How to generate serialVersionUID programmatically in Java?

Question

I am working on a project that generates Java files. I'd like to be able to optionally add the serialVersionUID as you would with the serialver tool.

Is there a way to do this when I generate the Java code, or will I need to ask the user of the tool to provide UIDs manually? To be clear, I'm not looking to do this automatically through Eclipse or the serialver tool, but to do it via Java itself.

Answer 1

There is a version of the serialver tool source available from OpenJDK. It all comes down to this call:

ObjectStreamClass c = ObjectStreamClass.lookup(MyClass.class);
long serialID = c.getSerialVersionUID();
System.out.println(serialID);

In JDK 6 at least it returns the same number with serialver tool.

Answer 2

From ObjectStreamClass:

/**
 * Computes the default serial version UID value for the given class.
 */
private static long computeDefaultSUID(Class<?> cl) {
    if (!Serializable.class.isAssignableFrom(cl) || Proxy.isProxyClass(cl))
    {
        return 0L;
    }

    try {
        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        DataOutputStream dout = new DataOutputStream(bout);

        dout.writeUTF(cl.getName());

        int classMods = cl.getModifiers() &
            (Modifier.PUBLIC | Modifier.FINAL |
             Modifier.INTERFACE | Modifier.ABSTRACT);

        /*
         * compensate for javac bug in which ABSTRACT bit was set for an
         * interface only if the interface declared methods
         */
        Method[] methods = cl.getDeclaredMethods();
        if ((classMods & Modifier.INTERFACE) != 0) {
            classMods = (methods.length > 0) ?
                (classMods | Modifier.ABSTRACT) :
                (classMods & ~Modifier.ABSTRACT);
        }
        dout.writeInt(classMods);

        if (!cl.isArray()) {
            /*
             * compensate for change in 1.2FCS in which
             * Class.getInterfaces() was modified to return Cloneable and
             * Serializable for array classes.
             */
            Class<?>[] interfaces = cl.getInterfaces();
            String[] ifaceNames = new String[interfaces.length];
            for (int i = 0; i < interfaces.length; i++) {
                ifaceNames[i] = interfaces[i].getName();
            }
            Arrays.sort(ifaceNames);
            for (int i = 0; i < ifaceNames.length; i++) {
                dout.writeUTF(ifaceNames[i]);
            }
        }

        Field[] fields = cl.getDeclaredFields();
        MemberSignature[] fieldSigs = new MemberSignature[fields.length];
        for (int i = 0; i < fields.length; i++) {
            fieldSigs[i] = new MemberSignature(fields[i]);
        }
        Arrays.sort(fieldSigs, new Comparator<MemberSignature>() {
            public int compare(MemberSignature ms1, MemberSignature ms2) {
                return ms1.name.compareTo(ms2.name);
            }
        });
        for (int i = 0; i < fieldSigs.length; i++) {
            MemberSignature sig = fieldSigs[i];
            int mods = sig.member.getModifiers() &
                (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED |
                 Modifier.STATIC | Modifier.FINAL | Modifier.VOLATILE |
                 Modifier.TRANSIENT);
            if (((mods & Modifier.PRIVATE) == 0) ||
                ((mods & (Modifier.STATIC | Modifier.TRANSIENT)) == 0))
            {
                dout.writeUTF(sig.name);
                dout.writeInt(mods);
                dout.writeUTF(sig.signature);
            }
        }

        if (hasStaticInitializer(cl)) {
            dout.writeUTF("<clinit>");
            dout.writeInt(Modifier.STATIC);
            dout.writeUTF("()V");
        }

        Constructor[] cons = cl.getDeclaredConstructors();
        MemberSignature[] consSigs = new MemberSignature[cons.length];
        for (int i = 0; i < cons.length; i++) {
            consSigs[i] = new MemberSignature(cons[i]);
        }
        Arrays.sort(consSigs, new Comparator<MemberSignature>() {
            public int compare(MemberSignature ms1, MemberSignature ms2) {
                return ms1.signature.compareTo(ms2.signature);
            }
        });
        for (int i = 0; i < consSigs.length; i++) {
            MemberSignature sig = consSigs[i];
            int mods = sig.member.getModifiers() &
                (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED |
                 Modifier.STATIC | Modifier.FINAL |
                 Modifier.SYNCHRONIZED | Modifier.NATIVE |
                 Modifier.ABSTRACT | Modifier.STRICT);
            if ((mods & Modifier.PRIVATE) == 0) {
                dout.writeUTF("<init>");
                dout.writeInt(mods);
                dout.writeUTF(sig.signature.replace('/', '.'));
            }
        }

        MemberSignature[] methSigs = new MemberSignature[methods.length];
        for (int i = 0; i < methods.length; i++) {
            methSigs[i] = new MemberSignature(methods[i]);
        }
        Arrays.sort(methSigs, new Comparator<MemberSignature>() {
            public int compare(MemberSignature ms1, MemberSignature ms2) {
                int comp = ms1.name.compareTo(ms2.name);
                if (comp == 0) {
                    comp = ms1.signature.compareTo(ms2.signature);
                }
                return comp;
            }
        });
        for (int i = 0; i < methSigs.length; i++) {
            MemberSignature sig = methSigs[i];
            int mods = sig.member.getModifiers() &
                (Modifier.PUBLIC | Modifier.PRIVATE | Modifier.PROTECTED |
                 Modifier.STATIC | Modifier.FINAL |
                 Modifier.SYNCHRONIZED | Modifier.NATIVE |
                 Modifier.ABSTRACT | Modifier.STRICT);
            if ((mods & Modifier.PRIVATE) == 0) {
                dout.writeUTF(sig.name);
                dout.writeInt(mods);
                dout.writeUTF(sig.signature.replace('/', '.'));
            }
        }

        dout.flush();

        MessageDigest md = MessageDigest.getInstance("SHA");
        byte[] hashBytes = md.digest(bout.toByteArray());
        long hash = 0;
        for (int i = Math.min(hashBytes.length, 8) - 1; i >= 0; i--) {
            hash = (hash << 8) | (hashBytes[i] & 0xFF);
        }
        return hash;
    } catch (IOException ex) {
        throw new InternalError();
    } catch (NoSuchAlgorithmException ex) {
        throw new SecurityException(ex.getMessage());
    }
}

Answer 3

If your tool is generating brand new code you don't have any need to compute it the way serialver does. Just use 1, or -1, or whatever you like.

Answer 4

Try the hashcode of the class name of the class being generated.

There could be collisions as a hashcode is not unique, but those collisions are statistically unlikely.

Here's documentation on how serialVersionUID values are generated. Its much more complex than I'd have guessed.

Because of its complexity, I'd either have the user type in the UID themselves, or just use a simple hash of the full classname.

Answer 5

This is an old thread but, I guess, serialVersionUID is still one of the hot topics.

When I started to write Java codes, it was too hard to find and assign a unique value to serlialVersionUID variable for me. After a while, simple date time format (yyyy-MM-ddTHH:mm:ss) gave me the idea: Why don't I generate a value from current date and time?

So, I started to generate (of course manually) values according to current date and time.

Let's say current date and time is 01/09/2015 11:00pm. I would assign 201601091100L value to serialVersionUID.

I hope this idea helps you for further improvements in your projects.