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Exam Tip By default, Windows XP manages the paging file size, but you can designate a file size for special circumstances. The recommended paging file size is equal to 1.5 times the total amount of RAM.

To configure the paging file, in the Performance Options dialog box, click the Change button in the Virtual Memory section. The Virtual Memory dialog box (see Figure 5-10) identifies the drives in which the paging files reside and allows you to modify the paging file size for the selected drive.

Of course, there are other kinds of persistence layers. XML persistence is a variation on the serialization theme; this approach addresses some of the limitations of byte-stream serialization by allowing easy access to the data through a standardized tool interface. However, managing data in XML would expose you to an object/hierarchical mismatch. Furthermore, there is no additional benefit from the XML itself, because it s just another text file format and has no inherent capabilities for data management. You can use stored procedures (even writing them in Java, sometimes) and move the problem into the database tier. So-called object-relational databases have been marketed as a solution, but they offer only a more sophisticated datatype system providing only half the solution to our problems (and further muddling terminology). We re sure there are plenty of other examples, but none of them are likely to become popular in the immediate future. Political and economic constraints (long-term investments in SQL databases), data independence, and the requirement for access to valuable legacy data call for a different approach. ORM may be the most practical solution to our problems.

Figure 5-10

5-26

Now that we ve looked at the alternative techniques for object persistence, it s time to introduce the solution we feel is the best, and the one we use with Hibernate: ORM. Despite its long history (the first research papers were published in the late 1980s), the terms for ORM used by developers vary. Some call it object relational mapping, others prefer the simple object mapping; we exclusively use

5

Only users with administrative rights can use the Performance Options dialog box to increase the paging file size.

the term object/relational mapping and its acronym, ORM. The slash stresses the mismatch problem that occurs when the two worlds collide. In this section, we first look at what ORM is. Then we enumerate the problems that a good ORM solution needs to solve. Finally, we discuss the general benefits that ORM provides and why we recommend this solution.

Paging files never decrease below the value found in the Initial Size text box that was set during installation. Unused space in the paging file remains available to the internal Windows XP Professional Virtual Memory Manager (VMM). As needed, a paging file grows from its initial size to the maximum configured size, which is listed in the Maximum Size text box. When a paging file reaches the maximum size, but a running program still needs to allocate more virtual memory, Windows XP Professional will refuse that allocation, which can cause an error, or even a crash, in applications. When you restart a computer running Windows XP Professional, the system resizes all paging files to the initial size.

7. 8.

Data Execution Prevention (DEP) is a set of hardware and software technologies that perform additional checks on memory to help prevent malicious code from running on a computer. In Windows XP Professional, DEP can be enforced by compatible hardware and by software.

In a nutshell, object/relational mapping is the automated (and transparent) persistence of objects in a Java application to the tables in a relational database, using metadata that describes the mapping between the objects and the database. ORM, in essence, works by (reversibly) transforming data from one representation to another. This implies certain performance penalties. However, if ORM is implemented as middleware, there are many opportunities for optimization that wouldn t exist for a hand-coded persistence layer. The provision and management of metadata that governs the transformation adds to the overhead at development time, but the cost is less than equivalent costs involved in maintaining a hand-coded solution. (And even object databases require significant amounts of metadata.)