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NEWER EDITION AVAILABLE · Java Persistence with Hibernate, Second Edition is now available. An eBook of the previous edition is included at no additional. MANNING. Greenwich "Hello World" with Hibernate Understanding the a persastmt object. Using transitive persistence in Hibernate Hibernate. Almost all applications require persistent data. Persistence is one of the fundamental concepts in application development. If an information system.

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He has been using Hibernate in numerous production applications for almost two years. He has more than nine years of commercial development experience, and he has developed software products and solutions for multiple Fortune clients as well as media, lobbying, and government organizations.

Janissaries were soldiers of the Ottoman Turkish Army, loyal to the sultan, rather than to tribal leaders. These New Soldiers, which is what the name means in Turkish, were the elite troops of the Ottoman Empire, renowned for their bravery and skills.

The title page is missing from the collection and we have been unable to track it down to date. The book s table of contents identifies the figures in both English and French, and each illustration bears the names of two artists who worked on it, both of whom would no doubt be surprised to find their art gracing the front cover of a computer programming book The collection was downloadd by a Manning editor at an antiquarian flea market in the Garage on West 26th Street in Manhattan.

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Six weeks into a cumbersome project, updated requirements are received from the client that result in massive changes to your application code. Weeks of work have to be scrapped or changed to comply with the new requirements. Updating the web pages or GUI is relatively simple, but hundreds or thousands of lines of database code, including your beautiful, handcrafted SQL, have to be updated and tested.

There needs to be a better way to build database-backed applications. Hibernate provides the bridge between the database and the application by storing application objects in the database for the developer, rather than requiring the developer to write and maintain mountains of code to store and retrieve objects.

You may wonder why Manning decided to publish a second book on Hibernate. After all, Hibernate in Action is the authoritative source, written by the project founders and widely regarded as the best reference on the topic. Manning feels there are two distinct needs. One calls for a focused and comprehensive book on the subject of Hibernate.

This is the book you are likely to turn to if you want to get up and running quickly. Since Hibernate is a persistence service, it s rarely the only framework or tool used in an application.

After covering the basics of Hibernate in the first few chapters, we ll move on to discuss development tools like Ant and XDoclet. The majority of applications using Hibernate will be web applications, so we ll look at how Hibernate integrates with three popular web frameworks: Struts, Webwork, and Tapestry.

We ll also look at some support frameworks and utilities, like Spring and JUnit. We start by introducing the concept of object persistence and some of the difficult problems encountered when storing objects in a relational database.

After reviewing the popular methods used to persist objects, we discuss how Hibernate solves most, if not all, of these issues. Chapter goals The goals for this chapter are as follows: Explain the concept of object persistence. Introduce object persistence with Hibernate. When data is persistent, you can obtain the data at a later point in time and it will be the same as when you left it, assuming an outside process didn t change it. There are a few kinds of persistence. When you re editing a source file, that file is persisted to disk for later retrieval and use.

Files stored on disk are probably the most common form of persistence. When we refer to persistence in this book, we re referring to storing application data in a relational database.

Applications, such as an online shopping cart, typically persist data in a relational database. Relational databases are a popular choice for storing data for a number of reasons. They re relatively easy to create and access, using the SQL.

Hibernate Quickly.pdf

Vendors also offer relational databases with a variety of features, allowing you to select the ideal database for your application s needs. Because relational databases are so common, finding developers with relevant experience is less difficult than for niche technologies. The model used by relational databases, called the relational model, represents data in two-dimensional tables.

This logical view of the data is how database users see the contained data. Tables can relate to each other through the use of primary and foreign keys. Primary keys uniquely identify a given row in a table, whereas foreign keys refer to a primary key stored in another table. Relational databases are designed to manage the data they contain, and they re very good at it. However, when you re working with object-oriented applications, you may encounter a problem when you try to persist objects to a relational model.

Java Persistence with Hibernate

As just stated, relational databases manage data. Object-oriented applications are designed to model a business problem. With two radically different purposes in mind, getting the models to work together can be challenging.

The impedance mismatch is caused by the differences between object and relational schemas. Most developers who have used direct JDBC to store objects are aware the mismatch exists, even if they don t know the name for it.

We ll look at a few areas of the impedance mismatch next Identity One of the more significant areas of the impedance mismatch is in regard to identity. If two objects are identical, they re the same object. If the objects are equal, they contain the same values. These different notions of identity don t exist in relational models. Rows in a relational database are only identified by the values they contain. How can you identify objects with their relational counterparts?

A common way to overcome this problem in relational models is to introduce a unique identifier column, typically called a sequence or identifier. The relational identifier is also represented in the object model and becomes a surrogate for identity checking. Figure 1. Relational databases don t support the notion of inheritance, so persisting a rich object hierarchy to a relational schema can be complex. Since inheritance is difficult to translate to a relational model, why can t you just design your object model without hierarchies?

Object-oriented languages were developed to model real-world problems. Inheritance is vital because it allows you to create a precise model of the problem while allowing shared properties and behavior to cascade down the hierarchy. You shouldn t be forced into sacrificing this feature because the relational model doesn t support it Associations The last portion of the impedance mismatch we ll look at is the differences in associations between object and relational models.

Associations are probably one of the easiest portions of the mismatch to overcome since both models support this notion. The relational model understands only one type of association: a foreign key reference to a Event id:long name:string startdate:date duration:int location:location Location id:long name:string NetworkingEvent ConferenceEvent Figure 1. One-to-one and one-to-many associations are direct relationships between two tables.

Compare that to the associations available in an object model: one-to-one, one-to-many and many-to-many. Converting an object-based one-to-one association to a relational schema is simple: Reference the primary key of the associated objects. If you have a one-to-many association, you can repeat this process for each row on the many side. The only problem is that the database doesn t understand the one-to-many association it only knows that a foreign key refers to a primary key.

Let s look at a diagram of one-toone and one-to-many associations in a relational schema, shown in figure 1. Mapping a many-to-many association is typically done with a join table.

The join table contains two foreign key columns referencing two tables, allowing multiple entries for each side of the association. Our goal was simply Many-to-many associations use a join table. ORM bridges the gap between object and relational schemas, allowing your application to persist objects directly without requiring you to convert objects to and from a relational format. There are many types of ORM solutions, offering varying levels of mapping support.

Some ORM frameworks require that persistent objects inherit from a base class or perform post-processing of bytecode. Hibernate, on the other hand, requires a small amount of metadata for each persistent object. Hibernate is a noninvasive ORM service.

It doesn t require bytecode processing or a base persistent class. Hibernate operates independently of application architecture, allowing it to be used in various applications. After all, JDBC provides a simple and complete interface to relational databases. Using JDBC directly is ideal for basic applications, since you can quickly persist objects with well-understood code. However, JDBC can get out of hand with larger applications or when requirements change.

If an object changes, the code that persists the object must be changed and tested, as well as all the SQL used to manage the object s state.

In section 1.

There are instances where ORM, and therefore Hibernate, isn t the best solution. For instance, if tables refer to each other through columns other than primary keys, your chosen ORM solution may have trouble adapting to the legacy schema. Let s look at a good example of a bad design, shown in figure 1. Admittedly, this schema is a contrived example; but before you dismiss it, realize that schemas even more poorly designed than this exist in enterprise applications.

Just what s wrong with this schema? None of the tables have a surrogate primary key. By a surrogate key, we mean that the table s primary key has no business significance. All the columns in the tables are relevant to the business problem.

You can certainly map these tables to objects with an ORM solution, but that may not be the best way to handle the domain model. You may spend more time working around how your ORM framework manages the data than is desirable. Alternative solutions, such as ibatis, may be a better candidate for persisting legacy databases.

Hibernate 3, discussed in chapter 11, adds many new features to support legacy database schemas. Before diving into what Hibernate offers, let s take a brief look at why using JDBC is so painful for large applications.

If you re still clinging to direct JDBC for application persistence, the next section is for you. For instance, if you want to persist a new instance of the Event class to the database, you must first convert the Event object to a SQL statement that can be executed on the underlying database.

Similarly, when rows are returned from the database, you must convert each result row into an instance of Event. Let s look at some of the difficulties presented when converting objects and graphs of objects between the relational and object models.

When working with objects, you re generally using a number of connected objects. This is called an object graph. An object graph represents an internally consistent view of application data. Internally consistent means that a change made to one object is reflected throughout the graph. The objects within a graph are typically connected using one-to-one or one-to-many associations. Using direct JDBC for persistence presents distinct problems for each of the persistence operations: creation, updating, retrieval, and deletion.

We examine those problems in detail in a moment, using an example application that will reappear throughout this book Example application To address the drawbacks of traditional application persistence with JDBC, we must first introduce an example that we ll use as the basis of comparison. The application that we use throughout the book is an event-management application used to manage a conference with speakers, attendees, and various locations, among other things.

To demonstrate the problems with JDBC, we ll discuss persisting one of the central objects in the domain model, Event. We use the term domain model frequently. A domain model is the collection of related objects describing the problem domain for an application. Each object in the domain model represents some significant component in the application, such as an Event or Speaker. Diagrams of the Event object and the relational table used to store the Event are shown in figure 1.

The parallel between the Event object and the relational table is clear. Each property in the Event class is reflected as a column in the events table.

The id column in the events table is the primary key for the table. We ve intentionally kept the Event object simple for the opening discussion. With the object and relational table defined, we can move forward with examining the drawbacks of application persistence with direct JDBC. Event id:long name:string startdate:java.

Problems arise when we want to retrieve a complete object graph from a relational schema. The Event class has three associations: a one-to-one association to the Location object and two one-to-many associations to the Speaker and Attendee objects.

The speakers table is omitted since it s identical to the attendees table. Let s look at the one-to-one association of the events and locations tables. The tables can be joined to retrieve the data from the locations table for a given row in the events table with the following SQL: select e. Object associations are directional, from one object to another object.

To navigate from both ends of the association, you must define the association in both objects.

Listing 1. When translating this association to the relational database, you ll realize that whereas the database has the notion of the association through the use of foreign keys, it lacks the notion of association multiplicity. The database doesn t know that there are multiple rows in the speakers table linked to the events table. I'm speaking approximately, of course - little popups would share the conversation of the parent, larger child windows might be a nested conversation, etc.

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Concurrency within a single conversation would be handled by serializing event processing before it reaches the business layer - ie. Events in different conversations do not need to be serialized.

But access to shared "global" state needs explicit synchronization never, ever use "synchronized" for this, it has no deadlock detection - see Seam's Synchronized annotation for the Right Way to do it. So, given that model, let's see where it breaks: 0 It breaks if I try to modify global state in the conversation: you should never, ever try to associate any globally visible object with a conversation-scope persistence context!

If you have to change a global object, modify a copy, and replace the globally-visible object after the update was successful. IMO, as long as you get your conversation model correct, and you follow 0 , this should be very, very rare to need to do this. Problems relating to associations. The problem of object graph navigation. Persistence layers and alternatives 1. Layered architecture. Object-oriented database systems.

What is ORM? Introducing and integrating Hibernate 2. Understanding the architecture 2. The core interfaces. Basic configuration 2. Creating a SessionFactory. Configuration in non-managed environments. Configuration in managed environments.

Advanced configuration settings 2. Using XML-based configuration. Mapping persistent classes 3. The CaveatEmptor application 3. Analyzing the business domain. The CaveatEmptor domain model. Implementing the domain model 3. Addressing leakage of concerns. Transparent and automated persistence. Implementing POJO associations. Adding logic to accessor methods. Defining the mapping metadata 3. Metadata in XML. Basic property and class mappings.

Attribute-oriented programming. Manipulating metadata at runtime. Understanding object identity 3. Identity versus equality. Database identity with Hibernate.

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Fine-grained object models 3. Entity and value types. Mapping class inheritance 3. Table per concrete class. Introducing associations 3. Managed associations? The simplest possible association. Making the association bidirectional. Working with persistent objects 4. The persistence lifecycle 4. Transient objects. Implementing equals and hashCode.

The persistence manager 4. Making an object persistent. Updating the persistent state of a detached instance. Retrieving a persistent object. Making a persistent object transient.

Making a detached object transient. Using transitive persistence in Hibernate 4. Persistence by reachability.

Hibernate in Action.pdf

Cascading persistence with Hibernate. Distinguishing between transient and detached instances. Retrieving objects 4.If you have to change a global object, modify a copy, and replace the globally-visible object after the update was successful. Selecting a fetching strategy in mappings.

Spring integrates extremely well with Hibernate; it streamlines the DAO pattern along with some of the productive boosting features. Hibernate, on the other hand, requires a small amount of metadata for each persistent object.

Let s look at the one-to-one association of the events and locations tables. The driver is the piece of software that allows Java code to access and send SQL commands to the database server. A direct JDBC persistence layer would need to provide the ability to return specific portions of the object graph, including the ability to populate collections of associations when required. This book blends the insights of the Hibernate Search leaddeveloper with the practical techniques required to index and manipulate data,assemble and execute search queries, and create smart filters for better searchresults.

Rows in a relational database are only identified by the values they contain.

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