Instance methods and thread-safety of instance variables

Question

I would like to known if each instance of a class has its own copy of the methods in that class?

Lets say, I have following class MyClass:

public MyClass {      private String s1;      private String s2;       private String method1(String s1){     ...     }      private String method2(String s2){     ...     } } 

So if two differents users make an instance of MyClass like:

MyClass instanceOfUser1 = new MyClass(); MyClass instanceOfUser2 = new MyClass(); 

Does know each user have in his thread a copy of the methods of MyClass? If yes, the instance variables are then thread-safe, as long as only the instance methods manipulate them, right?

I am asking this question because I often read that instance variables are not thread-safe. And I can not see why it should be like that, when each user gets an instance by calling the new operator?

Answer 1

Each object gets its own copy of the class's instance variables - it's static variables that are shared between all instances of a class. The reason that instance variables are not necessarily thread-safe is that they might be simultaneously modified by multiple threads calling unsynchronized instance methods.

class Example {     private int instanceVariable = 0;      public void increment() {         instanceVariable++;     } } 

Now if two different threads call increment at the same then you've got a data race - instanceVariable might increment by 1 or 2 at the end of the two methods returning. You could eliminate this data race by adding the synchronized keyword to increment, or using an AtomicInteger instead of an int, etc, but the point is that just because each object gets its own copy of the class's instance variables does not necessarily mean that the variables are accessed in a thread-safe manner - this depends on the class's methods. (The exception is final immutable variables, which can't be accessed in a thread-unsafe manner, short of something goofy like a serialization hack.)