Instantiating subclass of JNA Structure throws IllegalAccessException

Question

I was having trouble getting com.sun.jna.Structure to work, so I tried copying the first JNA test I found and I can't even get that to work.

For convenience, try it online.

import com.sun.jna.Structure;
import java.util.Arrays;
import java.util.List;

public class MyClass {
    public void testSimpleSize() throws Exception {
        class TestStructure extends Structure {
            public int field;
            @Override
            protected List<String> getFieldOrder() {
                return Arrays.asList("field");
            }
        }
        Structure s = new TestStructure();
        //assertEquals("Wrong size", 4, s.size());
    }

    public static void main(String[] args)
    {
        try
        {
            new MyClass().testSimpleSize();
        }
        catch (Exception e)
        {
            System.out.println(e);
        }
    }
}

The code compiles ok, but when running I get

Exception in thread "main" java.lang.Error: Exception reading field 'field' in class MyClass$1TestStructure
    at com.sun.jna.Structure.getFieldValue(Structure.java:639)
    at com.sun.jna.Structure.deriveLayout(Structure.java:1285)
    at com.sun.jna.Structure.calculateSize(Structure.java:1159)
    at com.sun.jna.Structure.calculateSize(Structure.java:1111)
    at com.sun.jna.Structure.allocateMemory(Structure.java:414)
    at com.sun.jna.Structure.<init>(Structure.java:205)
    at com.sun.jna.Structure.<init>(Structure.java:193)
    at com.sun.jna.Structure.<init>(Structure.java:180)
    at com.sun.jna.Structure.<init>(Structure.java:172)
    at MyClass$1TestStructure.<init>(MyClass.java:8)
    at MyClass.testSimpleSize(MyClass.java:15)
    at MyClass.main(MyClass.java:23)
Caused by: java.lang.IllegalAccessException: class com.sun.jna.Structure cannot access a member of class MyClass$1TestStructure with modifiers "public"
    at java.base/jdk.internal.reflect.Reflection.newIllegalAccessException(Reflection.java:361)
    at java.base/java.lang.reflect.AccessibleObject.checkAccess(AccessibleObject.java:591)
    at java.base/java.lang.reflect.Field.checkAccess(Field.java:1075)
    at java.base/java.lang.reflect.Field.get(Field.java:416)
    at com.sun.jna.Structure.getFieldValue(Structure.java:636)
    ... 11 more

Am I missing something?

Answer 1

As you've discovered in your answer, the root cause of the problem (as indicated by the stack trace) was lack of reflective access to your class. While your answer solves your specific issue, I wanted to expand on more possible reasons this could occur, which may help others.

The reason the JNA test case you linked to works is because the test resides in the same package as the Structure class, which gives the Structure class access to package private classes. In your case, the inner class TestStructure does not have the public modifier, and resides in a different package than com.sun.jna.Structure.

You solved the problem by moving the class to an accessible location, which works. Most often in JNA implementations, structures are declared in interfaces (and thus are by default public).

A second possibility of getting this error is if you are using Java Modules (JPMS). Java Modules do not permit reflective access by default, so it must be explicitly allowed in the module-info.java class, either with opens your.package to com.sun.jna; (to allow runtime-only reflection including private members) or exports your.package to com.sun.jna; (to allow compile-time and run-time public access). While this does not apply in your case (you're using the default package which would be in the unnamed module) this may be something you may encounter in the future if you create a modular project.

Answer 2

I have found a solution and it was much simpler than I thought.

According to the documentation,

An IllegalAccessException is thrown when an application tries to reflectively create an instance (other than an array), set or get a field, or invoke a method, but the currently executing method does not have access to the definition of the specified class, field, method or constructor.

Which means that com.sun.jna.Structure needs to have access to TestStructure, in order to use reflection (which it needs in order to get the struct fields). The simplest way to solve that is to move TestStructure out of the function and make it public. Like this:

import com.sun.jna.Structure;
import java.util.Arrays;
import java.util.List;

public class MyClass {
    public class TestStructure extends Structure {
        public int field;
        @Override
        protected List<String> getFieldOrder() {
            return Arrays.asList("field");
        }
    }
    
    public void testSimpleSize() throws Exception {
        Structure s = new TestStructure();
        //assertEquals("Wrong size", 4, s.size());
    }

    public static void main(String[] args)
    {
        try
        {
            new MyClass().testSimpleSize();
        }
        catch (Exception e)
        {
            System.out.println(e);
        }
    }
}

There may be a better solution, but this is the simplest and works for me.