Java Reflection API
Java Reflection is a process of examining or modifying the run time behavior of a class at run time.
java.lang.Class class provides many methods that can be used to get metadata, examine and change the run time behavior of a class.
The java.lang and java.lang.reflect packages provide classes for java reflection.
Where it is used
The Reflection API is mainly used in:
IDE (Integrated Development Environment) e.g. Eclipse, MyEclipse, NetBeans etc.
Test Tools etc.
Do You Know ?
How many ways we can get the instance of Class class ?
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How to access the private method from outside the class ?
The java.lang.Class class performs mainly two tasks:
provides methods to get the metadata of a class at run time.
provides methods to examine and change the run time behavior of a class.
Commonly used methods of Class class:
1) public String getName() returns the class name
2) public static Class forName(String className)throws ClassNotFoundException loads the class and returns the reference of Class class.
3) public Object newInstance()throws InstantiationException,IllegalAccessException creates new instance.
4) public boolean isInterface() checks if it is interface.
5) public boolean isArray() checks if it is array.
6) public boolean isPrimitive() checks if it is primitive.
7) public Class getSuperclass() returns the superclass class reference.
8) public Field getDeclaredFields()throws SecurityException returns the total number of fields of this class.
9) public Method getDeclaredMethods()throws SecurityException returns the total number of methods of this class.
10) public Constructor getDeclaredConstructors()throws SecurityException returns the total number of constructors of this class.
11) public Method getDeclaredMethod(String name,Class parameterTypes)throws NoSuchMethodException,SecurityException returns the method class instance.
How to get the object of Class class?
There are 3 ways to get the instance of Class class. They are as follows:
forName() method of Class class
getClass() method of Object class
the .class syntax
1) forName() method of Class class
is used to load the class dynamically.
returns the instance of Class class.
It should be used if you know the fully qualified name of class.This cannot be used for primitive types.
Let's see the simple example of forName() method.
2) getClass() method of Object class
It returns the instance of Class class. It should be used if you know the type. Moreover, it can be used with primitives.
3) The .class syntax
If a type is available but there is no instance then it is possible to obtain a Class by appending ".class" to the name of the type.It can be used for primitive data type also.
Determining the class object
Following methods of Class class is used to determine the class object:
1) public boolean isInterface(): determines if the specified Class object represents an interface type.
2) public boolean isArray(): determines if this Class object represents an array class.
3) public boolean isPrimitive(): determines if the specified Class object represents a primitive type.
Let's see the simple example of reflection api to determine the object type.