public void setPartitionExpression(String partitionExpression) {
		this.partitionExpression = partitionExpression;
	}
Sets the partition expression. This expression is used to determine
if this factory creates an instance of {@link PartitionTextFileWriter}
or {@link TextFileWriter}. Validity of this spel expression
is not checked in this factory, thus any non empty string will
result creation of {@link PartitionTextFileWriter}.

@param partitionExpression the new partition expression
spring-xd-hadoop/src/main/java/org/springframework/xd/integration/hadoop/config/StoreWriterFactoryBean.java