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OS06: Monitors in Java

Based on Chapter 4 of [Hai19]

(Usage hints for this presentation)

Computer Structures and Operating Systems 2021
Dr. Jens Lechtenbörger (License Information)

1 Introduction

1.1 OS Plan

OS course plan, summer 2021

1.2 Today’s Core Question

  • How to achieve MX with monitors in Java?

1.3 Learning Objectives

  • Apply and explain MX and cooperation based on monitor concept in Java
    • Give example
    • Discuss incorrect attempts

1.4 Retrieval Practice

1.4.1 Thread Terminology

1.4.2 Thread States

1.4.3 Java Threads

1.4.4 Races

1.4.5 Mutual Exclusion

Table of Contents

2 Monitors

2.1 Monitor Idea

  • Monitor ≈ instance of class with methods and attributes
  • Equip every object (= class instance) with a lock
    • Automatically
      • Call lock() when method is entered
        • As usual: Thread is blocked if lock is already locked
        • Thus, automatic MX
        • We say that executing thread entered the monitor or executes inside the monitor when it has passed lock() and executes a method
      • Call unlock() when method is left
        • Thread leaves the monitor

2.2 Monitor Origin

  • Monitors proposed by Hoare; 1974
  • Abstract data type
    • Methods encapsulate local variables
      • Just like methods in Java classes
    • Thread enters monitor via method
      • Built-in MX: At most one thread in monitor
    • In addition: Methods for cooperation
      • cwait(x): Blocks calling thread until csignal(x)
        • Monitor free then
      • csignal(x): Starts at most one thread waiting for x
        • If existing; otherwise, nothing happens
        • Difference to semaphore: signal may get lost

3 MX in Java

3.1 Monitors in Java: Overview

  • In Java, classes and objects come with built-in locks
    • Which are ignored by default
  • Keyword synchronized activates locks
    • Automatic locking of this object during execution of method
      • Automatic MX for method’s body
      • Useful if (large part of) body is a CS
    • E.g., for sample code from [Hai19] (for which you found races previously):

      public synchronized void sell() {
        if (seatsRemaining > 0) {
          seatsRemaining = seatsRemaining - 1;
        } else displaySorrySoldOut();

3.1.1 Java, synchronized, this

  • Java basics, hopefully clear
    • Method sell() from previous slides invoked on some object, say theater
      • Each theater has its own attribute seatsRemaining
      • seatsRemaining is really this.seatsRemaining, which is the same as theater.seatsRemaining
        • Inside the method, the name theater is unknown, theater is the this object, which is used implicitly
  • Without synchronized, races arise when two threads invoke sell() on the same object theater
    • With synchronized, only one of the threads obtains the lock on theater, so races are prevented

3.1.2 Possible Sources of Confusion

  • With synchronized, locks for objects are activated
    • For synchronized methods, thread needs to acquire lock for this object
  • Methods cannot be locked
  • Individual attributes of the this object (e.g., seatsRemaining) are not locked
    • (Which is not a problem as object-orientation recommends to encapsulate attributes, i.e., they cannot be accessed directly but only through synchronized methods)

3.1.3 JiTT Assignment

  1. Inspect and understand, compile, and run this sample program, which embeds the code to sell tickets, for which you found races previously.
  2. Change sell() to use the monitor concept, recompile, and run again. Observe the expected outcome.

(Nothing to submit here; maybe ask questions online.)

3.2 Java Monitors in Detail

  • MX based on monitor concept
  • Every Java object (and class) comes with
    • Monitor with lock (not activated by default)
      • Keyword synchronized activates lock
      • For method
        • public synchronized methodAsCS(...) {…}
        • Thread acquires lock for this object upon call (Class object for static methods)
      • Or for block
        • synchronized (syncObj) {…}
        • Thread acquires lock for syncObj
      • First thread acquires lock for duration of method/block
      • Further threads get blocked
    • Wait set (set of threads; wait() and notify(), explained later; ignore for now)

3.3 Recall: synchronized Example

public synchronized void sell() {
  if (seatsRemaining > 0) {
    seatsRemaining = seatsRemaining - 1;
  } else displaySorrySoldOut();
  • As you observed above, synchronized avoids races
    • Method executed under MX
    • Threads need to acquire lock on this object before executing method
  • Really, it is that simple!

4 Cooperation in Java

4.1 General Idea

  • Threads may work with different roles on shared data structures
  • Some may find that they cannot continue before others did their work
    • The former call wait() and hope for notify() by the latter
    • Cooperation (orthogonal to and not necessary for MX!)

4.2 wait() and notify() in Java

  • Waiting via blocking
    • wait(): thread unlocks and leaves monitor, enters wait set
      • Thread enters state blocked (no busy waiting)
      • Called by thread that cannot continue (without work/help of another thread)
  • Notifications
    • notify()
      • Remove one thread from wait set (if such a thread exists)
        • Change state from blocked to runnable
      • Called by thread whose work may help another thread to continue
    • notifyAll()
      • Remove all threads from wait set
        • Only one can lock and enter the monitor, of course
        • Only after the notifying thread has left the monitor, of course
        • Overhead (may be avoidable with appropriate synchronization objects or with semaphores as seen previously)

5 BoundedBuffer in Java

5.1 Bounded Buffers

  • A buffer is a data structure to store items, requests, responses, etc.
    • Lots of buffer variants exist
  • A bounded buffer has a limited size
  • As with any other data structure, MX is necessary when buffers are shared
    • In following
      • Java arrays used as underlying, shared data structure to implement buffer
      • Java’s monitor concept for MX and cooperation

5.2 Sample Java Method

// Based on Fig. 4.17 of [Hai17]
public synchronized void insert(Object o)
  throws InterruptedException
// Called by producer thread
  while(numOccupied == buffer.length)
      // block thread as buffer is full;
      // cooperation from consumer required to unblock
  buffer[(firstOccupied + numOccupied) % buffer.length] = o;
  // in case any retrieves are waiting for data, wake/unblock them

(Part of

5.2.1 Some Comments

  • Previous method in larger program:
    • SynchronizedBoundedBuffer as shared resource
    • Different threads (Producer instances and Consumer instances) call synchronized methods on that bounded buffer
      • Before methods are executed, lock of buffer needs to be acquired
        • This enforces MX for methods insert() and retrieve()
      • In methods, threads call wait() on buffer if unable to continue
        • this object used implicitly as target of wait()
        • Thread enters wait set of buffer
        • Until notifyAll() on same buffer
      • Note that thread classes contain neither synchronized nor wait/notify

5.3 JiTT: Questions and Feedback

  • What did you find difficult or confusing about the contents of the presentation? Please be as specific as possible. For example, you could describe your current understanding (which might allow us to identify misunderstandings), ask questions that allow us to help you, or suggest improvements (maybe on GitLab). Please use the session’s shared document or MoodleOverflow. Most questions turn out to be of general interest; please do not hesitate to ask and answer where others can benefit. If you created additional original content that might help others (e.g., a new exercise, an experiment, explanations concerning relationships with different courses, …), please share.

6 Conclusions

6.1 Summary

  • Java objects can act as monitors
    • Keyword synchronized
      • MX for CS (method/block of code)
        • No flags, no explicit locks!
    • Cooperation via wait() and notify()


License Information

This document is part of an Open Educational Resource (OER) course on Operating Systems. Source code and source files are available on GitLab under free licenses.

Except where otherwise noted, the work “OS06: Monitors in Java”, © 2017-2021 Jens Lechtenbörger, is published under the Creative Commons license CC BY-SA 4.0.

No warranties are given. The license may not give you all of the permissions necessary for your intended use.

In particular, trademark rights are not licensed under this license. Thus, rights concerning third party logos (e.g., on the title slide) and other (trade-) marks (e.g., “Creative Commons” itself) remain with their respective holders.