Implementation of domain models using dynamic proxies Bjørn Vidar Bøe, JavaZone 2005

1. Implementation of domain models using dynamic proxies Bjørn Vidar Bøe, JavaZone 2005

2. Agenda • Rationale • Overall design • Handling business logic • Conclusion

3. Rationale • The new mantra for many new frameworks is simplification(Hibernate, Spring, EJB3, Prevayler, Ruby on Rails…) • AOP getting popular for handling cross-cutting concerns like logging, security, caching… • How far can we take this?

4. Simple Patient class and implementation Patient public class Patient { public String firstName; public String lastName; public Date dob; } +firstName : String +lastName : String +dob : Date

5. Correct implementation of the Patient class public class Patient { private String firstName; private String lastName; public Date dob; public String getFirstName() { return this.firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } public String getLastName() { return this.lastName; } public void setLastName(String lastName) { this.lastName = lastName; } ... }

6. Reverse engineered UML from the new class Patient -firstName : String -lastName : String -dob : Date +getFirstName() : String +setFirstName(in firstName : String) +getLastName() : String +setLastName(in lastName : String) +getDob() : Date +setDob(in dob : Date)

7. Patient class from a user’s perspective public interface Patient { public String getFirstName(); public void setFirstName(String firstName); public String getLastName(); public void setLastName(String lastName); public Date getDob(); public void setDob(Date dob); }

8. What’s interesting about this? • What is the minimum of information needed to define a Patient-class? • Should the implementation matter? • Should the user need to know about the implementation? • New opportunities?

9. What if the interface was all that’s needed? • No implementation, just the interface to specify the model • Let a framework implement the model • Other frameworks do this in varying degree, but not in this way

10. Dynamic proxies Patient (dynamic proxy) Invocation handler +getFirstName() : String +setFirstName(in firstName : String) +getLastName() : String +setLastName(in lastName : String) +getDob() : Date +setDob(in dob : Date) public Object invoke(Object proxy, Method m, Object[] args) Patient patient = (Patient) Proxy.newProxyInstance(Patient.class.getClassLoader(), new Class[] { Patient.class }, myInvocationHandlerImpl);

11. What about business logic? public String sayHello() { return ”Hello! My name is ” + firstName; }

12. Split data and business logic Data definition Business service definition BS Impl Domain object

13. Patient with sayHello() PatientData PatientService +getFirstName() : String +setFirstName(in firstName : String) +getLastName() : String +setLastName(in lastName : String) +getDob() : Date +setDob(in dob : Date) +sayHello() : String PatientServiceImpl -patient : Patient + sayHello() : String Patient

14. Implementation of PatientServiceImpl public class PatientImpl implements PatientService { private Patient patient; public PatientImpl(Patient patient) { this.patient = patient; } public String sayHello() { return ”Hello! My name is ” + patient.getFirstName(); } }

15. Alternate solution to the business logic problem – CGLIB! • Similar to dynamic proxies but is also able to extend classes • Used in frameworks like Spring and Hibernate

16. CGLIB code-example Enhancer enhancer = new Enhancer(); enhancer.setSuperclass(Patient.class); enhancer.setCallback(myMethodInterceptor); Patient patient = (Patient)e.create(); public Object intercept(Object proxy, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable { if (!Modifier.isAbstract(method.getModifiers())) { return methodProxy.invokeSuper(proxy, objects); } else { ... } }

17. Alternative implementation public abstract class Patient { public abstract String getFirstName(); public abstract void setFirstName(String firstName); public abstract String getLastName(); public abstract void setLastName(String lastName); public abstract Date getDob(); public abstract void setDob(Date dob); public String sayHello() { return ”Hello! My name is ” + getFirstName(); } }

18. Pros and cons • Easy to change the model • Code-wise extremely lightweight • Not ment to have a database as a back-end • Interface-based solution gets complicated when business logic is added • Haven’t we seen the abstract solution before? • Where will we see this?