public Expression getPartitionKeyExpression() {
		String partionKeyExpression = getProperty(PARTITION_KEY_EXPRESSION);
		Expression expression = null;
		if (partionKeyExpression != null) {
			expression = spelExpressionParser.parseExpression(partionKeyExpression);
		}
		return expression;
	}
The expression to determine the partition key, evaluated against the
message as the root object.
@return The key.
spring-xd-messagebus-spi/src/main/java/org/springframework/xd/dirt/integration/bus/AbstractBusPropertiesAccessor.java