이렇게 해도 되는 것인지 모르겠다. -_-

졸작을 하다가 네트워크 프로그래밍에서 쓰레드를 닫으려고 쓰레드의 stop()을 호출했는데, 이클립스에서 deprecated라는 경고가 떴다. 왜그런가 해서 뒤져봤더니 흠냐 -_-

Why is Thread.stop deprecated?

Because it is inherently unsafe. Stopping a thread causes it to unlock all the monitors that it has locked. (The monitors are unlocked as the ThreadDeath exception propagates up the stack.) If any of the objects previously protected by these monitors were in an inconsistent state, other threads may now view these objects in an inconsistent state. Such objects are said to be damaged. When threads operate on damaged objects, arbitrary behavior can result. This behavior may be subtle and difficult to detect, or it may be pronounced. Unlike other unchecked exceptions, ThreadDeath kills threads silently; thus, the user has no warning that his program may be corrupted. The corruption can manifest itself at any time after the actual damage occurs, even hours or days in the future.

Couldn't I just catch the ThreadDeath exception and fix the damaged object?

In theory, perhaps, but it would vastly complicate the task of writing correct multithreaded code. The task would be nearly insurmountable for two reasons:

1. A thread can throw a ThreadDeath exception almost anywhere. All synchronized methods and blocks would have to be studied in great detail, with this in mind.
2. A thread can throw a second ThreadDeath exception while cleaning up from the first (in the catch or finally clause). Cleanup would have to repeated till it succeeded. The code to ensure this would be quite complex.

In sum, it just isn't practical.

What about Thread.stop(Throwable)?

In addition to all of the problems noted above, this method may be used to generate exceptions that its target thread is unprepared to handle (including checked exceptions that the thread could not possibly throw, were it not for this method). For example, the following method is behaviorally identical to Java's throw operation, but circumvents the compiler's attempts to guarantee that the calling method has declared all of the checked exceptions that it may throw:

    static void sneakyThrow(Throwable t) {
        Thread.currentThread().stop(t);
    }

What should I use instead of Thread.stop?

Most uses of stop should be replaced by code that simply modifies some variable to indicate that the target thread should stop running. The target thread should check this variable regularly, and return from its run method in an orderly fashion if the variable indicates that it is to stop running. To ensure prompt communication of the stop-request, the variable must be volatile (or access to the variable must be synchronized).

For example, suppose your applet contains the following start, stop and run methods:

    private Thread blinker;

    public void start() {
        blinker = new Thread(this);
        blinker.start();
    }

    public void stop() {
        blinker.stop();  // UNSAFE!
    }

    public void run() {
        Thread thisThread = Thread.currentThread();
        while (true) {
            try {
                thisThread.sleep(interval);
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

You can avoid the use of Thread.stop by replacing the applet's stop and run methods with:

    private volatile Thread blinker;

    public void stop() {
        blinker = null;
    }

    public void run() {
        Thread thisThread = Thread.currentThread();
        while (blinker == thisThread) {
            try {
                thisThread.sleep(interval);
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

How do I stop a thread that waits for long periods (e.g., for input)?

That's what the Thread.interrupt method is for. The same "state based" signaling mechanism shown above can be used, but the state change (blinker = null, in the previous example) can be followed by a call to Thread.interrupt, to interrupt the wait:

    public void stop() {
        Thread moribund = waiter;
        waiter = null;
        moribund.interrupt();
    }

For this technique to work, it's critical that any method that catches an interrupt exception and is not prepared to deal with it immediately reasserts the exception. We say reasserts rather than rethrows, because it is not always possible to rethrow the exception. If the method that catches the InterruptedException is not declared to throw this (checked) exception, then it should "reinterrupt itself" with the following incantation:

    Thread.currentThread().interrupt();

This ensures that the Thread will reraise the InterruptedException as soon as it is able.

What if a thread doesn't respond to Thread.interrupt?

In some cases, you can use application specific tricks. For example, if a thread is waiting on a known socket, you can close the socket to cause the thread to return immediately. Unfortunately, there really isn't any technique that works in general. It should be noted that in all situations where a waiting thread doesn't respond to Thread.interrupt, it wouldn't respond to Thread.stop either. Such cases include deliberate denial-of-service attacks, and I/O operations for which thread.stop and thread.interrupt do not work properly.

Why are Thread.suspend and Thread.resume deprecated?

Thread.suspend is inherently deadlock-prone. If the target thread holds a lock on the monitor protecting a critical system resource when it is suspended, no thread can access this resource until the target thread is resumed. If the thread that would resume the target thread attempts to lock this monitor prior to calling resume, deadlock results. Such deadlocks typically manifest themselves as "frozen" processes.

What should I use instead of Thread.suspend and Thread.resume?

As with Thread.stop, the prudent approach is to have the "target thread" poll a variable indicating the desired state of the thread (active or suspended). When the desired state is suspended, the thread waits using Object.wait. When the thread is resumed, the target thread is notified using Object.notify.

For example, suppose your applet contains the following mousePressed event handler, which toggles the state of a thread called blinker:

    private boolean threadSuspended;

    Public void mousePressed(MouseEvent e) {
        e.consume();

        if (threadSuspended)
            blinker.resume();
        else
            blinker.suspend();  // DEADLOCK-PRONE!

        threadSuspended = !threadSuspended;
    }

You can avoid the use of Thread.suspend and Thread.resume by replacing the event handler above with:

    public synchronized void mousePressed(MouseEvent e) {
        e.consume();

        threadSuspended = !threadSuspended;

        if (!threadSuspended)
            notify();
    }

and adding the following code to the "run loop":

                synchronized(this) {
                    while (threadSuspended)
                        wait();
                }

The wait method throws the InterruptedException, so it must be inside a try ... catch clause. It's fine to put it in the same clause as the sleep. The check should follow (rather than precede) the sleep so the window is immediately repainted when the the thread is "resumed." The resulting run method follows:

    public void run() {
        while (true) {
            try {
                Thread.currentThread().sleep(interval);

                synchronized(this) {
                    while (threadSuspended)
                        wait();
                }
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

Note that the notify in the mousePressed method and the wait in the run method are inside synchronized blocks. This is required by the language, and ensures that wait and notify are properly serialized. In practical terms, this eliminates race conditions that could cause the "suspended" thread to miss a notify and remain suspended indefinitely.

While the cost of synchronization in Java is decreasing as the platform matures, it will never be free. A simple trick can be used to remove the synchronization that we've added to each iteration of the "run loop." The synchronized block that was added is replaced by a slightly more complex piece of code that enters a synchronized block only if the thread has actually been suspended:

                if (threadSuspended) {
                    synchronized(this) {
                        while (threadSuspended)
                            wait();
                    }
                }

In the absence of explicit synchronization, threadSuspended must be made volatile to ensure prompt communication of the suspend-request.

The resulting run method is:

    private boolean volatile threadSuspended;

    public void run() {
        while (true) {
            try {
                Thread.currentThread().sleep(interval);

                if (threadSuspended) {
                    synchronized(this) {
                        while (threadSuspended)
                            wait();
                    }
                }
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

Can I combine the two techniques to produce a thread that may be safely "stopped" or "suspended"?

Yes; it's reasonably straightforward. The one subtlety is that the target thread may already be suspended at the time that another thread tries to stop it. If the stop method merely sets the state variable (blinker) to null, the target thread will remain suspended (waiting on the monitor), rather than exiting gracefully as it should. If the applet is restarted, multiple threads could end up waiting on the monitor at the same time, resulting in erratic behavior.

To rectify this situation, the stop method must ensure that the target thread resumes immediately if it is suspended. Once the target thread resumes, it must recognize immediately that it has been stopped, and exit gracefully. Here's how the resulting run and stop methods look:

    public void run() {
        Thread thisThread = Thread.currentThread();
        while (blinker == thisThread) {
            try {
                thisThread.sleep(interval);

                synchronized(this) {
                    while (threadSuspended && blinker==thisThread)
                        wait();
                }
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

    public synchronized void stop() {
        blinker = null;
        notify();
    }

If the stop method calls Thread.interrupt, as described above, it needn't call notify as well, but it still must be synchronized. This ensures that the target thread won't miss an interrupt due to a race condition.

What about Thread.destroy?

Thread.destroy has never been implemented. If it were implemented, it would be deadlock-prone in the manner of Thread.suspend. (In fact, it is roughly equivalent to Thread.suspend without the possibility of a subsequent Thread.resume.) We are not implementing it at this time, but neither are we deprecating it (forestalling its implementation in future). While it would certainly be deadlock prone, it has been argued that there may be circumstances where a program is willing to risk a deadlock rather than exit outright.

Why is Runtime.runFinalizersOnExit deprecated?

Because it is inherently unsafe. It may result in finalizers being called on live objects while other threads are concurrently manipulating those objects, resulting in erratic behavior or deadlock. While this problem could be prevented if the class whose objects are being finalized were coded to "defend against" this call, most programmers do not defend against it. They assume that an object is dead at the time that its finalizer is called.

Further, the call is not "thread-safe" in the sense that it sets a VM-global flag. This forces every class with a finalizer to defend against the finalization of live objects!

(Wikipedia 원문 보기)

Threads are distinguished from traditional multitasking operating system processes in that processes are typically independent, carry considerable state information, have separate address spaces, and interact only through system-provided inter-process communication mechanisms. Multiple threads, on the other hand, typically share the state information of a single process, and share memory and other resources directly. Context switching between threads in the same process is typically faster than context switching between processes. Systems like Windows NT and OS/2 are said to have "cheap" threads and "expensive" processes; in other operating systems there is not so great a difference.

Multithreading is a popular programming and execution model that allows multiple threads to exist within the context of a single process, sharing the process' resources but able to execute independently. The threaded programming model provides developers with a useful abstraction of concurrent execution. However, perhaps the most interesting application of the technology is when it is applied to a single process to enable parallel execution on a multiprocessor system.

This advantage of a multithreaded program allows it to operate faster on computer systems that have multiple CPUs, CPUs with multiple cores, or across a cluster of machines. This is because the threads of the program naturally lend themselves to truly concurrent execution. In such a case, the programmer needs to be careful to avoid race conditions, and other non-intuitive behaviors. In order for data to be correctly manipulated, threads will often need to rendezvous in time in order to process the data in the correct order. Threads may also require atomic operations (often implemented using semaphores) in order to prevent common data from being simultaneously modified, or read while in the process of being modified. Careless use of such primitives can lead to deadlocks.

Operating systems schedule threads in one of two ways. Preemptive multithreading is generally considered the superior approach, as it allows the operating system to determine when a context switch should occur. Cooperative multithreading, on the other hand, relies on the threads themselves to relinquish control once they are at a stopping point. This can create problems if a thread is waiting for a resource to become available. The disadvantage to preemptive multithreading is that the system may make a context switch at an inappropriate time, causing priority inversion or other bad effects which may be avoided by cooperative multithreading.

Traditional mainstream computing hardware did not have much support for multithreading as switching between threads was generally already quicker than full process context switches. Processors in embedded systems, which have higher requirements for real-time behaviors, might support multithreading by decreasing the thread switch time, perhaps by allocating a dedicated register file for each thread instead of saving/restoring a common register file. In the late 1990s, the idea of executing instructions from multiple threads simultaneously has become known as simultaneous multithreading. This feature was introduced in Intel's Pentium 4 processor, with the name Hyper-threading.

**
2학년 2학기 System S/W 중간고사에, 쓰레드와 프로세스의 차이점에 대하여 쓰라는 문제가 생각나서 위키피디아를 검색하였다. 시험당시에는 쓰지 못했지만 역시나 큰 차이점은 프로세스는 리소스를 공유하기 위해 IPC라는 수단을 이용하지만, 쓰레드는 싱글 프로세스내에서 다른 수단이 필요없이 리소스 공유가 가능하다는 점이 아닐런지...