Reader Writer monitor in JAVA

From: Spliff Monkey (spliffmonkey_at_iname.com)
Date: 03/29/05 

Date: 29 Mar 2005 08:32:36 -0800

I am trying to program a simple monitor for a multiple reader single
writer problem in JAVA based on the pseudo code given at the bottom of
this post. A link to my code is also given bellow.

I get deadlock. I basically have producer and consumer classes which
do nothing except call the Start_Read/Write and End_Read/Write from
the monitor. I also have created a condition class.

It seems as if, say, a consumer finishes its synchronized start_read.
Then, say, a producer tries to write but is told to wait. Then the
consumer just stops and never enters its end_read.

The consumer should enter the end_read and then signal the producer.

Does the producer waiting in the synchronized start_write, prevent the
consumer entering the synchronized end_read?

If so how do I get around it.

My code is here:

http://www.maths.tcd.ie/~z/MainClass.java

Based in the following pseudo code for a monitor:

  monitor Reader_Writer_Monitor is
  Readers: Integer := 0;
  Writing: Boolean := False;
  OK_to_Read, OK_to_Write: Condition;

  procedure Start_Read is
  begin
    if Writing or Non_Empty(OK_to_Write) then
      Wait(OK_to_Read);
    end if;
    Readers := Readers + 1;
    Signal(OK_to_Read);
  end Start_Read;

  procedure End_Read is
  begin
    Readers := Readers - 1;
    if Readers = 0 then Signal(OK_to_Write); end if;
  end End_Read;

  procedure Start_Write is
  begin
    if Readers /= 0 or Writing then
      Wait(OK_to_Write);
    end if;
    Writing := True;
  end Start_Write;

  procedure End_Write is
  begin
    Writing := False;
    if Non_Empty(OK_to_Read) then
      Signal(OK_to_Read);
    else
      Signal(OK_to_Write);
    end if;
  end End_Write;
end Reader_Writer_Monitor;