Reflection

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Reflection

• Jim Cai

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Motivation the Class class

• Why reflection?
• We need a way to access class information at run
  time
• Fields, methods, constructors, interfaces..
• JRE maintains an immutable Class object for each
  class.
• We say the Class object reflect the class.
• Not that the Class object can also reflect
  interfaces (with restrictions)

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Summary

• Examine classes
• Getting class name, retrieve Class object.
  Examine methods, fields,
• Manipulating objects
• Invoking methods
• Change fields values

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Examine classes
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Retrieving Class object

• If an instance of the object is available
• Object.getClass()
• E.g. Class c myMestery.getClass
• If an instance of the object is not available
• If name of the class is unknown until run time
• Use forName method.
• E.g. Class cClass.forName (classname)
• If name of the class is known
• Append .class to the class name
• E.g. Class c java.awt.Button.class

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Getting the class name

• Every class in the Java programming language has
  a name
• public class Point int x, y
• Use getName() method to get the fully qualified
  name of the Class object.

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What is the output?

• import java.lang.reflect.
• import java.awt.
• class SampleName
• public static void main(String args)
• Button b new Button()
• printName(b)
• static void printName(Object o)
• Class c o.getClass()
• String s c.getName()
• System.out.println(s)

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Identify class fields

• Find out what fields belongs to what class
• getFields() method returns an array of Field
  objects, each one corresponds to one accessible
  public field
• Now what if I want information about private (or
  protected) field
• getDeclaredFields()

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Example

• static void printFieldNames(Object o)
• Class c o.getClass()
• Field publicFields c.getFields()
• for (int i 0 i lt publicFields.length i)
• String fieldName publicFieldsi.getName()
  Class typeClass publicFieldsi.getType()
  String fieldType typeClass.getName()
  System.out.println("Name " fieldName ",
  Type " fieldType)

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Obtaining methods information

• Find out what public method belong to a class
• getMethods() return an array of Method object

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• static void showMethods(Object o)
• Class c o.getClass()
• Method theMethods c.getMethods()
• for (int i 0 i lt theMethods.length i)
• String methodString theMethodsi.getName()
  System.out.println("Name " methodString)
• String returnString theMethodsi.getReturnTyp
  e().getName() System.out.println(" Return Type
  " returnString)
• Class parameterTypes theMethodsi.getParame
  terTypes() System.out.print(" Parameter
  Types")
• for (int k 0 k lt parameterTypes.length k
  )
• String parameterString parameterTypesk.getNa
  me()
• System.out.print(" " parameterString)
• System.out.println()

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Other class information

• Discover class modifier
• Discover class constructor
• Finding superclass
• Java API is your friend

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Manipulating objects
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Creating objects

• What? Isnt it trivial?
• Rectangle r new Rectangle()
• create an object with the no-argument constructor
  
• Class classDefinition Class.forName(className)
  
• object classDefinition.newInstance()
• create an object with a constructor that has
  arguments
• Create a Class object for the object you want to
  create.
• Create a constructor object (getConstructor())
• Create an object by invoking this constructor

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Example

• Class rectangleDefinitionClass.forName("java.awt.
  Rectangle")
• Class intArgsClass new Class int.class,
  int.class
• Object intArgs new Object height, width
• intArgsConstructor rectangleDefinition.getConstr
  uctor(intArgsClass)
• object intArgsConstructor.newInstance(intArgs)
  

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Invoking methods

• Create a Class object that corresponds to the
  object whose method you want to invoke.
• How? Anyone?
• Create a Method object by invoking getMethod()
• Invoke the method by calling invoke()

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Example

• //Append one string to another concat()
• public static String append(String firstWord,
  String secondWord)
• String result null
• Class c String.class
• Class parameterTypes new Class
  String.class
• Method concatMethod
• Object arguments new Object secondWord
• try
• concatMethod c.getMethod("concat",
  parameterTypes)
• result (String) concatMethod.invoke(firstWord,
  arguments)
• catch (NoSuchMethodException e)
• System.out.println(e)
• catch (IllegalAccessException e)
• System.out.println(e)
• catch (InvocationTargetException e)
• System.out.println(e)
• return result

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Alter field values

• Create a Class object
• Create a Field object by using getField()
• Invoke get() to get the value
• Invoke set() to set the value
• Public fields ONLY!

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Example

• static void modifyWidth(Rectangle r)
• Field widthField
• Class c r.getClass()
• try
• widthField c.getField("width")
• heightFieldc.getField(height)
• heightValue (Integer) heightField.get(r)
• widthField.set(r, widthValue)
• catch (NoSuchFieldException e)
• System.out.println(e)
• catch (IllegalAccessException e)
• System.out.println(e)

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Key points

• There is no magic
• A class is just a data structure
• A method is just a data structure, too
• It just happens to contain bytes that look like
  instructions for the interpreter
• The call stack is another data structure
• With libraries to give you access to it at
  runtime
• Many programming tools make use of reflection
• Junit, your project, debugger etc.