Overview
- What is Java Reflection?
- Class Object
- Constructors
- Fields
- Methods
- Annotations
What Is Java Reflection?
- Powerful and helpful feature which helps to inspect the classes, interfaces, methods etc, at runtime without knowing about these at compile time.
- E.g. Methods[] m = testObject.getClass().getMethods()
Class Object
- JVM creates a class object for all the objects of that class.
- It is shared by all these objects.
- E.g. Class test= testObject.class;
- Or one can use Class.forName(String);
- From the class, we can get the information about class name, modifiers, package information, superclass, constructors, methods, fields and annotations.
Constructors
- Public Constructors for a class could be obtained by
Constructor[] c = myClass.getConstructors(); - A constructor with specific type of parameter can be obtained by
Constructor constructor =testClass.getConstructor(new Class[] {String.class}) - A new object could be created using these constructors.
java.lang.reflect Package: Reflection in Java is primarily implemented through the java.lang.reflect package, which contains classes like Class, Method, Field, Constructor, etc., that provide the necessary methods and APIs for reflective operations.
Fields
- We can get all public fields of the object by
: Field[] f = myClass.getFields(); - All fields including private ones could be accessed by
Field[] f = myClass.getDeclaredFields(); - A particular field could be accessed by
Field f = myClass.getField(“fieldName”); - For accessing and setting values on fields, we can use
- f.getValue(Object);
- f.set(Object, Object);
Methods
- We can get all public methods of the object by :
Method[] m = myClass.getMethods(); - All methods including private ones could be accessed by
Method[] m = myClass.getDeclaredMethods(); - A particular method could be accessed by
Method m = myClass.getMethod(“methodName”, Class parameters); - For invoking a method, we can use
m.invoke(Object, Object);
Annotations
- We can access the annotations defined at object, method, field and parameter level.
- @Retention:- This property should be set as RetentionPolicy.RUNTIME for accessing it using reflection.
- E.g. Annotation[] a= myClass.getAnnotations()
