8' ContainerManaged Persistent Entity Beans

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8. Container-Managed Persistent Entity Beans

• CMP
• Container-Managed Persistent
• in this chapter EJB 2.0 CMP2.0 (CMP1.0)
• The EJB container performs storage operations
  for us, we dont write any JDBC or SQL/J codes.

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Features of CMP Entity Beans

• The difference between BMP and CMP
• CMP Entity Beans Are Subclassed
• database-independent
• clear separation between an entity bean and its
  persistent representation (logic methods and the
  JDBC)
• superclass our job
• subclass container generates them by
  subclassing our entity bean class
• //The generated subclass inherits from our
  entiity bean class

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Features of CMP Entity Beans

• CMP Entity Beans Have No Declared Fields
• The persistent fields are kept in the subclass.

//CMP Subclass, generated by Container Vendors
tool public class AccountBeanSubClass extends
AccountBean public String accountID//PK
public String ownerName public double
balance ...methods...
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CMP Class Hierarchy
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Features of CMP Entity Beans

• CMP Get/Set Methods Are Defined in the Subclass
• The subclass implements the get/set methods, the
  super-class defines the declaration (abstract) of
  the get/set methods.

public abstract class CartBean implements
EntityBean ... public float
getTotal() return this.getSubtotal()thi
s.getTaxes() ... //These methods,
similar with getTotal(), can not be automatically
generated by the container, we need to write
these methods in the super-class by ourselves.
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Features of Entity Beans

• CMP Entity Beans Have an Abstract Persistent
  Schema
• When deploy the J2EE application, we will set a
  abstract persistent name for each CMP Bean.
• The container inspects the deployment
  descriptors to figure out what to generate.
• (See also ejb-jar.xml in .ear package)

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Features of Entity Beans

• CMP Entity Beans Have a Query Language
• EJB-QL EJB Query Language, object-oriented SQL
  syntax
• CMP 2.0, EJB 2.0
• finder (used by external entity bean clients)
• select (used internally by the entity bean itself
  )
• SELECT, FROM, WHERE clause

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EJB-QL Terminology

• Abstract schema The part of an entity bean's
  deployment descriptor that defines the bean's
  persistent fields and relationships.
• Abstract schema name A logical name that is
  referenced in EJB QL queries. You specify an
  abstract schema name for each entity bean with
  container-managed persistence.
• Abstract schema type All EJB QL expressions
  evaluate to a type. If the expression is an
  abstract schema name, by default its type is the
  local interface of the entity bean for which the
  abstract schema name is defined.
• Backus-Naur Form (BNF) A notation that describes
  the syntax of high-level languages. The syntax
  diagrams in this chapter are in BNF notation.
• Navigation The traversal of relationships in an
  EJB QL expression. The navigation operator is a
  period.
• Path expression An expression that navigates to
  a related entity bean.
• Persistent field A virtual field of an entity
  bean with container-managed persistence it is
  stored in a database.
• Relationship field A virtual field of an entity
  bean with container-managed persistence it
  identifies a related entity bean.

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EJB QL

• EJB-QL is a standard and portable language.
• EJB-QL SQL
• EJB-QL OQL
• EJB-QL Key Words
• AND AS BETWEEN DISTINCT EMPTY FALSE FROM IN IS
  LIKE MEMBER NOT NULL OBJECT OF OR SELECT TRUE
  UNKNOW WHERE
• EJB-QL Data Type
• String in EJB-QL quoted by single quotes, for
  example, OK
• Integer long
• Float float, double
• Boolean TRUE, FALSE

Translate into
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EJB QL

• EJB-QL Parameters
• Only used in WHERE clause
• ?1, ?2, Indexed from 1 instead of 0
• Parameters data type should be identical with
  the parameters in finder or select methods.
• EJB-QL Comparison Operator
• ! in Java
• ltgt in EJB-QL
• EJB-QL Other Symbols
• zero or more characters
• _ a single character

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EJB QL

• Abstract Schema Name
• EJB-QL Functions
• CONCAT(String, String) return String
• SUBSTRING(String, start, length) return String
• LOCATE(String, String, start) return int
• LENGTH(String) return int
• ABS(number) return int, float or double
• SQRT(double) return double

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EJB QL
Abstract Schema Name

• A Simple Example
• SELECT OBJECT(p) FROM Product AS p WHERE
  p.productID?1
• SELECT OBJECT(p) FROM Product AS p WHERE
  p.productPrice BETWEEN 100 AND 300
• SELECT OBJECT(p) FROM Product AS p WHERE
  p.productPrice gt ?1
• SELECT OBJECT(p) FROM Product AS p WHERE
  p.productName LIKE Mobile
• SELECT DISTINCT OBJECT(p) FROM Product AS p
  WHERE p.productName ?1 AND p.productPrice gt ?2

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EJB-QL Relationship

• Examples
• SELECT DISTINCT OBJECT(p) FROM Player p, IN
  (p.teams) AS t WHERE t.city ?1
• Data retrieved
• The players whose teams belong to the specified
  city.
• Finder method
• findByCity(String city)
• Description
• p represents Player Entity Bean
• t represents Teams Entity Bean
• p.teams navigates from Player bean to its
  related Teams bean, navigation operator
• IN(p.teams) signifies that teams is a
  collection of related beans
• t.city delimiter, city is a persistent field
• p.teams.city ERROR, because the teams is a
  collection

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EJB QL

• EJB-QL Effective Range
• EJB-QL is useful for both home interfaces and
  local home interfaces. A single EJB-QL definition
  will inform the container about how to implement
  the SQL code for both a home object and local
  home object that have identically named finder
  methods.
• SELECT Method
• In Bean Class
• public abstract Collection ejbSelectAllProducts()
  
• Whether should the ejbSelect method return EJB
  local objects or EJB objects
• public abstract String ejbSelectProductName()
• ejbSelect methods can return container managed
  fields, not does finder methods

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EJB-QL Restriction

• Comments are not allowed.
• Date and time values are in milliseconds and use
  a Java long. A date or time literal should be an
  integer literal. To generate a millisecond value,
  you may use the java.util.Calendar class.
• Currently, container-managed persistence does not
  support inheritance. For this reason, two entity
  beans of different types cannot be compared

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Features of Entity Beans

• CMP Entity Beans Can Have ejbSelect Methods
• A ejbSelect method is like a finder method, but
  is not directly exposed to the client. ejbSelect
  methods is used internally within an entity bean.

//ejbSelect() public abstract Collection
ejbSelectAllAccountBalances() throws
FinderException //ejbHome() public double
ejbHomeGetTotalBankValue() throws Exception
//Get a collection of bank account
balances Collection c this.ejbSelectAllAccountBa
lances() //Loop through collection and return
sum
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Java Sources

• J2SDKEE developing
• (See also demo 1, Chapter8/cmp/readme.txt)
• (See also demo 2, Chapter8/cmpNoPK/readme.txt
  ,
• no primary key class)
• WSAD
• (See also demo 3, Chapter8/WASD/.avi and
  .ear)