8198249: Remove deprecated Runtime::runFinalizersOnExit and System::runFinalizersOnExit
Reviewed-by: dholmes, alanb, smarks
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<title>Java Thread Primitive Deprecation</title>
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<h2>Java Thread Primitive Deprecation</h2>
<hr>
<h3>Why is <code>Thread.stop</code> deprecated?</h3>
<p>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 <code>ThreadDeath</code> 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
<i>damaged</i>. 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,
<code>ThreadDeath</code> 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.</p>
<hr>
<h3>Couldn't I just catch the <code>ThreadDeath</code> exception
and fix the damaged object?</h3>
<p>In theory, perhaps, but it would <em>vastly</em> complicate the
task of writing correct multithreaded code. The task would be
nearly insurmountable for two reasons:</p>
<ol>
<li>A thread can throw a <code>ThreadDeath</code> exception
<i>almost anywhere</i>. All synchronized methods and blocks would
have to be studied in great detail, with this in mind.</li>
<li>A thread can throw a second <code>ThreadDeath</code> exception
while cleaning up from the first (in the <code>catch</code> or
<code>finally</code> clause). Cleanup would have to be repeated till
it succeeded. The code to ensure this would be quite complex.</li>
</ol>
In sum, it just isn't practical.
<hr>
<h3>What about <code>Thread.stop(Throwable)</code>?</h3>
<p>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
<code>throw</code> operation, but circumvents the compiler's
attempts to guarantee that the calling method has declared all of
the checked exceptions that it may throw:</p>
<pre>
static void sneakyThrow(Throwable t) {
Thread.currentThread().stop(t);
}
</pre>
<hr>
<h3>What should I use instead of <code>Thread.stop</code>?</h3>
<p>Most uses of <code>stop</code> 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
<code>volatile</code> (or access to the variable must be
synchronized).</p>
<p>For example, suppose your applet contains the following
<code>start</code>, <code>stop</code> and <code>run</code>
methods:</p>
<pre>
private Thread blinker;
public void start() {
blinker = new Thread(this);
blinker.start();
}
public void stop() {
blinker.stop(); // UNSAFE!
}
public void run() {
while (true) {
try {
Thread.sleep(interval);
} catch (InterruptedException e){
}
repaint();
}
}
</pre>
You can avoid the use of <code>Thread.stop</code> by replacing the
applet's <code>stop</code> and <code>run</code> methods with:
<pre>
private volatile Thread blinker;
public void stop() {
blinker = null;
}
public void run() {
Thread thisThread = Thread.currentThread();
while (blinker == thisThread) {
try {
Thread.sleep(interval);
} catch (InterruptedException e){
}
repaint();
}
}
</pre>
<hr>
<h3>How do I stop a thread that waits for long periods (e.g., for
input)?</h3>
<p>That's what the <code>Thread.interrupt</code> method is for. The
same "state based" signaling mechanism shown above can be used, but
the state change (<code>blinker = null</code>, in the previous
example) can be followed by a call to
<code>Thread.interrupt</code>, to interrupt the wait:</p>
<pre>
public void stop() {
Thread moribund = waiter;
waiter = null;
moribund.interrupt();
}
</pre>
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 <em>reasserts</em>
rather than <em>rethrows</em>, because it is not always possible to
rethrow the exception. If the method that catches the
<code>InterruptedException</code> is not declared to throw this
(checked) exception, then it should "reinterrupt itself" with the
following incantation:
<pre>
Thread.currentThread().interrupt();
</pre>
This ensures that the Thread will reraise the
<code>InterruptedException</code> as soon as it is able.
<hr>
<h3>What if a thread doesn't respond to
<code>Thread.interrupt</code>?</h3>
<p>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. <em>It should be noted that in all situations where a
waiting thread doesn't respond to <code>Thread.interrupt</code>, it
wouldn't respond to <code>Thread.stop</code> either.</em> Such
cases include deliberate denial-of-service attacks, and I/O
operations for which thread.stop and thread.interrupt do not work
properly.</p>
<hr>
<h3>Why are <code>Thread.suspend</code> and
<code>Thread.resume</code> deprecated?</h3>
<p><code>Thread.suspend</code> 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 <code>resume</code>, deadlock results. Such deadlocks
typically manifest themselves as "frozen" processes.</p>
<hr>
<h3>What should I use instead of <code>Thread.suspend</code> and
<code>Thread.resume</code>?</h3>
<p>As with <code>Thread.stop</code>, 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 <code>Object.wait</code>. When
the thread is resumed, the target thread is notified using
<code>Object.notify</code>.</p>
<p>For example, suppose your applet contains the following
mousePressed event handler, which toggles the state of a thread
called <code>blinker</code>:</p>
<pre>
private boolean threadSuspended;
Public void mousePressed(MouseEvent e) {
e.consume();
if (threadSuspended)
blinker.resume();
else
blinker.suspend(); // DEADLOCK-PRONE!
threadSuspended = !threadSuspended;
}
</pre>
You can avoid the use of <code>Thread.suspend</code> and
<code>Thread.resume</code> by replacing the event handler above
with:
<pre>
public synchronized void mousePressed(MouseEvent e) {
e.consume();
threadSuspended = !threadSuspended;
if (!threadSuspended)
notify();
}
</pre>
and adding the following code to the "run loop":
<pre>
synchronized(this) {
while (threadSuspended)
wait();
}
</pre>
The <code>wait</code> method throws the
<code>InterruptedException</code>, so it must be inside a <code>try
... catch</code> clause. It's fine to put it in the same clause as
the <code>sleep</code>. The check should follow (rather than
precede) the <code>sleep</code> so the window is immediately
repainted when the thread is "resumed." The resulting
<code>run</code> method follows:
<pre>
public void run() {
while (true) {
try {
Thread.sleep(interval);
synchronized(this) {
while (threadSuspended)
wait();
}
} catch (InterruptedException e){
}
repaint();
}
}
</pre>
Note that the <code>notify</code> in the <code>mousePressed</code>
method and the <code>wait</code> in the <code>run</code> method are
inside <code>synchronized</code> blocks. This is required by the
language, and ensures that <code>wait</code> and
<code>notify</code> are properly serialized. In practical terms,
this eliminates race conditions that could cause the "suspended"
thread to miss a <code>notify</code> and remain suspended
indefinitely.
<p>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:</p>
<pre>
if (threadSuspended) {
synchronized(this) {
while (threadSuspended)
wait();
}
}
</pre>
<p>In the absence of explicit synchronization,
<code>threadSuspended</code> must be made <code>volatile</code> to ensure
prompt communication of the suspend-request.</p>
The resulting <code>run</code> method is:
<pre>
private volatile boolean threadSuspended;
public void run() {
while (true) {
try {
Thread.sleep(interval);
if (threadSuspended) {
synchronized(this) {
while (threadSuspended)
wait();
}
}
} catch (InterruptedException e){
}
repaint();
}
}
</pre>
<hr size="3" noshade="noshade" />
<h3>Can I combine the two techniques to produce a thread that may
be safely "stopped" or "suspended"?</h3>
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 <code>stop</code> method merely sets
the state variable (<code>blinker</code>) 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.
<p>To rectify this situation, the <code>stop</code> 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
<code>run</code> and <code>stop</code> methods look:</p>
<pre>
public void run() {
Thread thisThread = Thread.currentThread();
while (blinker == thisThread) {
try {
Thread.sleep(interval);
synchronized(this) {
while (threadSuspended && blinker==thisThread)
wait();
}
} catch (InterruptedException e){
}
repaint();
}
}
public synchronized void stop() {
blinker = null;
notify();
}
</pre>
If the <code>stop</code> method calls <code>Thread.interrupt</code>, as
described above, it needn't call <code>notify</code> as well, but it
still must be synchronized. This ensures that the target thread
won't miss an interrupt due to a race condition.
<hr>
<h3>What about <code>Thread.destroy</code>?</h3>
<code>Thread.destroy</code> was never implemented and has been
deprecated. If it were implemented, it would be deadlock-prone in
the manner of <code>Thread.suspend</code>. (In fact, it is roughly
equivalent to <code>Thread.suspend</code> without the possibility
of a subsequent <code>Thread.resume</code>.)
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