How to override ObjectOutputStream.writeStreamHeader()?

Question

The method ObjectOutputStream.writeStreamHeader() can be overridden to prepend or append data to the header. However, if that data is based on an argument passed to the derived class's constructor like:

public class MyObjectOutputStream extends ObjectOutputStream {

    public MyObjectOutputStream( int myData, OutputStream out ) throws IOException {
        super( out );
        m_myData = myData;
    }

    protected void writeStreamHeader() throws IOException {
        write( m_myData );            // WRONG: m_myData not initialized yet
        super.writeStreamHeader();
    }

    private final int m_myData;
}

it doesn't work because super() is called before m_myData is initialized and super() calls writeStreamHeader(). The only way I can think to work around this is by using ThreadLocal like:

public class MyObjectOutputStream extends ObjectOutputStream {

    public MyObjectOutputStream( int myData, OutputStream out ) throws IOException {
        super( thunk( myData, out ) );
    }

    protected void writeStreamHeader() throws IOException {
        write( m_myData.get().intValue() );
        super.writeStreamHeader();
    }

    private static OutputStream thunk( int myData, OutputStream out ) {
        m_myData.set( myData );
        return out;
    }

    private static final ThreadLocal<Integer> m_myData = new ThreadLocal<Integer>();
}

This seems to work, but is there a better (less clunky) way?

Answer 1

There is a general way to solve this sort of problem. Make the class and inner class and reference a variable in the outer scope. (Note, this only works with -target 1.4 or greter, which is the default in current versions of javac. With -target 1.3 you will get an NPE.)

public static ObjectOutputStream newInstance(
    final int myData, final OutputStream out
) throws IOException {
    return new ObjectOutputStream(out) {
        @Override
        protected void writeStreamHeader() throws IOException {
            write(myData);
            super.writeStreamHeader();
        }
    };
}

But, it's probably easier just to write the data out before constructing the ObjectOuputStream.