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Naturally, you use a loop here and iterate through the results, but the problem this code exposes is the same. You see one SQL SELECT to retrieve all the Item objects, and an additional SELECT for every seller of an Item as soon as you process it. All associated User objects are proxies. This is one of the worst-case scenarios we ll describe later in more detail: the n+1 selects problem. This is what the SQL looks like:

The key to writing a task in Ant as efficiently as possible is to understand the infrastructure that Ant provides to tasks. Datatypes can be used easily in tasks with very little code or effort, and nested datatypes implicitly support references without your task being aware of it. We have not provided an in-depth API reference to Ant. Ant ships with a complete Javadoc set and Ant s own source code is readily available. We encourage you to consult these indispensable references when you begin writing your own tasks. Simply coding by example, cutting and pasting relevant pieces from Ant s own tasks that are similar to what you need to accomplish, is the quickest method to writing sophisticated tasks. With the information in this chapter, which you cannot easily glean from reading source code, you now have an understanding of how Ant provides data to tasks. Java introspection and method naming conventions are the secret. A lot of complex details are buried under the hood of these mechanisms, allowing you to write tasks at a much higher, domain-specific level.

select items... select u.* from USERS u where u.USER_ID = select u.* from USERS u where u.USER_ID =

20.1 20.2 20.3 20.4 20.5 20.6 Scripting within Ant 499 Listeners and loggers 502 Developing a custom mapper 514 Creating custom selectors 515 Implementing a custom filter 517 Summary 520

Hibernate offers some algorithms that can prefetch User objects. The first optimization we now discuss is called batch fetching, and it works as follows: If one proxy of a User must be initialized, go ahead and initialize several in the same SELECT. In other words, if you already know that there are three Item instances in the persistence context, and that they all have a proxy applied to their seller association, you may as well initialize all the proxies instead of just one. Batch fetching is often called a blind-guess optimization, because you don t know how many uninitialized User proxies may be in a particular persistence context. In the previous example, this number depends on the number of Item objects returned. You make a guess and apply a batch-size fetching strategy to your User class mapping:

Ant s extensibility does not end with custom Java tasks. Ant allows for extensibility and customization in several more ways: Scripting ad-hoc tasks using many popular scripting languages with the <script> task Monitoring or logging of the build process with custom build listeners and loggers Custom <mapper> implementations Powerful fileset filtering with specialized selectors Filters to transform text streams Regardless of whether or not you use these features, it is useful to know that these exist in case you encounter them in a build you inherit or come across a situation that could benefit from them.

You re telling Hibernate to prefetch up to 10 uninitialized proxies in a single SQL SELECT, if one proxy must be initialized. The resulting SQL for the earlier query and procedure may now look as follows:

The <script> task supports multiple script language commands to invoke tasks and manipulate other Ant objects. The script can be written in JavaScript (Mozilla s Rhino implementation, to be precise), or a number of other languages. It is therefore a simple way to extend Ant from inside a build file, without having to distribute extra libraries or even add another source file to the project. The basis of the <script> task is the Bean Scripting Framework from IBM: their bsf.jar needs to be downloaded and dropped into ANT_HOME/lib. The URL for this file is listed in the library dependencies section of the Ant documentation. The framework is a foundational package to bridge arbitrary scripting languages to Java applications; you also need to provide the implementation of the scripting language you wish to use. The Mozilla Rhino version of JavaScript is a popular choice, but Python is another possibility, as are others. Again, the URLs to these libraries are listed in the Ant documentation. The example problem chosen to demonstrate the value of the script task is that of assigning a random number to a property. This number can then be used as a parameter to another task, as shown in listing 20.1.

select items... select u.* from USERS u where u.USER_ID in ( , , )