Monitors

A monitor is a synchronization tool designed to make a programmer's life simple. A monitor can be thought of as a conceptual box. A programmer can put functions/procedures/methods into this box and the monitor makes him a very simple guarantee: only one function within the monitor will execute at a time -- mutual exclusion will be guaranteed.

Furthermore, the monitor can protect shared data. Data items declared within the monitor can only be accessed by functions/procedures/methods within the monitor. Therefore mutual exclusion is guaranteed for these data items. Functions/procedures/methods outside of the monitor can not corrupt them.

If nothing is executing within the monitor, a thread can execute one of its procedures/methods/functions. Otherwise, the thread is put into the entry queue and put to sleep. As soon as a thread exits the monitor, it wakes up the next process in the entry queue.

The picture gets a bit messier when we consider that threads executing within the monitor may require an unavailable resource. When this happens, the thread waits for this resource, using the wait operation of a condition variable. At this point, another thread is free to enter the monitor.

Now let me suggest that while a second thread is running in the monitor, it frees a resource required by the first. It signals that the resource that the first thread is waiting for becomes available. What should happen? Should the first thread be immediately awakened or should the second thread finish first? This situation gives rise to different versions of monitor sematics.

Monitors In Java

The Java programming language provides support for monitors via synchronized methods within a class. We are assured that at most one synchronized method within a particular class can be active at any particular time, even in multi-threaded applications. Java does not require that all methods within a class be synchronized, so every method of the class is not necessarily part of the monitor -- only synchronized methods of a class are protected from concurrent execution. This is obviously an opportunity for a programmer to damage an appendage with a massive and rapidly moving projectile.

Java monitors are reasonably limited -- especially when contrasted with monitors using Hoare or Mesa semantics. In Java, there can only be one reason to wait (block) within the monitor, not multiple conditions. When a thread waits, it is made unrunnable. When it has been signaled to wake-up, it is made runnable -- it will next run whenever the scheduler happens to run it. Unlike BH monitors, a signal can occur anywhere in the code. Unlike Hoare semantics, the signaling thread doesn't immediately yield to the signaled thread. Unlike all three, there can only be one reason to wait/signal. In this way, they offer simplified Mesa sematics.

To wait for a condition, a Java thread invokes wait(). To signal a waiting thread to tell it that it can run (the condition upon which it is waiting is satisfied), a Java thread invokes notify(). Notify is actually a funny name -- normally this operation is called signal.

Monitor Examples in Java

In class we walked through these examples in Java from Concurrent Programming: The Java Language by Stephen Hartley and published by Oxford Univerity Press in 1998:

The first example is a solution to the Bounded Buffer problem, also known as the Producer-Consumer Problem. This solution supports one producer thread and one consumer thread.

Please notice that the producer signals a waiting consumer if it fills the first slot in the buffer -- this is because the consumer might have blocked because there were no full buffers. The consumer follows a similar practice if it takes the last item in the buffer -- there could be a producer blocked waiting for an available slot in a buffer.

class BoundedBuffer {        // designed for a single producer thread
                             // and a single consumer thread
   private int numSlots = 0;
   private double[] buffer = null;
   private int putIn = 0, takeOut = 0;
   private int count = 0;

   public BoundedBuffer(int numSlots) {
      if (numSlots <= 0) throw new IllegalArgumentException("numSlots<=0");
      this.numSlots = numSlots;
      buffer = new double[numSlots];
      System.out.println("BoundedBuffer alive, numSlots=" + numSlots);
   }

   public synchronized void deposit(double value) {
      while (count == numSlots)
         try {
            wait();
         } catch (InterruptedException e) {
            System.err.println("interrupted out of wait");
         }
      buffer[putIn] = value;
      putIn = (putIn + 1) % numSlots;
      count++;                   // wake up the consumer
      if (count == 1) notify();  // since it might be waiting
      System.out.println(" after deposit, count=" + count
         + ", putIn=" + putIn);
   }

   public synchronized double fetch() {
      double value;
      while (count == 0)
         try {
            wait();
         } catch (InterruptedException e) {
            System.err.println("interrupted out of wait");
         }
      value = buffer[takeOut];
      takeOut = (takeOut + 1) % numSlots;
      count--;                           // wake up the producer
      if (count == numSlots-1) notify(); // since it might be waiting
      System.out.println(" after fetch, count=" + count
         + ", takeOut=" + takeOut);
      return value;
   }
}
  

Fair Reader-Writer Solution

What follows is a fair solution to the reader-writer problem -- it allows the starvation of neither the producer, nor the consumer. The only tricky part of this code is realizing that starvation of writers by readers is avoided by yielding to earlier requests.

  class Database extends MyObject {

   private int numReaders = 0;
   private int numWriters = 0;
   private int numWaitingReaders = 0;
   private int numWaitingWriters = 0;
   private boolean okToWrite = true;
   private long startWaitingReadersTime = 0;

   public Database() { super("rwDB"); }

   public synchronized void startRead(int i) {
      long readerArrivalTime = 0;
      if (numWaitingWriters > 0 || numWriters > 0) {
         numWaitingReaders++;
         readerArrivalTime = age();
         while (readerArrivalTime >= startWaitingReadersTime)
            try {wait();} catch (InterruptedException e) {}
         numWaitingReaders--;
      }
      numReaders++;
   }
  
   public synchronized void endRead(int i) {
      numReaders--;
      okToWrite = numReaders == 0;
      if (okToWrite) notifyAll();
   }
  
   public synchronized void startWrite(int i) {
      if (numReaders > 0 || numWriters > 0) {
         numWaitingWriters++;
         okToWrite = false;
         while (!okToWrite)
            try {wait();} catch (InterruptedException e) {}
         numWaitingWriters--;
      }
      okToWrite = false;
      numWriters++;
   }
  
   public synchronized void endWrite(int i) {
      numWriters--;              // ASSERT(numWriters==0)
      okToWrite = numWaitingReaders == 0;
      startWaitingReadersTime = age();
      notifyAll();
   }
}