hibernate

[ Hibernate in 60 Minutes ]

Eric M. Burke
www.ericburke.com

April 14, 2005 Hibernate in 60 Minutes 1

[ My JDBC Experience ]
 Frustration drove me to investigate
Hibernate
 Custom JDBC frameworks and idioms
○ Usually a class called JdbcUtil with static methods
 Custom code generators
○ Database metadata is too cool to resist
 Custom strategy for optimistic locking
 Database-specific ID generation


[ Typical JDBC Problems ]
 Massive duplication  Relationships are
 Tied to specific really hard
database ○ N+1 selects problem
○ parent/child updates
 Error-prone
 delete all then add
try/catch/finally
 Screen-specific
DAOs for
optimization


[ Hibernate Questions ]
 How hard is it to learn and use?
 How invasive is it to my code?

 What are the configuration headaches?
○ Yet another framework with XML files, JAR files,
additional build complexity?
 Does it simplify my life?


[ What is Hibernate? ]
 Java Object/Relational Mapping
○ Open source – LGPL
○ http://www.hibernate.org/ (JBoss Group)
 Lets you avoid SQL
 Works with (almost) all relational DBs
○ DB2, FrontBase, HSQLDB, Informix, Ingres, Interbase,
Mckoi, MySQL, Oracle, Pointbase, PostgreSQL, Progress,
SAP DB, SQL Server, Sybase, etc...
○ Or you can extend one of the Dialect classes


[ What is Hibernate? (Cont'd) ]
 Has a huge feature list
○ Go to their web site – we don't have time
 Maps JavaBeans (POJOs) to tables
○ XML defines the mappings
○ Very few bean requirements
 SQL is generated at app startup time
○ As opposed to bytecode manipulation
○ New database? No problem! Change a few props


[ Avoiding SQL ]
 Hibernate Query Language approach
public User getByScreenName(String screenName) {
try {
return (User) getSession().createQuery(
"from User u where u.screenName=:screenName")
.setString("screenName", screenName)
.uniqueResult();
} catch (HibernateException e) {
throw new DaoException(e); JavaBeans
} Property Name
}

 Code-based query API
public User getByScreenName(String screenName) {
return getUnique(Expression.eq("screenName", screenName));
}


[ Sample Application Code ]
Session session = getSessionFactory().openSession();

// assume we know the root category id is 1
Category rootCategory = (Category) session.load(
Category.class, new Long(1));

// we can immediately traverse the collection of messages
// without any additional Hibernate code
for (Object msgObj : rootCategory.getMessages()) {
Message msg = (Message) msgObj; // generics+XDoclet==bad
System.out.println(“Subject: “ + msg.getSubject());
System.out.println(“Body: “ + msg.getBody());
System.out.println();
}

session.close();


[ Sample Code with Transaction ]
Session sess = ...
Transaction tx = sess.beginTransaction();

// find the root category using Criteria
Criteria criteria = sess.createCriteria(Category.class);
criteria.add(Expression.isNull("parentCategory"));
Category rootCategory = (Category) criteria.uniqueResult();

// change something
rootCategory.setDescription(“The Root Category”);

tx.commit();
sess.close();


[ Development Options ]
 Start with the database
○ Use middlegen to generate mapping files
○ Use hbm2java to generate JavaBeans
 Start with JavaBeans C
○ Use XDoclet to generate mapping files
○ Use hbm2ddl to generate the database
 Start with the XML mapping files
○ Hand-code the mapping files
○ Use both hbm2java and hbm2ddl

[ Hibernate Tutorial ]
 Design the database
 Code some persistent classes

 Write an Ant buildfile
○ Generate the mapping files and hibernate.cfg.xml
 Choose a strategy for sessions and
transactions (Spring framework)
 Write and test the DAOs

 Add business logic and helper methods


[ Database Design ]
 Design your database first

Categories Surrogate Keys Resources
Work Best PK id
PK id

name name
url
description many-to-one description
FK1 parentCategoryId
createdOn

CategoriesResources
PK,FK1 categoryId
PK,FK2 resourceId
many-to-many


[ Persistent Objects ]
 POJOs - Plain Old Java Objects
○ No base class or interface requirement
 Must have public no-arg constructor
 Should have getter/setters
○ Or Hibernate can access fields directly
○ Hibernate uses JavaBeans property names
 For mappings as well as in queries
 Serializable is recommended
 XDoclet tags are useful but not required

[ My Base Class ]
public class PersistentClass implements Serializable {
private static final long serialVersionUID = 1;
private Long id;

/**
* @hibernate.id
* column="id"
* unsaved-value="null"
* generator-class="native"
* access="field"
*/
public Long getId() { Hibernate assigns the id
return id;
}

private void setId(Long id) {
this.id = id;
}
}


[ Category Class ]
public class Category extends NameDescription {
private static final long serialVersionUID = 1;
My base
private Set childCategories = new HashSet();
private Set resources = new HashSet();
class, not
private Category parentCategory; Hibernate's
public Category() {
}

public Category(String name, String description) {
super(name, description);
}

public Set getChildCategories() {
return childCategories;
}

public void setChildCategories(Set childCategories) {
this.childCategories = childCategories;
}

[ Category (Cont'd) ]
public Category getParentCategory() {
return parentCategory;
}

public void setParentCategory(Category parentCategory) {
this.parentCategory = parentCategory;
}

public Set getResources() {
return resources;
}

public void setResources(Set resources) {
this.resources = resources;
}
}


[ Mapping Files
Or Java 5
Annotations ]
 Generated by XDoclet, 1 per persistent bean
<hibernate-mapping>
<class name="com.ericburke.linkblog.model.Category"
table="Categories">
<id name="id" column="id" type="java.lang.Long"
unsaved-value="null">
<generator class="native"/>
</id>

<set name="childCategories" lazy="true"
inverse="true" cascade="save-update">
<key column="parentCategoryId"/>
<one-to-many class="com.ericburke.linkblog.model.Category"/>
</set>

<many-to-one name="parentCategory"
class="com.ericburke.linkblog.model.Category"
cascade="none" column="parentCategoryId" not-null="false"/>


[ Mapping Files (Cont'd) ]
<set name="resources" table="CategoriesResources"
lazy="true" cascade="save-update">
<key column="categoryId"/>
<many-to-many column=”resourceId”
class="com.ericburke.linkblog.model.Resource"/>
</set>

<property name="name" type="java.lang.String"
column="name" length="255"/>

<property name="description" type="java.lang.String"
column="description" length="2000"/>
</class>
</hibernate-mapping>


[ Xdoclet Tag Example ]
/**
* @hibernate.class table="Categories"
*/
public class Category extends NameDescription {

/**
* @hibernate.set cascade="save-update" inverse="true"
* lazy="true"
* @hibernate.collection-key column="parentCategoryId"
* @hibernate.collection-one-to-many
* class="com.ericburke.linkblog.model.Category"
*/
public Set getChildCategories() {
return childCategories;
}

// no tags on the setters
public void setChildCategories(Set childCategories) { }


[ hibernate.cfg.xml ]
Generated by XDoclet, or
<hibernate-configuration> use Spring's XML file
<session-factory>
<property name="dialect">
net.sf.hibernate.dialect.HSQLDialect</property>
<property name="show_sql">true</property>
<property name="use_outer_join">false</property>
<property name="connection.username">sa</property>
<property name="connection.driver_class">
org.hsqldb.jdbcDriver</property>
<property name="connection.url">
jdbc:hsqldb:hsql://localhost/linkblog</property>
<property name="connection.pool_size">5</property>

<mapping
resource="com/ericburke/linkblog/model/Category.hbm.xml"/>
<mapping
resource="com/ericburke/linkblog/model/Resource.hbm.xml"/>
</session-factory>
</hibernate-configuration>


[ Ant Buildfile ]
 Invokes XDoclet
○ XDoclet ships with the Hibernate tasks
○ Generates mapping files and hibernate.cfg.xml
 Hibernate includes tasks for
○ hbm2java – generates the persistent classes
○ hbm2ddl – generates the database Requires the persistent
.class files in the
taskdef's classpath
 Classpath needs
○ Hibernate Jars, Hibernate extensions, XDoclet,
JDBC driver JAR file

[ HibernateUtil ]
 Trivial examples show this class
○ Creates a SessionFactory in the static initializer
Configuration cfg = new Configuration();
SessionFactory sessionFactory =
cfg.configure("/hibernate.cfg.xml")
.buildSessionFactory();
 Provides access to a Session ThreadLocal
public static Session getSession() {
Session s = (Session) threadSession.get();
if (s == null) {
s = sessionFactory.openSession();
threadSession.set(s);
}
return s;
 APIs to begin/commit/rollback transactions

[ DAOs ]
 Insulate your code from Hibernate specifics
 CRUD operations
○ I use Java 5 generics heavily
○ You can also use Spring for DAO support
 Locating persistent objects
○ Use Hibernate queries for efficiency
○ From there, walk the graph using normal Java
collections semantics
 Detached objects can be challenging


[ Servlet Filter ]
public class HibernateFilter implements Filter {
public void init(FilterConfig fc) throws ServletException {
}

public void destroy() {
}

public void doFilter(ServletRequest req, ServletResponse res,
FilterChain filterChain)
throws IOException, ServletException {
try {
filterChain.doFilter(req, res);
HibernateUtil.commitTransaction();
} finally { Hibernate in Action,
HibernateUtil.closeSession(); page 304
}
}
}


[ Business Objects ]
 A layer between your application code and
the DAOs
○ Only allow business objects to hit the DAOs
○ Business objects and app code use persistent objects
 Provide a consistent place to use AOP
○ try/catch/finally for transactions
 Risk duplicating DAO code
○ Sometimes business object methods just delegate


[ Architecture Summary ]
AOP for
Transactions

Business
DAOs Hibernate Database
Objects

Application
Code Persistent XML
Objects Mappings

Keep an
Open Session


[ Gotchas ]
 Must always have an open session
○ Seemingly innocent code like fetching a lazy
collection bombs
○ Swing – open a session on event thread
○ Servlets – use a filter or Spring
 Hibernate tasks do not abort Ant
○ No failonerror attribute
○ Watch the build output carefully
 Test associations and collections heavily

[ When Not to Use Hibernate ]
 A horrible database design is forced on you
○ Many composite keys
 If your application is mostly mass
operations
○ See pg. 181 “Hibernate in Action”
○ You might exhaust available heap space
 ??


[ 10 Reasons to Love Hibernate ]
1.Dynamic UPDATE 6.No more DTOs
generation – figure out 7.Query result paging
which columns
changed 8.Automatic dirty
checking
2.Trying new databases
9.Good price
3.Traversing
associations 10.Good documentation
4.Optimistic Locking
5.ID generation

hibernate

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