ThreadLocal for thread saferty

Some code I saw recently that is really a kind of concurrency pattern in Java

Problems and Constraints 

There is an implementation of an interface that is not thread safe. You would like to use it in a multi threaded scenario without modifying the interface.

Example:

public interface Calculator {
    public int calculate(int input) ;
}

public class NonThreadSafeCalculatorImpl implements Calculator {
    public int calculate(int input) {
        //...
        // non thread safe logic here
        // ...
    }
}

An instance of NonThreadSafeCalculatorImplcan't be safely shared by multiple threads.

Solution
Implement the interface by delegating to a ThreadLocal that wraps the unsafe implementation

Example:

public class ThreadSafeCalculatorImpl implements Calculator {

    private static final ThreadLocal threadLocal = new ThreadLocal() {
        @Override
        protected Calculator initialValue() {
            return new NonThreadSafeCalculatorImpl();
        }
    };

    public int calculate(int input) {
        return threadLocal.get().calculate(input);
    }
}

An instance of ThreadSafeCalculatorImpl can be safely shared by multiple threads.