Websocket Servlet thread-safe - multithreading

Question

I'am playing around with WebSocketServlet (tomcat) and I have some question about doing it properly without race condition problems.
I have an instance variable (so non thread-safe) that will keep track of all the websocket connections
HashMap<String,MyServers> myNonThreadSafeVariable = HashMap<String,MyServers>
This is what the HashMap will contain (roughly...)
private final class MyServers extends MessageInbound {
final Set<MyClients> clients = new CopyOnWriteArraySet<MyClients>();
private String serverName;
#Override
protected void onOpen(WsOutbound outbound) {}
#Override
protected void onClose(WsOutbound outbound) {}
#Override
protected void onText(WsOutbound outbound) {}
}
private final class Clients extends MessageInbound {
private int clientID;
#Override
protected void onOpen(WsOutbound outbound) {}
#Override
protected void onClose(WsOutbound outbound) {}
#Override
protected void onText(WsOutbound outbound) {}
}
So now.. during my servlet life time, I am looping through myNonThreadSafeVariable and then maybe also will loop through myNonThreadSafeVariable.clients and then maybe also modified or add a clients or server etc...
For example when a server connect, in his onOpen there will be something like
myNonThreadSafeVariable.put(key,this);
or When a client connects in his onOpen (quit concern about this one)
server = myNonThreadSafeVariable,get(key);
sever.clients.add(this);
Or sometimes when I have to ping all the clients of all the servers:
for (Entry<String, MyServers> entry : myNonThreadSafeVariable.entrySet()) {
MyServers server = entry.getValue();
server.sendMessage("ping||");
for (MyClients member : entry.getValue().clients) {
client.sendMessage("")
}
}
So If I undertand correctly as myNonThreadSafeVariable is global so will myNonThreadSafeVariable.clients etc..
So my question is what is a good practice to avoid race condition in this scenario ?
Using mutex and synchronized on them when access ether the myNonThreadSafeVariable and myNonThreadSafeVariable.clients ? Or should I avoid using an instance variable at all ? But how ?
thanks !

Answer 1

You could use a ReadWriteLock: you block readers and writers when writing, you block only writers when reading:
private final ReadWriteLock lock = new ReentrantReadWriteLock();
private final Lock readLock = lock.readLock();
private final Lock writeLock = lock.writeLock();
...
writeLock.lock();
try
{
myNonThreadSafeVariable.put(key,this);
}
finally
{
writeLock.unlock();
}
...
writeLock.lock();
try
{
server = myNonThreadSafeVariable,get(key);
sever.clients.add(this);
}
finally
{
writeLock.unlock();
}
...
readLock.lock();
try
{
for (Entry<String, MyServers> entry : myNonThreadSafeVariable.entrySet())
{
MyServers server = entry.getValue();
server.sendMessage("ping||");
for (MyClients member : entry.getValue().clients)
{
client.sendMessage("")
}
}
}
finally
{
readLock.unlock();
}
Moreover if you want to avoid the read lock you can copy the whole collection and scan the copy, letting the possibility for the original collection to be changed while notifying.