Here's the snippet:
public class LogService {
public void stop() {
synchronized (this) { isShutdown = true; }
loggerThread.interrupt(); /* Is it necesarry? */
}
public void log(String msg) throws InterruptedException {
synchronized (this) {
if (isShutdown)
throw new IllegalStateException(...);
++reservations;
}
queue.put(msg);
}
private class LoggerThread extends Thread {
public void run() {
try {
while (true) {
try {
synchronized (LogService.this) {
if (isShutdown && reservations == 0)
break;
}
String msg = queue.take();
synchronized (LogService.this) {
--reservations;
}
writer.println(msg);
} catch (InterruptedException e) { } /* Do nothing */
}
} finally {
writer.close();
}
}
}
}
As the code above, Even if we put LoggerThread.interrupt() in stop() method, the interruption just be caught by thread and do nothing.
So is LoggerThread.interrupt() necessary?
Yes it it necessary. If the queue is empty, this statement String msg = queue.take(); will block until an element is put in the queue or it is is interrupted.
If you want to guarantee that the thread does not hang you need to interrupt it.
However there seems to be a glitch: if reservations is not 0 when you call the close method AND the queue is empty, it seems that your loop will keep going and hang on queue.take() at the while loop iteration following the interruption.
Related
I am working on java version upgrade project and I am on the work where I need to replace deprecated methods.
this.stop();
Code USed this method are in ::
ThreadedTestGroup.java::
package utmj.threaded;
import junit.framework.*;
public class ThreadedTestGroup extends ThreadGroup {
private Test test;
private TestResult testResult;
public ThreadedTestGroup(Test test) {
super("ThreadedTestGroup");
this.test = test;
}
public void interruptThenStop() {
this.interrupt();
if (this.activeCount() > 0) {
this.stop(); // For those threads which won't interrupt
}
}
public void setTestResult(TestResult result) {
testResult = result;
}
public void uncaughtException(Thread t, Throwable e) {
if (e instanceof ThreadDeath) {
return;
}
if (e instanceof AssertionFailedError) {
testResult.addFailure(test, (AssertionFailedError) e);
} else {
testResult.addError(test, e);
}
this.interruptThenStop();
}
}
CobcyrrentTestCase.java
package utmj.threaded;
import java.util.*;
import junit.framework.*;
/
public class ConcurrentTestCase extends TestCase {
private TestResult currentResult;
private ThreadedTestGroup threadGroup;
private Hashtable threads = new Hashtable();
private boolean deadlockDetected = false;
private Vector checkpoints = new Vector();
class ConcurrentTestThread extends Thread {
private volatile boolean hasStarted = false;
private volatile boolean hasFinished = false;
ConcurrentTestThread(
ThreadGroup group,
Runnable runnable,
String name) {
super(group, runnable, name);
}
public void run() {
hasStarted = true;
super.run();
finishThread(this);
}
}
public ConcurrentTestCase(String name) {
super(name);
}
public ConcurrentTestCase() {
super();
}
protected void addThread(String name, final Runnable runnable) {
if (threads.get(name) != null) {
fail("Thread with name '" + name + "' already exists");
}
ConcurrentTestThread newThread =
new ConcurrentTestThread(threadGroup, runnable, name);
threads.put(name, newThread);
}
public synchronized void checkpoint(String checkpointName) {
checkpoints.addElement(checkpointName);
this.notifyAll();
}
public boolean checkpointReached(String checkpointName) {
return checkpoints.contains(checkpointName);
}
public boolean deadlockDetected() {
return deadlockDetected;
}
private synchronized void finishThread(ConcurrentTestThread thread) {
thread.hasFinished = true;
this.notifyAll();
}
private ConcurrentTestThread getThread(String threadName) {
return (ConcurrentTestThread) threads.get(threadName);
}
/**
* Returns true if the thread finished normally, i.e. was not inerrupted or stopped
*/
public boolean hasThreadFinished(String threadName) {
ConcurrentTestThread thread = this.getThread(threadName);
if (thread == null) {
fail("Unknown Thread: " + threadName);
}
return thread.hasFinished;
}
public boolean hasThreadStarted(String threadName) {
ConcurrentTestThread thread = this.getThread(threadName);
if (thread == null) {
fail("Unknown Thread: " + threadName);
}
return thread.hasStarted;
}
private void interruptAllAliveThreads() {
threadGroup.interruptThenStop();
}
/**
* Wait till all threads have finished. Wait maximally millisecondsToWait.
* Should only be called after startThreads().
*/
protected void joinAllThreads(long millisecondsToWait) {
Enumeration enum1 = threads.elements();
long remainingMilliseconds = millisecondsToWait;
while (enum1.hasMoreElements()) {
long before = System.currentTimeMillis();
ConcurrentTestThread each =
(ConcurrentTestThread) enum1.nextElement();
try {
each.join(remainingMilliseconds);
} catch (InterruptedException ignored) {
}
long spent = System.currentTimeMillis() - before;
if (millisecondsToWait != 0) {
remainingMilliseconds = remainingMilliseconds - spent;
if (remainingMilliseconds <= 0) {
deadlockDetected = true;
break;
}
}
}
}
public void joinThread(String threadName) throws InterruptedException {
this.joinThread(threadName, 0);
}
public void joinThread(String threadName, long millisecondsToTimeout)
throws InterruptedException {
ConcurrentTestThread thread = this.getThread(threadName);
if (thread == null) {
fail("Unknown Thread: " + threadName);
}
thread.join(millisecondsToTimeout);
}
/**
* Stores the current result to be accessible during the test
*/
public void run(TestResult result) {
currentResult = result;
super.run(result);
}
protected void setUp() throws Exception {
threadGroup = new ThreadedTestGroup(this);
}
/**
* Sleep and ignore interruption
*/
public void sleep(long milliseconds) {
try {
Thread.sleep(milliseconds);
} catch (InterruptedException ignored) {
}
}
/**
* Run all threads and wait for them to finish without timeout
*/
protected void startAndJoinAllThreads() {
this.startAndJoinThreads(0);
}
protected void startThreads() {
threadGroup.setTestResult(currentResult);
Enumeration enum1 = threads.elements();
while (enum1.hasMoreElements()) {
ConcurrentTestThread each =
(ConcurrentTestThread) enum1.nextElement();
each.start();
each.hasStarted = true;
}
Thread.yield();
}
protected void tearDown() throws Exception {
this.interruptAllAliveThreads();
threads = new Hashtable();
checkpoints = new Vector();
deadlockDetected = false;
threadGroup = null;
currentResult = null;
}
public synchronized void waitForCheckpoint(String checkpointName) {
while (!this.checkpointReached(checkpointName)) {
try {
this.wait();
} catch (InterruptedException ignored) {
}
}
}
public synchronized void waitUntilFinished(String threadName) {
while (!this.hasThreadFinished(threadName)) {
try {
this.wait();
} catch (InterruptedException ignored) {
}
}
}
}
I tried to search lot about this but did not got suitable solution so is there anyone who can help me out to replace this.stop() method which is deprecated.
IDE message: The method stop() from the type ThreadGroup is deprecated
There is no single method that replaces stop() from Thread Group but rather a design approach
From the oracle documentation it says
Many 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
Looking at the samples on What should I use instead of Thread.stop?
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();
}
}
Throughout your thread, you need to check on a thread safe variable (in the example above its blinker) ... when stop is called, it sets the thread to null breaking out of the while loop and returning from run... thereby "stopping" the thread
Well I red a bit of the documentation about why stop() is deprecated and here is the most relevant part :
This method is inherently unsafe. Stopping a thread with Thread.stop causes it to unlock all of the monitors that it has locked (as a natural consequence of the unchecked ThreadDeath exception propagating up the stack). If any of the objects previously protected by these monitors were in an inconsistent state, the damaged objects become visible to other threads, potentially resulting in arbitrary behavior. Many uses of stop should be replaced by code that simply m>odifies 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. If the target thread waits for long periods (on a condition variable, for example), the interrupt method should be used to interrupt the wait.
With those details, I think there is no more a simple way to stop all the threads as stop() did. You might need to modifie the threads so that you have a way to stop them (if it is possible for you).
The Oracle Java SE Docs recommend doing this:
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 {
Thread.sleep(interval);
} catch (InterruptedException e){
}
repaint();
}
}
Is there a way to do the same thing for a class blinker implements Runnable ?
As you would have to use blinker thisClass = this; or similar, wouldn't the (blinker == thisClass) always evaluate as true?
Or will this code suffice:
class blinker implements Runnable {
boolean stop = false;
#override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
// code
// ...
if (stop) { Thread.currentThread().interrupt(); }
// ...
}
}
}
You could do something like that:
class Blinker implements Runnable {
Runnable blinker = this;
public void stop() {
blinker = null;
}
public void run() {
while(blinker == this) {
}
}
}
But it would be pretty pointless. I don't think you're understanding the point the documentation is trying to get across, which is don't use an infinite loop to keep threads alive, using Thread#stop() to terminate them. Instead, use a condition, then set it to false when you want to end the loop which is keeping the thread alive.
You do not need to constantly check Thread#isInterrupted() to keep the thread alive.
while(!stop) {
}
Would do just fine. You also should not interrupt the thread from within the thread. The purpose of interrupting is to end tasks that halt the thread. These tasks are surround within try/catch which catches an InterruptedException. Other threads are usually the ones in charge of interrupting.
The documentation is referring to allowing the thread to die gracefully.
In the first example, the run() method was handled via an infinite loop: while(true). The only way to stop the thread would be to forcing some kind of stop, such as usong Thread#stop:
public void run() {
while (true) {
try {
Thread.sleep(interval);
} catch (InterruptedException e){
}
repaint();
}
}
But it's not recommended to use Thread#stop. Instead, the loop should depend on a boolean, which another thread (or the current one) could set to true or false:
private volatile boolean running;
public void stop() {
running = false;
}
public void run() {
while (running) {
try {
Thread.sleep(interval);
} catch (InterruptedException e){
}
repaint();
}
}
Instead of using a running boolean, they used blinker == thisThread, then changed the value of blinker when they wanted to end the loop:
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();
}
}
Essentially, what I want to do is start all my threads, pause them all, then resume them all, using the multithreading approach. I am just looking for a simple solution to this. I'm not sure if I have to use a timer or what. Right now when I run it, the threads are like being executed in random order (I guess the PC is just randomly picking which ones it wants to run at a certain time).
class ChoppingThread extends Thread
{
public void run()
{
for(int j=40;j!=0;j-=10)
System.out.println("Chopping vegetables...("+j+" seconds left)");
}
}
class MixingThread extends Thread
{
public void run()
{
for(int k=60;k!=0;k-=10)
System.out.println("Mixing sauces...("+k+" seconds left)");
}
}
class TenderizingThread extends Thread
{
public void run()
{
for(int j=50;j!=0;j-=10)
System.out.println("Tenderizing meat...("+j+" seconds left)");
}
}
class MultiThreadTasking
{
public static void main (String [] args)
{
ChoppingThread ct = new ChoppingThread();
MixingThread mt = new MixingThread();
TenderizingThread tt = new TenderizingThread();
System.out.println("\nWelcome to the busy kitchen.");
//putting threads into ready state
ct.start();
mt.start();
tt.start();
}
}
There are probably other ways to achieve the same result, but this is the simplest I can come up with off the top of my head (I know, sad isn't it)...
Basically, this is a special Runnable with some additional management functionality.
This basically contains a state flag that indicates the state of the task and a monitor lock
public class ThreadFun {
public static void main(String[] args) {
MyTask task = new MyTask();
Thread thread = new Thread(task);
thread.start();
try {
Thread.sleep(1000);
} catch (InterruptedException ex) {
}
task.pauseTask();
try {
Thread.sleep(1000);
} catch (InterruptedException ex) {
}
task.resumeTask();
try {
Thread.sleep(1000);
} catch (InterruptedException ex) {
}
task.stopTask();
}
public enum TaskState {
Running,
Stopped,
Paused
}
public static class MyTask implements Runnable {
private static final Object PAUSED_LOCK = new Object();
private volatile TaskState state = TaskState.Running;
public void pauseTask() {
if (state == TaskState.Running) {
System.out.println("Paused...");
state = TaskState.Paused;
}
}
public void resumeTask() {
if (state == TaskState.Paused) {
state = TaskState.Running;
synchronized (PAUSED_LOCK) {
PAUSED_LOCK.notifyAll();
}
System.out.println("Resumed...");
}
}
public void stopTask() {
if (state == TaskState.Running || state == TaskState.Paused) {
state = TaskState.Stopped;
System.out.println("Stopped...");
}
}
public boolean isStopped() {
return state == TaskState.Stopped;
}
public boolean isPaused() {
return state == TaskState.Paused;
}
protected void doPause() {
synchronized (PAUSED_LOCK) {
while (isPaused()) {
try {
PAUSED_LOCK.wait();
} catch (InterruptedException ex) {
}
}
}
}
#Override
public void run() {
int index = 0;
while (!isStopped() && index < 1000) {
try {
Thread.sleep(25);
} catch (InterruptedException ex) {
}
doPause();
index++;
System.out.println(index);
}
stopTask(); // Make sure the task is marked as begin stopped ;)
}
}
}
The main criteria is you will need to pool isStopped and doPause at appropriate points to ensure that they are begin implemented as required...
To coordinate them use a CyclicBarrier.
To launch them all at the same time use a CountDownLatch.
Google the two classes above for many examples and explanations.
To fully understand what is happening read the Java Concurrency In Practice book.
I believe you can accomplish this by using Object.wait and Thread.interrupt.
Object.wait blocks until notify is called. So
private boolean paused;
private Object waitObject;
...
public void run() {
for ... {
if (this.paused) { this.waitObject.wait(); }
...
public void pause() { this.paused = true; }
public void resume() { this.paused = false; this.waitObject.notify(); }
Then you can call pause to pause the thread.
Thread.interrupt can help with stopping.
private boolean paused;
...
public void run() {
for ... {
// interrupted() is different from interrupt()!
if (this.iterrupted()) { break; }
...
To stop it, you would call interrupt() from another thread.
This is the basic idea, but there's a lot of details to worry about here. For example, wait can throw an InterruptedException you'll need to handle. Also, wait is not guaranteed to return only after a notify. It can return randomly. Here is a pair of tutorials:
Wait: http://docs.oracle.com/javase/tutorial/essential/concurrency/guardmeth.html
Interrupt: http://docs.oracle.com/javase/tutorial/essential/concurrency/interrupt.html
I'm testing a Java multi-threading sample code but the thread started in the for loop of qB.start() is blocked because it's waiting for entry of qB monitor. What is the cause of this blockage?
Thank you.
import java.util.*;
class QA {
public synchronized void open() throws Exception {
Thread o = new Thread() {
public void run() {
QB qB = new QB();
qB.start();
}
};
o.start();
}
public static void main(String args[]) throws Exception {
new QA().open();
}
public class QB {
private boolean shutdown;
private Vector<Thread> tList;
private final Object waitingLock = new Object();
public QB() {
tList = new Vector<Thread>();
}
public synchronized void start() {
for(int i = 0; i < 1; i++) {
final int id = i;
Thread t = new Thread("Thread " + id) {
public void run() {
load(id);
}
};
tList.add(i, t);
t.start();
}
tMonitor();
waitUntilFinished();
}
private void tMonitor() {
Thread cmt = new Thread("T Monitor Thread") {
public void run() {
synchronized(waitingLock) {
while(tList.size() > 0) {
try {
sleep(10000);
} catch(Exception e) {
e.printStackTrace();
}
}
waitingLock.notifyAll();
}
}
};
cmt.start();
}
private void waitUntilFinished() {
synchronized(waitingLock) {
while(!isShutDown()) {
try {
waitingLock.wait();
} catch(Exception e) {
e.printStackTrace();
}
}
}
}
private synchronized void load(int id) {
try {
System.out.println("blocked here");
// some work done here
removeFromTList(id);
} catch(Exception e) {
e.printStackTrace();
}
}
public synchronized boolean isShutDown() {
return shutdown;
}
}
}
The first problem I see is that QB#start() is synchronized on the instance of QB.
Inside the thread t that you are trying to spawn, load(id) is also synchronized on the same instance of QB. So when you call t.start() the t thread blocks until QB#start() finishes.
Presumably, at the end of the QB#start() method, QB#waitUntilFinished() is supposed to wait for all the t threads to finish, but they can't even enter the QB#load method because they're still waiting for the QB#start() method to release the lock on the QB instance.
So, circular deadlock.
Edit:
Ok, now that we see how the threads are removed from tList the bug is fully revealed.
If the index 0 thread finishes first then it will remove itself from the list. That means when the index 1 thread finishes, it will remove the 1th position from the Vector but that does not point to itself anymore. It is removing the #2 thread. Sooner or later you are going to get an exception when the remove happens because it is going to be removing an invalid index.
You need to remove items from the Vector by address and not by position:
tList.remove(this);
That will remove the current thread from the list. You should also just do an add(t) instead of an add(i t) in the start loop:
tList.add(t);
You now don't need the id position passed into your thread at all.
I don't see where you are removing the finished threads from your tList. I see a definition (not that you edited your OP) of a removeFromTList() method but I don't see it used anywhere. In tMonitor you are in a while loop here:
while(tList.size() > 0) {
try {
sleep(10000);
} catch(Exception e) {
e.printStackTrace();
}
}
// you never get to this line
waitingLock.notifyAll();
But I don't see anything that removes the thread from the list. Maybe when the threads each finish they are supposed to remove themselves?
If tMonitor thread never gets out of that loop then it never calls:
waitingLock.notifyAll();
So the main thread will hang forever in waitUntilFinished();.
synchronized(waitingLock) {
while(!isShutDown()) {
try {
waitingLock.wait();
} catch(Exception e) {
e.printStackTrace();
}
}
Also, you don't want to do a sleep in tMonitor() because you are in a synchronized block. You should be doing a:
waitingLock.wait(10000);
Nothing will ever notify it but it's bad form to hold the lock like that in a sleep.
We require a piece of code to control a thread. For example, use three buttons like start, stop and pause, press one of them and perform the action against it. Like press start then start the thread, press stop actually stops thread and pause perform pause action respectively.
Starting a thread is simple with Thread.start(). Stopping a thread can be as simple as setting a flag that is checked asychronously in the run method, but may need to include a call to Thread.interrupt(). Pausing a thread is more problematic, but could also be done using a flag that cauases the run method to yield instead of process. Here is some (untested) code:
class MyThread extends Thread {
private final static int STATE_RUN = 0, STATE_PAUSE = 2, STATE_STOP = 3;
private int _state;
MyThread() {
_state = STATE_RUN;
}
public void run() {
int stateTemp;
synchronized(this) {
stateTemp = _state;
}
while (stateTemp != STATE_STOP) {
switch (stateTemp) {
case STATE_RUN:
// perform processing
break;
case STATE_PAUSE:
yield();
break;
}
synchronized(this) {
stateTemp = _state;
}
}
// cleanup
}
public synchronized void stop() {
_state = STATE_STOP;
// may need to call interrupt() if the processing calls blocking methods.
}
public synchronized void pause() {
_state = STATE_PAUSE;
// may need to call interrupt() if the processing calls blocking methods.
// perhaps set priority very low with setPriority(MIN_PRIORITY);
}
public synchronized void unpause() {
_state = STATE_RUN;
// perhaps restore priority with setPriority(somePriority);
// may need to re-establish any blocked calls interrupted by pause()
}
}
As you can see it can quite quickly get complex depending on what you are doing in the thread.
I would like to add on Richard's answer to address a few issues:
Needless cycles when paused
Needless extra cycle when state changed
yield() used where wait() needed
Single instance
Stopping the thread waits for the thread to finish
This is my altered code:
class MyThread extends Thread {
private final static int STATE_RUN = 0, STATE_PAUSE = 2, STATE_STOP = 3;
private int _state;
private static MyThread thread;
public static MyThread getInstance() {
if (thread == null || !thread.isAlive()) {
thread = new MyThread();
}
return thread;
}
private MyThread() {
_state = STATE_RUN;
}
public static void main(String[] args) {
MyThread t = MyThread.getInstance();
try {
t.start();
Thread.sleep(500);
t.pause();
Thread.sleep(500);
t.unpause();
Thread.sleep(500);
t.end();
} catch (InterruptedException e) {
// ignore; this is just an example
}
}
public void run() {
int i = 0;
while (_state != STATE_STOP) {
if (_state == STATE_PAUSE) {
System.out.println(this + " paused");
synchronized (this) {
try {
this.wait();
} catch (InterruptedException e) {
}
}
}
if (_state == STATE_STOP) {
break;
}
// this is where the actual processing happens
try {
// slow output down for this example
Thread.sleep(100);
} catch (InterruptedException e) {
// state change handled next cycle
}
System.out.println(this + " cycle " + i);
i++;
}
System.out.println(this + " finished");
// cleanup
}
public synchronized void end() {
_state = STATE_STOP;
try {
this.interrupt();
this.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public synchronized void pause() {
_state = STATE_PAUSE;
}
public synchronized void unpause() {
_state = STATE_RUN;
synchronized (this) {
this.notify();
}
}
}