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 <[email protected]> wrote: > Hi, > > What is the justification for this extra level? > > Gary > > > -------- Original message -------- > From: [email protected] > Date:08/11/2014 22:04 (GMT-05:00) > To: [email protected] > 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 <[email protected]>
