# PersistedSequencingMixin.java

    /*
     * Copyright 2008 Niclas Hedhman. All rights Reserved.
     *
     * Licensed  under the  Apache License,  Version 2.0  (the "License");
     * you may not use  this file  except in  compliance with the License.
     * You may obtain a copy of the License at
     *
     *   http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed  under the  License is distributed on an "AS IS" BASIS,
     * WITHOUT  WARRANTIES OR CONDITIONS  OF ANY KIND, either  express  or
     * implied.
     *
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */
    package org.qi4j.library.uid.sequence;

    import org.qi4j.api.configuration.Configuration;
    import org.qi4j.api.injection.scope.Structure;
    import org.qi4j.api.injection.scope.This;
    import org.qi4j.api.property.Property;
    import org.qi4j.api.unitofwork.ConcurrentEntityModificationException;
    import org.qi4j.api.unitofwork.UnitOfWork;
    import org.qi4j.api.unitofwork.UnitOfWorkCompletionException;
    import org.qi4j.api.unitofwork.UnitOfWorkFactory;

    public class PersistedSequencingMixin
        implements Sequencing
    {
        @Structure private UnitOfWorkFactory uowf;
        @This private Configuration<Sequence> sequence;

        @Override
        public Long newSequenceValue()
            throws SequencingException
        {
            synchronized( this )
            {
                ConcurrentEntityModificationException exc = null;
                UnitOfWork uow = uowf.newUnitOfWork();
                try
                {
                    for( int i = 0; i < 3; i++ )
                    {
                        try
                        {
                            Property<Long> property = sequence.get().currentValue();
                            long value = property.get();
                            value = value + 1;
                            property.set( value );
                            uow.complete();
                            return value;
                        }
                        catch( ConcurrentEntityModificationException e )
                        {
                            // Ignore;
                            exc = e;
                        }
                    }
                    throw new SequencingException( "Unable to update sequence value.", exc );
                }
                catch( UnitOfWorkCompletionException e )
                {
                    throw new SequencingException( "Unable to update sequence value.", exc );
                }
            }
        }

        @Override
        public Long currentSequenceValue()
        {
            synchronized( this )
            {
                return sequence.get().currentValue().get();
            }
        }
    }