How to create generic MVEL expression

Question

I am writing generic MVEL expression for Java objects. So for understanding purpose I am taking one example.

class Student {

   String name;

   String rollNo;

   List<Course> courses;
}

class Course {
   
   String courseName;

   String facultyName;

   String fee;

   String classRoom;
}

I can write MVEL expression If my query is: Check If a student has given name and rollNo

MVEL.eval(" name == 'XYZ' && rollNo == '3456' ", Student)

But If the query is: Check If a student attends all the courses in the same classroom?

MVEL.eval(" courses[0].classRoom == 'A' ", Student);

But this checks classRoom in one course only. But I want to check If all courses of a student happens in classRoom 'A'. I couldn't find any resource to solve this problem. I am completely new in MVEL. If you have any doubt please ask me.

Thank you

Answer 1

To do this requires a few steps - you will need to make use of MVEL variables and its foreach operator.

Here is the approach:

import org.mvel2.MVEL;
import java.util.List;
import java.util.ArrayList;
import java.util.Map;
import java.util.HashMap;

public class App {
    public static void main(String[] args) {
        
        List<Course> courses = new ArrayList<>();
        courses.add(new Course("Math", "123"));
        courses.add(new Course("Physics", "123"));
        courses.add(new Course("Chemistry", "123"));
        
        Student student = new Student("XYZ", "3456", courses);
                
        // Check If a student has given name and rollNo:
        boolean b1 = MVEL.eval(" name == 'XYZ' && rollNo == '3456' ", student, Boolean.class);
        boolean b2 = MVEL.eval(" name == 'XYP' && rollNo == '3456' ", student, Boolean.class);
        //System.out.println(b1);
        //System.out.println(b2);
        
        // Check If a student attends all the courses in the same classroom:
        boolean allClassesInSameRoom = true;
        String prevRoom = null;
        
        Map vars = new HashMap();
        vars.put( "sameRoom", allClassesInSameRoom );
        vars.put( "prevRoom", prevRoom );
        
        String expression = String.join("\n", 
                "sameRoom = true;",
                "prevRoom = null;",
                "foreach (course : courses) {",
                "  if (sameRoom == true && prevRoom != null && course.classRoom != prevRoom) {",
                "    sameRoom = false;",
                "  }",
                "  prevRoom = course.classRoom;",
                "}",
                "sameRoom");
        
        allClassesInSameRoom = MVEL.eval(expression, student, vars, Boolean.class);
        
        System.out.println(allClassesInSameRoom);
        
    }
    
}

First we define two Java variables, which are needed in the MVEL script. They are passed to the MVEL eval operator via a Java map:

boolean allClassesInSameRoom = true;
String prevRoom = null;

Map<String, Object> vars = new HashMap<>();
vars.put( "sameRoom", allClassesInSameRoom );
vars.put( "prevRoom", prevRoom );

Then we build a string containing the MVEL script we need to use:

String expression = String.join("\n", 
        "sameRoom = true;",
        "prevRoom = null;",
        "foreach (course : courses) {",
        "  if (sameRoom == true && prevRoom != null && course.classRoom != prevRoom) {",
        "    sameRoom = false;",
        "  }",
        "  prevRoom = course.classRoom;",
        "}",
        "sameRoom");

The final line returns the sameRoom variable.

The eval statement assembles the items we need:

allClassesInSameRoom = MVEL.eval(expression, student, vars, Boolean.class);

Note that the Course class needs to be defined as public:

public class Course {

    private final String courseName;
    private final String classRoom;

    public Course (String courseName, String classRoom) {
        this.courseName = courseName;
        this.classRoom = classRoom;
    }
    
    public String getCourseName() {
        return courseName;
    }

    public String getClassRoom() {
        return classRoom;
    }

}

It is possible to create MVEL scripts as templates, which I think can help avoid the somewhat messy process of building a Java String line-by-line, as I do here.

This may all add up to more work than simply checking the student object without using MVEL. But MVEL supports functions and lambda expressions, also - so there are probably ways to streamline my approach and simplify the MVEL script.

---

Update

Here is an example which is a bit more compact:

Map<String, String> rooms = new HashMap<>();
Map<String, Object> vars2 = new HashMap<>();
vars2.put( "rooms", rooms );
        
String expression2 = String.join("\n", 
        "foreach (course : courses) {",
        "  rooms.put(course.classRoom, course.classRoom);",
        "}",
        "(rooms.size() == 1) ? true : false;");
        
allClassesInSameRoom = MVEL.eval(expression2, student, vars2, Boolean.class);
System.out.println(allClassesInSameRoom);

In this case, the script adds each room name to a map. Duplicate room names will have the same map key, and therefore will not increase the size of the map. If we end up with a map containing only one entry, that means there is only one room being used for all courses.

---

As an alternative, you can count the number of different room names in the student's list of courses using Java (Java 8 or higher):

import static java.util.stream.Collectors.toList;

...

long roomsCount = courses.stream()
        .map(Course::getClassRoom)    // get the room name from each course
        .collect(toList())            // build a list of these room names
        .stream().distinct().count(); // count the number of unique room names

That's one line of code. But since your goal is to explore and understand MVEL, this is just a side note.