java.lang.IllegalAccessException: can not access a member of class java.util.Collections$UnmodifiableCollection with modifiers "public"

Question

I have classes PrimitiveProperty and ComplexProperty and the interface Property. I want to create an implementation of Property which enforces an empty unmodifiable Set of Property instances as return value of Property.getProperties, e.g.

public interface Property {
    Set<Property> getProperties();
}

public class ComplexProperty implements Property {
    private Set<Property> properties; 
    //getter overrides the interface method
}

public class PrimitiveProperty implements Property {
    private final static Set<Property> EMPTY_PROPS = Collections.unmodifiableSet(new HashSet<Property>(1));

    @Override
    public Set<Property> getProperties() {
        return EMPTY_PROPS;
    }
}

With Glassfish 4.0 and I'm getting

java.lang.IllegalAccessException: Class javax.el.ELUtil can not access a member of class java.util.Collections$UnmodifiableCollection with modifiers "public"

when I access the property in the leaf attribute of a Richfaces tree, e.g.

<r:tree id="aTree" 
        toggleType="ajax" var="item" >
  <r:treeModelRecursiveAdaptor roots="#{aCtrl.roots}" 
                               nodes="#{item.properties}" leaf="#{item.properties.isEmpty()}">
    <r:treeNode>
      #{item.name}
    </r:treeNode>
    <r:treeModelAdaptor nodes="#{item.properties}" leaf="#{item.properties.isEmpty()}"/>
  </r:treeModelRecursiveAdaptor>
</r:tree>

The issue disappears if I make the EMPTY_PROPS constant modifiable (by assigning an instance of HashSet instead of the return value of Collections.unmodifiableSet) which is not my intention.

Is it possible to achieve what I want to do? Do I have to invent or use an implementation of what Collections$UnmodifiableCollection ('s subclasses) do(es) which is compatible with the JSF access needs?

Mousey

You forgot to show how you're accessing it (in the view side). – Cough

I've used the Arrays#asList utility, which returns an unmodifiable List quite frequently, with no problem. Don't know why you have that error for the Set. – Improvised

The clue is in the way how he's accessing it. – Cough

Answer 1

Here is the problem:

leaf="#{item.properties.isEmpty()}"

You're attempting to invoke a method directly on the UnmodifiableSet instance. Although it implements Collection, which is public, the UnmodifiableSet implementation itself, where EL (read: Reflection API) is trying to find the method on the class, is not public.

Exactly this problem is reproducible in plain Java (a main() method) as follows:

Set<Object> set = Collections.unmodifiableSet(new HashSet<>());
for (Method method : set.getClass().getMethods()) {
    if ("isEmpty".equals(method.getName())) {
        method.invoke(set); // IllegalAccessException.
    }
}

This is essentially a bug in the EL implementation used.

You'd better just use EL's own empty operator:

leaf="#{empty item.properties}"

Cough

... and once more, a JSF abstraction turns out to leak implementation details: to use EL, one should not have to be aware of the specifics of how it interacts with the Reflection API (a public method implemented in a private class can be invoked through reflection by using setAccessible(true) or through the method object of the public super type that declared the method). – Scrapple

It's just a bug in EL impl used. – Cough

@Cough What would be an approprieate El implementation - I experimented with different versions of javax.el:el-api and org.glassfish.web:el-impl (I'm using and deploying to glassfish)[ocpsoft.org/java/jsf2-java/… and combinations of them, but I guess I don't know what I'm doing nor where to find infos. – Mousey

Apache's EL implementation (as used in my Tomcat 7.0.55) doesn't expose this bug. But I wouldn't worry much about this. Just use the empty operator. There it is for. – Cough

Is it a bug in EL or in Java? How would you modify your java code above so that is doesn't throw the exception? – Neckar

@Maurice: From the answer: "This is essentially a bug in the EL implementation used" and "You'd better just use EL's own empty operator". – Cough

@Cough OK. Now I've had the same problem in a different context (not with EL). Again, How would you modify your java code above so that is doesn't throw the exception? If I call set.isEmpty() (or set.size()) in Java, it works without problem. How can we make if work with reflection? – Neckar

@Cough (I've tryed method.setAccessible(true);, it doesn't work) – Neckar

@Maurice: Oh? Just do instanceof Collection or instanceof Iterable (depending on your coverage) and then simply downcast and perform Collection#isEmpty() or Iterator#hasNext() rather than doing reflection trickery. – Cough

@Cough There are times when you need "reflection trickery". Thanks anyway. – Neckar

@Maurice: I have yet to find a real world use case where instanceof and downcast (or inherently Class#isAssignableFrom() or Class#isInstance() followed by Class#cast()) is impossible and therefore "reflection trickery" is required. – Cough

@Cough I guess your "real world" doesn't include say, an implementation of "EL". – Neckar

@Maurice: even in EL the base is readily available in evaluation context. Therefore, "This is essentially a bug in the EL implementation used" and "You'd better just use EL's own empty operator". – Cough