Oh my bad, I mixed up lazy singleton and eager singleton.
On 12 August 2014 17:43, Ralph Goers <ralph.go...@dslextreme.com> wrote: > Finally, see > http://en.wikipedia.org/wiki/Singleton_pattern#Eager_initialization. > This was the prior pattern which is clearly thread safe. > > Ralph > > On Aug 12, 2014, at 3:40 PM, Ralph Goers <ralph.go...@dslextreme.com> > wrote: > > Let me put it another way. > > If your intent is that the factory should only be created when getManager > is called vs when AbstractDataManager is loaded then your change is fine. > However, I am not sure there is any benefit to that since I would expect > every access to AbstractDataManager to be immediately followed by a call to > getManager. > > From what I can see the previous code was not vulnerable to any race > conditions. > > Ralph > > On Aug 12, 2014, at 3:33 PM, Ralph Goers <ralph.go...@dslextreme.com> > wrote: > > So you are saying static initializers in a class can be executed multiple > times? If so I certainly wasn’t aware of that and I would think a few > pieces of code would need modification. Before I go searching can you > point to something that says it works that way? > > When I read > http://en.wikipedia.org/wiki/Initialization-on-demand_holder_idiom it > indicates that static initialization IS guaranteed to be serial, in which > case the holder pattern is not needed for this. That pattern is for when > you want lazy initialization, not for guaranteeing a static variable is > only initialized once. > > Ralph > > On Aug 12, 2014, at 1:13 PM, Ralph Goers <ralph.go...@dslextreme.com> > wrote: > > What race condition? Static variables are initialized when the class is > constructed. As far as I know there is no race condition on that or Java > would be hosed. > > Ralph > > On Aug 12, 2014, at 12:07 PM, Matt Sicker <boa...@gmail.com> wrote: > > Prevents multiple threads from constructing the field if they access the > class concurrently. Basically, it prevents a race condition. The > alternative is to make the field volatile and do a double-checked lock > which we do in another class somewhere. > > > On 12 August 2014 13:53, Gary Gregory <garydgreg...@gmail.com> wrote: > >> Hi, >> >> What is the justification for this extra level? >> >> Gary >> >> >> -------- Original message -------- >> From: mattsic...@apache.org >> Date:08/11/2014 22:04 (GMT-05:00) >> To: comm...@logging.apache.org >> Subject: svn commit: r1617397 - >> /logging/log4j/log4j2/trunk/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/db/jdbc/JdbcDatabaseManager.java >> >> >> Author: mattsicker >> Date: Tue Aug 12 02:04:38 2014 >> New Revision: 1617397 >> >> URL: http://svn.apache.org/r1617397 >> Log: >> Singleton pattern >> >> Modified: >> >> logging/log4j/log4j2/trunk/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/db/jdbc/JdbcDatabaseManager.java >> >> Modified: >> logging/log4j/log4j2/trunk/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/db/jdbc/JdbcDatabaseManager.java >> URL: >> http://svn.apache.org/viewvc/logging/log4j/log4j2/trunk/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/db/jdbc/JdbcDatabaseManager.java?rev=1617397&r1=1617396&r2=1617397&view=diff >> >> ============================================================================== >> --- >> logging/log4j/log4j2/trunk/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/db/jdbc/JdbcDatabaseManager.java >> (original) >> +++ >> logging/log4j/log4j2/trunk/log4j-core/src/main/java/org/apache/logging/log4j/core/appender/db/jdbc/JdbcDatabaseManager.java >> Tue Aug 12 02:04:38 2014 >> @@ -36,7 +36,6 @@ import org.apache.logging.log4j.core.uti >> * An {@link AbstractDatabaseManager} implementation for relational >> databases accessed via JDBC. >> */ >> public final class JdbcDatabaseManager extends AbstractDatabaseManager { >> - private static final JDBCDatabaseManagerFactory FACTORY = new >> JDBCDatabaseManagerFactory(); >> >> private final List<Column> columns; >> private final ConnectionSource connectionSource; >> @@ -174,10 +173,19 @@ public final class JdbcDatabaseManager e >> final >> ColumnConfig[] columnConfigs) { >> >> return AbstractDatabaseManager.getManager( >> - name, new FactoryData(bufferSize, connectionSource, >> tableName, columnConfigs), FACTORY >> + name, new FactoryData(bufferSize, connectionSource, >> tableName, columnConfigs), getFactory() >> ); >> } >> >> + // the usual lazy singleton >> + private static class Holder { >> + private static final JDBCDatabaseManagerFactory INSTANCE = new >> JDBCDatabaseManagerFactory(); >> + } >> + >> + private static JDBCDatabaseManagerFactory getFactory() { >> + return Holder.INSTANCE; >> + } >> + >> /** >> * Encapsulates data that {@link JDBCDatabaseManagerFactory} uses to >> create managers. >> */ >> >> >> > > > -- > Matt Sicker <boa...@gmail.com> > > > > > > -- Matt Sicker <boa...@gmail.com>
