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

Here SpEL will return the countries as a list of strings from the list of employees.

Following example shows the various use cases.

Example - Usage of Projection in Collection in SpEL

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

• Employee.java − Employee Class.

• Dept.java − Department Class.

• MainApp.java − Main application to run and test.

Employee.java

Here is the content of Employee.java file −

package com.tutorialspoint;

public class Employee {
   private String name;
   private String country;

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

Dept.java

Here is the content of Dept.java file −

package com.tutorialspoint;

import java.util.List;

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

MainApp.java

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;

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

      ExpressionParser parser = new SpelExpressionParser();

      Employee employee1 = new Employee("Robert", "USA");
      Employee employee2 = new Employee("Julie", "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 list of countries
      List<String> list = (List<String>)
         parser.parseExpression("employees.![country]").getValue(deptContext);
      System.out.println(list);
   }
}

Output

Once you are done creating the source and bean configuration files, let us run the application. If everything is fine with your application, it will print the following message −

[USA, USA, India]