· java

Java: Work out the serialVersionUID of a class

Earlier in the week I wanted to work out the serialVersionUID of a serializable class so that I could override its toString method without breaking everything.

I came across Frank Kim’s blog post which suggested using the serialver tool which comes with the JDK.

I created a little Maven project to test this tool out on a very simple class:

import java.io.Serializable;

public class SerialiseMe implements Serializable
{

}

If we compile that class into a JAR and then run the serialver tool we see the following output:

$ serialver -classpath target/serialiser-0.0.1-SNAPSHOT.jar SerialiseMe
SerialiseMe:    static final long serialVersionUID = -6060222249255158490L;

I wanted to quickly confirm that I could serialise and deserialise this class using this value so I wrote the following bit of code to serialise the class (when it didn’t have a serial version UID):

public class Serialiser
{
    public static void main( String[] args ) throws IOException, ClassNotFoundException
    {
        ByteArrayOutputStream bout = new ByteArrayOutputStream(  );
        ObjectOutputStream oout = new ObjectOutputStream( bout );

        Object value = new SerialiseMe();

        oout.writeObject( value );
        oout.close();
        byte[] bytes = bout.toByteArray();

        FileOutputStream fileOuputStream = new FileOutputStream("/tmp/foo.txt");
        fileOuputStream.write(bytes);
        fileOuputStream.close();
    }
}

After I’d done that, I wrote the following bit of code to deserialise the file:

public class Deserialiser
{
    public static void main( String[] args ) throws IOException, ClassNotFoundException
    {
        FileInputStream fileInputStream = new FileInputStream( new File( "/tmp/foo.txt" ) );
        byte[] bytes = IOUtils.toByteArray( fileInputStream );

        ByteArrayInputStream in = new ByteArrayInputStream( bytes, 0, bytes.length );
        ObjectInputStream oin = new ObjectInputStream( in );
        Object object = oin.readObject();
    }
}

I plugged the serial version UID into the class and was able to deserialise it correctly. I tried changing one of the digits just to check it would blow up and indeed it did:

import java.io.Serializable;

public class SerialiseMe implements Serializable
{
    static final long serialVersionUID = -6060222249255158491L;
}
Exception in thread "main" java.io.InvalidClassException: SerialiseMe; local class incompatible: stream classdesc serialVersionUID = -6060222249255158490, local class serialVersionUID = -6060222249255158491
	at java.io.ObjectStreamClass.initNonProxy(ObjectStreamClass.java:604)
	at java.io.ObjectInputStream.readNonProxyDesc(ObjectInputStream.java:1620)
	at java.io.ObjectInputStream.readClassDesc(ObjectInputStream.java:1515)
	at java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1769)
	at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1348)
	at java.io.ObjectInputStream.readObject(ObjectInputStream.java:370)
	at Deserialiser.main(Deserialiser.java:18)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:601)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

serialver #ftw!

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