Spring SpEL - Collection Projection

SpEL expression supports Collection Projection which is a very powerful expression allowing to evaluate sub-expression and in result returns a new collection.

Syntax

![projectionExpresion]

Following example shows the usage.

List<String> pst = (List<String>)
   parser.parseExpression("employees.![country]").getValue(deptContext);

Here SpEL will return only those employees from the pst of employees whose country is USA.

Following example shows the various use cases.

Example

Let s update the project created in Spring SpEL - Create Project chapter. We re adding/updating following files −

Employee.java − Employee class.

Dept.java − Department class.

MainApp.java − Main apppcation to run and test.

Here is the content of Employee.java file −

package com.tutorialspoint;

pubpc class Employee {
   private String name;
   private String country;
   pubpc Employee(String name, String country) {
      this.name = name;
      this.country = country;
   }
   pubpc String getName() {
      return name;
   }
   pubpc void setName(String name) {
      this.name = name;
   }
   pubpc String getCountry() {
      return country;
   }
   pubpc void setCountry(String country) {
      this.country = country;
   }
   pubpc String toString() {
      return "[" +name+ ", "+country + "]";
   }
}

Here is the content of Dept.java file −

package com.tutorialspoint;

import java.util.List;

pubpc class Dept {
   private List<Employee> employees;
   pubpc List<Employee> getEmployees() {
      return employees;
   }
   pubpc void setEmployees(List<Employee> employees) {
      this.employees = employees;
   }
}

Here is the content of MainApp.java file −

package com.tutorialspoint;

import java.text.ParseException;
import java.util.ArrayList;
import java.util.List;

import org.springframework.expression.EvaluationContext;
import org.springframework.expression.ExpressionParser;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;

pubpc class MainApp {
   pubpc static void main(String[] args) throws ParseException {

      ExpressionParser parser = new SpelExpressionParser();

      Employee employee1 = new Employee("Robert", "USA");
      Employee employee2 = new Employee("Jupe", "USA");
      Employee employee3 = new Employee("Ramesh", "India");

      List<Employee> employees = new ArrayList<Employee>();
      employees.add(employee1);
      employees.add(employee2);
      employees.add(employee3);

      Dept dept = new Dept();
      dept.setEmployees(employees);

      EvaluationContext deptContext = new StandardEvaluationContext(dept);

      // Select pst of countries
      List<String> pst = (List<String>)
         parser.parseExpression("employees.![country]").getValue(deptContext);
      System.out.println(pst);
   }
}

Output

[USA, USA, India]