android java qr code generator <class name="Location" proxy="com.manning.hq.ch03.Location"...>... </class> in Java

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<class name="Location" proxy="com.manning.hq.ch03.Location"...>... </class>
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The second method is to use the lazy attribute. Setting lazy="true" is a shorthand way of defining the persistent class as the proxy. Let s assume the Location class is defined as lazy:
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<class name="Location" lazy="true"...>...</class>
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The lazy attribute is true by default in Hibernate 3. An easy way to disable all proxies, including lazy collections, is to set the default-lazy attribute to true in the hibernate-mapping element for a given mapping file. Let s look at an example of using a proxied Location instance:
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Session session = factory.openSession(); Event ev = (Event) session.load(Event.class, myEventId); Location loc = ev.getLocation(); String name = loc.getName(); session.close();
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The returned Location instance is a proxy. Hibernate populates the Location instance when getName() is called. You ll be dealing with a proxy of Location generated by CGLIB until you call an instance method.1 What happens when you retrieve the
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CGLIB is a code generation library used by Hibernate. You can find out more about it at http://cglib.sourceforge.net/.
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Creating mapping definitions
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instance from the database All the properties for the Event are retrieved, along with the ID of the associated Location instance. The generated SQL looks something like this:
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select event0_.id as id0_, event0_.name as name0_, event0_.location_id as location_id0_ from events event0_ where event0_.id=
When you call loc.getName(), the following generated SQL is executed:
select location0_.id as id0_ as id0_, location0_.name as name0_ from locations location0_ where location0_.id=
If you ve guessed that you can call loc.getId() without invoking a call to the database, you re correct. The proxied object already contains the ID value, so it can be safely accessed without retrieving the full object from the database. Next, we ll look at collections of persistent objects. Like proxies, collections can also be lazily populated.
3.2.5 Collections
The mapping file defines the collections for Speakers and Attendees. Since the two collections are essentially the same, we re just going to look at the Speaker collection here. The collections are defined as sets, meaning Hibernate manages the collections with the same semantics as a java.util.Set:
<set name="speakers"> <key column="event_id"/> <one-to-many class="Speaker"/> </set>
This definition declares that the Event class has a property named speakers, and that it s a Set containing instances of the Speaker class. The Event class has the corresponding property:
Hibernate basics
public class Event { private Set speakers; ... public void setSpeakers(Set speakers) { This.speakers = speakers; } public Set getSpeakers() { return this.speakers; } ... }
The key element defines the foreign key from the collection table to the parent table. In this case, the speakers table has an event_id column referring to the id column in the events table. The one-to-many element defines the association to the Speaker class. We ve only touched on persisting collections with Hibernate. In addition to Sets, Hibernate also supports persistent Maps and Lists, as well as arrays of objects and primitive values. Persistent collections are covered in detail in chapter 5.
ORGANIZING YOUR MAPPING FILES
Let s take a quick break from discussing Hibernate s persistence features and discuss a matter of practice: the location of mapping files. After you create mapping files for each persistent class, where should they be stored so the application can access them Ideally, mapping files should be stored in the same JAR file as the classes they describe. Suppose the class file for the Event object is stored in the com/manning/hq directory and therefore in the com.manning.hq package. The Event.hbm.xml file should also be stored in the com/manning/hq directory inside the JAR archive.
3.2.6 Cascades
If you ve worked with relational databases, you ve no doubt encountered cascades. Cascades propagate certain operations on a table (such
Creating mapping definitions
as a delete) to associated tables. (Remember that tables are associated through the use of foreign keys.) Suppose that when you delete an Event, you also want to delete each of the Speaker instances associated with the Event. Instead of having the application code perform the deletion, Hibernate can manage it for you. Hibernate supports ten different types of cascades that can be applied to many-to-one associations as well as collections. The default cascade is none. Each cascade strategy specifies the operation or operations that should be propagated to child entities. The cascade types that you are most likely to use are the following:
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