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Java implementation of Monitor Concurrency Pattern


Monitor is a synchronization construct that allows threads to have both thread-safe access to underlying monitored object or value (hereinafter monitored entity) and wait for it to mutate (if it is a mutable object) or change (if it is an immutable value) to a desirable state.

understanding monitor

Monitor<Entity> class allows threads to access monitored entity in either exclusive (write) or shared (read) mode. Entity here refers to type of monitored entity.

For accessing monitored entity Monitor class has several overloaded read and write methods.

  • all read methods take Consumer<Entity> as a first argument. It could be a lambda function that reads monitored entity but must not mutate it. The lambda function takes monitored entity as its only argument and must return no value.

  • all write methods take Function<Entity, Entity> as a first argument. It could be a lambda function that accesses monitored value exclusively. The lambda function must return a reference to the monitored entity. This could be the same value that received by the lambda function in its argument (if the lambda function mutates monitored entity) or a new value (if the lambda function's intent is to change the reference to monitored entity).

  • read and write methods might take Predicate<Entity> as a second argument. It could be a lambda function that evaluates monitored entity and indicates if the desirable state is achieved. Concurrent predicates examine monitored entity exclusively.

  • read and write methods might take a maximum time to wait as a third argument. If the time detectably elapsed the methods return false and monitored entity is not accessed.

type safety note

Unfortunately, Java type system cannot guarantee through Monitor's contract that consumers and predicates do not mutate monitored entity. Monitor's behavior in this case is undefined.

Consumers and predicates must not mutate monitored entity even if it is an instance of Atomic classes (see java.util.concurrent.atomic) or synchronized in some other way. Monitor is not able to detect changes in this case and will not re-evaluate predicates of other waiting threads.


Sandbox class allows threads to create a local copy of monitored entity and then Compare and Swap the mutated or changed entity back to monitor. The Entity must be Cloneable in order to create its local copy.

Sandbox instance is not reentrant. Each thread must create a local instance of Sandbox.


For more information see Project website or JavaDoc.


 Monitor<Queue<String>> outputQueue =
    new Monitor<>(new LinkedList<String>());

 // ... monitoring thread
 while(true) {
    // wait for a new string in queue
       queue -> {

             // Write access lambda must return its argument
             // to preserve reference to the queue
             return queue;

       // wake up when the queue is not empty
       queue -> !queue.isEmpty()

 // ... somewhere in some other thread
    queue -> {
          queue.add("Hello Monitor!");
          return queue;

Monitor compared to other synchronization constructs

compared to synchronized

Monitor is different from Java synchronized keyword in several crucial ways:

  • Unlike synchronized keyword Monitor allows threads to wait for monitored entity to mutate or change to a desirable state.

  • Unlike synchronized keyword Monitor allows threads to access monitored entity in either exclusive or shared mode.

compared to Object.wait()

Object.wait() method allow threads to wait for a desirable event to occur. The event occurrence is determined by actor thread and signaled by invoking Object.notify() or Object.notifyAll() methods.

Unlike Object.wait() method Monitor allows threads to wait for desirable state which is defined by a Predicate. The actor thread behavior is decoupled from waiting threads intentions.

compared to Atomic classes

  • Even though Monitor provides an ability to perform Compare and Swap over monitored entity it cannot be considered lock-free or wait-free construct and thus it is not recommended for use in lock-free or wait-free algorithms.

  • Atomic classes could have less overhead and be more effective when used for short-term non-blocking spin lock or wait constructs. Even though Monitor has an overhead for blocking and resuming threads it could be more effective it case of mid- or long-term blocks and waits as blocked and waiting threads do not consume CPU time.

installation and usage


Java 8 or higher


Apache License Version 2.0

declaring dependency in pom.xml


project health

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