Serializing arbitrary Java objects

Question

I am currently in need to serialize arbitrary Java objects since I would like to use the Hash as a key for a hash table. After I read various warnings that the default hashCode creates collisions way to often, I wanted to switch to hashing via MessageDigest to use alternative algorithms (e.g. SHA1, ...) that are said to allow more entries without collisions. [As a sidenote: I am aware that even here collisions can occur early on, yet I want to increase the likelihood to remain collision free.]

To achieve this I tried a method proposed in this StackOverflow post. It uses the following code to obtain a byte[] necessary for MessageDigest:

public static byte[] convertToHashableByteArray(Object obj) {
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutput out = null;
    byte[] byteOutput = null;

    try {
        out = new ObjectOutputStream(bos);
        out.writeObject(obj);
        byteOutput = bos.toByteArray();
    } catch (IOException io) {
        io.printStackTrace();
    } finally {
        try {
            if(out != null) { out.close(); }
        } catch(IOException io) {
            io.printStackTrace();
        }

        try {
            bos.close();
        } catch(IOException io) {
            io.printStackTrace();
        }
    }

    return byteOutput;
}

This, however, causes the problem that only objects implementing the serializable interface will be serialized/converted into a byte[]. To circumvent this issue I applied toString() to the given obj in the catch clause to enforce getting a byte[] in all cases:

public static byte[] convertToHashableByteArray(Object obj) {
    ByteArrayOutputStream bos = new ByteArrayOutputStream();
    ObjectOutput out = null;
    byte[] byteOutput = null;

    try {
        out = new ObjectOutputStream(bos);
        out.writeObject(obj);
        byteOutput = bos.toByteArray();
    } catch (IOException io) {
        String stringed = obj.toString();
        byteOutput = stringed.getBytes();
    } finally {
        try {
            if(out != null) { out.close(); }
        } catch(IOException io) {
            io.printStackTrace();
        }

        try {
            bos.close();
        } catch(IOException io) {
            io.printStackTrace();
        }
    }

    return byteOutput;
}

However, this still feels utterly wrong for me. So my question is, whether there is a better alternative to convert arbitrary objects to byte[] to be able to compute hashes. Preferably a solution that works without using additional libraries or one using well established ones like Apache Commons. (Beside that I am also open for other approaches to obtain SHA1/SHA512 hashes of arbitrary Java objects.)

Answer 1

Perhaps you can use UUIDs for your objects as immutable unique identifiers?

Answer 2

There are so many things wrong here...

1. You should have proper key classes with equals and hashCode implemented, instead of using random objects.
2. Serialization performance overhead can easily mean that such map will be slower than even trivial iteration search.
3. Default hashcode should not be used in most cases, as it might be different for objects which are 'equal' from business point of view. You should reimplement hashcode together with equals (which comes back to point 1). Whenever it has collisions due to pointer aliasing is irrelevant if it won't work poperly
4. Way overcomplicated method of closing in-memory streams. Just close them one after another, it is not external resource - if it fails, just let it fail, you don't need to close everything 100% in case of failures. You can also use one of closeable utilities (or try/catch with resources) to avoid some overhead
5. You don't need complicated digest of that byte array - use Arrays.hashCode, it WILL be good enough for your use cases (remember - don't do it anyway, point 1)

If you are still reading and still not willing to implement point 1, go back to point 1. And again. And again.

And to finally answer you question, use hessian serialization.

http://hessian.caucho.com/doc/hessian-overview.xtp

It is very similar to java one, just faster, shorter output and allows serializing objects which do not implement Serializable interface (at the risk of messing things up, you need to set special flag to allow that).

Answer 3

If you want to serialize a given object, i suggest you change your méthod like this :

public static byte[] convertToHashableByteArray(Serializable obj){
     ..........
     ..........
}