Question

I am trying to debug a Java application that is relying on Reflection. Right now the error I get is the following:

java.lang.IllegalArgumentException: Can not set int field DataStructures.StackAr.topOfStack to java.lang.Integer
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:146)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:150)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:37)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.getInt(UnsafeIntegerFieldAccessorImpl.java:38)
    at sun.reflect.UnsafeIntegerFieldAccessorImpl.get(UnsafeIntegerFieldAccessorImpl.java:18)
    at java.lang.reflect.Field.get(Field.java:358)

THe last lines of the application running are:

Field f = classUnderTest.getDeclaredField(processFieldName(var));
f.setAccessible(true);
Long value = (Long) f.get(runtimeInstance);

The error message is a bit misleading and I am not sure why it is mentioning a set operation whereas I am trying to preform a get.

I am suspecting that the runtimeInstance is not an object of the expected class. But that error message is throwing me away.

Has anyone encountered this issue before? Any clues?

PS1: The exact line causing the exception is this one:

Long value = (Long) f.get(runtimeInstance);

PS2: processFieldName(var) processes the correct name of the field, i.e. it removes some artefacts from a string with the field name like this. and so on.

Answer 1

From the source of those accessors it seems that the class declaring the field is not assignable from the runtimeInstance‘s class:

if (!(this.field.getDeclaringClass().isAssignableFrom(paramObject.getClass())))

  throwSetIllegalArgumentException(paramObject);

field seems to be the field you want to get from the instance, paramObject is your runtimeInstance.

Thus, if the declaring class of the field isn’t the class or a super class of the paramObject you’d get that message.

Any chance your paramObject is an Integer here?

Edit: here’s some source code from OpenJDK (should be similar to Oracle’s), to explain the message:

  protected String getSetMessage(String attemptedType, String attemptedValue) {
         String err = "Can not set";
         if (Modifier.isStatic(field.getModifiers()))
             err += " static";
         if (isFinal)
             err += " final";
         err += " " + field.getType().getName() + " field " + getQualifiedFieldName() + " to ";
         if (attemptedValue.length() > 0) {
             err += "(" + attemptedType + ")" + attemptedValue;
         } else {
             if (attemptedType.length() > 0)
                 err += attemptedType;
             else
                 err += "null value";
         }
         return err;
     }

Taking your message java.lang.IllegalArgumentException: Can not set int field DataStructures.StackAr.topOfStack to java.lang.Integer we find that:

• the field is of type int
• the field’s name is topOfStack in class DataStructures.StackAr
• attemptedType is java.lang.Integer

Since attemptedType is the type of your runtimeInstance I suspect classUnderTest is DataStructures.StackAr whereas runtimeInstance is of type java.lang.Integer.