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| <title>Chapter 2. Why JPA?</title><base href="display"><link rel="stylesheet" type="text/css" href="css/docbook.css"><meta name="generator" content="DocBook XSL Stylesheets V1.79.1"><link rel="home" href="manual.html" title="Apache OpenJPA 3.0 User's Guide"><link rel="up" href="jpa_overview.html" title="Part 2. Java Persistence API"><link rel="prev" href="jpa_overview_intro_transpers.html" title="2. Lightweight Persistence"><link rel="next" href="jpa_overview_arch.html" title="Chapter 3. Java Persistence API Architecture"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter 2. |
| Why JPA? |
| </th></tr><tr><td width="20%" align="left"><a accesskey="p" href="jpa_overview_intro_transpers.html">Prev</a> </td><th width="60%" align="center">Part 2. Java Persistence API</th><td width="20%" align="right"> <a accesskey="n" href="jpa_overview_arch.html">Next</a></td></tr></table><hr></div><div class="chapter" id="jpa_overview_why"><div class="titlepage"><div><div><h2 class="title">Chapter 2. |
| Why JPA? |
| </h2></div></div></div> |
| |
| <a class="indexterm" name="d5e127"></a> |
| <p> |
| Java developers who need to store and retrieve persistent data already have |
| several options available to them: serialization, JDBC, JDO, proprietary |
| object-relational mapping tools, object databases, and EJB 2 entity beans. Why |
| introduce yet another persistence framework? The answer to this question is that |
| with the exception of JDO, each of the aforementioned persistence solutions has |
| severe limitations. JPA attempts to overcome these limitations, as illustrated |
| by the table below. |
| </p> |
| <div class="table" id="d5e131"><p class="title"><b>Table 2.1. |
| Persistence Mechanisms |
| </b></p><div class="table-contents"> |
| |
| <table class="table" summary="
 Persistence Mechanisms
 " border="1"><colgroup><col align="left" class="sup"><col align="left" class="ser"><col align="left" class="jdbc"><col align="left" class="or"><col align="left" class="objdb"><col align="left" class="ejb2"><col align="left" class="jdo"><col align="left" class="jpa"></colgroup><thead><tr><th align="left"> |
| Supports: |
| </th><th align="left"> |
| Serialization |
| </th><th align="left"> |
| JDBC |
| </th><th align="left"> |
| ORM |
| </th><th align="left"> |
| ODB |
| </th><th align="left"> |
| EJB 2 |
| </th><th align="left"> |
| JDO |
| </th><th align="left"> |
| JPA |
| </th></tr></thead><tbody><tr><td align="left"> |
| Java Objects |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Advanced OO Concepts |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Transactional Integrity |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Concurrency |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Large Data Sets |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Existing Schema |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Relational and Non-Relational Stores |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td></tr><tr><td align="left"> |
| Queries |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Strict Standards / Portability |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr><tr><td align="left"> |
| Simplicity |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| No |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td><td align="left"> |
| <span class="bold"><strong> |
| Yes |
| </strong></span> |
| </td></tr></tbody></table> |
| </div></div><br class="table-break"> |
| <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"> |
| <p> |
| <a class="indexterm" name="d5e298"></a> |
| <a class="indexterm" name="d5e300"></a> |
| <span class="emphasis"><em>Serialization</em></span> is Java's built-in mechanism for transforming |
| an object graph into a series of bytes, which can then be sent over the network |
| or stored in a file. Serialization is very easy to use, but it is also very |
| limited. It must store and retrieve the entire object graph at once, making it |
| unsuitable for dealing with large amounts of data. It cannot undo changes that |
| are made to objects if an error occurs while updating information, making it |
| unsuitable for applications that require strict data integrity. Multiple threads |
| or programs cannot read and write the same serialized data concurrently without |
| conflicting with each other. It provides no query capabilities. All these |
| factors make serialization useless for all but the most trivial persistence |
| needs. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <a class="indexterm" name="d5e306"></a> |
| <a class="indexterm" name="d5e309"></a> |
| <a class="indexterm" name="d5e311"></a> |
| Many developers use the <span class="emphasis"><em>Java Database Connectivity</em></span> (JDBC) |
| APIs to manipulate persistent data in relational databases. JDBC overcomes most |
| of the shortcomings of serialization: it can handle large amounts of data, has |
| mechanisms to ensure data integrity, supports concurrent access to information, |
| and has a sophisticated query language in SQL. Unfortunately, JDBC does not |
| duplicate serialization's ease of use. The relational paradigm used by JDBC was |
| not designed for storing objects, and therefore forces you to either abandon |
| object-oriented programming for the portions of your code that deal with |
| persistent data, or to find a way of mapping object-oriented concepts like |
| inheritance to relational databases yourself. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <a class="indexterm" name="d5e317"></a> |
| <a class="indexterm" name="d5e320"></a> |
| <a class="indexterm" name="d5e322"></a> |
| There are many proprietary software products that can perform the mapping |
| between objects and relational database tables for you. These <span class="emphasis"><em> |
| object-relational mapping</em></span> (ORM) frameworks allow you to focus on the |
| object model and not concern yourself with the mismatch between the |
| object-oriented and relational paradigms. Unfortunately, each of these product |
| has its own set of APIs. Your code becomes tied to the proprietary interfaces of |
| a single vendor. If the vendor raises prices, fails to fix show-stopping bugs, |
| or falls behind in features, you cannot switch to another product without |
| rewriting all of your persistence code. This is referred to as vendor lock-in. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <a class="indexterm" name="d5e328"></a> |
| <a class="indexterm" name="d5e331"></a> |
| <a class="indexterm" name="d5e333"></a> |
| <a class="indexterm" name="d5e336"></a> |
| Rather than map objects to relational databases, some software companies have |
| developed a form of database designed specifically to store objects. These |
| <span class="emphasis"><em>object databases</em></span> (ODBs) are often much easier to use than |
| object-relational mapping software. The Object Database Management Group (ODMG) |
| was formed to create a standard API for accessing object databases; few object |
| database vendors, however, comply with the ODMG's recommendations. Thus, vendor |
| lock-in plagues object databases as well. Many companies are also hesitant to |
| switch from tried-and-true relational systems to the relatively unknown object |
| database technology. Fewer data-analysis tools are available for object database |
| systems, and there are vast quantities of data already stored in older |
| relational databases. For all of these reasons and more, object databases have |
| not caught on as well as their creators hoped. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <a class="indexterm" name="d5e341"></a> |
| <a class="indexterm" name="d5e344"></a> |
| <a class="indexterm" name="d5e346"></a> |
| The Enterprise Edition of the Java platform introduced entity Enterprise Java |
| Beans (EJBs). EJB 2.x entities are components that represent persistent |
| information in a datastore. Like object-relational mapping solutions, EJB 2.x |
| entities provide an object-oriented view of persistent data. Unlike |
| object-relational software, however, EJB 2.x entities are not limited to |
| relational databases; the persistent information they represent may come from an |
| Enterprise Information System (EIS) or other storage device. Also, EJB 2.x |
| entities use a strict standard, making them portable across vendors. |
| Unfortunately, the EJB 2.x standard is somewhat limited in the object-oriented |
| concepts it can represent. Advanced features like inheritance, polymorphism, and |
| complex relations are absent. Additionally, EBJ 2.x entities are difficult to |
| code, and they require heavyweight and often expensive application servers to |
| run. |
| </p> |
| </li><li class="listitem"> |
| <p> |
| <a class="indexterm" name="d5e351"></a> |
| <a class="indexterm" name="d5e353"></a> |
| The JDO specification uses an API that is strikingly similar to JPA. JDO, |
| however, supports non-relational databases, a feature that some argue dilutes |
| the specification. |
| </p> |
| </li></ul></div> |
| <p> |
| <a class="indexterm" name="d5e357"></a> |
| JPA combines the best features from each of the persistence mechanisms listed |
| above. Creating entities under JPA is as simple as creating serializable |
| classes. JPA supports the large data sets, data consistency, concurrent use, and |
| query capabilities of JDBC. Like object-relational software and object |
| databases, JPA allows the use of advanced object-oriented concepts such as |
| inheritance. JPA avoids vendor lock-in by relying on a strict specification like |
| JDO and EJB 2.x entities. JPA focuses on relational databases. And like JDO, JPA |
| is extremely easy to use. |
| </p> |
| <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Note</h3> |
| <p> |
| OpenJPA typically stores data in relational databases, but can be customized for |
| use with non-relational datastores as well. |
| </p> |
| </div> |
| <p> |
| JPA is not ideal for every application. For many applications, though, it |
| provides an exciting alternative to other persistence mechanisms. |
| </p> |
| </div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="jpa_overview_intro_transpers.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="jpa_overview.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="jpa_overview_arch.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">2. |
| Lightweight Persistence |
| </td><td width="20%" align="center"><a accesskey="h" href="manual.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 3. |
| Java Persistence API Architecture |
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