为何Scala无法保留字段注解？附Java注解与类示例

Great question! The root of this issue boils down to fundamental differences between how Scala and Java handle class fields and annotation targeting, plus some default behavior in the Scala compiler that catches many developers off guard. Let’s break this down step by step:

1. Scala’s Field Model Isn’t Identical to Java’s #

In Java, when you annotate a field, that annotation is directly attached to the field’s runtime metadata—no surprises. But Scala does things differently: when you define a non-private val or var, the compiler automatically generates getter (and setter, for var) methods behind the scenes. By default, any annotation you slap on that Scala field gets applied to these generated getter/setter methods, not the actual underlying field. Since your @When annotation is marked with @Target(ElementType.FIELD), it doesn’t belong on methods—so the runtime ignores it entirely when looking at the field.

2. Fixing It: Tell Scala Where to Apply the Annotation #

To make Scala respect your Java annotation’s target and attach it directly to the field, you need to use Scala’s @field meta-annotation to explicitly override the default behavior. Here’s how your Scala class should look:

class TestScalaClass {
  @(When @field)(event = "something")
  var testField: String = _
}

The @(When @field) syntax tells the compiler: "Hey, apply this @When annotation to the actual field, not the generated accessor methods."

3. Why Your Java Class Works (But Scala Doesn’t) #

Your TestJavaClass works exactly as expected because Java’s compiler strictly adheres to the @Target(FIELD) directive. There’s no automatic getter/setter generation interfering here—your annotation goes straight to the field’s metadata, so reflection can pick it up at runtime without any extra steps.

4. Verify It with Reflection #

Want to confirm this works? Here’s a quick reflection snippet to test:

import java.lang.reflect.Field

object AnnotationChecker extends App {
  val field = classOf[TestScalaClass].getDeclaredField("testField")
  val annotation = field.getAnnotation(classOf[When])
  
  annotation match {
    case null => println("No annotation found on field")
    case when => println(s"Annotation found! Event: ${when.event()}")
  }
}

Without the @field meta-annotation, this will print "No annotation found on field". With it, you’ll see the event string you specified.

内容的提问来源于stack exchange，提问作者missingfaktor