What causes "Cannot deserialize Proxy class ${type.getName()} as a Bean" in FasterXML/jackson-databind?

Deserialize into a concrete POJO class (the un-proxied class), not the proxy class.

FasterXML/jackson-databind · error · IllegalArgumentException

Cannot deserialize Proxy class ${type.getName()} as a Bean

Error message

Cannot deserialize Proxy class ${type.getName()} as a Bean

What it means

Thrown by BeanDeserializerFactory.isPotentialBeanType when the target class is a JDK dynamic proxy (ClassUtil.isProxyType true -- class name starts with the proxy prefix). Jackson cannot instantiate or populate a java.lang.reflect.Proxy-backed class as a bean because its invocation handler is opaque, so it rejects it up front.

Source

Thrown at src/main/java/tools/jackson/databind/deser/BeanDeserializerFactory.java:1071

    /**********************************************************
     */

    /**
     * Helper method used to skip processing for types that we know
     * cannot be (i.e. are never consider to be) beans:
     * things like primitives, Arrays, Enums, and proxy types.
     *<p>
     * Note that usually we shouldn't really be getting these sort of
     * types anyway; but better safe than sorry.
     */
    protected boolean isPotentialBeanType(Class<?> type)
    {
        String typeStr = ClassUtil.canBeABeanType(type);
        if (typeStr != null) {
            throw new IllegalArgumentException("Cannot deserialize Class "+type.getName()+" (of type "+typeStr+") as a Bean");
        }
        if (ClassUtil.isProxyType(type)) {
            throw new IllegalArgumentException("Cannot deserialize Proxy class "+type.getName()+" as a Bean");
        }
        // [databind#3229]: Local/anonymous classes cannot be instantiated but
        //   can still be updated via `readerForUpdating()`. So let them through
        //   here; if actual instantiation is attempted, `ValueInstantiator` will
        //   fail with a clear error.
        return true;
    }

    /**
     * Helper method that will check whether given raw type is marked as always ignorable
     * (for purpose of ignoring properties with type)
     */
    protected boolean isIgnorableType(DeserializationContext ctxt, BeanPropertyDefinition propDef,
            Class<?> type, Map<Class<?>,Boolean> ignoredTypes)
    {
        Boolean status = ignoredTypes.get(type);
        if (status != null) {
            return status.booleanValue();

View on GitHub (pinned to 87876ca5c0)

Solutions

Deserialize into a concrete POJO class (the un-proxied class), not the proxy class.
Unwrap the proxy (e.g. Spring's AopProxyUtils.getSingletonTarget / HibernateProxyHelper) before serializing/deserializing.
Register a mix-in or custom deserializer that targets the real class behind the proxy.
Avoid using proxy classes as Jackson target types; use the real interface/implementation.

Example fix

// before: target is a proxied class
Object read = mapper.readValue(json, proxiedInstance.getClass());

// after: target the real class behind the proxy
Class<?> real = AopProxyUtils.proxiedUserInterfaces(proxiedInstance)[0];
Object read = mapper.readValue(json, real);

Defensive patterns

Strategy: type-guard

Validate before calling

Class<?> type = instance.getClass();
if (ClassUtil.isProxyType(type)) {
    throw new IllegalArgumentException(type + " is a Proxy; use the real class");
}
mapper.readValue(json, type);

Type guard

boolean isProxiable(Class<?> c) {
    return !ClassUtil.isProxyType(c);
}

Prevention

Deserialize into concrete POJO classes, not proxy classes.
Unwrap framework proxies (Spring/Hibernate) before binding.
Register mixins targeting the real interface/class.

When it happens

Trigger: Calling readValue(..., MyClass.class) (or forcing bean deserialization) where the runtime class is a Proxy generated by java.lang.reflect.Proxy, or a custom AbstractTypeResolver/introspector pointing at a proxy class.

Common situations: Passing a proxy instance's class (e.g., from a framework AOP/interceptor) as the target type; binding to an interface that has been proxied; Spring/Hibernate proxies leaking into Jackson as target types.

Related errors

AI-assisted analysis of FasterXML/jackson-databind@87876ca5c0 (2026-08-11). Data as JSON: /api/errors/682f4d69d225467b. Report an issue: GitHub.