Overriding methods dynamically

In the previous recipe, Dynamically extending classes with new methods, we learned how to dynamically add a method to a class through one of the metaprogramming features of Groovy, named ExpandoMetaClass.

In this recipe, we will see how to intercept and replace a call to an existing method of a class. This technique can be very handy when writing unit tests for classes that have dependencies to other classes.

Getting ready

Let's introduce three classes for which we want to write a unit test:

• Customer.groovy:

  package org.groovy.cookbook
  class Customer {
    String name
  }

• CustomerDao.groovy:

  package org.groovy.cookbook
  class CustomerDao {
    Customer getCustomerById(Long id) {
      // DO SOME DATABASE RELATED QUERY
      ...
    }
  }

• CustomerService.groovy:

  package org.groovy.cookbook
  class CustomerService {
    CustomerDao dao
    void setCustomerDao(CustomerDao dao) {
      this.dao = dao
    }
    Customer getCustomer(Long id) {
      dao.getCustomerById(id)
    }
  }

The fictional CustomerService...