Looking for JEXL Filter feature

I know, I can do several things in JEXL, but unable to find Filter feature in it, which is indeed very useful.

How can I do something like

 var x=[{a:11,b=5},{a:1,b=15},{a:12,b=25},{a:4,b=35},{a:7,b=45}];

 return x[.a>10].b; // Which filters to {a:11,b=5} & {a:12,b=25}
                   // & hence returns [5,25]

First of all your syntax is not valid JEXL. I assume you meant this:

var x = [{'a':11,'b':5}, {'a':1,'b':15}, {'a':12,'b':25}, {'a':4,'b':35}, {'a':7,'b':45}];

Since you can call any Java method on any object in a JEXL script, you have (at least theoretically) full access to the Java Stream API.

However, the Stream API isn't directly available from a raw array and we can't just call Arrays.stream(x); without some effort. The easiest way around this is to create a set instead:

var x = {{'a':11,'b':5}, {'a':1,'b':15}, {'a':12,'b':25}, {'a':4,'b':35}, {'a':7,'b':45}};

Now we can simply call stream() and work from there:

x.stream();

What we want now is something like this:

x.stream().filter(function(m){m['a']>10});

Unfortunately the method resolver in JEXL will not be able to correctly resolve Stream.filter(Predicate) with a JEXL function, as it doesn't know how to turn a JEXL function into a Predicate. A JEXL function is of type org.apache.commons.jexl3.internal.Closure.

Thus the very least you need to do is to provide your own Predicate implementation in Java and then create a new instance in your script:

public class MyCustomFilterPredicate implements Predicate<HashMap<String, Integer>> {
    @Override
    public boolean test(final HashMap<String, Integer> m)
    {
        return m.get("a") > 10;
    }
}

You can then create a new instance in your JEXL script:

var filterPredicate = new('my.custom.filter.predicate.MyCustomFilterPredicate');

The same goes for Stream.map(Function):

public class MyCustomMapFunction implements Function<HashMap<String, Integer>, Integer> {
    @Override
    public Integer apply(final HashMap<String, Integer> m)
    {
        return m.get("b");
    }
}

And again create a new instance in your script:

var mapFunction = new('my.custom.map.function.MyCustomMapFunction');

Your entire script will then look like this:

var x = {{'a':11,'b':5}, {'a':1,'b':15}, {'a':12,'b':25}, {'a':4,'b':35}, {'a':7,'b':45}};

var filterPredicate = new('my.custom.filter.predicate.MyCustomFilterPredicate');
var mapFunction = new('my.custom.map.function.MyCustomMapFunction');

return x.stream().filter(filterPredicate).map(mapFunction).toArray();

Of course you might have noticed that the reusability of your predicate and function implementations are rather limited. This is why I'd recommend creating implementations that wrap a JEXL Closure:

public class MyCustomFilterPredicate implements Predicate<Object> {
    private final Closure closure;
    public MyCustomFilterPredicate(final Closure closure) {
        this.closure = closure;
    }
    @Override
    public boolean test(final Object o)
    {
        return (boolean) closure.execute(JexlEngine.getThreadContext(), o);
    }
}

public class MyCustomMapFunction implements Function<Object, Object> {
    private final Closure closure;
    public MyCustomMapFunction(final Closure closure) {
        this.closure = closure;
    }
    @Override
    public Object apply(final Object o)
    {
        return closure.execute(JexlEngine.getThreadContext(), o);
    }
}

Now you can change your script as follows and reuse these Java classes in various ways:

var x = {{'a':11,'b':5}, {'a':1,'b':15}, {'a':12,'b':25}, {'a':4,'b':35}, {'a':7,'b':45}};

var filterPredicate = new('my.custom.filter.predicate.MyCustomFilterPredicate', function(m){m['a']>10});
var mapFunction = new('my.custom.map.function.MyCustomMapFunction', function(m){m['b']});

return x.stream().filter(filterPredicate).map(mapFunction).toArray();