Monday, 3 October 2016

Understanding static for Database interaction and when we should use static

This is very common practice when a developer thinks whether a method should be static or not during development and generally an average developer always use static for Utility data but when it comes to Database call, most of developers confuse when should we use static ????

Understanding of Static

Why we declare static ??

Ans : Those method which consist the data which is common for multiple objects, never changes or infrequent changes.

Q. When static method and when static block gets initialize the data ?
Ans : static method gets initialize the data when we invoke it.Static block initialize the data during class loading time.

static blocks invoked and gets initialize the data after a first call of the mehod and their state never changes after that during program execution.

This is the reason that the data which actually changes shouldn't be accessed through static block. Therefore static blocks used in caching.

For ex- If a static block invoked already and fetch some data from Database and now accessing the data from static block will return same data which actually we got on first call. Therefore if a UI which contains data from DB will result stale data.

So now the next question is :

Q. So we shouldn't use static block for Database interaction ?
Ans : Generally we shouldn't use for database interaction but in some special cases for ex- In caching, we can use static methods to gets stale data from cache and after certain amount of time (caching refresh interval) we can have static method call.

Or the data which never changes in the database can be accessed via static block.

Similarly the Connection to a Database itself never changes annd therefore we can access it via static block or method..

Q. What is the difference between static block and static method ?

Ans : Static block initialize the data during class loading in which the block declared but static method gets initialized the data only when we called the method.

Q. Can't we use static block for read only data ?
Ans : First we need to understand the difference between Read only data and data never changes. Read only data is the data which a particular user or entity has not permisssion. Data never changes is the data which never changes throughout its life. For ex- Age of a person - we can declare static because it never changes throughout the object life cycle and similarly historical data.

Thefore static has no relation with read only data but it can be used to access infrequent or never changing data.

Q. When should use static block ?
Ans : When we need data during application initialization, caching purpose etc

Q. When should we use static method ?
Ans : Define static methods in the following scenarios only:
  1. If you are writing utility classes and they are not supposed to be changed.
  2. If the method is not using any instance variable.
  3. If any operation is not dependent on instance creation.
  4. If there is some code that can easily be shared by all the instance methods, extract that code into a static method.
  5. If you are sure that the definition of the method will never be changed or overridden. As static methods can not be overridden.
In general : One rule-of-thumb: ask yourself "does it make sense to call this method, even if no Obj has been constructed yet?" If so, it should definitely be static.
So in a class Car you might have a method double convertMpgToKpl(double mpg) which would be static, because one might want to know what 35mpg converts to, even if nobody has ever built a Car. But void setMileage(double mpg) (which sets the efficiency of one particular Car) can't be static since it's inconceivable to call the method before any Car has been constructed.
(Btw, the converse isn't always true: you might sometimes have a method which involves two Carobjects, and still want it to be static. E.g. Car theMoreEfficientOf( Car c1, Car c2 ). Although this could be converted to a non-static version, some would argue that since there isn't a "privileged" choice of which Car is more important, you shouldn't force a caller to choose one Car as the object you'll invoke the method on. This situation accounts for a fairly small fraction of all static methods, though.)

Q. Then why we use static method for utility classes ?
Ans : A utility class is one whose behaviour never changes and remain same irrespective of the application state.

Q. Can we use static method for Database call ?

Ans : Yes but if we really need it .For ex- if two or more objects sharing a data accessed from Database then in that case we can access the data through static method.

Q. Why in MVC - service classes shouldn't contain static method if multiple controller classes may need the same method and generally request routing through the controller return stateless data ?
Ans : Straight reason : It violate the service spec because we cant put the service method in in the service interface.

but why we generally dont need because of the following reasons :

(I) you can't replace Service with a mock easily during tests (tools like PowerMock aside)
(ii) static methods do not participate in standard, proxy-based AOP (no transactions, security, custom aspects)
(iii) you can no longer use fancy injection techniques, like injecting HTTP request (request scoped) into singleton scoped services (poor design anyway, but...)
But to be complete, there are also advantages:

static method is actually closer to your intent, Spring beans are very rarely stateful, thus they don't really need an instance to work
static call might be faster

Q. In Transaction which one should use ?
Ans : no one transaction data never access in static...i.e. why I said static always for caching and never for transaction in application infrastructure.