不可序列化对象异常

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

Maybe an object inside your MemcachedClient class can't be serialized. You have 2 options to solve the problem:

1. Make sure all the classes there implement the Serializable interface.

    public class MemcachedClient { //AnotherClass must implement Serializable too. private AnotherClass anotherClassInstance; } 

2. If the other class doesn't implement Serializable (and you can't modify those classes to make them Serializable ), then add the transient keyword so those objects won't be serialized

    public class MemcachedClient { //AnotherClass doesn't implement Serializable. private transient AnotherClass anotherClassInstance; } 

If you must have an instance of AnotherClass when you deserialize your MemcachedClient object, then you can write the readObject(ObjectInputStream is) to create an instance based on the needs:

public class MemcachedClient {

    //AnotherClass doesn't implement Serializable.
    private transient AnotherClass anotherClassInstance;

    private void writeObject(ObjectOutputStream os) throws IOException, ClassNotFoundException {
        os.defaultWriteObject();
    }

    private void readObject(ObjectInputStream is) throws IOException, ClassNotFoundException {
        is.defaultReadObject();
        anotherClassInstance = new AnotherClass();
        //more logic to get a consistant AnotherClass instance.
    }
}

---

Based in Brian's comment, assuming this MemcachedClient class is from a third party library and you can't modify it, you would have an scenario like described in point 2:

public class MyClass {

    //MemcachedClient can't be modified and doesn't implement Serializable interface
    //the only solution would be using the transient modifier
    private transient MemcachedClient memcachedClient;

    //MyClass attributes and methods...

    //If you need to keep a MemcachedClient instance when deserializing the object
    //just write the readObject method

    private void writeObject(ObjectOutputStream os) throws IOException, ClassNotFoundException {
        os.defaultWriteObject();
    }

    private void readObject(ObjectInputStream is) throws IOException, ClassNotFoundException {
        is.defaultReadObject();
        memcachedClient= new MemcachedClient();
        //more logic to get a consistant MemcachedClient instance.
    }
}

Answer 2

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.

Answer 3

确保您的对象实现可Serializable （甚至从对象引用的所有对象）

Answer 4

I had a similar issue with serializing lists. My issue turned out to be that I was using List.subList, and even though the List only contains serializable items, the List returned by subList was not serializable. A quick solution was to do something like

List mySublist = new ArrayList().addAll(originalList.subList(...));