在视图模式中打开会话[英] Open Session In View Pattern

问题描述

I'm asking this question given my chosen development frameworks of JPA (Hibernate implementation of), Spring, and <insert MVC framework here - Struts 1, Struts 2, Spring MVC, Stripes...>.

I've been thinking a bit about relationships in my entity layer - for example I have an order entity that has many order lines. I've set up my app so that it eagerly loads the order lines for every order. Do you think this is a lazy way to get around the lazy initialization problems that I would come across if I was to set the fetch strategy to false?

The way I see it, I have the following alternatives when retrieving entities and their associations:

1. Use the Open Session In View pattern to create the session on each request and commit the transaction before returning the response.

2. Implement a DTO (Data Transfer Object) layer such that every DAO query I execute returns the correctly initialized DTO for my purposes. I don't really like this option much because in my experience I've found that it creates a lot of boilerplate copying code and becomes messy to maintain.

3. Don't map any associations in JPA so that every query I execute returns only the entities I'm interested in - this will probably require me to have DTOs anyway and will be a pain to maintain and I think defeats the purpose of having an ORM in the first place.

4. Eagerly fetch all (or most associations) - in the example above, always fetch all order lines when I retrieve an order.

So my question is, when and under what circumstances would you use which of these options? Do you always stick with one way of doing it?

I would ask a colleague but I think that if I even mentioned the term 'Open Session in View' I would be greeted with blank stares :( What I'm really looking for here is some advice from a senior or very experienced developer.

Thanks guys!

推荐答案

I've successfully solved all my lazy initialization problems with Open Session In View -pattern (ie. the Spring implementation). The technologies I used were the exact same as you have.

Using this pattern allows me to fully map the entity relationships and not worry about fetching child entities in the dao. Mostly. In 90% of the cases the pattern solves the lazy initialization needs in the view. In some cases you'll have to "manually" initialize relationships. These cases were rare and always involved very very complex mappings in my case.

When using Open Entity Manager In View pattern it's important to define the entity relationships and especially propagation and transactional settings correctly. If these are not configured properly, there will be errors related to closed sessions when some entity is lazily initialized in the view and it fails due to the session having been closed already.

I definately would go with option 1. Option 2 might be needed sometimes, but I see absolutely no reason to use option 3. Option 4 is also a no no. Eagerly fetching everything kills the performance of any view that needs to list just a few properties of some parent entities (orders in tis case).

N+1 Selects

During development there will be N+1 selects as a result of initializing some relationships in the view. But this is not a reason to discard the pattern. Just fix these problems as they arise and before delivering the code to production. It's as easy to fix these problems with OEMIV pattern as it's with any other pattern: add the proper dao or service methods, fix the controller to call a different finder method, maybe add a view to the database etc.

其他推荐答案

Open Session in View has some problems.

For example, if the transaction fails, you might know it too late at commit time, once you are nearly done rendering your page (possibly the response already commited, so you can't change the page !) ... If you had know that error before, you would have followed a different flow and ended up rendering a different page...

Other example, reading data on-demand might turn to many "N+1 select" problems, that kill your performance.

Many projects use the following path:

1. Maintain transactions at the business layer ; load at that point everything you are supposed to need.
2. Presentation layer runs the risk of LazyExceptions : each is considered a programming error, caught during tests, and corrected by loading more data in the business layer (you have the opportunity to do it efficiently, avoiding "N+1 select" problems).

To avoid creating extra classes for DTOs, you can load the data inside the entity objects themselves. This is the whole point of the POJO approach (uses by modern data-access layers, and even integration technologies like Spring).

其他推荐答案

I have successfully used the Open-Session-in-View pattern on a project. However, I recently read in "Spring In Practice" of an interesting potential problem with non-repeatable reads if you manage your transactions at a lower layer while keeping the Hibernate session open in the view layer.

We managed most of our transactions in the service layer, but kept the hibernate session open in the view layer. This meant that lazy reads in the view were resulting in separate read transactions.

We managed our transactions in our service layer to minimize transaction duration. For instance, some of our service calls resulted in both a database transaction and a web service call to an external service. We did not want our transaction to be open while waiting for a web service call to respond.

As our system never went into production, I am not sure if there were any real problems with it, but I suspect that there was the potential for the view to attempt to lazily load an object that has been deleted by someone else.