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build_files.html
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<!DOCTYPE html>
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<h1 id="build-files">BUILD files</h1>
<p>A large, well-organized codebase divides into many small components. These components,
and the code dependencies between them, form a directed graph.</p>
<p>In Pants parlance, these components are called <em>targets</em>. The information about your
targets and their dependencies lives in files named <code>BUILD</code>, scattered throughout your
source tree. A <code>BUILD</code> file in a given directory describes targets that own the
source files in or under that directory.</p>
<ul>
<li>See <a href="first_tutorial.html">tutorial</a> for an easy introduction to <code>BUILD</code> files.</li>
<li>See the <a href="build_dictionary.html">BUILD Dictionary</a> for an exhaustive list of all <code>BUILD</code> file syntax.</li>
</ul>
<h2 id="target-granularity">Target Granularity</h2>
<p>A target can encapsulate any amount of code, from a single source file to
(as a silly hypothetical) the entire codebase. In practice, you'll want to
pick a granularity that reflects a "sensible" level at which to express
dependencies: Too coarse, and you lose the benefits of fine-grained invalidation.
Too fine, and you drown in BUILD boilerplate.</p>
<p>Many programming languages (E.g., Java, Python, Go) have a concept of a <em>package</em>, usually
corresponding to a single filesystem directory. It turns out that this is often the appropriate level
of granularity for targets. The idiom of having one target per directory, representing
a single package, is sometimes referred to as the <em>1:1:1 rule</em>. It's by no means required,
but has proven in practice to be a good rule of thumb. And of course there are always exceptions.</p>
<p>Note that Pants forbids circular dependencies between targets. That is, the dependency graph must
be a DAG. In fact, this is good codebase hygiene in general. So if you have any tightly-bound
cross dependencies between certain packages, they will all have to be part of a single target until
you untangle the dependency "hairball".</p>
<h2 id="target-definitions">Target Definitions</h2>
<p>A target definition in a <code>BUILD</code> file looks something like</p>
<div class="codehilite"><pre><span class="n">java_library</span><span class="p">(</span>
<span class="n">name</span><span class="o">=</span><span class="s">'util'</span><span class="p">,</span>
<span class="n">dependencies</span> <span class="o">=</span> <span class="p">[</span>
<span class="s">'3rdparty:commons-math'</span><span class="p">,</span>
<span class="s">'3rdparty:thrift'</span><span class="p">,</span>
<span class="s">'src/java/org/pantsbuild/auth'</span><span class="p">,</span>
<span class="s">':base'</span>
<span class="p">],</span>
<span class="n">sources</span><span class="o">=</span><span class="n">globs</span><span class="p">(</span><span class="s">'*.java'</span><span class="p">,</span> <span class="n">exclude</span><span class="o">=</span><span class="p">[[</span><span class="s">'Base.java'</span><span class="p">]]),</span>
<span class="p">)</span>
</pre></div>
<p><strong>java_library</strong><br>
Each target will have a different <em>target type</em>, which is <code>java_library</code> in this case.
This tells Pants tasks what can be done with the target.</br></p>
<p><strong>name</strong><br>
The target's name, along with the path to its BUILD file, forms its <em>address</em>.
The address has two important roles:</br></p>
<ul>
<li>It's used on the command line to specify which targets to operate on.</li>
<li>It's used in other <code>BUILD</code> files to reference the target as a dependency.</li>
</ul>
<p><strong>dependencies</strong><br>
List of targets that this target depends upon. If this target's code imports
or otherwise depends on code in other targets, list those targets here.</br></p>
<ul>
<li>To reference a target <code>target</code> defined in <code>path/to/BUILD</code> use <code>path/to:target</code>.</li>
<li>If the target has the same name as the BUILD file's directory, you can omit the repetition:
<br><code>path/to/target</code> instead of <code>path/to/target:target</code>.</br></li>
<li>If the target is defined in the same BUILD file, you can omit the path:
<br><code>:target</code> instead of <code>path/to:target</code>.</br></li>
<li><a href="target_addresses.html">More details</a> on how to address targets in a list of dependencies.</li>
</ul>
<p><strong>sources</strong><br>
The source files in this target. These are usually specified in one of two ways:</br></p>
<ul>
<li>Enumerating the files: <code>sources=['FileUtil.java', 'NetUtil.java']</code>.</li>
<li>Globbing over the files in the BUILD file's directory: <code>sources=globs('*.java')</code>.
<br>This means you don't have to modify your BUILD file when you add a new source file.</br></li>
</ul>
<p>You can exclude files from the results of a glob. For example, to glob over unit tests
but not integration tests you could use something like this:
<br><code>sources=globs('*.py', exclude=[globs('*_integration.py')])</code>.
<br>The value of <code>exclude=</code> is a list of things that evaluate to lists of source files,
i.e., globs or literal lists. This is why there are double-brackets around <code>Base.java</code> in
the example target above.</br></br></p>
<p>You can also recursively glob over files in all subdirectories of the BUILD file's directory: <code>sources=rglobs('*.java')</code>.
However this is discouraged as it tends to lead to coarse-grained dependencies, and Pants's
advantages come into play when you have many fine-grained dependencies.</p>
<h2 id="build-files_1"><code>BUILD.*</code> files</h2>
<p>BUILD files are usually just named <code>BUILD</code>, but they can also be named <code>BUILD.ext</code>, with any
extension. Pants considers all files matching <code>BUILD(.*)</code> in a single directory to be a single
logical BUILD file. In particular, they share a single namespace, so target names must be
distinct across all such files.</p>
<p>This has various uses, such as the ability to separate internal-only BUILD definitions from those
that should be pushed to an open-source mirror of an internal repo: You can put the former
in <code>BUILD.internal</code> files and the latter in <code>BUILD.oss</code> files.</p>
<h2 id="debugging-build-files">Debugging BUILD Files</h2>
<p>If you're curious to know how Pants interprets your <code>BUILD</code> files, these
techniques can be especially helpful:</p>
<p><em>What targets does a BUILD file define?</em> Use the <code>list</code> goal:</p>
<div class="codehilite"><pre><span class="nv">$ </span>./pants list examples/src/java/org/pantsbuild/example/hello/greet
examples/src/java/org/pantsbuild/example/hello/greet:greet
</pre></div>
<p><em>Are any BUILD files broken?</em>
List <strong>every</strong> target to see if there are any errors:
Use the recursive wildcard <code>::</code> with the list goal:</p>
<div class="codehilite"><pre><span class="nv">$ </span>./pants list ::
...lots of output...
File <span class="s2">"pants/commands/command.py"</span>, line 79, in __init__
File <span class="s2">"pants/commands/goal_runner.py"</span>, line 144, in setup_parser
File <span class="s2">"pants/base/build_graph.py"</span>, line 351, in inject_address_closure
TransitiveLookupError: great was not found in BUILD file examples/src/java/org/pantsbuild/example/h
ello/greet/BUILD. Perhaps you meant:
:greet
referenced from examples/src/scala/org/pantsbuild/example/hello/welcome:welcome
</pre></div>
<p><em>Do I pull in the transitive dependencies I expect?</em> Use <code>depmap</code>:</p>
<div class="codehilite"><pre><span class="nv">$ </span>./pants depmap examples/tests/java/org/pantsbuild/example/hello/greet
internal-examples.tests.java.org.pantsbuild.example.hello.greet.greet
internal-3rdparty.junit
internal-3rdparty.hamcrest-core
org.hamcrest-hamcrest-core-1.3
junit-junit-dep-4.11
internal-examples.src.java.org.pantsbuild.example.hello.greet.greet
internal-examples.src.resources.org.pantsbuild.example.hello.hello
junit-junit-dep-4.11
org.hamcrest-hamcrest-core-1.3
</pre></div>
<p><em>What source files do I depend on?</em> Use <code>filedeps</code>:</p>
<div class="codehilite"><pre><span class="nv">$ </span>./pants filedeps examples/src/java/org/pantsbuild/example/hello/main
~archie/workspace/pants/examples/src/java/org/pantsbuild/example/hello/greet/BUILD
~archie/workspace/pants/examples/src/java/org/pantsbuild/example/hello/main/config/greetee.txt
~archie/workspace/pants/examples/src/resources/org/pantsbuild/example/hello/BUILD
~archie/workspace/pants/examples/src/java/org/pantsbuild/example/hello/main/HelloMain.java
~archie/workspace/pants/examples/src/resources/org/pantsbuild/example/hello/world.txt
~archie/workspace/pants/examples/src/java/org/pantsbuild/example/hello/main/BUILD
~archie/workspace/pants/examples/src/java/org/pantsbuild/example/hello/greet/Greeting.java
</pre></div>
<p>Use the <code>-h</code> flag to get help on these commands and their various options.</p>
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