Library for evaluating logical expressions on Java objects

Question

Let's say I have the following class:

class Person {
    int age;
    String city;
    Collection<Person> friends;
    Person spouse;
}

I need a library which would allow me to evaluate whether a logical expression is true on a given Person object. The expression would look something like this:

((age>25 OR spouse.age>27) AND city=="New-York" AND size(friends)>100)

So, the requirements are:

Ability to use basic logical operators
Access properties of the given object
Access properties of an internal object
Use simple mathematical\sql-like functions such as size,max,sum

Suggestions?

Can you say why you can't use plain Java to do this? Does the expression have to come from a dynamic String? — Peter Lawrey, Oct 02 '12 at 13:40
Why do you need a library for anything? Because it is more complicated than it looks (mainly the parsing issues) and there are probably are tons of pitfalls you can run into which a solid library already eliminated. — Alex, Oct 02 '12 at 14:03

assylias · Answer 1 · 2012-10-02T13:51:48.343

You could use a ScriptEngine + reflection:

access all the fields in your object and create variable that have those values
evaluate the expression

Here is a contrived example which outputs:

age = 35
city = "London"
age > 32 && city == "London" => true
age > 32 && city == "Paris" => false
age < 32 && city == "London" => false

It could become quite messy if you want to deal with non primitive types, such as collections.

public class Test1 {

    public static void main(String[] args) throws Exception{
        Person p = new Person();
        p.age = 35;
        p.city = "London";

        ScriptEngineManager factory = new ScriptEngineManager();
        ScriptEngine engine = factory.getEngineByName("JavaScript");

        Class<Person> c = Person.class;
        for (Field f : c.getDeclaredFields()) {
            Object o = f.get(p);
            String assignement = null;
            if (o instanceof String) {
                assignement = f.getName() + " = \"" + String.valueOf(o) + "\"";
            } else {
                assignement = f.getName() + " = " + String.valueOf(o);
            }
            engine.eval(assignement);
            System.out.println(assignement);
        }

        String condition = "age > 32 && city == \"London\"";
        System.out.println(condition + " => " + engine.eval(condition));

        condition = "age > 32 && city == \"Paris\"";
        System.out.println(condition + " => " + engine.eval(condition));

        condition = "age < 32 && city == \"London\"";
        System.out.println(condition + " => " + engine.eval(condition));
    }

    public static class Person {

        int age;
        String city;
    }
}

score 3 · Accepted Answer · answered Oct 03 '12 at 10:38

OK, think I found what I wanted, it's a variation on assylias' answer but I prefer it since it's more standardized (woudn't want to depend specifically on Javascript expressions, or run Javascript at all for that matter).

Apparently there is a Unified Expression Language for evaluating expressions, it was originally designed for JSP but can be used for other stuff as well. There are several parser implementations, I'll probably go either with Spring EL or JUEL

score 0 · Answer 3 · answered Jun 15 '22 at 12:05

You can use rhino library from Mozilla. https://github.com/mozilla/rhino

Example :

@Test
public void shouldCheckLogicalOperations(){
    Context cx = Context.enter();
    try {
        Scriptable scope = cx.initStandardObjects();
        ScriptableObject.putProperty(scope, "form_admin_checked", true);
        ScriptableObject.putProperty(scope, "form_limit_value", 50000);

        String script = "((form_admin_checked && form_limit_value<2000) || (form_admin_checked && !form_admin_checked) || form_admin_checked && form_limit_value==50000)";
        String expectedResponse = "true";

        Object result = cx.evaluateString(scope, script, "<cmd>", 1, null);

        Assert.assertEquals(expectedResponse, result.toString());

    } finally {
        // Exit from the context.
        Context.exit();
    }
}

Library for evaluating logical expressions on Java objects

3 Answers3