当我们使用类实例调用静态方法/函数时会发生什么？

Question

Static variables and method belong to class rather than it's instances(Objects). generally they are accessed using className.staticMethod() or className.staticVariable but we can also access or call them using class instances eg classInstance.staticMethod() and classInstance.staticVariable .

My 1st question is

Why allow accessing static methods/functions through class instances in the 1st place? Does it have any use case?

Next question is as follows.Consider the following class

public class Counter{
private static int count = 0;

public static synchronized int getCount()
{
  return count;
}

public synchronized setCount(int count)
{
   this.count = count;
}

}

In a multi threaded environment if these functions are called as below

Counter myCounter = new Counter();
myCounter.setCount(10);
System.out.println(myCounter.getCount());

2nd question is

Will both functions have separate lock or the same lock(considering they are both called by myCounter object)? If thread processing static method still acquiring class level lock how does it internally figure out what lock it has to take?

Answer 1

Question1 : An class instance always has information of a class, so it is valid to call a static method from instance as it will have information regarding class. Other way round is not possible as class will not have info regarding its instances, so you can't call instance level method using class name.

If you try to call a static method via an instance you will get warning as you should avoid it, so you wont get a valid case but yes its logical to allow static call through instance.

Question2 : Coming to 2nd question, moment a thread enters a static block or method, jvm knows which lock to take ie class level. User has no control over it. So instance method will take object lock and static method will take class lock as thread works on it irrespective of how call is made.

Answer 2

Q1: you can reed the following link

http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

Q2: First of all the second method will show up a warning that it should declared as static otherwise the field should not be static. How ever they Absolutely have different locks; Instance method have object lock and static object have class level lock