Documentation Version: 3.4 - Publication Date : Dec 24, 2014

CacheWriter

A javax.cache.integration.CacheWriter is used to update an external backend resource. If the cache is configured to be write-through this process is executed transparently to the users code otherwise at the current state there is no way to trigger writing changed entries to the external resource to a user defined point in time.

If bulk operations throw an exception, java.util.Collection has to be cleaned of all successfully written keys so the cache implementation can determine what keys are written and can be applied to the cache state.

public class UserCacheWriter
    implements CacheWriter<Integer, User>, Serializable {

  private final UserDao userDao;

  public UserCacheWriter( UserDao userDao ) {
    // Store the dao instance created externally
    this.userDao = userDao;
  }

  @Override
  public void write( Cache.Entry<? extends Integer, ? extends User> entry )
      throws CacheWriterException {

    // Store the user using the dao
    userDao.storeUser( entry.getKey(), entry.getValue() );
  }

  @Override
  public void writeAll( Collection<Cache.Entry<...>> entries )
      throws CacheWriterException {

    // Retrieve the iterator to clean up the collection from
    // written keys in case of an exception
    Iterator<Cache.Entry<...>> iterator = entries.iterator();
    while ( iterator.hasNext() ) {
      // Write entry using dao
      write( iterator.next() );
      // Remove from collection of keys
      iterator.remove();
    }
  }

  @Override
  public void delete( Object key ) throws CacheWriterException {
    // Test for key type
    if ( !( key instanceof Integer ) ) {
      throw new CacheWriterException( "Illegal key type" );
    }
    // Remove user using dao
    userDao.removeUser( ( Integer ) key );
  }

  @Override
  public void deleteAll( Collection<?> keys ) throws CacheWriterException {
    // Retrieve the iterator to clean up the collection from
    // written keys in case of an exception
    Iterator<?> iterator = keys.iterator();
    while ( iterator.hasNext() ) {
      // Write entry using dao
      delete( iterator.next() );
      // Remove from collection of keys
      iterator.remove();
    }
  }
}

Again the implementation is pretty straight forward and also as above all exceptions thrown by the external resource, like java.sql.SQLException has to be wrapped into a javax.cache.integration.CacheWriterException. Note this is a different exception from the one thrown by CacheLoader.