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<META name=vsisbn content="1571690433"><META name=vstitle content="Black Art of Java Game Programming"><META name=vsauthor content="Joel Fan"><META name=vsimprint content="Sams"><META name=vspublisher content="Macmillan Computer Publishing"><META name=vspubdate content="11/01/96"><META name=vscategory content="Web and Software Development: Programming, Scripting, and Markup Languages: Java"><TITLE>Black Art of Java Game Programming:The Internet MahJong Server</TITLE>
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<font face="arial, helvetica" size="-1" color="#336633"><b>Black Art of Java Game Programming</b></font>
<br>
<font face="arial, helvetica" size="-1"><i>by Joel Fan</i>
<br>
Sams, Macmillan Computer Publishing
<br>
<b>ISBN:</b> 1571690433<b> Pub Date:</b> 11/01/96</font>
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<P><BR></P>
<H4 ALIGN="LEFT"><A NAME="Heading19"></A><FONT COLOR="#000077">The Table Class: A Simple Version</FONT></H4>
<P>For the basic server, we only need to record which players are on a table, and provide methods for players to join and leave tables. As with players, we keep a list of all the tables on the server in an array. The following is a sketch of the Table class.
</P>
<!-- CODE //-->
<PRE>
package server;
class Table {
public static Table[] global = new Table[25];
int id = -1;
Player[] players = new Player[8]; // including 4 spectators
public Table () throws FullServerException {
// assign a unique table id
synchronized (global) { // critical section
for (int i = 0; i < global.length; i++) {
if (global[i] == null) {
id = i;
global[i] = this;
break;
}
}
}
if (id == -1) throw new FullServerException();
}
public synchronized void outputTable (byte[] p) {
for (int i = 0; i < players.length; i++) {
if (players[i] != null)
players[i].output(p);
}
}
public synchronized void join (Player him, boolean play) {
join:
{
// seats 0 - 3 are for players, the rest spectators
int s0 = play ? 0 : 4;
int s1 = play ? 4 : players.length;
for (int i = s0; i < s1; i++) {
if (players[i] == null) {
players[i] = him;
him.table = this;
him.seat = i;
Player.outputAll(Packet.SPTablePlayer(id, i, him.id));
break join;
}
}
him.output(Packet.SPMessage(Packet.MSG_GOD, 0,
"Table is full."));
return;
}
// MahJong-related stuff here
}
public void leave (Player him) {
synchronized (global) { // critical section
synchronized (this) {
if (him.table != this) return;
// announce to all; player id of -1 means leaving table
Player.outputAll(Packet.SPTablePlayer(id, him.seat, -1));
players[him.seat] = null;
him.table = null;
him.seat = -1;
for (int i = 0; i < 4; i++)
if (players[i] != null)
return;
// now the table has only spectators, so we close it
for (int i = 4; i < players.length; i++) {
if (players[i] != null) {
Player.outputAll(Packets.SPTablePlayer(id, i, -1));
players[i].table = null;
players[i].seat = -1;
players[i] = null;
}
}
global[id] = null;
}
}
}
public synchronized void update (Player him) {
for (int i = 0; i < players.length; i++) {
if (players[i] != null)
him.output(Packet.SPTablePlayer(id, i, t.players[i].id));
}
}
// other methods...
}
</PRE>
<!-- END CODE //-->
<P>I will say more about the leave() method later.
</P>
<H4 ALIGN="LEFT"><A NAME="Heading20"></A><FONT COLOR="#000077">Handling Login Requests</FONT></H4>
<P>There is one last loose end to tie up: login. For the basic server, we let a player log in whenever he isn’t on the server already. Passwords can be added later.
</P>
<P>After a player logs in, we need to send him updates on all the players and tables currently on the server. This is done in the hdLogin() method that handles login requests:</P>
<!-- CODE //-->
<PRE>
void hdLogin () throws IOException {
String n = in.readUTF(); // read in packet data
synchronized (global) { // critical section
for (int i = 0; i < global.length; i++) {
if (global[i] != null && n.equals(global[i].name)) {
output(Packet.SPLogin(Packet.LOGIN_TWICE));
return; // reject his login
}
}
name = n; // accept his login
}
output(Packet.SPLogin(Packet.LOGIN_OK));
// send updates on all the players
synchronized (global) { // critical section
for (int i = 0; i < global.length; i++) {
if (global[i] != null && global[i].name != null)
output(Packet.SPJoin(i, global[i].name));
}
}
// send updates on all the tables
synchronized (Table.global) { // critical section
for (int i = 0; i < Table.global.length; i++) {
if (Table.global[i] != null) {
output(Packet.SPTableOpt(i, null)); // update this table
Table.global[i].update(this); // update players
}
}
}
}
</PRE>
<!-- END CODE //-->
<H4 ALIGN="LEFT"><A NAME="Heading21"></A><FONT COLOR="#000077">Synchronization and Deadlock Prevention</FONT></H4>
<P>As you’ve seen in the code I’ve given so far, in the Player and Table classes we often need to acquire monitors on certain objects—four kinds of them, to be exact:
</P>
<DL>
<DD><B>•</B> The Player.global array
<DD><B>•</B> The Table.global array
<DD><B>•</B> A particular Player instance
<DD><B>•</B> A particular Table instance
</DL>
<P>Often, two or more monitors have to be acquired. For instance, in the process of updating all the tables to a certain player, we need to acquire the monitor of Table.global first and then of each individual Table instance. As the server code gets complicated, we face potential deadlock situations.
</P>
<P>A deadlock occurs when several threads are competing for monitors, yet each holds a monitor that another thread needs to acquire in order to proceed. As an example, suppose I defined Table.leave() like this (which is actually more “natural” than the first version):</P>
<!-- CODE SNIP //-->
<PRE>
// an alternative problematic Table.leave()
public synchronized leave (Player him) {
// get him off the table here, and check for empty table
// suppose table is empty:
synchronized (global) { // remove table
global[id] = null;
}
}
</PRE>
<!-- END CODE SNIP //-->
<P>Now imagine two players, A and B. A just logged in, so we are informing him about all the tables. We are about to reach table T, on which B just happens to be the only player, and he happens to decide to leave at this very moment. Say T.leave() is called first, then T.update() must wait for it to finish. In T.leave() we find out that the table should be removed because it is now empty. However, to do this we need to acquire the monitor on Table.global, which is currently held by player A. Thus, these two threads are now in a deadlock. Granted, the chance of this happening is rare, but it’s likely to happen after an extended period of time, and we have to deal with it to get a robust server.
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