wicket-safemodel

A typesafe and refactoring-safe way to build Wicket PropertyModels.

Note: This is no longer maintained. A faster implementation based partially on these ideas is available at wicketstuff/LazyModel. The API is nearly identical. I am keeping this implementation here because LazyModel only works with Wicket 6 and up, whereas SafeModel works with Wicket 1.5.

Current version: 1.3.4.1

New in 1.3.4.1:

Fixed bug where model creation would fail when the last getter in the from-chain returned null.
Note: This version has been available for years, I only forgot to update this README. SafeModel continues to be unmaintained, see the notice about LazyModel above.

New in 1.3.3:

Fixed bug where under some circumstances reflection on a model's object type didn't work. Now the getObject().getClass() will be used if getObject() isn't null.

New in 1.3.2:

Fixed bug where fromService couldn't work with java.lang.reflect.Proxy based instances either.

New in 1.3.1:

Fixed bug where fromService could not work with CGlib-proxied service instances, such as those generated by wicket-inject.

New in 1.3:

Can now work with IModels as targets. Reflection and proxying is done on a best-effort basis.
In addition to PropertyModels, SafeModel can now also build LoadableDetachableModels to wrap backend service calls. See below.

Why do I need this?

There are (vastly simplified for the sake of the argument) two ways to build models in Wicket. One is typesafe and refactoring-safe:

SomeBean myBean = ...
IModel<String> childNameModel = new Model<String>() {
    public void setObject(String s) {
        myBean.getChild().setName(s);
    }
    public String getObject() {
        return myBean.getChild().getName();
    }
};

As you can see, this way is also utterly verbose.

The other way is much better in this regard:

IModel<String> childNameModel = new PropertyModel<String>(
    myBean, "child.name");

A one-liner, basically. Of course, this will embarrassingly fail at runtime if you refactor your bean and rename even just one of the properties along the path to what your model is pointing to. It will also fail at runtime if you have a typo in the path string. There is no way to catch any of this at compile time.

Or is there?

What do we want instead?

What we really want are functional models like in the Scala extensions for Wicket. Unfortunately, most of us are stuck in Java world where proper functions are still considered science fiction like flying cars and World Peace.

We still want something concise, typesafe, refactoring-safe and (mostly) compilesafe. This can be achieved with something like metagen. Unfortunately, this requires an additional build step since it uses bytecode instrumentation. On the plus side, it gives you good compile-safety at almost no runtime cost. On the downside, you have to get it working in your main (probably Maven) build, and most likely separately in your IDE build settings. This can be awkward, especially for third-party developers.

If you feel comfortable with configuring annotation processors, use metagen, since it will give you more than a reflection-based technique ever can.

If you don't want to fiddle with your build, yet still want more safety for almost all use cases, read on.

Introducing SafeModel

How about this?

SomeBean myBean = ...
IModel<String> childNameModel = model(from(myBean).getChild().getName());

Note that there is no casting, nor any type annotation on the right hand side. There is no string literal either, yet everything is fully typesafe. Cool, huh?

How do I use it?

Add my maven repo to your pom.xml:

<repositories>
  <repository>
    <id>duesenklipper</id>
    <url>http://duesenklipper.github.com/maven/releases</url>
    <snapshots>
      <enabled>false</enabled>
    </snapshots>
    <releases>
      <enabled>true</enabled>
    </releases>
  </repository>
</repositories>

Add a dependency on safemodel1.4 if you use Wicket 1.4.x...

<dependency>
  <groupId>de.wicketbuch.safemodel</groupId>
  <artifactId>safemodel1.4</artifactId>
  <version>1.3.3</version>
</dependency>

...or on safemodel1.5 if you use Wicket 1.5.x:

<dependency>
  <groupId>de.wicketbuch.safemodel</groupId>
  <artifactId>safemodel1.5</artifactId>
  <version>1.3.3</version>
</dependency>

Add a static import to your class to get the from and model methods into your scope:
```
import static de.wicketbuch.safemodel.SafeModel.*;
```
Then use it as shown above - use from(my-root-object) to start and walk through your getters to the property you need. Wrap the whole thing in model(...) and you get a properly typechecked IModel instance. You can use any IModel<root-type> as a root object. In this case, reflection and proxying will be more difficult, so if you have truly strange generics, this might fail. It will not creep up on you though - it either works the first time, or it doesn't.
To get a LoadableDetachableModel to wrap backend calls, use fromService instead of from:
```
IModel<User> userModel = model(fromService(userEJB).loadUser(42));
```
This will give you a LoadableDetachableModel that will load the user with the ID 42. Note that in this case no arbitrary chaining of method calls is possible - just fromService(<service>.<methodcall>). This should cover most use cases.

Currently this works only with non-final JavaBean-style objects with standard getter methods. It also supports java.util.List<T>s and java.util.Map<String, V>s. Note that only string keys are allowed for maps.

This does not cover all imaginable use cases, but for the 90% of cases where you have simple JavaBean-style domain classes, this should be useful.

Enjoy!

Carl-Eric

Can I use this? What's the license?

It's only locally-tested code, so take it with a grain of salt for now.

The license is Apache 2.0, so you can do almost anything you like.

How does it work?

Under the hood, the from method uses jMock's ClassImposteriser to create a proxy around the given bean. The following calls each record the property name they are at and return further proxies. The model method captures the whole affair and constructs an old-fashioned property path string from the recorded property names. It then creates a new PropertyModel using the originally given bean and the recorded property path.

Types work out because the type parameter of the model will be the type of the whole expression passed to model - and that is the type of the last getter in the chain, which is exactly what we want. Thus we can safely use the normally unsafe PropertyModels, because the creation process is stricter than just writing a string. Type errors and refactorings will be caught by this.

There may be runtime errors only if you have odd class hierarchies that end up confusing SafeModel. With any "normal" JavaBean-style classes this should work just fine. See the source for SafeModelTest for further examples.

Acknowledgements

The idea for this was lifted wholesale from the LambdaJ-based proposal on the Wicket wiki. Instead of pulling in LambdaJ as a dependency, SafeModel uses jMock's ClassImposteriser directly (both LambdaJ and Mockito use that as well). The code was also simplified to not require the root object's class to be passed in.

duesenklipper/wicket-safemodel