A simple scenario using wait and notify in java

Question

Can I get a complete simple scenario i e  tutorial that suggest how this should be used  specifically with a Queue

User · Answer

Have you taken a look at this Java Tutorial?

Further, I'd advise you to stay the heck away from playing with this kind of stuff in real software. It's good to play with it so you know what it is, but concurrency has pitfalls all over the place. It's better to use higher level abstractions and synchronized collections or JMS queues if you are building software for other people.

That is at least what I do. I'm not a concurrency expert so I stay away from handling threads by hand wherever possible.

User · Answer

Example   public class myThread extends Thread        override      public void run            while true              threadCondWait      Circle waiting                 bla bla bla bla                       public synchronized void threadCondWait            while myCondition              wait     Comminucate with notify                       public class myAnotherThread extends Thread        override      public void run              Bla Bla bla         notify     Trigger wait   Next Step

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The wait   and notify   methods are designed to provide a mechanism to allow a thread to block until a specific condition is met  For this I assume you re wanting to write a blocking queue implementation  where you have some fixed size backing-store of elements   The first thing you have to do is to identify the conditions that you want the methods to wait for  In this case  you will want the put   method to block until there is free space in the store  and you will want the take   method to block until there is some element to return   public class BlockingQueue lt T gt         private Queue lt T gt  queue   new LinkedList lt T gt         private int capacity       public BlockingQueue int capacity            this capacity   capacity             public synchronized void put T element  throws InterruptedException           while queue size      capacity                wait                       queue add element           notify       notifyAll   for multiple producer consumer threads            public synchronized T take   throws InterruptedException           while queue isEmpty                  wait                       T item   queue remove            notify       notifyAll   for multiple producer consumer threads         return item            There are a few things to note about the way in which you must use the wait and notify mechanisms    Firstly  you need to ensure that any calls to wait   or notify   are within a synchronized region of code  with the wait   and notify   calls being synchronized on the same object   The reason for this  other than the standard thread safety concerns  is due to something known as a missed signal    An example of this  is that a thread may call put   when the queue happens to be full  it then checks the condition  sees that the queue is full  however before it can block another thread is scheduled  This second thread then take   s an element from the queue  and notifies the waiting threads that the queue is no longer full  Because the first thread has already checked the condition however  it will simply call wait   after being re-scheduled  even though it could make progress   By synchronizing on a shared object  you can ensure that this problem does not occur  as the second thread s take   call will not be able to make progress until the first thread has actually blocked   Secondly  you need to put the condition you are checking in a while loop  rather than an if statement  due to a problem known as spurious wake-ups  This is where a waiting thread can sometimes be re-activated without notify   being called  Putting this check in a while loop will ensure that if a spurious wake-up occurs  the condition will be re-checked  and the thread will call wait   again     As some of the other answers have mentioned  Java 1 5 introduced a new concurrency library  in the java util concurrent package  which was designed to provide a higher level abstraction over the wait notify mechanism  Using these new features  you could rewrite the original example like so   public class BlockingQueue lt T gt         private Queue lt T gt  queue   new LinkedList lt T gt         private int capacity      private Lock lock   new ReentrantLock        private Condition notFull   lock newCondition        private Condition notEmpty   lock newCondition         public BlockingQueue int capacity            this capacity   capacity             public void put T element  throws InterruptedException           lock lock            try               while queue size      capacity                    notFull await                               queue add element               notEmpty signal              finally               lock unlock                         public T take   throws InterruptedException           lock lock            try               while queue isEmpty                      notEmpty await                               T item   queue remove                notFull signal                return item            finally               lock unlock                          Of course if you actually need a blocking queue  then you should use an implementation of the  BlockingQueue interface   Also  for stuff like this I d highly recommend Java Concurrency in Practice  as it covers everything you could want to know about concurrency related problems and solutions

User · Answer

Not a queue example  but extremely simple     class MyHouse       private boolean pizzaArrived   false       public void eatPizza            synchronized this               while  pizzaArrived                   wait                                    System out println  yumyum                 public void pizzaGuy            synchronized this                this pizzaArrived   true               notifyAll                        Some important points  1  NEVER do   if  pizzaArrived        wait         Always use while condition   because    a  threads can sporadically awake from waiting state without being notified by anyone   even when the pizza guy didn t ring the chime  somebody would decide try eating the pizza     b  You should check for the condition again after acquiring the synchronized lock  Let s say pizza don t last forever  You awake  line-up for the pizza  but it s not enough for everybody  If you don t check  you might eat paper      probably better example would be while  pizzaExists   wait         2  You must hold the lock  synchronized  before invoking wait nofity  Threads also have to acquire lock before waking     3  Try to avoid acquiring any lock within your synchronized block and strive to not invoke alien methods  methods you don t know for sure what they are doing   If you have to  make sure to take measures to avoid deadlocks     4  Be careful with notify    Stick with notifyAll   until you know what you are doing   5 Last  but not least  read Java Concurrency in Practice

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Example for wait   and notifyall   in Threading   A synchronized static array list is used as resource and wait   method is called if the array list is empty  notify   method is invoked once a element is added for the array list   public class PrinterResource extends Thread     resource public static List lt String gt  arrayList   new ArrayList lt String gt      public void addElement String a         System out println  Add element method   this getName         synchronized  arrayList            arrayList add a           arrayList notifyAll             public void removeElement          System out println  Remove element method    this getName         synchronized  arrayList            if arrayList size      0               try                   arrayList wait                  catch  InterruptedException e                       TODO Auto-generated catch block                 e printStackTrace                           else              arrayList remove 0                      public void run        System out println  Thread name --   this getName         if  this getName   equalsIgnoreCase  p4             this removeElement              this addElement  threads        public static void main String   args        PrinterResource p1   new PrinterResource        p1 setName  p1        p1 start         PrinterResource p2   new PrinterResource        p2 setName  p2        p2 start          PrinterResource p3   new PrinterResource        p3 setName  p3        p3 start          PrinterResource p4   new PrinterResource        p4 setName  p4        p4 start              try          p1 join            p2 join            p3 join            p4 join         catch InterruptedException e           e printStackTrace              System out println  Final size of arraylist    arrayList size

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Even though you asked for wait   and notify   specifically  I feel that this quote is still important enough   Josh Bloch  Effective Java 2nd Edition  Item 69  Prefer concurrency utilities to wait and notify  emphasis his       Given the difficulty of using wait and notify correctly  you should use the higher-level concurrency utilities instead       using wait and notify directly is like programming in  concurrency assembly language   as compared to the higher-level language provided by java util concurrent  There is seldom  if ever  reason to use wait and notify in new code

[java] A simple scenario using wait() and notify() in java

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