ObjectOutput throws java.io.NotSerializableException

Question

This is the class that's about to be serialized into a byte array.

public class DummyClass implements Serializable
{
    private static transient final long serialVersionUID    = -8483859843874771619L;
    public String   y;

    public DummyClass(String y)
    {
        this.y = y;
    }
    public String getY()
    {
        return this.y;
    }
}

This is the Serialization execution test

    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutput out = null;
    byte[] bs = null;
    try
    {
        DummyClass dummyClass = new DummyClass("World I Salute you");
        out = new ObjectOutputStream(bos);
        out.writeObject(dummyClass); // <--- Throws exception here
        bs = bos.toByteArray();
    }
    finally
    {
        out.close();
        bos.close();
    }

and regarding the stack trace:

java.io.NotSerializableException: tests.DummyClassTest
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.writeObject(Unknown Source)
at tests.DummyClassTest.dummyTest(DummyClassTest.java:109)

The Obvious question is what am i doing wrong?

Cheers.

EDIT:

The whole Class

public class DummyClassTest
{
    public class DummyClass implements Serializable
    {
        private static transient final long serialVersionUID    = -8483859843874771619L;
        public String   y;

        public DummyClass(String y)
        {
            this.y = y;
        }
        public String getY()
        {
            return this.y;
        }
    }

    @Test
    public void dummyTest() throws IOException
    {
        ByteArrayOutputStream bos = new ByteArrayOutputStream();
        ObjectOutput out = null;
        byte[] bs = null;
        try
        {
            DummyClass dummyClass = new DummyClass("World I Salute you");
            out = new ObjectOutputStream(bos);
            out.writeObject(dummyClass);
            bs = bos.toByteArray();
        }
        finally
        {
            out.close();
            bos.close();
        }
    }
}

Answer 1

A non-static inner class has an implicit reference to its outer class instance. So when you serialize DummyClass, you're also serializing this implicit reference to the DummyClassTest instance owning it.

Define the DummyClass as a top-level class (non nested), or as a static inner class, and everything will work as expected.

The following:

public class DummyClassTest {
    public class DummyClass {
    }
}

is more or less equivalent to

public class DummyClassTest {
}

public class DummyClass {
    private DummyClassTest outerInstance;
}