Advanced Java Course

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Advanced Java Course

• Reflection

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Reflection API

• What if you want to access information not just
  about the Object, but about that Objects Class?
• What if you want to access a method, field, or
  constructor whose name you dont know at compile
  time?

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Example WebCrawler timer

• To optimize your WebCrawler, first find out how
  long each big operation is taking.
• Getting content from URLs
• Parsing pages
• Setting up Database Connection Statement
• Deleting from the Database
• Inserting into the Database
• Other?

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How to time an operation?

• Without reflection
• int startTime System.currentTimeMillis()
• doOperation(...)
• int endTime System.currentTimeMillis()
• int duration endTime startTime
• Problem
• Code is repeated every place you want to time an
  operation! Not clean at all...

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A better way to time an operation

• Object timeMethod(String methodToCall, Class
  argTypes, Object args, String
  timeToUpdateField) ...
• timeMethod(methodName, new Class ..., new
  Object ..., timeStoringField)
• No code repetition, less clutter!

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Classes in the Reflection API

• Class Class
• Class Constructor
• Class Field
• Class Method

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Getting an Object of type Class

• If you have an instance of the class
• Class classObject instanceObject.getClass()
• If you know the name of the class at compile
  time
• Class classObject ClassName.class
• If you know the name of the class at run time
• Class.forName(...)
• If you want a primitive type class, and you know
  the name of the wrapper class at compile time
• Integer.TYPE, Long.TYPE, etc

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some methods in the Class class

• getName()
• getSuperClass()
• getComponentType() null if !isArray()
• isArray()
• isInterface()
• isPrimitive()
• isInstance(Object o)
• Object newInstance()

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Getting Constructors/Methods/Fields

• ask the Class Object for them
• classObject.getConstructors()
• classObject.getConstructor( Class argTypes)
• classObject.getDeclaredConstructors()
• classObject.getDeclaredConstructors( Class
  argTypes)
• Methods and Fields have analogous getters in
  class Class.
• you can get an array of all the c/m/f in the
  class, or just one by name, and if you ask for
  Declared it gives you the non-public ones as
  well.

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Objects of Type Method

• calling a method dynamically
• Object methodObject.invoke(Object o, Object
  args)
• getting info about it
• Class getParameterTypes()
• Class getReturnType()
• Class getExceptionTypes()

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Objects of type Field

• Object fieldObject.get( Object o )
• also getInt(Object), getBoolean(Object), etc.
• void fieldObject.set( Object o, Object value )
• also setInt(Object o, int i), etc.
• Class fieldObject.getType() compile time, of
  course
• Fields of type int will have Integer.TYPE as the
  result of getType(), and its a Class Object.

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Objects of type Constructor

• Object constructorObject.newInstance( Object
  args )
• Class constructorObject.getExceptionTypes()
• Class constructorObject.getParameterTypes()

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Member Interface

• Method, Field, and Constructor classes all extend
  the Member Interface
• Class getDeclaringClass()
• String getName()
• int getModifiers()

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Modifier class (static methods)

• How do you know if something is static, final,
  etc.?
• int getModifiers() from Member Interface
• Modifer.isStatic(int)
• Modifer.isFinal(int)
• Modifier.isProtected(int)
• etc.

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Array class (static methods)

• Object Array.get(Object arrayObject, int i)
• void Array.set(Object arrayObject, int index,
  Object value)
• int Array.getLength(Object arrayObject)
• Object newInstance(Class type, int length)
• Object newInstance(Class type, int dimensions)

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Reflection Group Task 1

• Fill in example from beginning of lecture
• Object timeMethod(String methodToCall, Class
  argTypes, Object args, String
  timeToUpdateField)
• ...
• Please fill this in!
• ...

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Solution to Reflection Group Task 1

• Object timeMethod(String methodToCall, Class
  argTypes, Object args, String
  timeToUpdateField)
• throws NoSuchMethodException,
  NoSuchFieldException, InvocationTargetException,
  IllegalAccessException
• Class c this.getClass()
• long startTime System.currentTimeMillis()
• Method m c.getMethod(methodToCall,
  argTypes)
• Object returnValue m.invoke(this, args)
• long endTime System.currentTimeMillis()
• Field f c.getField(timeToUpdateField)
• f.setLong(this, f.getLong(this) (endTime -
  startTime))
• return returnValue

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Reflection Group Task 2

• Fill in the class definition to call all getX
  methods and print their values in the form
• ClassName propertyName value
• import java.lang.reflect.
• public class PropertyValueFinder
• final String GET "get"
• public void printProperties(Object o) /fill
  this in/
• private Property propertyGottenBy(Method
  method) /fill this in/
• private boolean isGetter(Method method)
  /fill this in/
• public class Property
• String name
• Class type
• Object value
• public Property(String name, Class type)
• this.name name
• this.type type
• public void setValue(Object value)
  this.value value
• public String toString() return type"
  "name" "value

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Group Task 2 Solutions

• private boolean isGetter(Method m)
• boolean nameOK m.getName().startsWith(GET)
• boolean noArgs m.getParameterTypes().length
  0
• boolean nonVOID m.getReturnType() !
  Void.TYPE
• return nameOK noArgs nonVOID
• private Property propertyGottenBy(Method m)
• if (!isGetter(m)) return null
• Class propertyType m.getReturnType()
• String propertyName m.getName().substring
  (GET.length())
• return new Property(propertyName,
  propertyType)

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Group Task 2 Solutions

• public void printProperties(Object o)
• Class oClass o.getClass()
• Method methods oClass.getMethods()
• for (int i 0 i lt methods.length i)
• Method m methodsi
• if (isGetter(m))
• try
• Object value m.invoke(o, new
  Object0)
• Property p propertyGottenBy(m)
• p.setValue(value)
• System.out.println(p)
• catch (IllegalAccessException e)
• throw new RuntimeException()
  //programmer error
• catch (IllegalArgumentException e)
• throw new RuntimeException()
  //programmer error
• catch (InvocationTargetException e)
• throw new RuntimeException()
  //programmer error

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Discussion Topic

• Restriction to Serializer project
• I must promise that the Objects I test your code
  on will have a no-args constructor that
• sets the value of any final variables that are
  not set in the class definition
• does not throw any exceptions.
• If you could add functionality to the Reflection
  API, how would you fix this without compromising
  the integrity of the keyword final?
• Work in teams, and then well exchange ideas.
• Great, now go start a JSR (Java Specification
  Request)!
• http//www.jcp.org/en/participation/membership