//*********************email 0****************************************
Marc Krisjanous wrote:
> [...]
> I have a class that manages a log file which all objects within the system
> can write messages to. The class utilises the Singleton Instance design
> pattern so only one instance is accessible. The main method that writes the
> message to the log file is synchronized. Here is the skeleton class:
>
> public class EventLogController {
>
> public static EventLogController getInstance()
> {
> //Singleton implementation
> }
>
> public synchronized boolean writeToEventLog(String fileName, String
> message)
> {
> //write to file
> }
>
> }//end class
>
> Question:
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Should I still use this approach or should I create the writeToEventLog as
> static thus removing the need to kept an instance of the class.
>
> Best Regards
>
> Marc
> [...]
//*********************email 1****************************************
Make sure, that getInstance() is synchronized or it could
break that design pattern, depending on the implementation.
Greetings
Stefan
//*********************email 2****************************************
Hello,
from my understanding of what you said I will need to wrap every call to the
EventLogController.getInstance() method every time an object calls it?
Would that mean that I then do not need to have the writeToEventLog method
synchronized??
Best Regards
Marc
Hi :-) I also think you need to make your getInstance() synchronized
if you want to use SingleTon. and the following is just my guessing:
0
IMO, if we want to manage a *log file/DBConeection pool/or other* with Servlet,
perhaps we need the following three things:
* Controller/Holder, for example -> EventLogController or
DBConnectionPoolHolder
* locker which is used for synchronized some code
* Servlet class
I guess the most important things are the following two things:
* we need to make our code thread-safe, SO we need to find something
which must be *only one As Sure As Possible* in our Sevlet container,
and we can use it to lock our code.
* perhaps we need to reduce the loading/unloading times of our
Controller/Holder, for example, we need to avoid opening/closing/
reopening/reclosing our DBConnection many times.
SingleTon is just one way to do the above two things, I guess we also can do
it by other ways.
1
# Controller/Holder
normally it is a class, it could be a SingleTon, but it also could
not be
a SingleTon.
# locker
it could be:
[a] * a static field in Servlet class
* if we only have one Servlet definition, it also could be a
instance field
in Servlet class
* Servlet class itself
* if we only have one Servlet definition, it also could be the
instance of
Servlet class -> *this* in Servlet class
[b] * a static field in another class ->locker class
* a instance field in another class ->locker class, if now
locker class
is a SingleTon, or we use other way to make sure we only
have only one
instance of locker class.
* locker class itself
* the only one instance of another class ->locker class, if
now locker class
is a SingleTon, or we use other way to make sure we only
have only one
instance of locker class.
[c] * a static field in Controller/Holder class
* a instance field in Controller/Holder class, if now
Controller/Holder class
is a SingleTon, or we use other way to make sure we only
have only one
instance of Controller/Holder class.
* Controller/Holder class itself
* the only one instance of Controller/Holder class, if now
Controller/Holder
is a SingleTon, or we use other way to make sure we only
have only one
instance of Controller/Holder class.
2 for example, I wrote a simple DBConnection pool, if it is wrong, please
correct it
thanks in advance :-)
[a] ******************** the simple DBConnection pool
*********************
public class outsideDBCPHoder implements java.io.Serializable {
private static outsideDBCPHoder onlyOneInstance;
private java.util.Vector pool=new java.util.Vector();
private outsideDBCPHoder() {}
public static synchronized outsideDBCPHoder getOnlyOneInstance() {
if (onlyOneInstance == null) {
onlyOneInstance = new outsideDBCPHoder();
}
return onlyOneInstance;
}
public synchronized java.sql.Connection getDBConnection() {
java.sql.Connection con = null;
if ( pool.isEmpty() ) {
con = ...; //make a new DBConnection
} else {
con = (java.sql.Connection) pool.elementAt(0);
pool.removeElementAt(0);
}
return con;
}
public synchronized void putbackDBConnection( java.sql.Connection con ) {
if (pool.size() < 8) { // 8 is the MAX number of my pool, it also can be
100, 101...
pool.addElement(con);
}else{
... //close this con
con=null;
}
return;
}
public synchronized void closeAllDBConnection() {
String con=null;
if ( !pool.isEmpty() ) {
con=(java.sql.Connection)pool.elementAt(0);
...// cole the con
con=null;
pool.removeElementAt(0);
}
}
protected synchronized void finalize(){
closeAllDBConnection();
super.finalize();
}
}
[b] ********how to use this simple DBConnection pool in Servlet
class********
public class servlet_outsideDBCPHoder extends HttpServlet {
private static outsideDBCPHoder myDBCP
=outsideDBCPHoder.getOnlyOneInstance();
//init ...
_service/doGet/doPost{
java.sql.Connection con= myDBCP.getDBConnection();
...// use con
myDBCP.putbackDBConnection(con);
}
protected synchronized void finalize(){
myDBCP.closeAllDBConnection();
super.finalize();
}
}
[c] outsideDBCPHoder is a *SingleTonBean* ->
* the reason for making it as a SingleTon is:
try to avoid the problem led by loading/unloading MyServlet several
times, but perhaps it is not necessary, I am not sure.
* the reason for that make it Serializable is:
from the emails in Servlet-List, bean class will be loaded by
the same ClassLoader, I also don't know if it is nessary.
Bo
Nov.23, 2000
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