java-native-access/jna · error · Error

Exception reading field '" + f.getName() + "' in " + getClas

Error message

Exception reading field '" + f.getName() + "' in " + getClass()

What it means

While initializing/pre-reading fields before a native read, reflective access f.get(this) threw. JNA wraps any such exception in java.lang.Error naming the field and Structure class, since it indicates reflective access failure rather than an expected condition.

Source

Thrown at src/com/sun/jna/Structure.java:1492

    }

    /**
     * Initialize any null-valued fields that should have a non-null default
     * value.
     */
    @SuppressWarnings("UseSpecificCatch")
    private void initializeFields() {
        // Get the full field list, don't care about sorting
        List<Field> flist = getFieldList();
        for (Field f : flist) {
            try {
                Object o = f.get(this);
                if (o == null) {
                    initializeField(f, f.getType());
                }
            }
            catch (Exception e) {
                throw new Error("Exception reading field '" + f.getName() + "' in " + getClass(), e);
            }
        }
    }

    private Object initializeField(Field field, Class<?> type) {
        Object value = null;
        if (Structure.class.isAssignableFrom(type)
            && !(ByReference.class.isAssignableFrom(type))) {
            try {
                value = newInstance((Class<? extends Structure>) type, PLACEHOLDER_MEMORY);
                setFieldValue(field, value);
            }
            catch(IllegalArgumentException e) {
                String msg = "Can't determine size of nested structure";
                throw new IllegalArgumentException(msg, e);
            }
        }
        else if (NativeMapped.class.isAssignableFrom(type)) {

View on GitHub (pinned to d036ad9781)

Solutions

1. Make the failing field public and ensure its type has an accessible no-arg constructor
2. Fix exceptions thrown in the field's initializer or its class's constructor (check the cause chain)
3. Ensure initializeFields runs only when fields are writable; avoid SecurityManager restrictions blocking reflection
4. If a custom TypeMapper's fromNative path throws during initialization, fix that converter

Example fix

// before
class S extends Structure {
    private X x = new X(); // X ctor throws or field inaccessible
}
// after
class S extends Structure {
    public X x; // X has a safe no-arg constructor
}

for (Field f : S.class.getFields()) {
    try { f.setAccessible(true); } catch (SecurityException e) { throw new IllegalStateException("Cannot access " + f, e); }
}

try {
    s.read();
} catch (Error e) {
    if (e.getMessage() != null && e.getMessage().startsWith("Exception reading field")) {
        Throwable cause = e.getCause();
        // fix reflective access or the field initializer that threw
        throw new IllegalStateException("Fix field access/initializer: " + e.getMessage(), cause);
    }
    throw e;
}

When it happens

Trigger: Field.get(this) fails during ensureInitialized/initializeFields — e.g. IllegalAccessException on non-public fields on restricted VMs, or exceptions thrown from field initializers/constructors of field objects (including Structure subclass constructors that themselves fail).

Common situations: Structures with private fields under strict access control; field objects whose constructors throw (recursive structure initialization, OOM); J2ME/embedded VMs lacking access override support.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• This VM does not support read-only fields (field '" + field.
• Can't determine size of nested structure
• Can't instantiate " + type
• Instantiation of " + type + " (Pointer) not allowed, is it p
• Exception thrown while instantiating an instance of " + type

AI-assisted analysis of java-native-access/jna@d036ad9781 (2026-09-12). Data as JSON: /api/errors/ee931a9f83eef9b2. Report an issue: GitHub.