Thread Confinement

Question

I am reading Java Concurrency in Practice and kind of confused with the thread confinement concept. The book says that

When an object is confined to a

Answer 1

One way is "stack confinement" in which the object is a local variable confined to the thread's stack, so no other thread can access it. In the method below, the list is a local variable and doesn't escape from the method. The list doesn't have to be threadsafe because it is confined to the executing thread's stack. No other thread can modify it.

public String foo(Item i, Item j){
    List list = new ArrayList();
    list.add(i);
    list.add(j);
    return list.toString();
}

Another way of confining an object to a thread is through the use of a ThreadLocal variable which allows each thread to have its own copy. In the example below, each thread will have its own DateFormat object and so you don't need to worry about the fact that DateFormat is not thread-safe because it won't be accessed by multiple threads.

private static final ThreadLocal df
                 = new ThreadLocal(){
    @Override
    protected DateFormat initialValue() {
        return new SimpleDateFormat("yyyyMMdd");
    }
  };

Further Reading