科特林。如何通过反射检查字段是否可为空？

Question

我正在开发一个代码生成器，它在运行时从类中获取数据。此生成器旨在仅与 Kotlin 一起使用。此刻，我遇到了这个问题，因为我不知道如何检查该字段是否可以为空。

所以主要的问题是如何通过反射实现这个检查？

Answer 1

您可以使用 isMarkedNullable 检查可空性。以下代码：

class MyClass(val nullable: Long?, val notNullable: MyClass)
MyClass::class.declaredMemberProperties.forEach {
    println("Property $it isMarkedNullable=${it.returnType.isMarkedNullable}")
}

将打印：

Property val MyClass.notNullable: stack.MyClass isMarkedNullable=false
Property val MyClass.nullable: kotlin.Long? isMarkedNullable=true

摘自documentation（强调我的）：

For Kotlin types, it means that null value is allowed to be represented by this type. In practice it means that the type was declared with a question mark at the end. For non-Kotlin types, it means the type or the symbol which was declared with this type is annotated with a runtime-retained nullability annotation such as javax.annotation.Nullable.

Note that even if isMarkedNullable is false, values of the type can still be null. This may happen if it is a type of the type parameter with a nullable upper bound:
fun <T> foo(t: T) {
    // isMarkedNullable == false for t's type, but t can be null here 
}

科特林。如何通过反射检查字段是否可为空？

Kotlin. How to check if the field is nullable via reflection?

java

reflection

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kotlin