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OS06: MX in Java

Based on Chapter 4 of [Hai19]

(Usage hints for this presentation)

Computer Structures and Operating Systems 2023
Dr. Jens Lechtenbörger (License Information)

1. Introduction

1.1. OS Plan

OS course plan, summer 2022

1.2. Today’s Core Question

  • How to achieve MX with monitors in Java?
  • (Also: Semaphores revisited with Java as alternative)

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. Self-Study Task

  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

  • 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 with Monitors 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 capacity
      • E.g., a Java array or any other data structure of limited capacity
  • As with any other data structure, MX is necessary when buffers are shared
    • Subsequently, two alternative buffer implementations with MX
      • Java’s monitor concept (with array as underlying, shared data structure)
      • Java Semaphore (with list as underlying, shared data structure)

5.2. Sample synchronized 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. Comments on synchronized

  • 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. Sample Semaphore Use in Java

import java.util.concurrent.Semaphore;
  This code is based on Figure 4.18 of the following book:
  Max Hailperin, Operating Systems and Middleware – Supporting
  Controlled Interaction, revised edition 1.3, 2017.

  In Figure 4.18, synchronizedList() is used, whereas here a
  plain LinkedList is used, which is protected by the additional
  semaphore mutex.
  Also, the class here is renamed and implements a new interface.
public class SemaphoreBoundedBuffer implements BoundedBuffer {
  private java.util.List<Object> buffer =
      new java.util.LinkedList<Object>();

  private static final int SIZE = 20; // arbitrary

  private Semaphore mutex = new Semaphore(1);
  private Semaphore occupiedSem = new Semaphore(0);
  private Semaphore freeSem = new Semaphore(SIZE);

  /* invariant: occupiedSem + freeSem = SIZE
     buffer.size() = occupiedSem
     buffer contains entries from oldest to youngest */

  public void insert(Object o) throws InterruptedException {
    // Called by producer thread

  public Object retrieve() throws InterruptedException {
    // Called by consumer thread
    Object retrieved = buffer.remove(0);
    return retrieved;

  public int size() {
    return buffer.size();

5.3.1. Comments on Java Semaphore

  • Java provides java.util.concurrent.Semaphore
    • Implements semaphore concept
      • Constructor with integer argument to track resources
      • Methods acquire() and release()
  • SemaphoreBoundedBuffer implements same interface as SynchronizedBoundedBuffer
    • Use whichever you want with
    • Bounded buffer uses three semaphores
      • One (initialized to 1) acting as mutex
        • Note acquire() and release() around buffer accesses for MX
      • Other two counting occupied and free places

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: MX in Java”, © 2017-2023 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.