Kotlin 的注解类详解及实例

annotation class Fancy

@Fancy class Foo {
  @Fancy fun baz(@Fancy foo: Int): Int {
    return (@Fancy 1)
  }
}

class Foo @Inject constructor(dependency: MyDependency) {
  // ……
}

class Foo {
  var x: MyDependency? = null
    @Inject set
}

annotation class Special(val why: String)

@Special("example") class Foo {}

annotation class ReplaceWith(val expression: String)

annotation class Deprecated(
    val message: String,
    val replaceWith: ReplaceWith = ReplaceWith(""))
@Deprecated("This function is deprecated, use === instead", ReplaceWith("this === other"))

import kotlin.reflect.KClass

annotation class Ann(val arg1: KClass<*>, val arg2: KClass<out Any?>)

@Ann(String::class, Int::class) class MyClass

annotation class Suspendable

val f = @Suspendable { Fiber.sleep(10) }

class Example(@field:Ann val foo,  // 标注 Java 字段
       @get:Ann val bar,   // 标注 Java getter
       @param:Ann val quux)  // 标注 Java 构造函数参数

@file:JvmName("Foo")

package org.jetbrains.demo

class Example {
   @set:[Inject VisibleForTesting]
   var collaborator: Collaborator
}

fun @receiver:Fancy String.myExtension() { }

import org.junit.Test
import org.junit.Assert.*
import org.junit.Rule
import org.junit.rules.*

class Tests {
  // 将 @Rule 注解应用于属性 getter
  @get:Rule val tempFolder = TemporaryFolder()

  @Test fun simple() {
    val f = tempFolder.newFile()
    assertEquals(42, getTheAnswer())
  }
}

// Java
public @interface Ann {
  int intValue();
  String stringValue();
}
// Kotlin
@Ann(intValue = 1, stringValue = "abc") class C

// Java
public @interface AnnWithValue {
  String value();
}
// Kotlin
@AnnWithValue("abc") class C

// Java
public @interface AnnWithArrayValue {
  String[] value();
}
// Kotlin
@AnnWithArrayValue("abc", "foo", "bar") class C

// Java
public @interface AnnWithArrayMethod {
  String[] names();
}
// Kotlin
@AnnWithArrayMethod(names = arrayOf("abc", "foo", "bar")) class C

// Java
public @interface Ann {
  int value();
}
// Kotlin
fun foo(ann: Ann) {
  val i = ann.value
}