Friday, April 24, 2009

Bean properties without hard-coded names?

The problem

Java Beans specifications have been around for more than a decade and, although they were good for tools (IDE), their full capabilities (bound properties in particular) were not much used in 3nd party libraries until only recently (e.g. JSR-295 "Beans binding", JSR-303 "Beans Validation", Glazed Lists...)

I have never been a big fan of Bean properties for many reasons:
  1. they incur a lot of boilerplate code (in particular for bound properties)
  2. there's no way to refer to a bean property by anything else than its name, a hard-coded String, hence this doesn't bear refactoring
  3. they offer no compile-time safety (for the same reason as above, e.g. how can you be sure that "SSID" property is a String, it might be an int, or even an instance of a custom class!)
OK, I know some points are easily worked around:
  • Issue 1, for instance, is not a problem for most IDE: they will generate getter/setter for all your properties in no time; however, you have to remember that software maintenance cost increases with the number of lines of code to maintain (whether manually produced or automatically generated).
  • Refactoring of properties (issue 2) can be correctly handled by your IDE as well (with just some little extra effort)
Nevertheless, I see no workaround to issue 3, no IDE -as far as I know- will check that a hard-coded name refers to a property of some given type!

For developers working on Swing applications in particular (but developers in other architectures may be concerned as well), beans usage is a must, so they face those issues everyday.

The solution

I will further demonstrate a proof of concept of how we can solve issues 2 & 3 above. The complete prototype also includes a solution to issue 1 (i.e. transforming "normal" bean properties into bound properties) but I won't discuss it here because many solutions have been blogged about for a couple of years. There's even an example of that in cglib snippets.

Before getting to the solution, I'd like to describe how I came to it. It's quite simple. For unit testing, I like to use EasyMock. In EasyMock, here is how you create a mock and define its expectations:
import static org.easymock.EasyMock.expect;
import static org.easymock.EasyMock.createMock;
CustomerService mock = createMock(CustomerService.class);
expect(mock.getNextAppointment()).andReturn(new Date());

What happens here is that EasyMock generates a mock implementation of CustomerService interface so that any call to any method is recorded. Then the generic method expect(T) allows EasyMock to have compile-time safety in the andReturn(T) call.

Hence I had the thought of using the same kind of API to get access to bean property:
import static net.sf.beanutils.BeanUtils.mock;
import static;
import net.sf.beanutils.Property;
MyBean mock = mock(MyBean.class);
Property<MyBean, String> property = property(mock.getCustomerId());

Here, I would introduce a specific generic class Property<T, U> that encapsulates description of a given property of a specific bean class.

Of course, in the example above, you've probably seen that -technically- this is different from EasyMock because EasyMock deals with interfaces while we have to deal with beans (non abstract classes). But there is an EasyMock extension that supports just that, it is based on cglib.

I have decided to also use cglib for my proof of concept, because its API seems quite easy and I found the provided examples quite straightforward.

First of all, let me introduce Bean<T> class, which is the main factory for Property instances of a given bean:
public class Bean<T>
// The only way to get a Bean<T> instance is to use this factory method
public static <T> Bean<T> create(Class<T> clazz) {...}

// Initializes all members (uses cglib)
protected Bean(Class<T> clazz)
_clazz = clazz;
_properties = ReflectUtils.getBeanProperties(_clazz);
_mockInterceptor = new MockInterceptor(_properties);

// Create a mock immediately with cglib
Enhancer enhancer = new Enhancer();
_mock = clazz.cast(enhancer.create());

public Class<T> type() {...}

// Returns a T mock, used in conjunction with property() method (as argument)
public T mock() {...}

// Returns a beans property reference without using its hard-coded string name
// This pattern will always survive bean refactoring (compile-safe)!
// The returned reference can be used to get/set property value or get its
// name (in a safe way)
public<U> Property<T, U> property(U mockCall)
PropertyDescriptor property = _mockInterceptor.lastUsedProperty();
checkType(mockCall, property);
return Property.create(property);

// Create a new bean that delegates to this one but makes all its
// properties bound
public T proxy(T source) {...}

// Global cache of Bean objects (each class T should have only one
// Bean<T> instance)
private static final Map<Class<?>, Bean<?>> _cache =
new HashMap<Class<?>, Bean<?>>();

private final Class<T> _clazz;
private final PropertyDescriptor[] _properties;
private final MockInterceptor _mockInterceptor;
private final T _mock;

Only the relevant methods & API are shown above. The "meat" of the code is essentially in Bean<T> constructor and the property() method. The proxy() method is not shown here but is also interesting.

The second interesting piece of code is the MockInterceptor class, which is automatically called by cglib for any call to a method of _mock:
class MockInterceptor implements MethodInterceptor
public MockInterceptor(PropertyDescriptor[] properties)
_properties = properties;

public PropertyDescriptor lastUsedProperty()
PropertyDescriptor property = _lastUsedProperty;
_lastUsedProperty = null;
return property;

public Object intercept(
Object target, Method method, Object[] args, MethodProxy proxy)
throws Throwable
_lastUsedProperty = null;
// Check this is a getter
for (PropertyDescriptor descriptor: _properties)
if (method.equals(descriptor.getReadMethod()))
_lastUsedProperty = descriptor;
//TODO try to return something non-null when possible
//TODO should we call super method if not abstract of course)?
return null;

private final PropertyDescriptor[] _properties;
//FIXME should be in a ThreadLocal no?
private PropertyDescriptor _lastUsedProperty = null;

The principles are quite simple actually: every time a getter of _mock is called, the matching java.beans.PropertyDescriptor is stored in _lastUsedProperty. The latest is used in Bean<T>.property() method to create a new Property<T, U> instance:
public class Property<T, U>
static<T, U> Property<T, U> create(PropertyDescriptor descriptor)
return new Property<T, U>(descriptor);

protected Property(PropertyDescriptor descriptor)
_descriptor = descriptor;

public Class<?> type() {...}
public U get(T bean) {...}
public void set(T bean, U value) {...}

public String name()
return _descriptor.getName();

private final PropertyDescriptor _descriptor;

As you can see, Property<T, U> is merely a wrapper to a java.beans.PropertyDescriptor instance, with additional type information (T: type of the bean, U: type of the bean property) added as generic parameters, making it typesafe.

With this little design, here is what we have achieved:
import static net.sf.beanutils.Bean.*;
Bean<MyBean> helper = create(MyBean.class);
MyBean mock = mock();
Property<MyBean, String> prop1 =;
Property<MyBean, Integer> prop2 =;
System.out.println(; // prints "myFirstProperty"
MyBean bean = new MyBean();
prop1.set(bean, "Something");
prop2.set(bean, 123);
prop1.set(bean, 123); // Compile-time error!

This is not exactly like the foreseen use shown at the beginning of this post, but that's not very far!

Evaluation & Limitations

All initial issues have been solved:
  1. no boilerplate code for bound properties (performed by Bean<T>.proxy(T source) method)
  2. no need for hard-coded property names thanks to Property<T, U>.name() that will return the right name
  3. thanks to heavy use of Java 5 generics, this proof of concept provides compile-time safety

There are still some limitations with the current design:
  1. this works only with classes that comply to java-beans specifications (but that was a pre-requisite of the proof of concept)
  2. this won't work with final classes or final getters of bean classes (limitation of cglib)
  3. the current prototype is not thread-safe (that one is very easy to fix, with a ThreadLocal)
  4. write-only properties (no getter) are not supported (that could be added if needed)
  5. type-safety may be not guaranteed in case of misuse of the API (e.g. not following the usage example shown above)


The current proof of concept just demonstrated what was feasible. Some possible next steps would be:
  1. fix limitations #3 & #4 above (quite easy)
  2. add possibility to add/remove property listeners directly in Property<T, U> (would ease creation of new frameworks such as binding or validation)
  3. provide a work-around to limitation #2 (like logging a warning that some methods are final)
  4. check that generated proxy can work with Hibernate and Beans-bindings
  5. create an Open Source project somewhere (I thought about SourceForge which I am familiar with)

For step #5, I am not sure I am willing to start such a project alone (I already have a bunch of other OSS projects currently and I never can find enough time for them), so if someone is interested, send me a note!

You can find the complete project (maven 2 and eclipse) here.

Any comments are appreciated. Have fun with this prototype!