## Depend

A task to manage Java class file dependencies.

### Description

The depend task works by determining which classes are out of date with respect to their source and then removing the class files of any other classes which depend on the out-of-date classes.

To determine the class dependencies, the depend task analyses the class files of all class files passed to it. Depend does not parse your source code in any way but relies upon the class references encoded into the class files by the compiler. This is generally faster than parsing the Java source.

To learn more about how this information is obtained from the class files, please refer to the Java Virtual Machine Specification

Since a class' dependencies only change when the class itself changes, the depend task is able to cache dependency information. Only those class files which have changed will have their dependency information re-analysed. Note that if you change a class' dependencies by changing the source, it will be recompiled anyway. You can examine the dependency files created to understand the dependencies of your classes. Please do not rely, however, on the format of the information, as it may change in a later release.

Once depend discovers all of the class dependencies, it "inverts" this relation to determine, for each class, which other classes are dependent upon it. This "affects" list is used to discover which classes are invalidated by the out of date class. The class files of the invalidated classes are removed, triggering the compilation of the affected classes.

The depend task supports an attribute, "closure" which controls whether depend will only consider direct class-class relationships or whether it will also consider transitive, indirect relationships. For example, say there are three classes, A, which depends on B, which in-turn depend on C. Now say that class C is out of date. Without closure, only class B would be removed by depend. With closure set, class A would also be removed. Normally direct relationships are sufficient - it is unusual for a class to depend on another without having a direct relationship. With closure set, you will notice that depend typically removes far more class files.

### Performance

The performance of the depend task is dependent on a number of factors such as class relationship complexity and how many class files are out of date. The following tests have been run when build Ant itself, on the author's machine using JDK1.3

Building Ant from clean using Javac | 11 seconds
Building Ant from clean using Jikes | 5 seconds
Running <depend> when up-to-date and no cache | 4 seconds (Depend takes 2 seconds)
Running <depend> when up-to-date and with a cache | 2 seconds (Depend takes 1 seconds)

This involves compiling 177 files. The above figures are indicative only. The decision about whether it is cheaper to just recompile all classes or to use the depend task will depend on the size of your project and how interrelated your classes are.

### Limitations

There are some dependencies which depend will not detect.

- If the Java compiler optimizes away a class relationship, there can be a source dependency without a class dependency.
- Non public classes cause two problems. Firstly depend cannot relate the class file to a source file. In the future this may be addressed using the source file attribute in the classfile. Secondly, neither depend nor the compiler tasks can detect when a non public class is missing. Inner classes are handled by the depend task.

### Parameters

Attribute | Description | Required
srcDir | This is the directory where the source exists. depend will examine this to determine which classes. If you use multiple source directories you can pass this a path of source directories. | Yes
destDir | This is the root directory of the class files which will be analysed. If this is not present, the srcdir is used. | No
cache | This is a directory in which depend can store and retrieve dependency information. If this is not present, depend will not use a cache | No
closure | This attribute controls whether depend only removes classes which directly depend on out of date classes. If this is set to true, depend will traverse the class dependency graph deleting all affected classes. Defaults to false | No

### Examples

>         <depend srcdir="${java.dir}"
>         destdir="${build.classes}"
>         cache="depcache"
>         closure="yes"/>

In this example classes in the ${build.classes} directory will be removed if they depend on out-of-date classes. Classes are considered out of date with respect to the source in the ${java.dir} directory using the same mechanism as the javac task. In this instance the depend task caches its dependency information in the depcache directory.

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