On Oct 20, 2009, at 2:16 PM, Asiri Rathnayake wrote:

Hi,

I don't understand why you've used "transient". Can you explain?

...

Sorry, "mutable servlet field" was not the error. It was "A non- serializable value is stored into a non-transient field of a serializable class". My mistake.

This is why I added the transient field thinking it would fix the problem. It did fix the problem but I wasn't sure if that is the correct thing to do.

ok that's better indeed. The reason is that according to the spec a Servlet can be serialized (saved on disk) if the memory becomes low and thus must be able to be loaded back into memory at a later point (the same need for load-balancing). However your change has introduced another problem: transient means the field will not be saved when the servlet is serialized. Thus when it's deserialized these fields will be null and the code will fail... Whereas before the servlet would have failed on serialization, it'll now fail on deserialization... Note: Deserialization will not call the constructor. Solution: either make the field objects serializable or capture deserialization to set up the transient field values. Note that this last solution may potentially cause problems with threading so this might need to be synchronized I think. The best solution is to make the field Objects Serializable IMO. WDYT? Thanks -Vincent

IMO the problem is the design of XWikiDavServlet. It shouldn't contain any field. For example why not use the component manager to load a component to perform the handling of DAV requests? The CM would be retrieved from the servlet context. Thanks -Vincent On Oct 20, 2009, at 2:36 PM, Asiri Rathnayake wrote:

Hi,

ok that's better indeed. The reason is that according to the spec a

...

Servlet can be serialized (saved on disk) if the memory becomes low and thus must be able to be loaded back into memory at a later point (the same need for load-balancing).

However your change has introduced another problem: transient means the field will not be saved when the servlet is serialized. Thus when it's deserialized these fields will be null and the code will fail...

Whereas before the servlet would have failed on serialization, it'll now fail on deserialization...

Note: Deserialization will not call the constructor.

I was hoping upon de serialization it will call init() where i initialize these variables. But I can't find a reference that says so (yet).

...

Solution: either make the field objects serializable or capture deserialization to set up the transient field values. Note that this last solution may potentially cause problems with threading so this might need to be synchronized I think. The best solution is to make the field Objects Serializable IMO.

One instance variable used is provided by jackrabbit library which is not serializable. I can do a custom serialization or,

I can create these instance variable for each request (get***) which is a waste.

Looking around to see if there is another way.

Thanks.

- Asiri

...

WDYT?

Thanks -Vincent

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On Oct 20, 2009, at 3:04 PM, Asiri Rathnayake wrote:

Hi Vincent,

On Tue, Oct 20, 2009 at 6:18 PM, Vincent Massol <[email protected]> wrote:

...

IMO the problem is the design of XWikiDavServlet. It shouldn't contain any field.

For example why not use the component manager to load a component to perform the handling of DAV requests?

jackrabbit DavServlet has following abstract methods:

<code> /** * Checks if the precondition for this request and resource is valid. * * @param request * @param resource * @return */ abstract protected boolean isPreconditionValid(WebdavRequest request, DavResource resource);

/** * Returns the <code>DavSessionProvider</code>. * * @return the session provider */ abstract public DavSessionProvider getDavSessionProvider();

/** * Returns the <code>DavSessionProvider</code>. * * @param davSessionProvider */ abstract public void setDavSessionProvider(DavSessionProvider davSessionProvider);

/** * Returns the <code>DavLocatorFactory</code>. * * @return the locator factory */ abstract public DavLocatorFactory getLocatorFactory();

/** * Sets the <code>DavLocatorFactory</code>. * * @param locatorFactory */ abstract public void setLocatorFactory(DavLocatorFactory locatorFactory);

/** * Returns the <code>DavResourceFactory</code>. * * @return the resource factory */ abstract public DavResourceFactory getResourceFactory();

/** * Sets the <code>DavResourceFactory</code>. * * @param resourceFactory */ abstract public void setResourceFactory(DavResourceFactory resourceFactory);

/** * Returns the value of the 'WWW-Authenticate' header, that is returned in * case of 401 error. * * @return value of the 'WWW-Authenticate' header */ abstract public String getAuthenticateHeaderValue(); </code>

Now I have few doubts about why there are setXXX() methods but the point is we need those instance variables if we are to implement these methods.

We could still either make them components and look them up in getXXX() or have a manager component that returns them. Thanks -Vincent

Thanks.

- Asiri

...

The CM would be retrieved from the servlet context.

Thanks -Vincent

On Oct 20, 2009, at 2:36 PM, Asiri Rathnayake wrote:

...

Hi,

ok that's better indeed. The reason is that according to the spec a

...

Servlet can be serialized (saved on disk) if the memory becomes low and thus must be able to be loaded back into memory at a later point (the same need for load-balancing).

However your change has introduced another problem: transient means the field will not be saved when the servlet is serialized. Thus when it's deserialized these fields will be null and the code will fail...

Whereas before the servlet would have failed on serialization, it'll now fail on deserialization...

Note: Deserialization will not call the constructor.

I was hoping upon de serialization it will call init() where i initialize these variables. But I can't find a reference that says so (yet).

...

Solution: either make the field objects serializable or capture deserialization to set up the transient field values. Note that this last solution may potentially cause problems with threading so this might need to be synchronized I think. The best solution is to make the field Objects Serializable IMO.

One instance variable used is provided by jackrabbit library which is not serializable. I can do a custom serialization or,

I can create these instance variable for each request (get***) which is a waste.

Looking around to see if there is another way.

Thanks.

- Asiri

...

WDYT?

Thanks -Vincent

On Oct 20, 2009, at 2:32 PM, Sergiu Dumitriu wrote:

On 10/20/2009 02:16 PM, Asiri Rathnayake wrote:

...

Hi,

I don't understand why you've used "transient". Can you explain?

...

Sorry, "mutable servlet field" was not the error. It was "A non- serializable value is stored into a non-transient field of a serializable class". My mistake.

This is why I added the transient field thinking it would fix the problem. It did fix the problem but I wasn't sure if that is the correct thing to do.

Thanks.

Why don't you make them static? Instance members of a singleton class is roughly equivalent to static members.

Please don't use statics. This is an antipattern. BTW I'm pretty sure I've read that statics can be lost when an object is serialized if the JVM unloads the class. Thanks -Vincent