Compass是建立在Lucene基礎之上的一款開放源碼的JAVA搜索引擎框架。關於Compass的介紹在此我不多講了，更多瞭解請直接參考http://www.opensymphony.com/compass。
很多看了Compass的人多少對Compass的配置文件都有點雲裏霧裏的感覺。由於Compass有很多地方都借鑑了Hibernate的思想，在此我結合Hibernate中的思想來幫助我們對Compass的配置文件進行理解。
Hibernate是一個O/R Mapping工具，它可以將實體對象和數據庫中的表進行映射。最終通過訪問實體對象來達到訪問數據庫的目的。
Compass是一個搜索引擎框架，它可以將common meta data與實體對象進行映射。最終通過訪問common meta data來達到訪問對象的目的。
綜上所述，Hibernate是對象到數據庫。而Compass是common meta data 到對象。
Compass的配置文件主要分成三類：
第一類：*.cmd.xml文件
*.cmd.xml文件是對common meta data進行定義，我們就可以把它是當作Hibernate中的POJO來理解。裏面定義了最終搜索的結果中的最基本的元數據。下面是一個*.cmd.xml文件的片斷：
< compass - core - meta - data > < meta - data - group id = " petclinic " displayName = " Petclinic Meta Data " > < descrīption > Petclinic Meta Data </ descrīption > < uri > http: // compass/sample/petclinic</uri> < alias id = " vet " displayName = " Vet " > < descrīption > Vet alias </ descrīption > < uri > http: // compass/sample/petclinic/alias/vet</uri> < name > vet </ name > </ alias > ………… < meta - data id = " petType " displayName = " Pet Type " > < descrīption > The type of a pet </ descrīption > < uri > http: // compass/sample/petclinic/petType</uri> < name > petType </ name > </ meta - data > </ meta - data - group > </ compass - core - meta - data > 在這裏
< meta - data - group id = " petclinic " displayName = " Petclinic Meta Data " > 可當作是POJO的package來理解，petclinic可以看成是package name。
< alias id = " vet " displayName = " Vet " > 可當作是POJO的Class來理解，vet 可以看成是class name。
< meta - data id = " petType " displayName = " Pet Type " > 可當作是POJO的property來理解，petType可以看成是property name。
上面xml中所定義的meta data 和alias就是compass需要查詢的所有的基本元素組件。
然而common meta data的定義與POJO所不同的是：POJO是用java代碼來體現，而common meta data使用xml來體現。POJO的class中定義了property，common meta data中的alias和meta-data是分開定義。
當然上面都是一個類比而已, 其目的是爲了便於讓我們理解，實際生成的代碼並非如上所述的一一對應。compass提供了一個ant task可以將common meta data生成一個class。代碼片斷如下：
public final class Petclinic {
/** 
* Petclinic Meta Data
*/ public static final class Group { public static final String Id = " petclinic " ; public static final String DispayName = " Petclinic Meta Data " ; public static final String Uri = " http://compass/sample/petclinic " ; } public static final class Alias { /** * Owner alias */ public static final class Owner { public static final String Id = " owner " ; public static final String Name = " owner " ; public static final String DisplayName = " Owner " ; public static final String Uri = " http://compass/sample/petclinic/alias/owner " ; public static final String GroupId = " petclinic " ; } } ……
} 第二類：*.cpm.xml文件
*.cpm.xml就是Object/Search Engine Mapping了。 我們也可以拿她當作同Hibernate中的*.hbm.xml來理解。他的作用就是提供了POJO到common meta data的映射。下面是一個*.cpm.xml文件的片斷：
< compass - core - mapping package = " org.compass.sample.petclinic " > < contract alias = " entity " > < id name = " id " /> </ contract > < contract alias = " person " extends = " entity " > < property name = " firstName " > < meta - data > $ {petclinic.firstName} </ meta - data > </ property > < property name = " lastName " > < meta - data > $ {petclinic.lastName} </ meta - data > </ property > < property name = " address " > < meta - data > $ {petclinic.address} </ meta - data > </ property > < property name = " city " > < meta - data > $ {petclinic.city} </ meta - data > </ property > < property name = " telephone " > < meta - data > $ {petclinic.telephone} </ meta - data > </ property > </ contract > < class name = " Specialty " alias = " ${petclinic.specialty} " root = " false " > < property name = " name " > < meta - data > $ {petclinic.specialty} </ meta - data > </ property > </ class > < class name = " Owner " alias = " ${petclinic.owner} " extends = " person " > < property name = " firstName " > < meta - data > $ {petclinic.firstName} </ meta - data > </ property > < property name = " lastName " > < meta - data > $ {petclinic.lastName} </ meta - data > </ property > < property name = " address " > < meta - data > $ {petclinic.address} </ meta - data > </ property > < property name = " city " > < meta - data > $ {petclinic.city} </ meta - data > </ property > < property name = " telephone " > < meta - data > $ {petclinic.telephone} </ meta - data > </ property > < reference name = " petsInternal " ref - alias = " ${petclinic.pet} " /> </ class > </ compass - core - mapping > 上面 package對應了POJO的包名，class對應了POJO類名，contract爲POJO中一些較爲Base類, property對應了POJO的屬性。上面看到的像ANT中的${*}就是*.cmd.xml 中所定義的common meta data。通常來說*.cmd.xml中的alias同POJO的Class進行映射。Meta data 同Class中的property進行映射。更多的映射的細節在此不多講。可以參考 Compass的中自帶的Sample Petclinic來理解。
第三類：*.cfg.xml文件
Compass的*.cfg.xml文件就和Hibernate的*.cfg.xml有些類似了。下面是一個*.cfg.xml文件的內容：
< compass - core - configuration > < compass > < setting name = " compass.engine.connection " > target / index </ setting > < meta - data resource = " org/compass/sample/library/library.cmd.xml " /> < mappings > < class name = " test.Author " /> </ mappings > </ compass > </ compass - core - configuration > 上面
< setting name = " compass.engine.connection " > target / index </ setting > 指定了索引文件存放的路徑。
< meta - data resource = " org/compass/sample/library/library.cmd.xml " /> 指定了*.cmd.xml文件存放的路徑。
< mappings >< class name = " test.Author " /></ mappings > 是對compass的mapping文件進行指定。Test.Author對應的文件是test/Author.cpm.xml.
<compass>是對compass中的compass類進行的一些參數設置，這個compass是一個重量級的類，類似於Hibernate中SessionFactory.
當Hibernate同Spring進行整合後，Hibernate就不需要*.cfg.xml這個文件了，借而代之的是用Spring的配置文件來進行配置。同樣，Compass同Spring進行整合後，*.cfg.xml也可以不需要了。比如如下的Spring配置。
< bean id = " compass " class = " org.compass.spring.LocalCompassBean " > < property name = " resourceLocations " > < list > < value > classpath:org / compass / sample / petclinic / petclinic.cmd.xml </ value > < value > classpath:petclinic.cpm.xml </ value > </ list > </ property > < property name = " compassSettings " > < props > < prop key = " compass.engine.connection " > file: // ${user.home}/compass/petclinic</prop> < prop key = " compass.transaction.factory " > org.compass.spring.transaction.SpringSyncTransactionFactory </ prop > </ props > </ property > < property name = " transactionManager " > < ref local = " transactionManager " /> </ property > </ bean > < bean id = " hibernateGpsDevice " class = " org.compass.spring.device.hibernate.SpringHibernate3GpsDevice " > < property name = " name " >< value > hibernateDevice </ value ></ property > < property name = " sessionFactory " >< ref local = " sessionFactory " /></ property > </ bean > < bean id = " compassGps " class = " org.compass.gps.impl.SingleCompassGps " init - method = " start " destroy - method = " stop " > < property name = " compass " >< ref bean = " compass " /></ property > < property name = " gpsDevices " > < list > < ref local = " hibernateGpsDevice " /> </ list > </ property > </ bean >
在上訴的配置文件中又引出了兩個新的知識點，CompassGps和CompassGpsDevice。
1 ：CompassGps像是一個Service，他需要在application startup時啓動服務， application shutdown停止服務，
2 ：CompassGpsDevice不能獨立的存在，他需要依賴CompassGps，
CompassGps爲CompassGpsDevice提供Compass對象，他們一起爲程序提供Index的實時更新。 Compass整合Hibernate 等等 persitance framework的代碼就在CompassGpsDevice裏，你需要提供不同的Device，如HibernateDevice， JDODevice。你也可以實現自己的Device， CompassGpsDevice會把domain object的更新事件通過CompassGps去通知Compass去實時更新索引文件。