFirstName"));
projectionList.add(Projections.alias(Projections.property("salary"), "empSalary"));
projectionList.add(Projections.alias(Projections.property("_department.deptName"), "empDeptName"));
criteria.setProjection(projectionList);

criteria.setResultTransformer(Transformers.aliasToBean(EmployeeDetail.class));
List<EmployeeDetail> employeeDetails = criteria.list();

EmployeeDetail employeeDetail = employeeDetails.get(0);
System.out.println(employeeDetail.toString());

Output

The output will be as follows:

EmployeeDetail
 Employee id: 1
 Employee FirstName: yogesh
 Employee Salary: 50000.0
 Employee DepartmentName : developement

To use this feature in hibernate, we need to match a resultant column alias with a fieldname in the bean. For example, here we gave empId as an alias of the id field.

Actually, here you will notice a new term known as projection, which we will discuss in the next section.

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