Reflection

Reflection in java provides ability to inspect and modify the runtime behavior of applications. Reflection is one of the advance topic of core java. Using reflection we can inspect a class, interface, enums, get their structure, methods and fields information at runtime even though class is not accessible at compile time. We can also use reflection to instantiate an object, invoke it’s methods, change field values.

Fundamentally, the Reflection API consists of two components: objects that represent the various parts of a class file, and a means for extracting those objects in a safe and secure way. The latter is very important, as Java provides many security safeguards, and it would not make sense to provide a set of classes that invalidated those safeguards.

The first component of the Reflection API is the mechanism used to fetch information about a class. This mechanism is built into the class named Class. The special class Class is the universal type for the meta information that describes objects within the Java system. Class loaders in the Java system return objects of type Class. Up until now the three most interesting methods in this class were:

• forName, which would load a class of a given name, using the current class loader
• getName, which would return the name of the class as a String object,which was useful for identifying object references by their class name
• newInstance, which would invoke the null constructor on the class (if it exists) and return you an object instance of that class of object

The Reflection API is symmetric, which means that if you are holding a Class object, you can ask about its internals, and if you have one of the internals, you can ask it which class declared it. Thus you can move back and forth from class to method to parameter to class to method, and so on. One interesting use of this technology is to find out most of the inter-dependencies between a given class and the rest of the system.

We should not use reflection in normal programming where we already have access to the classes and interfaces because of following drawbacks.

• Poor Performance – Since reflection resolve the types dynamically, it involves processing like scanning the classpath to find the class to load, causing slow performance.
• Security Restrictions – Reflection requires runtime permissions that might not be available for system running under security manager. This can cause you application to fail at runtime because of security manager.
• Security Issues – Using reflection we can access part of code that we are not supposed to access, for example we can access private fields of a class and change it’s value. This can be a serious security threat and cause your application to behave abnormally.
• Exposure of Internals - Since reflection allows code to perform operations that would be illegal in non-reflective code, such as accessing private fields and methods, the use of reflection can result in unexpected side-effects, which may render code dysfunctional and may destroy portability. Reflective code breaks abstractions and therefore may change behavior with upgrades of the platform.
• High Maintenance – Reflection code is hard to understand and debug, also any issues with the code can’t be found at compile time because the classes might not be available, making it less flexible and hard to maintain.

Example : Invoke method on unknown object

By using reflection, the code can use the object and find out if the object has a method called "print" and then call it.

package reflection; import java.lang.reflect.Method;   public class Reflection { public static void main(String[] args){ Foo f = new Foo();   Method method; try { method = f.getClass().getMethod("print", new Class<?>[0]); method.invoke(f); } catch (Exception e) { e.printStackTrace(); } } }   class Foo { public void print() { System.out.println("Hello World"); } } Output :

Hello World

Read more information on Oracle website.