DDevelopment & Design

Use asInstanceOf[T] carefully!

BackgroundScala has nice static type checking engine but from time to time there are situations when we must downcast some general object. If this casting is not possible we expect that virtual machine will throw ClassCastExeption as fast as possible. …

Background

Scala has nice static type checking engine but from time to time there are situations when we must downcast some general object. If this casting is not possible we expect that virtual machine will throw ClassCastExeption as fast as possible. Although it is not always true. Consider code below.

Suprisingly when we run this test we will see:

Solution

Why this happens? Because type T is erasured during compile. The problem is that compiler doesn’t warn about it. Method asInstanceOf[T] is treated as any other regular generic method. If we want to be noticed about type erasure we should use pattern matching:

And then during compilation we will see:

But how to fix this? We can provide implicit evidence parameter:

But we will still have no error if we cast value to generic type e.g.:

With help comes shapeless with Type safe cast. Using this approach casting will be available in compile time only when exists evidence how it is possible.

Summary

Summarizing:
* Use pattern matching instead of asInstanceOf[T]
* If you are using asInstanceOf[T] make sure that target type is not erasured
* Use ClassTag implicit evidence parameter if you are casting only to not generic types
* Use shapeless Typeable in all other situations

# CodeCode with tests is available on

[GitHub][2]

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Simple trick to DRY your Grails controller

Grails controllers are not very DRY. It's easy to find duplicated code fragments in default generated controller. Take a look at code sample below. It is duplicated four times in show, edit, update and delete actions:

class BookController {
def show() {
def bookInstance = Book.get(params.id)
if (!bookInstance) {
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Solution

At TouK we've implemented a simple trick to resolve that situation:

  1. wrap everything with a simple withStoppingOnRender method,
  2. whenever you want to render or redirect AND stop controller execution - throw EndRenderingException.

We call it Big Return - return from a method and return from a controller at once. Here is how it works:

class BookController {
def show(Long id) {
withStoppingOnRender {
Book bookInstance = Book.get(id)
validateInstanceExists(bookInstance)
            [bookInstance: bookInstance]
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protected Object withStoppingOnRender(Closure closure) {
try {
return closure.call()
        } catch (EndRenderingException e) {}
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private void validateInstanceExists(Book instance) {
if (!instance) {
flash.message = message(code: 'default.not.found.message', args: [message(code: 'book.label', default: 'Book'), params.id])
redirect(action: "list")
throw new EndRenderingException()
        }
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}

class EndRenderingException extends RuntimeException {}

Example usage

For simple CRUD controllers, you can use this solution and create some BaseController class for your controllers. We use withStoppingOnRender in every controller so code doesn't look like a spaghetti, we follow DRY principle and code is self-documented. Win-win-win! Here is a more complex example:

class DealerController {
@Transactional
def update() {
withStoppingOnRender {
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validateInstanceExists(dealerInstance)
validateAccountInExternalService(dealerInstance)
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saveUpdatedInstance(dealerInstance)
redirectToAfterUpdate(dealerInstance)
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}