Kotlin Language Features Related to Null Handling

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Any software engineer with a Java background would find the null handling features in the Kotlin language interesting. Let's summarize this topic with some examples. Nullable types: In Kotlin, types are non-nullable by default. If you want a variable to be able to hold a null value, you need to explicitly declare its type as nullable using the Type? syntax. For example, String? denotes a nullable string, while String represents a non-nullable string. Safe calls (?.): Kotlin introduces the safe call operator (?.) for handling nullable types. It allows you to safely invoke a method or access a property on a nullable object. If the object is null, the expression returns null instead of throwing a NullPointerException. Example: data class Person(val name: String, val age: Int, val address: String?) fun main() {     // Create a person with a nullable address     val person1 = Person("John Doe", 25, "123 Main Street")     val person2 = Person("Jane Doe", 30,...
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"Open Session In View" kalıbı

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Bu kalıp Hibernate ile Java ağ uygulamaları geliştirirken ortaya çıkan bir probleme istinâden belirmiştir. Bir HTTP isteği web sunucusuna gelsin ve sunucumuzda yüklü olan kontrölcü servlet isteği ele alsın. Örneğin:

protected ModelAndView handleRequestInternal(HttpServletRequest request,
HttpServletResponse response) throws Exception {

String expenseId = request.getParameter("expenseId");

Expense expense = expenseService.loadWithId(expenseId);

return new ModelAndView("edit_expense", "expense", expense);

}

Burada expense nesnesi tembel bir şekilde elde edilmiş olabilir (lazily loaded). Bu durumda henüz ihtiyaç olmadığı için veritabanı tarafında bir SELECT sorgusu çalıştırılmış değildir. Fakat gösterim sayfasına (view) gönderdiğimiz nesnenin içindeki değerler, program akışı view kodlarına geldiğinde okunmak istenecektir. İşte burada bir sorun ortaya çıkıyor ki o da şu:

Bizim servis metodumuz yükleme (load) işlemini yaptıktan sonra Session nesnesini kapatacaktır. Böyle bir durumda view nesnesinin bizim expense varlığından değer okumaya çalışması "LazyInitializationException" ile sonuçlanır.

Bu sorunu çözmek için önerilen yol şudur:
Hibernate Session, sayfa gösterildikten sonra sonlandırılsın. Bunu gerçekleştirmek için Java Servlet Bildirgesinde yer alan "Filter" nesneleri kullanılır. Kendimiz bir Filter yazabileceğimiz gibi, Spring çatısının kullanıma sunduğu bir sınıfı da kullanabiliriz. Örneğin filter bildirimi web.xml içerisinde aşağıdaki biçimde yapılabilir:

< filter >
< filter-name > OpenSessionInViewFilter < /filter-name >
< filter-class >
org.springframework.orm.hibernate3.support.OpenSessionInViewFilter
< /filter-class >
< /filter >

< filter-mapping >
< filter-name > OpenSessionInViewFilter < /filter-name >
< servlet-name > myControllerServlet < /servlet-name >
< /filter-mapping >

Tabi ki Hibernate ayarları ile nesnelerin tembel olarak çekilmesini engelleyebiliriz. Fakat bunu her durumda uygulamak mümkün olmayabilir çünkü "lazy loading" bizi büyük performans kayıplarından kurtaran gerekli bir tekniktir. Hibernate gibi muazzam bir araç ile bu işleri çok ustaca yapmak kolay hale geliyor.

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