CODEWITHSUNDEEP
scala
user79074
user79074

Reputation: 5300

Scala reflection to access all public fields at runtime

Given the following class hierarchy:

class A {
  val x = 3
  val y = 4
}

class B extends A {
  val z = 5
}

And say I have an instance of B with no compile-time knowledge of its class hierarchy and would like to be able to report something like the following:

Class: B
val[x]: 3
val[y]: 4
val[z]: 5

What would be the best approach using Scala reflection?

Thanks Des

Upvotes: 12

Views: 10686

Answers (3)

jhegedus
jhegedus

Reputation: 20653

Here is a slightly other way to do it:

scala> val i = 10
i: Int = 10

scala> val ru=scala.reflect.runtime.universe
ru: scala.reflect.api.JavaUniverse = scala.reflect.runtime.JavaUniverse@2096fb9f

scala> val m = ru.runtimeMirror(getClass.getClassLoader)
m: ru.Mirror = JavaMirror with scala.tools.nsc.interpreter.IMain$TranslatingClassLoader@3ae5ce58 of type class scala.tools.nsc.interpreter.IMain$TranslatingClassLoader with classpath [(memory)] and parent being scala.tools.nsc.util.ScalaClassLoader$URLClassLoader@454e119d of type class scala.tools.nsc.util.ScalaClassLoader$URLClassLoader with classpath [file:/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/jre/lib/resources.jar,file:/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/jre/lib/rt.jar,file:/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/jre/lib/jsse.jar,file:/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/jre/lib/jce.jar,file:/Library/Java/JavaVirtualMachines/jdk1.7.0_45.jdk/Contents/Home/jre/lib/charsets.jar,file:...
scala> val im=m.reflect(i)
im: ru.InstanceMirror = instance mirror for 10

scala> val is =im.symbol
is: ru.ClassSymbol = class Int

scala> val ims=is.toType.members
ims: ru.MemberScope = Scopes(method getClass, constructor Int, method ##, method asInstanceOf, method isInstanceOf, method toString, method hashCode, method equals, method !=, method ==, method %, method %, method %, method %, method %, method %, method %, method /, method /, method /, method /, method /, method /, method /, method *, method *, method *, method *, method *, method *, method *, method -, method -, method -, method -, method -, method -, method -, method +, method +, method +, method +, method +, method +, method +, method ^, method ^, method ^, method ^, method ^, method &, method &, method &, method &, method &, method |, method |, method |, method |, method |, method >=, method >=, method >=, method >=, method >=, method >=, method >=, method >, method >, method >, met...
scala>

ims can then be further filtered any way you like.

Upvotes: 0

J Cracknell
J Cracknell

Reputation: 3547

In brief:

import scala.reflect.runtime.universe._

val a = new A
val rm = scala.reflect.runtime.currentMirror
val accessors = rm.classSymbol(a.getClass).toType.members.collect {
  case m: MethodSymbol if m.isGetter && m.isPublic => m 
}
val instanceMirror = rm.reflect(a)
for(acc <- accessors)
  println(s"$a: ${instanceMirror.reflectMethod(acc).apply()}")

Upvotes: 16

4lex1v
4lex1v

Reputation: 21567

You can you TypeTag to get an access:

scala> import scala.reflect.runtime.universe._
import scala.reflect.runtime.universe._

scala> class A {
     |   val x = 3
     |   val y = 4
     | }
defined class A

scala> class B extends A {
     |   val z = 5
     | }
defined class B

scala> typeOf[B]
res0: reflect.runtime.universe.Type = B

Now you have two options: members which shows all members (even inherited ones) and declarations (only defined in the current class), e.g:

scala> res0.declarations
res2: reflect.runtime.universe.MemberScope = SynchronizedOps(constructor B, value z, value z)

If you want to work with this field you need reflect them through InstanceMirror:

scala> val b = new B
b: B = B@6ebe10dd

scala> val currentMirror = runtimeMirror(getClass.getClassLoader)
.....

scala> val bMir = currentMirror.reflect(b)
bMir: reflect.runtime.universe.InstanceMirror = instance mirror for B@6ebe10dd

Now you just need to get a Symbol you need, e.g you want to get a value of z varible:

scala> val zt = typeOf[B].declaration("z": TermName).asMethod
zt: reflect.runtime.universe.MethodSymbol = value z

scala> bMir.reflectField(zt).get
res20: Any = 5

Upvotes: 1

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