Retaining backward compatibility

Before

You decide to extend a class and create new choosable classes, e.g. more browsers for a SCM-plugin.
The old data structure looked like this when you had only one class SCMBrowser:
```
    <browser>
      <url>http://yahoo.com/</url>
    </browser>
```

After

Now you decide to add a new NewSCMBrowser, all your SCMBrowsers are extending SCMBrowserBase and your XML suddenly looks like this:

    <browser class="hudson.plugins.foo.NewSCMBrowser">
      <url>http://yahoo.com/</url>
    </browser>

or

    <browser class="hudson.plugins.foo.SCMBrowser">
      <url>http://yahoo.com/</url>
    </browser>

With new jobs, no problem. Old jobs however will probably break.

In your SCMBrowserBase class add a method readResolve (see XStream FAQ):

   // compatibility with earlier plugins
    public Object readResolve() {        
        if (!this.getClass().equals(SCMBrowserBase.class)) {
            return this;
        }
        // make sure to return the default SCMBrowser only if we no class is given in config.
        try {
            return new SCMBrowser(url.toExternalForm());
        } catch (MalformedURLException e) {
            throw new RuntimeException(e);
        }
    }

Hint on retaining backward compatibility

Retaining backward compatibility

Before

After

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