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Thread Confinement

Asked 2011-06-07T07:28:56.013
46

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 thread, such usage is automatically thread-safe even if the confined object itself is not

So when an object is confined to a thread, no other thread can have access to it? Is that what it means to be confined to a thread? How does one keep an object confined to a thread?

Edit: But what if I still want to share the object with another thread? Let's say that after thread A finishes with object O, thread B wants to access O. In this case, can O still be confined to B after A is done with it?

Using a local variable is one example for sure but that just means you don't share your object with other thread (AT ALL). In case of JDBC Connection pool, doesn't it pass one connection from one thread to another once a thread is done with that connection (totally clueless about this because I never used JDBC).

java multithreading concurrency thread-safety thread-confinement
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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<Item> list = new ArrayList<Item>();
    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<DateFormat> df
                 = new ThreadLocal<DateFormat>(){
    @Override
    protected DateFormat initialValue() {
        return new SimpleDateFormat("yyyyMMdd");
    }
  };

Further Reading

answered 2011-06-07T07:41:50.760

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