I am trying to understand how ReentrantLock works in java.
Lets consider a simple example below :
private ReentrantLock lock;
public void foo() {
lock.lock();
try{
...
}finally {
lock.unlock();
}
}
I was trying to figure out the call hierarchy of lock() method.
public void lock() {
sync.lock();
}
For FairSync :
final void lock() {
acquire(1);
}
For NonFairSync :
final void lock() {
if (compareAndSetState(0, 1))
setExclusiveOwnerThread(Thread.currentThread());
else
acquire(1);
}
Both lock() methods call acquire() method with argument as 1.
In AbstractQueuedSynchronizer class :
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
static void selfInterrupt() {
Thread.currentThread().interrupt();
}
If current thread cannot acquire a resource (i.e. some another thread has acquired this resource) , then current thread has to wait. In this case ReentrantLock calls selfInterrupt() method.
Now my question is how interrupt() method can stop a thread which is equivalent to wait() method in synchronized ?
Also , after the resource has been released by another thread, how currentThread start automatically? ( After calling unlock() method by another thread which is internally calling sync.release(1); )
I also tried to figure out how interrupt() method works from here but unable to find answer to my questions.
If current thread cannot acquire a resource (i.e. some another thread
has acquired this resource) , then current thread has to wait. In this
case ReentrantLock calls selfInterrupt() method.
No, if tryAcquire returns false, acquireQueued will be called. Internally, that method uses LockSupport#park to unschedule the thread. The javadoc states
If the permit is available then it is consumed and the call returns
immediately; otherwise the current thread becomes disabled for thread
scheduling purposes and lies dormant until one of three things
happens:
- [..]
- Some other thread interrupts the current thread;
It doesn't actually throw an InterruptedException in this case, the method just returns. To check if the corresponding thread was woken up due to an interrupt, it has to use Thread#interrupted() which returns true or false depending, but also clears the interrupt flag on the corresponding Thread instance.
So the acquireQueued propagates that interrupted value back up and lets acquire reset the interrupt flag on the Thread if needed in that little piece of code
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
Also , after the resource has been released by another thread, how
currentThread start automatically?
Again, it makes uses of the LockSupport utility with unpark
Makes available the permit for the given thread, if it was not already
available. If the thread was blocked on park then it will unblock.
Related
Please note that this is not the actual scenario. I created a sample scenario based on my actual implementation, to make it easy to review. I am also already getting the expected output too. However, I need to clarify some concepts regarding the wait() and notifyAll() methods in Java. (In here both these threads will starts there run method at once in the main thread.) So according to my knowledge, since thread B is sleeping, because you can see at the initial stage reamingCount is 400.
So thread B will calls its MUTEX.wait() and continue its sleep until some other thread invokes a notify() or notifyAll(), then after the remainingCount decrements to 0, thread A will call MUTEX.notifyAll(); to awake the thread B and MUTEX.wait() to release its already granted lock, and go to sleep until thread B notifies it.
When I call MUTEX.notifyAll() through thread A, won't thread B wake up and continue its task before thread A calls MUTEX.wait()?
I mean, you can see when thread A calls the MUTEX.notifyAll(), thread B will awake and check again if the condition in the while loop is true or false. So, since the remainingCount is equal to 0, thread B will exit the while loop and continue its task before thread A calls wait(). Won't this scenario break the principle of wait()? According to my knowledge thread B can only continue its execution when thread A calls wait().
public class A implements Runnable{
public static volatile remainingCount =400;
private final Object MUTEX;//Both class A and B holds the same object mutex
private void methodA(){
synchronized(MUTEX){
while(remainingCount == 0){
MUTEX.notifyAll();
MUTEX.wait();
}
//Perform it's usual task.In here remaining count will decrement during the process.
}
#Override
public void run() {
while(true){
methodA();
}
}
}
}
public class B implements Runnable{
private final Object MUTEX;//Both class A and B holds the same object mutex
private void methodB(){
synchronized(MUTEX){
while (A.remainingCount != 0) {
try {
MUTEX.wait();
} catch (InterruptedException ex) {
Logger.getLogger(InkServiceImpl.class.getName()).log(Level.SEVERE, null, ex);
}
}
//incrementing the A.remainingCount
MUTEX.notifyAll();
}
#Override
public void run() {
while(true){
methodB();
}
}
}
When a thread holding a lock calls wait() on the locked object, the thread is added to the object's wait set and the lock is released.
When a thread holding a lock calls notify(), and the wait set is not empty, a thread in the wait set is selected and removed. Likewise, calling notifyAll() removes all threads from the wait set.
Note: threads can also be removed from the wait set by a call to thread.interrupt().
When a thread is removed from the wait set and begins to run, the first step is to reacquire the lock. This happens before the return from wait().
This will not happen until the thread that called notify() or notifyAll() releases the lock by either calling wait() or exiting the synchronized block.
So, while your thread B has been enabled to run, it won't actually return from wait() until thread A releases the lock by calling MUTEX.wait(). Likewise, thread A is enabled to run when B calls MUTEX.notifyAll(), but doesn't return from wait() until thread B exits the synchronized(MUTEX) block.
I am using multi-threading in java for my program.
I have run thread successfully but when I am using Thread.wait(), it is throwing java.lang.IllegalMonitorStateException.
How can I make a thread wait until it will be notified?
You need to be in a synchronized block in order for Object.wait() to work.
Also, I recommend looking at the concurrency packages instead of the old school threading packages. They are safer and way easier to work with.
EDIT
I assumed you meant Object.wait() as your exception is what happens when you try to gain access without holding the objects lock.
wait is defined in Object, and not it Thread. The monitor on Thread is a little unpredictable.
Although all Java objects have monitors, it is generally better to have a dedicated lock:
private final Object lock = new Object();
You can get slightly easier to read diagnostics, at a small memory cost (about 2K per process) by using a named class:
private static final class Lock { }
private final Object lock = new Lock();
In order to wait or notify/notifyAll an object, you need to be holding the lock with the synchronized statement. Also, you will need a while loop to check for the wakeup condition (find a good text on threading to explain why).
synchronized (lock) {
while (!isWakeupNeeded()) {
lock.wait();
}
}
To notify:
synchronized (lock) {
makeWakeupNeeded();
lock.notifyAll();
}
It is well worth getting to understand both Java language and java.util.concurrent.locks locks (and java.util.concurrent.atomic) when getting into multithreading. But use java.util.concurrent data structures whenever you can.
I know this thread is almost 2 years old but still need to close this since I also came to this Q/A session with same issue...
Please read this definition of illegalMonitorException again and again...
IllegalMonitorException is thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.
This line again and again says, IllegalMonitorException comes when one of the 2 situation occurs....
1> wait on an object's monitor without owning the specified monitor.
2> notify other threads waiting on an object's monitor without owning the specified monitor.
Some might have got their answers... who all doesn't, then please check 2 statements....
synchronized (object)
object.wait()
If both object are same... then no illegalMonitorException can come.
Now again read the IllegalMonitorException definition and you wont forget it again...
Based on your comments it sounds like you are doing something like this:
Thread thread = new Thread(new Runnable(){
public void run() { // do stuff }});
thread.start();
...
thread.wait();
There are three problems.
As others have said, obj.wait() can only be called if the current thread holds the primitive lock / mutex for obj. If the current thread does not hold the lock, you get the exception you are seeing.
The thread.wait() call does not do what you seem to be expecting it to do. Specifically, thread.wait() does not cause the nominated thread to wait. Rather it causes the current thread to wait until some other thread calls thread.notify() or thread.notifyAll().
There is actually no safe way to force a Thread instance to pause if it doesn't want to. (The nearest that Java has to this is the deprecated Thread.suspend() method, but that method is inherently unsafe, as is explained in the Javadoc.)
If you want the newly started Thread to pause, the best way to do it is to create a CountdownLatch instance and have the thread call await() on the latch to pause itself. The main thread would then call countDown() on the latch to let the paused thread continue.
Orthogonal to the previous points, using a Thread object as a lock / mutex may cause problems. For example, the javadoc for Thread::join says:
This implementation uses a loop of this.wait calls conditioned on this.isAlive. As a thread terminates the this.notifyAll method is invoked. It is recommended that applications not use wait, notify, or notifyAll on Thread instances.
Since you haven't posted code, we're kind of working in the dark. What are the details of the exception?
Are you calling Thread.wait() from within the thread, or outside it?
I ask this because according to the javadoc for IllegalMonitorStateException, it is:
Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.
To clarify this answer, this call to wait on a thread also throws IllegalMonitorStateException, despite being called from within a synchronized block:
private static final class Lock { }
private final Object lock = new Lock();
#Test
public void testRun() {
ThreadWorker worker = new ThreadWorker();
System.out.println ("Starting worker");
worker.start();
System.out.println ("Worker started - telling it to wait");
try {
synchronized (lock) {
worker.wait();
}
} catch (InterruptedException e1) {
String msg = "InterruptedException: [" + e1.getLocalizedMessage() + "]";
System.out.println (msg);
e1.printStackTrace();
System.out.flush();
}
System.out.println ("Worker done waiting, we're now waiting for it by joining");
try {
worker.join();
} catch (InterruptedException ex) { }
}
In order to deal with the IllegalMonitorStateException, you must verify that all invocations of the wait, notify and notifyAll methods are taking place only when the calling thread owns the appropriate monitor. The most simple solution is to enclose these calls inside synchronized blocks. The synchronization object that shall be invoked in the synchronized statement is the one whose monitor must be acquired.
Here is the simple example for to understand the concept of monitor
public class SimpleMonitorState {
public static void main(String args[]) throws InterruptedException {
SimpleMonitorState t = new SimpleMonitorState();
SimpleRunnable m = new SimpleRunnable(t);
Thread t1 = new Thread(m);
t1.start();
t.call();
}
public void call() throws InterruptedException {
synchronized (this) {
wait();
System.out.println("Single by Threads ");
}
}
}
class SimpleRunnable implements Runnable {
SimpleMonitorState t;
SimpleRunnable(SimpleMonitorState t) {
this.t = t;
}
#Override
public void run() {
try {
// Sleep
Thread.sleep(10000);
synchronized (this.t) {
this.t.notify();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
Thread.wait() call make sense inside a code that synchronizes on Thread.class object. I don't think it's what you meant.
You ask
How can I make a thread wait until it will be notified?
You can make only your current thread wait. Any other thread can be only gently asked to wait, if it agree.
If you want to wait for some condition, you need a lock object - Thread.class object is a very bad choice - it is a singleton AFAIK so synchronizing on it (except for Thread static methods) is dangerous.
Details for synchronization and waiting are already explained by Tom Hawtin.
java.lang.IllegalMonitorStateException means you are trying to wait on object on which you are not synchronized - it's illegal to do so.
Not sure if this will help somebody else out or not but this was the key part to fix my problem in user "Tom Hawtin - tacklin"'s answer above:
synchronized (lock) {
makeWakeupNeeded();
lock.notifyAll();
}
Just the fact that the "lock" is passed as an argument in synchronized() and it is also used in "lock".notifyAll();
Once I made it in those 2 places I got it working
I received a IllegalMonitorStateException while trying to wake up a thread in / from a different class / thread. In java 8 you can use the lock features of the new Concurrency API instead of synchronized functions.
I was already storing objects for asynchronous websocket transactions in a WeakHashMap. The solution in my case was to also store a lock object in a ConcurrentHashMap for synchronous replies. Note the condition.await (not .wait).
To handle the multi threading I used a Executors.newCachedThreadPool() to create a thread pool.
Those who are using Java 7.0 or below version can refer the code which I used here and it works.
public class WaitTest {
private final Lock lock = new ReentrantLock();
private final Condition condition = lock.newCondition();
public void waitHere(long waitTime) {
System.out.println("wait started...");
lock.lock();
try {
condition.await(waitTime, TimeUnit.SECONDS);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
lock.unlock();
System.out.println("wait ends here...");
}
public static void main(String[] args) {
//Your Code
new WaitTest().waitHere(10);
//Your Code
}
}
For calling wait()/notify() on object, it needs to be inside synchronized block. So first you have to take lock on object then would be possible to call these function.
synchronized(obj)
{
obj.wait()
}
For detailed explanation:
https://dzone.com/articles/multithreading-java-and-interviewspart-2
wait(), notify() and notifyAll() methods should only be called in syncronized contexts.
For example, in a syncronized block:
syncronized (obj) {
obj.wait();
}
Or, in a syncronized method:
syncronized static void myMethod() {
wait();
}
public class TwoThreads {
private static Object resource = new Object();
private static void delay(long n) {
try
{
Thread.sleep(n);
}
catch (Exception e)
{
e.printStackTrace();
}
}
public static void main(String[] args) {
System.out.print("StartMain ");
new Thread1().start();
delay(1000); //dealay 1
Thread t2 = new Thread2();
t2.start();
delay(1000); // delay 2
t2.interrupt(); //here its throwing exception
delay(1000); //delay 3
System.out.print("EndMain ");
}
static class Thread1 extends Thread {
public void run() {
synchronized (resource) {
System.out.print("Startl ");
delay(6000);
System.out.print("End1 ");
}
}
}
static class Thread2 extends Thread {
public void run() {
synchronized (resource) {
System.out.print("Start2 ");
delay(2000);
System.out.print("End2 ");
}
}
}
}
I just got confused here why t2.interrupt() is not throwing exception when t2 is waiting to acquire lock on resource object and interrupt() method might throw security exception then why compiler still allowing us to execute it without putting it into try catch block.
A synchronized block doesn't throw an InterruptedException and interrupting a thread blocking while attempting to acquire a monitor this way doesn't do anything.
If you want this functionality you need to use a Lock which has lockInterruptibly(), though this is not often used.
Acquires the lock unless the current thread is interrupted. Acquires
the lock if it is not held by another thread and returns immediately,
setting the lock hold count to one.
If the current thread already holds this lock then the hold count is
incremented by one and the method returns immediately.
If the lock is held by another thread then the current thread becomes
disabled for thread scheduling purposes and lies dormant until one of
two things happens:
The lock is acquired by the current thread; or Some other thread
interrupts the current thread. If the lock is acquired by the current
thread then the lock hold count is set to one.
If the current thread:
has its interrupted status set on entry to this method; or is
interrupted while acquiring the lock, then InterruptedException is
thrown and the current thread's interrupted status is cleared.
From Thread#interrupt():
If none of the previous conditions hold then this thread's interrupt status will be set.
If you checked t2.interrupted(), you'd see a true result, but the thread is blocking on entering the synchronized block, which doesn't trigger an InterruptedException.
A call to interrupt() may throw a SecurityException if the application's environment has set up restrictions on which threads can interact with others, but this doesn't apply in your simple example.
The question is unclear but I guess I understood it correct so I am attempting to answer.
syncrhonized blocks are NOT responsive to interrupts.
For that you can use explicit locks Lock, which has a method lockInterruptibly() which is responsive to interrupts.
lockInterruptibly() in Lock Interface
java.lang.Thread.interrupt() means Interrupts this thread.
Unless the current thread is interrupting itself, which is always permitted, the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown.
If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.
you have called sleep() on t2. that is the reason of getting interruptedException.
I used to write a synchronized block like:
synchronized(foobar) {
// do something
}
But, recently I saw someone write:
synchronized(foobar) {
// do something
foobar.notifyAll();
}
Is foobar.notifyAll(); necessary? What happens if I omit it?
The short answer is that is depends on what you are doing.
If the goal of the synchronized block is simply to ensure that access / updates to a data structure are performed safely, then notify() or notifyAll() serves no purpose.
On the other hand, if the goal is to implement a "condition variable" then the notify() or notifyAll() calls work with a wait call like this ... for example:
private boolean flag;
private final Object mutex = new Object();
public void awaitFlag(boolean flag) {
synchronized (mutex) {
while (this.flag != flag) {
mutex.wait();
}
}
}
public void setFlag(boolean flag) {
synchronized (mutex) {
this.flag = flag;
mutex.notifyAll();
}
}
The above implements a simple mechanism where threads call awaitFlag() to wait for the flag to become true or false. When another thread calls setFlag() to change the flag, all of the threads that are currently waiting for the flag to change will get woken up by the notifyAll(). This is an example where the notifyAll() is essential to the working of the code.
So to understand whether the notify or notifyAll code is necessary, you need to figure out if some other code might call wait on the same mutex / lock object.
You don't need to do this. You only have to do it if the object (here foobar) is waiting to be notified. Notify only Wakes up all threads that are waiting on this object's monitor.
In Java, you can use wait(), notify() and notifyAll() to achieve thread co-ordination. See How to use wait and notify in Java?
The notifyAll() is to tell any other thread sleeping in a foobar.wait() that the current thread is about to release the lock and they can compete for the resource again.
thread.join() will call thread.wait(), but who and when notifies (either with thread.notify() or notifyAll()) the thread.wait()?
As we know, thread join will wait for the thread to be completed, but who calls notify on it?
As for jdk7 for linux, you can get the answer from the source code of openjdk.
/jdk7/hotspot/src/os/linux/vm/os_linux.cpp
int ret = pthread_create(&tid, &attr, (void* (*)(void*)) java_start, thread);
static void *java_start(Thread *thread) {
...
thread->run();
return 0;
}
and when start thread in java, the thread will be instanceof JavaThread.
/jdk7/hotspot/src/share/vm/runtime/thread.cpp
void JavaThread::run() {
...
thread_main_inner();
}
void JavaThread::thread_main_inner() {
...
this->exit(false);
delete this;
}
void JavaThread::exit(bool destroy_vm, ExitType exit_type) {
...
// Notify waiters on thread object. This has to be done after exit() is called
// on the thread (if the thread is the last thread in a daemon ThreadGroup the
// group should have the destroyed bit set before waiters are notified).
ensure_join(this);
...
}
static void ensure_join(JavaThread* thread) {
// We do not need to grap the Threads_lock, since we are operating on ourself.
Handle threadObj(thread, thread->threadObj());
assert(threadObj.not_null(), "java thread object must exist");
ObjectLocker lock(threadObj, thread);
// Ignore pending exception (ThreadDeath), since we are exiting anyway
thread->clear_pending_exception();
// Thread is exiting. So set thread_status field in java.lang.Thread class to TERMINATED.
java_lang_Thread::set_thread_status(threadObj(), java_lang_Thread::TERMINATED);
// Clear the native thread instance - this makes isAlive return false and allows the join()
// to complete once we've done the notify_all below
java_lang_Thread::set_thread(threadObj(), NULL);
lock.notify_all(thread);
// Ignore pending exception (ThreadDeath), since we are exiting anyway
thread->clear_pending_exception();
}
so lock.notify_all(thread) will notify all threads whose wait for the thread to finish.
Edit:
Oh, you are talking about inside of the Thread object itself. Inside of join() we do see a wait(). Something like:
while (isAlive()) {
wait(0);
}
The notify() for this is handled by the Thread subsystem. When the run() method finishes, the notify() is called on the Thread object. I'm not sure if the code that actually calls notify() can be seen -- it seems to be done in native code.
No user code needs to call notify() on that Thread object. The Java Thread code handles this internally. Once the thread finishes, the join() call will return.
For example, the following code will execute fine and the join() call will return fine without any wait() or notify() calls.
Thread thread = new Thread(new Runnable() {
public void run() {
// no-op, just return immediately
}
});
thread.start();
thread.join();
It is important to note that this behavior should probably not be relied upon. The notify() call is internal to the thread system. You should use join() if you are waiting for a thread to finish.