OS06: Monitors in Java

Based on Chapter 4 of [Hai19]

(Usage hints for this presentation)

Computer Structures and Operating Systems 2022
Dr. Jens Lechtenbörger (License Information)

Data Science: Machine Learning and Data Engineering (Prof. Gieseke)
Dept. of Information Systems
WWU Münster, Germany

1. Introduction

1.1. OS Plan

MX in Java (Wk 25)

MX Challenges (Wk 25)
Virtual Memory I (Wk 26)
Virtual Memory II (Wk 26)
Processes (Wk 27)
Security (Wk 28)

OS course plan, summer 2022

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

1. Introduction
2. Monitors
3. MX in Java
4. Cooperation in Java
5. BoundedBuffer in Java
6. Conclusions

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

The basic idea of monitors is as follows: Think of a monitor as an instance of a special type of class, where each instance is automatically equipped with its own lock. The run-time system ensures that before a method of such a class is executed on a class instance (which is this in Java), the lock for that class instance needs to be acquired.

We say that a thread that has successfully executed lock() “entered the monitor” or “executes inside the monitor”.

Thus, monitors automatically provide MX for methods of the monitor class: If multiple threads share the same object (with a potential for race conditions), only one of them can execute inside the monitor at any point in time, while others are blocked.

Importantly, each object has its own lock. Thus, two threads that operate on different class instances can both acquire their different locks and execute monitor methods in parallel (without the danger of races as they do not not share resources).

The next slide explains the origin of monitors in terms of an abstract data type (instead of the more modern “class” formulation presented here). On that slide, you also see that monitors not only guarantee MX; in addition, they provide methods for cooperation of threads.

Subsequent slides then discuss how the monitor concept is implemented in Java with the keyword synchronized (which activates locking of the this object as explained here in general terms) and methods for cooperation.

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) {
    dispenseTicket();
    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

Inspect and understand, compile, and run this sample program, which embeds the code to sell tickets, for which you found races previously.
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
- See Java specification
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)

Java provides all methods for mutual exclusion discussed in the previous presentation, including the monitor concept, whose details can be found at the URL given here.
In essence, MX with Java is quite simple, as every Java object is equipped with a lock. By default, however, these locks are not used. Instead, you need to use the keyword synchronized if you want threads to acquire the locks for MX.

The simplest way to enforce MX is to declare methods operating on shared resources as synchronized. If a thread T1 wants to execute such a synchronized method on some object, then thread T1 will automatically try to acquire the lock for that object. If that lock has been taken, say by thread T0, then T1 will be blocked until T0 leaves the method and releases the lock.

Besides, you can also use other objects for synchronization if you want to turn blocks of code into critical sections. We will not use this, however.

Finally, the Java monitor concept includes a mechanism for cooperation of threads based on wait sets, which will be explained later.

3.3. Recall: `synchronized` Example

public synchronized void sell() {
  if (seatsRemaining > 0) {
    dispenseTicket();
    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
- E.g., producer/consumer problems seen earlier
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!)
  - General monitor concept
  - Wait set mentioned above and explained subsequently

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)

Cooperation between threads sharing resources can be managed with the methods wait() and notify() (or notifyAll()). A thread can only invoke these methods on an object if it has acquired the lock for that object, i.e., if it currently executes inside the object’s monitor. So, usually, you see invocations of wait and notify in synchronized methods.
If a thread finds that it cannot make use of the shared resource in the resource’s current state, it can invoke wait() to release the lock on that resource and leave its monitor. At that point in time, the thread’s state changes to blocked, and the thread is recorded in a special data structure associated with the object, called wait set. In the wait set, Java keeps track of all threads that have invoked wait() on the object. So, once a thread has executed wait(), the object’s lock is released, and other threads can acquire the object’s lock and modify the object’s state.
If a thread has modified the object’s state in such a way that there is reason to believe that waiting threads might now be able to continue, the thread invokes notify() on the object, which removes one thread from the wait set and makes it runnable. When that runnable thread is scheduled for execution later on, it can again try to enter to monitor by locking the object; once the lock has been acquired, the thread resumes execution after the wait() method. The method notifyAll() is an alternative to notify() that removes all threads from the wait set, not just one. You may want to think about advantages and disadvantage of notifying all waiting threads yourself.

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
- E.g., a Java array
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
      wait();
  buffer[(firstOccupied + numOccupied) % buffer.length] = o;
  numOccupied++;
  // in case any retrieves are waiting for data, wake/unblock them
  notifyAll();
}

(Part of SynchronizedBoundedBuffer.java)

5.2.1. Some Comments

Previous method in larger program: BBTest.java
- 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

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()

Bibliography

[Hai19] Hailperin, Operating Systems and Middleware – Supporting Controlled Interaction, revised edition 1.3.1, 2019. https://gustavus.edu/mcs/max/os-book/

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.

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.