The proxy pattern is a structural design pattern.
In the proxy pattern, a proxy class is used to control access to another class.
The reasons for this control can vary. As one example, a proxy may avoid instantiation of an object until the object is needed.
This can be useful if the object requires a lot of time or resources to create. Another reason to use a proxy is to control access rights to an object.
A client request may require certain credentials in order to access the object.
Another reason is to log the calls to the original method.
​It is also used by mocking frameworks for unit testing.

Using Java Reflection you create dynamic implementations of interfaces at runtime. You do so using the class java.lang.reflect.Proxy.

One example of proxies - Office internet.
We generally use proxies to access the internet in office & these proxies work as a filter/access to the sites being visited.
By these proxies also, admin can have the record of sites being visited by you.
Another example - Proxies during the class/lectures
We all have gone through this phase, where we used to work as a proxy for our friends during the lectures in college.
Means for attendance, that person is not required, so other person is working as a proxy for that missing person.

You create proxies using the Proxy.newProxyInstance() method. The newProxyInstance() methods takes 3 parameters:
The ClassLoader that is to "load" the dynamic proxy class.
An array of interfaces to implement.
An InvocationHandler to forward all methods calls on the proxy to.

Example Code : Note - We need interface for which we can have the proxies, on classes directly it doesn't work.

public interface Interface {
 public double findSqrRoot(int num);
}

Original class is calculating the square root of the number provided to its method findSqrRoot(). Lets assume its a heavy method.
public class Original implements Interface {
 
 public double findSqrRoot(int num) {
  return Math.sqrt(num);
 }
}

Handler class which implements InvocationHandler & manages calls to original method.
Here I have put condition, if number is greater than 100, then only original method will called else I will return 1.0 only.
Here, I can create some cache for the results, so that if I dont have the answer in my cache, then only will call the original method.
This way I can improve the performance further.
public class Handler implements InvocationHandler {
 private final Interface original;
 public Handler(Interface original) {
  this.original = original;
 }
 @Override
 public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  System.out.println("In handler");
  int num = (Integer)args[0];
  if(num <= 100)
   return 1d;
  else
   return method.invoke(original, args);
 }
}

Below is the Driver class, which will be used to check the functionality of proxies here. Check, I have used 2 ways to get the proxies below -

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Proxy;
public class Driver {
 public static void main(String[] args) throws InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, NoSuchMethodException, SecurityException {
  Interface original = new Original();
  Handler handler = new Handler(original);
  Interface f = (Interface) Proxy.newProxyInstance(Interface.class.getClassLoader(),
    new Class[] { Interface.class },
    handler);
  System.out.println(f.findSqrRoot(121));

  Class cl = Proxy.getProxyClass(Original.class.getClassLoader(), new Class[] {Interface.class});
  Interface in = (Interface) cl.getConstructor(new Class[] {InvocationHandler.class}).newInstance(new Object[] { handler });
  System.out.println(in.findSqrRoot(121));
 }
}