Json-lib vs. CGLib Proxies - 1:0

Posted on Feb 13, 2008

Sometimes I’m really blind. For the last couple of months I was all the time having some problems with serializing JavaBeans that I get out of Hibernate into JSON and often got an error like this:

java.lang.NoSuchMethodException: Property 'delegate' has no getter method

I always knew that it had to be something with the whole proxy-layer Hibernate uses for handling for instance lazy referencing, but for some reason I never really actually did anything against it but rewrite the code to build the JSONObjects more or less manually.

Explicitly setting lazy=“false” in the respective ForeignKey-column in the mapping file resolves this, but what if I want some laziness in my model and simply not serialize what’s behind that reference?

After posting on the Json-lib mailing-list and following Andres Almiray’s hint of taking a look at PropertyFilter, I wrote another one (I wrote one months ago when I first got this problem but filtered for the name “delegate”), still with no luck. Then I took a look at the source code of the Json-lib and noticed, that the PropertyFilter gets applied after all the properties are extracted using beanutils’ PropertyUtil, which is actually where the exception is thrown in the first place. So back to the source code and a few lines up ;-) Given that I’m already inside the bean, I thought the BeanProcessor must be where I actually want to add my modifications. And right I was:

So basically all you need is a custom JsonBeanProcessor that returns nothing but an empty JSONObject,

@@ java @@ import net.sf.json.JSONObject; import net.sf.json.JsonConfig; import net.sf.json.processor.JsonBeanProcessor;

public class HibernateJsonBeanProcessor implements JsonBeanProcessor {

public JSONObject processBean(Object obj, JsonConfig jsonConfig) {
    return new JSONObject();

} @@

a custom JsonBeanProcessorMatcher that maps the custom processor from above to the offending Bean,

@@ java @@ import net.sf.json.processor.JsonBeanProcessorMatcher; import java.util.Set; import org.apache.log4j.Logger;

public class HibernateJsonBeanProcessorMatcher extends JsonBeanProcessorMatcher {

private static Logger log = Logger.getLogger(HibernateJsonBeanProcessorMatcher.class);

public Object getMatch(Class target, Set set) {
    if (target.getName().contains("$$EnhancerByCGLIB$$")) {
        log.warn("Found Lazy-References in Hibernate object "
                + target.getName());
        return org.hibernate.proxy.HibernateProxy.class;
    return DEFAULT.getMatch(target, set);

} @@

and last but not least the JsonConfig that ties it all together:

@@ java @@ JsonConfig conf = new JsonConfig(); conf.registerJsonBeanProcessor(org.hibernate.proxy.HibernateProxy.class, new HibernateJsonBeanProcessor()); conf.setJsonBeanProcessorMatcher(new HibernateJsonBeanProcessorMatcher()); @@