Why `synchronized (lock)` was entered twice by different threads?
In this simple example I have two synchronized (theLock) that are accessed by different threads
public class Main {
public static void main(String args) throws InterruptedException {
System.out.println("start");
final Object theLock = new Object();
synchronized (theLock) {
System.out.println("main thread id : " + Thread.currentThread().getId());
new Thread(() -> {
System.out.println("new thread id : " + Thread.currentThread().getId() + ". Inside thread");
// before entering this section new thread should be blocked as `theLock` is already acquired
synchronized (theLock) {
System.out.println("inside synchronized");
theLock.notify();
}
}).start();
theLock.wait();
}
System.out.println("end");
}
}
Why the newly created thread can access to synchronized (theLock) section inside? As far as I understand, theLock is already acquired by the main thread and the new one should block forever. Instead I see that it enters to synchronized as well.
Here is an output
start
main thread id : 1
new thread id : 13. Inside thread
inside synchronized
end
java multithreading synchronized synchronized-block
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
add a comment |
In this simple example I have two synchronized (theLock) that are accessed by different threads
public class Main {
public static void main(String args) throws InterruptedException {
System.out.println("start");
final Object theLock = new Object();
synchronized (theLock) {
System.out.println("main thread id : " + Thread.currentThread().getId());
new Thread(() -> {
System.out.println("new thread id : " + Thread.currentThread().getId() + ". Inside thread");
// before entering this section new thread should be blocked as `theLock` is already acquired
synchronized (theLock) {
System.out.println("inside synchronized");
theLock.notify();
}
}).start();
theLock.wait();
}
System.out.println("end");
}
}
Why the newly created thread can access to synchronized (theLock) section inside? As far as I understand, theLock is already acquired by the main thread and the new one should block forever. Instead I see that it enters to synchronized as well.
Here is an output
start
main thread id : 1
new thread id : 13. Inside thread
inside synchronized
end
java multithreading synchronized synchronized-block
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
5
The original thread callstheLock.wait()which means it releases the lock and waits to be notified. Then the second thread obtains the lock, callsnotify, and releases the lock. Then the first thread wakes up, obtains the lock, and continues.
– Slaw
Nov 14 '18 at 13:27
add a comment |
In this simple example I have two synchronized (theLock) that are accessed by different threads
public class Main {
public static void main(String args) throws InterruptedException {
System.out.println("start");
final Object theLock = new Object();
synchronized (theLock) {
System.out.println("main thread id : " + Thread.currentThread().getId());
new Thread(() -> {
System.out.println("new thread id : " + Thread.currentThread().getId() + ". Inside thread");
// before entering this section new thread should be blocked as `theLock` is already acquired
synchronized (theLock) {
System.out.println("inside synchronized");
theLock.notify();
}
}).start();
theLock.wait();
}
System.out.println("end");
}
}
Why the newly created thread can access to synchronized (theLock) section inside? As far as I understand, theLock is already acquired by the main thread and the new one should block forever. Instead I see that it enters to synchronized as well.
Here is an output
start
main thread id : 1
new thread id : 13. Inside thread
inside synchronized
end
java multithreading synchronized synchronized-block
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
In this simple example I have two synchronized (theLock) that are accessed by different threads
public class Main {
public static void main(String args) throws InterruptedException {
System.out.println("start");
final Object theLock = new Object();
synchronized (theLock) {
System.out.println("main thread id : " + Thread.currentThread().getId());
new Thread(() -> {
System.out.println("new thread id : " + Thread.currentThread().getId() + ". Inside thread");
// before entering this section new thread should be blocked as `theLock` is already acquired
synchronized (theLock) {
System.out.println("inside synchronized");
theLock.notify();
}
}).start();
theLock.wait();
}
System.out.println("end");
}
}
Why the newly created thread can access to synchronized (theLock) section inside? As far as I understand, theLock is already acquired by the main thread and the new one should block forever. Instead I see that it enters to synchronized as well.
Here is an output
start
main thread id : 1
new thread id : 13. Inside thread
inside synchronized
end
java multithreading synchronized synchronized-block
java multithreading synchronized synchronized-block
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
asked Nov 14 '18 at 13:24
VitaliiVitalii
2,11962555
2,11962555
5
The original thread callstheLock.wait()which means it releases the lock and waits to be notified. Then the second thread obtains the lock, callsnotify, and releases the lock. Then the first thread wakes up, obtains the lock, and continues.
– Slaw
Nov 14 '18 at 13:27
add a comment |
5
The original thread callstheLock.wait()which means it releases the lock and waits to be notified. Then the second thread obtains the lock, callsnotify, and releases the lock. Then the first thread wakes up, obtains the lock, and continues.
– Slaw
Nov 14 '18 at 13:27
5
5
The original thread calls
theLock.wait() which means it releases the lock and waits to be notified. Then the second thread obtains the lock, calls notify, and releases the lock. Then the first thread wakes up, obtains the lock, and continues.– Slaw
Nov 14 '18 at 13:27
The original thread calls
theLock.wait() which means it releases the lock and waits to be notified. Then the second thread obtains the lock, calls notify, and releases the lock. Then the first thread wakes up, obtains the lock, and continues.– Slaw
Nov 14 '18 at 13:27
add a comment |
1 Answer
1
active
oldest
votes
The call to wait() releases the lock. Per wait() Javadoc (bolding mine):
Causes the current thread to wait until another thread invokes the
notify()method or thenotifyAll()method for this object. In
other words, this method behaves exactly as if it simply performs the
callwait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up either
through a call to thenotifymethod or thenotifyAllmethod. The
thread then waits until it can re-obtain ownership of the monitor and
resumes execution.
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
add a comment |
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1 Answer
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active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
The call to wait() releases the lock. Per wait() Javadoc (bolding mine):
Causes the current thread to wait until another thread invokes the
notify()method or thenotifyAll()method for this object. In
other words, this method behaves exactly as if it simply performs the
callwait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up either
through a call to thenotifymethod or thenotifyAllmethod. The
thread then waits until it can re-obtain ownership of the monitor and
resumes execution.
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
add a comment |
The call to wait() releases the lock. Per wait() Javadoc (bolding mine):
Causes the current thread to wait until another thread invokes the
notify()method or thenotifyAll()method for this object. In
other words, this method behaves exactly as if it simply performs the
callwait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up either
through a call to thenotifymethod or thenotifyAllmethod. The
thread then waits until it can re-obtain ownership of the monitor and
resumes execution.
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
add a comment |
The call to wait() releases the lock. Per wait() Javadoc (bolding mine):
Causes the current thread to wait until another thread invokes the
notify()method or thenotifyAll()method for this object. In
other words, this method behaves exactly as if it simply performs the
callwait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up either
through a call to thenotifymethod or thenotifyAllmethod. The
thread then waits until it can re-obtain ownership of the monitor and
resumes execution.
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
The call to wait() releases the lock. Per wait() Javadoc (bolding mine):
Causes the current thread to wait until another thread invokes the
notify()method or thenotifyAll()method for this object. In
other words, this method behaves exactly as if it simply performs the
callwait(0).
The current thread must own this object's monitor. The thread
releases ownership of this monitor and waits until another thread
notifies threads waiting on this object's monitor to wake up either
through a call to thenotifymethod or thenotifyAllmethod. The
thread then waits until it can re-obtain ownership of the monitor and
resumes execution.
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
answered Nov 14 '18 at 13:28
Andrew HenleAndrew Henle
19.8k21333
19.8k21333
add a comment |
add a comment |
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5
The original thread calls
theLock.wait()which means it releases the lock and waits to be notified. Then the second thread obtains the lock, callsnotify, and releases the lock. Then the first thread wakes up, obtains the lock, and continues.– Slaw
Nov 14 '18 at 13:27