Are Static Methods Thread Safe

How to ensure thread safety of utility static method?

It is well known that static methods with immutable objects as parameters are thread safe and mutable objects are not.

I would contest this. Arguments passed to a method are stored on a stack, which is a per-thread idiom.

If your parameter is a mutable object such as a Date then you need to ensure other threads are not modifying it at the same time elsewhere. But that's a different matter unrelated to the thread-safety of your method.

The method you posted is thread-safe. It maintains no state and operates only on its arguments.

I would strongly recommend you read Java Concurrency in Practice, or a similar book dedicated to thread safety in Java. It's a complex subject that cannot be addressed appropriately through a few StackOverflow answers.

Static methods and thread safety in Java

Yes, that method is thread-safe. The issue with thread-safety occurs is when the static method tries to do something more complex, like sharing data.

Going in more detail,

public static String convertToString(int i) {
    return String.valueOf(i);
}

the above method operates only on the parameter i, which resides on the stack. Stack is local to the thread, so it is perfectly thread-safe.

Are static methods thread safe

Static methods aren't inherently thread-safe. They're treated no differently by the CLR than instance methods. The difference is that one should generally try to make them thread-safe. (I can't think of any .NET BCL static methods which aren't thread-safe.) Instance methods are often not thread-safe because the typical pattern is to create an object and use it repeatedly from one thread, and if it does have to be used from multiple threads, the co-ordination involved includes making sure that the object is used safely. In very many cases that's more appropriate to do in the co-ordinating code than in the object itself. (Usually you want to make whole sequences of operations effectively atomic - something which can't be done within the object.)

Your Timer class is most definitely not thread-safe: two threads can stomp on each other's data with ease, and there's nothing to stop a thread from using "stale" data when calculating the duration.

Use the Stopwatch class instead - that's what it's there for. Admittedly if you want to use one instance from multiple threads you'll need to take the normal steps to ensure safety, but you'll be in a much better position in general. Admittedly Stopwatch is far from perfect too - see this question and the comment below for more details - but it is at least what the type is designed for. (Who knows, it may be fixed some time...)

Static or Non Static methods, thread safety is for the types not methods

are static method implementation threads safe?

No, if they modify shared data, then they are just as non-thread safe. Your example could be OK but only because the shared data is threadsafe itself being a ConcurrentDictionary of immutable types (ints).

instance methods are certainly thread safe, if we assign a separate instance to each thread

Well no, if an instance is accessed by a single thread then that doesn't make it threadsafe. That's just avoiding multi-thread issues.

In short static has nothing to do with multi-threading.

Are non-synchronised static methods thread safe if they don't modify static class variables?

This method is 100% thread safe, it would be even if it wasn't static. The problem with thread-safety arises when you need to share data between threads - you must take care of atomicity, visibility, etc.

This method only operates on parameters, which reside on stack and references to immutable objects on heap. Stack is inherently local to the thread, so no sharing of data occurs, ever.

Immutable objects (String in this case) are also thread-safe because once created they can't be changed and all threads see the same value. On the other hand if the method was accepting (mutable) Date you could have had a problem. Two threads can simultaneously modify that same object instance, causing race conditions and visibility problems.

How to make the static method thread safe in JAVA?

Don't think about making methods thread safe. Thread safety is about protecting the integrity of data. More specifically, it is about preventing threads from accessing data when some other thread is in the process of changing the data.

Your PersonUtils.addErrorMessage(person, message) method modifies List instances belonging to Person instances. Access to the lists should be synchronized If the same list can be modified by two different threads, or if it can be modified by one thread and accessed by other threads.

Adding an item to a list takes several steps, and the list almost certainly will appear to be in an illegal state if thread A is able to see it at a point when thread B has performed some, but not all of the steps. It's worse if two threads attempt to modify the list at the same time: That could leave the list in a permanent, illegal state.

You would still need synchronization even if threads that operate on the same Person instance don't actually do it at the same time. The reason is, without synchronization, the computer hardware and the operating system do not guarantee that changes made in memory by one thread will immediately be visible to other threads. But, synchronized comes to your rescue:

Whatever changes thread A makes before it leaves a synchronized(foo) block will be visible to thread B after thread B enters a synchronized(foo) block.

The simplest thing for you to do, again if different threads access the same Person instance, would be to synchronize on the Person object in your addErrorMessage(...) method:

  public static void addErrorMessage(Person person, String errorMessage){
      synchronized(person) {
          List<Message> msg = person.getMessageList();
          if(msg!=null){
             msg.add(buildMessage(errorMessage));
          }
      }
  }

Is my C# static method in a non static class thread safe?

If your static method does not change any member variables, and does not call mutating methods on its parameters, it is re-entrant, and is, therefore, thread-safe.

Static methods that perform mutating operations on their parameters may not be thread-safe when concurrent threads invoke the method with the same object as its parameter.

For example, if your method mutates o1 through methods, properties, or public variables, the method would no longer be thread-safe.

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