Reflection API

• An API is already developed set of classes which can be reused to enhance/add a functionality.
• We can have API for different concepts for example we have Collection API, JDBC API, Servlet API, Reflection API.
• In collection API we have classes like
• ArrayList Class.
• Hashmap class
• Hashset class
• TreeSet class
• Priorityqueue class
• In JDBC API we have classes like 
• Driver
• Connection
• Statement
• Prepared statement
• Callable statement
• Results set
• Results set metadata
• BLOB
• CLOB
• Save Point
• RawSet
• A set of classes meant for performing reflection operations is called as reflection API.
• Reflection API is used for reading and executing a class information/class meta-data at execution time.
• Reflection API is needed so that compiler, interpreter and decompiler can read a class information.
• Where ever in our project we need class information dynamically we should use reflection API.
• Reflection API is used for accessing the class members and executing member function dynamically.
• For example tomcat server reads our servlets defined in web.XML using reflection API and execute innate, service, destroy methods accordingly.
• For example a class cannot be declared as private/protected. Compiler throws error for this using Reflection API.
• Reflection API is used to read declarative information of a class.
• If we use expressions such as
• Class C = Class.fromName(“A”)
• A a = (A) C.newInstance();
• Here JVM will read metadata of class A using reflection API And it’s bite code will be loaded to the memory.
• The attribute will be stored in form of a glass object referred by C variable.
• Primary class of Java reflection API is class in java.lang.class which gathers the meta-data of a class.
• This class object in Java reflection API Consists of objects of classes that are defined in java.lang.reflect package. Some of the classes are as follows
• Field
• Gathers meta-data off variable
• Method
• Gathers meta-data of methods
• Constructor
• Gathers meta-data of constructor
• Modifier
• Gathers modifiers of class
• There are three ways to prepare class object in Java
• Class.forName(“employee”)
• Employee e = new Employee(); Class C = e.getClass();
• Method GetClass is in object class.
• Class C = Employee.class
• Class is a final variable define in “Class” class which holds class meta-data.
• Some methods defined in “Class” class are
• Public String getName()
• Gets name of the class
• Public class getSuperClass()
• Gets name of SuperClass
• Public class getInterfaces()
• Gets interfaces of class
• Public int getModifiers(); 
• Gets modifier ID
• Public Static String toString(int i) Of Modifier class gets Modifier.
• To get Fields we use two methods
• GetFields()
• Can be used to get all the public fields.
• GetDeclaredFields()
• Used to get all the fields of current class irrespective of acess modifier public.
• Once we have the field array we can use following methods for meta-data.
• Public string getName
• Public class getType
• Public vartype get (Field F)
• Gets the value of the field
• Public int getModifier(); Modifier.toString(i);
• To get methods we use
• Public Method[] getMethods();
• Public Method[] getDeclaredMethods();
• Exclamation of above is same as for Fields.
• Once we have Method array we can get method meta-data using
• Public string getName
• public class getReturnType
• Public int getModifiers
• Class[] getParameterTypes
• Class[] getExceptionTypes
• To getConstructors we use getConstructors and get DeclaredConstructors methods.
• This returns array of Constructors
• Once we have constructors array we can get method made a data using
• Public String getName
• Public int getModifiers
• Public Class[] getParameterTypes
• Class[] getExceptionTypes

Reflection API Example javacodes/reflexion_api at master · gauravmatta/javacodes (github.com)