Is there a Mutex in Java

Question

Is there a Mutex object in java or a way to create one  I am asking because a Semaphore object initialized with 1 permit does not help me  Think of this case   try      semaphore acquire         do stuff    semaphore release      catch  Exception e       semaphore release        if an exception happens at the first acquire  the release in the catch block will increase the permits  and the semaphore is no longer a binary semaphore   Will the correct way be    try      semaphore acquire         do stuff   catch  Exception e         exception stuff   finally      semaphore release        Will the above code ensure that the semaphore will be binary

User · Answer

Each object s lock is little different from Mutex Semaphore design  For example there is no way to correctly implement traversing linked nodes with releasing previous node s lock and capturing next one  But with mutex it is easy to implement   Node p   getHead    if  p    null    x    null  return false  p lock acquire        Prime loop by acquiring first lock     If above acquire fails due to interrupt  the method will      throw InterruptedException now  so there is no need for      further cleanup  for        Node nextp   null  boolean found  try     found   x equals p item     if   found        nextp   p next      if  nextp    null          try           Acquire next lock                       while still holding current         nextp lock acquire                  catch  InterruptedException ie           throw ie        Note that finally clause will                          execute before the throw                     finally          release old lock regardless of outcome     p lock release         Currently  there is no such class in java util concurrent  but you can find Mutext implementation here Mutex java  As for standard libraries  Semaphore provides all this functionality and much more

User · Answer

Any object in Java can be used as a lock using a synchronized block  This will also automatically take care of releasing the lock when an exception occurs   Object someObject         synchronized  someObject              You can read more about this here  Intrinsic Locks and Synchronization

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No one has clearly mentioned this  but this kind of pattern is usually not suited for semaphores  The reason is that any thread can release a semaphore  but you usually only want the owner thread that originally locked to be able to unlock  For this use case  in Java  we usually use ReentrantLocks  which can be created like this   import java util concurrent locks Lock  import java util concurrent locks ReentrantLock   private final Lock lock   new ReentrantLock true     And the usual design pattern of usage is     lock lock      try            do something     catch  Exception e             handle the exception     finally         lock unlock          Here is an example in the java source code where you can see this pattern in action    Reentrant locks have the added benefit of supporting fairness    Use semaphores only if you need non-ownership-release semantics

User · Answer

I think you should try with    While Semaphore initialization    Semaphore semaphore   new Semaphore 1  true     And in your Runnable Implementation  try       semaphore acquire 1         do stuff     catch  Exception e        Logging   finally      semaphore release 1

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Mistake in original post is acquire   call set inside the try loop   Here is a correct approach to use  binary  semaphore  Mutex    semaphore acquire    try        do stuff   catch  Exception e         exception stuff   finally      semaphore release

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See this page   http   www oracle com technetwork articles javase index-140767 html  It has a slightly different pattern which is  I think  what you are looking for   try     mutex acquire      try          do something     finally       mutex release          catch InterruptedException ie                 In this usage  you re only calling release   after a successful acquire

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To ensure that a Semaphore is binary you just need to make sure you pass in the number of permits as 1 when creating the semaphore  The Javadocs have a bit more explanation

User · Answer

import java util concurrent locks Lock  import java util concurrent locks ReentrantLock    private final Lock  mutex   new ReentrantLock true     mutex lock        your protected code here   mutex unlock

[java] Is there a Mutex in Java?

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