com.hazelcast.query.extractor

Class ValueExtractor<T>

java.lang.Object

com.hazelcast.query.extractor.ValueExtractor<T>

Type Parameters:

T - type of the target object to extract the value from

public abstract class ValueExtractor<T>
extends Object

Common superclass for all extractors that enable the user to define custom attributes and extract their values. The extraction logic may just extract the underlying value or group, reduce or transform it.

How to use a ValueExtractor?

First, extend this class and implement the @see com.hazelcast.query.extractor.ValueExtractor#extract method. Then, define a new attribute with the above-mentioned extractor in the configuration of the map.

How to define a new custom attribute? MapAttributeConfig attributeConfig = new MapAttributeConfig(); extractorConfig.setName("currency"); extractorConfig.setExtractor("com.bank.CurrencyExtractor");

How to register the newly-defined attribute in a configuration of a Map? MapConfig mapConfig = (...); mapConfig.addMapAttributeConfig(attributeConfig); Extractors may be also defined in the XML configuration.

Please, bear in mind that an extractor may not be added after the map has been instantiated. All extractor have to be defined upfront in the map's initial configuration.

Reflection-based extraction is the default mechanism - ValueExtractors are an alternative way of extracting attribute values from object.

Constructor Summary

Constructors

Constructor and Description
ValueExtractor()

Method Summary

Methods

Modifier and Type	Method and Description
abstract void	extract(T target, ValueCollector collector) Extracts a value from the given target object.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ValueExtractor

public ValueExtractor()

Method Detail

extract

public abstract void extract(T target,
           ValueCollector collector)

Extracts a value from the given target object. The method does not return any value, the extracted value is collected by the ValueCollector instead.

The extractor may extract:

• one 'single-value' result (not a collection)
• one 'multi-value' result (a collection)
• multiple 'single-value' or 'multi-value' results

In order to return multiple results from a single extraction just invoke the ValueCollector#collect method multiple times, so that the collector collects all results. MultiResult is an aggregate of results that is returned if the extractor returns multiple results due to a reducing / grouping operation executed on a hierarchy of values.

It sounds counter-intuitive, but a single extraction may return multiple values when arrays or collections are involved.

Let's have a look at the following data structure: class Swap { Leg legs[2]; }

class Leg { String currency; }

Let's assume that we want to extract currencies of all legs from a single Swap object. Each Swap has two Legs so there are two currencies too. In order to return both values from the extraction operation just collect them separately using the ValueCollector. Collecting multiple values in such a way allows us to operate on multiple "reduced" values as if they were single-values.

Let's have look at the following query 'swapCurrency = EUR', assuming that we registered a custom extractor to extract currencies from a swap under the name swapCurrency. It will return up to two currencies, but the default evaluation semantics is that it is enough if a single value evaluates the condition to true to return the Swap from the query.

Parameters:

target - object to extract the value from

collector - collector of the extracted value(s)

See Also:

ValueCollector