Question

I am trying to set a list of objects(serializable) in Memcached client. I am doing it using the set method. I am getting this error message:

Exception occurred in target VM: Non-serializable object

So it’s not allowing me to set the value in the MemcachedClient object. Here’s my code:

MemcachedClient client = CacheConnectionUtil.connectToCacheServer(this.applicationEnv);
client.set(generateCacheKey(namespace, key), expireInSeconds, value);
// value is a list of objects that implements serializable

Why am I getting this error message?

Answer 1

From the Serializable javadocs:

When traversing a graph, an object may be encountered that does not support the Serializable interface. In this case the NotSerializableException will be thrown and will identify the class of the non-serializable object.

Memcached seems to be hiding the rest of the exception message, so you’ll have to identify the non-serializable object yourself.

You say that values is a list of your own object. My guess is your object holds something that isn’t also Serializable, or it holds something that holds something else that isn’t Serializable. Go through the class of the objects in your list and make sure that every object underneath it implements Serializable.

As an example, if I have A implements Serializable:

public class A implements Serializable {
    private B b;
    // ...
}

And B doesn’t:

public class B { ... }

Then any attempt to serialize A using an ObjectOutputStream (which is probably what the default serializer in the Memcached library is doing) will throw a NotSerializableException because its field, private B b, is a non-serializable type, which is what you’re getting.