Sunday, April 05, 2009

Groovy beans

A typical Java Bean is a Java class which is serializable, has a public no-arg constructor, private attributes, and getter/setter methods. I'd say this is a lot of stuff for a simple pojo.

Groovy beans did to pojo's what Java could not. They are actually simple. A typical Groovy bean needs to implement java.io.Serializable and add properties to it. We do not need to add any getters/setters, they are added automatically for us.

Here is a simple Groovy bean.


class Customer implements java.io.Serializable {
String firstName //will be a private attribute in the bytecode
String lastName //will be a private attribute
def middleInitial //beans can even contain dynamic properties
}


See.... Groovy beans do not have any getters or setters, and you will also have noticed that the fields are not private. Even though the source does not contain getter/setter methods and private attributes, the bytecode does. When this bean is converted into bytecode all attributes which do not have an access modifier will be treated as properties. What this means is, they will become private attributes and public getter/setter methods will be generated for them.

Now let's create an instance of this bean.


def customer = new Customer()
//Even though this looks like we are accessing public fields
//the bytecode actually invokes the setter methods of these properties
customer.firstName = "John"
customer.lastName = "Gardner"
customer.middleInitial = "U"
println 'Customer's Last Name: ' + customer.lastName


Creating an instance of a Groovy bean is very similar to Java, but assigning values to it's properties seems to be a bit different. When we assign the value "John" to firstName, it seems as if we are assigning a value to a public field. But this is not what happens. Internally Groovy converts this into a call to the setter method for that field. Thus we get the same end result as in Java, but with less verbosity.

We can also use the bean constructor to instantiate and initialize the bean together.


def customer1 = new Customer(firstName: "Bob",
lastName: "Steele",
middleInitial: "Z")


In the above code this is what really happens under the hoods. First the constructor is invoked, followed by calls to the corresponding setter methods.

This is wonderful. Groovy Beans are just like Java Beans, but much simpler and smaller.

Now, you must be thinking, but what if I want to add custom code to the getter or setter? That is simple, simply create your own getter or setter and Groovy will use your code instead of automatically generating it's getter or setter. Your getter/setter can either be public or even protected if you so wish.

But what if we want to create a simple field without getters and setters. How do we prevent Groovy from automatically inserting getter/setters in the bytecode. Again simply create a field and give it an access modifier (like 'private') and Groovy will not auto generate anything for you.

One question that comes to mind, is how do we create package private attributes? Since Groovy 1.6, a package private field can be created by annotating it with @PackageScope

1 comment:

Abhilasha said...

hey it was a great information .
keep it up!!