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Checkstyle 5.3
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Last Published: 2010-10-19
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<strong>Writing checks</strong>
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<div class="section"><h2><a name="Overview"></a>Overview</h2>
<p>
OK, so you have finally decided to write your own Check. Welcome
aboard, this is really a fun thing to do. There are actually two
kinds of Checks, so before you can start, you have to find out
which kind of Check you want to implement.
</p>
<p>
The functionality of Checkstyle is implemented in modules that can
be plugged into Checkstyle. Modules can be containers for other
modules, i.e. they form a tree structure. The toplevel modules
that are known directly to the Checkstyle kernel (which is also a
module and forms the root of the tree) implement the <a href="apidocs/com/puppycrawl/tools/checkstyle/api/FileSetCheck.html">FileSetCheck</a>
interface. These are pretty simple to grasp: they take a set of
input files and fire error messages.
</p>
<p>
Checkstyle provides a few FileSetCheck implementations by default
and one of them happens to be the <a href="apidocs/com/puppycrawl/tools/checkstyle/TreeWalker.html">TreeWalker</a>. A
TreeWalker supports submodules that are derived from the <a href="apidocs/com/puppycrawl/tools/checkstyle/api/Check.html">Check</a>
class. The TreeWalker operates by separately transforming each of
the Java input files into an abstract syntax tree and then handing
the result over to each of the Check submodules which in turn have
a look at a certain aspect of the tree.
</p>
</div>
<div class="section"><h2><a name="Writing_Checks"></a>Writing Checks</h2>
<p>
Most of the functionality of Checkstyle is implemented as
Checks. If you know how to write your own Checks, you can extend
Checkstyle according to your needs without having to wait for the
Checkstyle development team. You are about to become a Checkstyle
Expert.
</p>
<p>
Suppose you have a convention that the number of methods in a
class should not exceed a certain limit, say 30. This rule makes
sense, a class should only do one thing and do it well. With a
zillion methods chances are that the class does more than one
thing. The only problem you have is that your convention is not
checked by Checkstyle, so you'll have to write your own Check
and plug it into the Checkstyle framework.
</p>
<p> This chapter is organized as a tour that takes you
through the process step by step and explains both the theoretical
foundations and the <a href="apidocs/com/puppycrawl/tools/checkstyle/api/package-summary.html">Checkstyle
API</a> along the way. </p>
</div>
<div class="section"><h2><a name="Java_Grammar"></a>Java Grammar</h2>
<p>
Every Java Program is structured into files, and each of these
files has a certain structure. For example, if there is a
package statement then it is the first line of the file that is
not comment or whitespace. After the package statement comes a
list of import statements, which is followed by a class or
interface definition, and so on.
</p>
<p>
If you have ever read an introductory level Java book you probably
knew all of the above. And if you have studied computer science,
you probably also know that the rules that specify the Java language
can be formally specified using a grammar (statement is simplified
for didactic purposes).
</p>
<p>
Tools exist which read a grammar definition and produce a parser
for the language that is specified in the grammar. In other
words, the output of the tool is a program that can transform a
stream of characters (a Java file) into a tree representation
that reflects the structure of the file. Checkstyle uses the
parser generator <a href="http://www.antlr.org" class="externalLink">ANTLR</a> but
that is an implementation detail you do not need to worry about
when writing Checks. Several other parser generators exist and
they all work well.
</p>
</div>
<div class="section"><h2><a name="The_Checkstyle_SDK_Gui"></a>The Checkstyle SDK Gui</h2>
<p>
Still with us? Great, you have mastered the basic theory so here
is your reward - a GUI that displays the structure of a Java
source file. To run it type
</p>
<div class="source"><pre>
java -classpath checkstyle-5.3-all.jar \
com.puppycrawl.tools.checkstyle.gui.Main
</pre>
</div>
<p>
on the command line. Click the button at the bottom of the frame
and select a syntactically correct Java source file. The frame
will be populated with a tree that corresponds to the structure
of the Java source code.
</p>
<p><img src="images/gui_screenshot.png" alt="screenshot" /></p>
<p> In the leftmost column you can open and close branches
of the tree, the remaining columns display information about each node
in the tree. The second column displays a token type for each node. As
you navigate from the root of the tree to one of the leafs, you'll
notice that the token type denotes smaller and smaller units of your
source file, i.e. close to the root you might see the token type <a href="apidocs/com/puppycrawl/tools/checkstyle/api/TokenTypes.html#CLASS_DEF">CLASS_DEF</a>
(a node that represents a class definition) while you will see token
types like IDENT (an identifier) near the leaves of the tree. </p>
<p>
We'll get back to the details in the other columns later, they
are important for implementing Checks but not for understanding
the basic concepts. For now it is sufficient to know that the
gui is a tool that lets you look at the structure of a Java
file, i.e. you can see the Java grammar 'in action'.
</p>
</div>
<div class="section"><h2><a name="Understanding_the_visitor_pattern"></a>Understanding the visitor pattern</h2>
<p>
Ready for a bit more theory? The last bit
that is missing before you can start writing Checks is understanding
the Visitor pattern.
</p>
<p>
When working with ASTs, a simple approach to define check operations
on them would be to add a <code>check()</code> method to the Class that defines
the AST nodes. For example, our AST type could have a method
<code>checkNumberOfMethods()</code>. Such an approach would suffer from a few
serious drawbacks. Most importantly, it does not provide an extensible
design, i.e. the Checks have to be known at compile time; there is no
way to write plugins.
</p>
<p> Hence Checkstyle's AST classes do not have any
methods that implement checking functionality. Instead,
Checkstyle's <a href="apidocs/com/puppycrawl/tools/checkstyle/TreeWalker.html">TreeWalker</a>
takes a set of objects that conform to a <a href="apidocs/com/puppycrawl/tools/checkstyle/api/Check.html">Check</a>
interface. OK, you're right - actually it's not an interface
but an abstract class to provide some helper methods. A Check provides
methods that take an AST as an argument and perform the checking process
for that AST, most prominently <a href="apidocs/com/puppycrawl/tools/checkstyle/api/Check.html#visitToken(com.puppycrawl.tools.checkstyle.api.DetailAST)"><code>visitToken()</code></a>. </p>
<p> It is important to understand that the individual
Checks do no drive the AST traversal. Instead, the TreeWalker initiates
a recursive descend from the root of the AST to the leaf nodes and calls
the Check methods. The traversal is done using a <a href="http://mathworld.wolfram.com/Depth-FirstTraversal.html" class="externalLink">depth-first</a>
algorithm. </p>
<p> Before any visitor method is called, the TreeWalker
will call <a href="apidocs/com/puppycrawl/tools/checkstyle/api/Check.html#beginTree(com.puppycrawl.tools.checkstyle.api.DetailAST)"><code>beginTree()</code></a> to give the Check a chance to do
some initialization. Then, when performing the recursive descend from
the root to the leaf nodes, the <code>visitToken()</code>
method is called. Unlike the basic examples in the pattern book, there
is a <code>visitToken()</code> counterpart called <a href="apidocs/com/puppycrawl/tools/checkstyle/api/Check.html#leaveToken(com.puppycrawl.tools.checkstyle.api.DetailAST)"><code>leaveToken()</code></a>. The TreeWalker will call that
method to signal that the subtree below the node has been processed and
the TreeWalker is backtracking from the node. After the root node has
been left, the TreeWalker will call <a href="apidocs/com/puppycrawl/tools/checkstyle/api/Check.html#finishTree(com.puppycrawl.tools.checkstyle.api.DetailAST)"><code>finishTree()</code></a>. </p>
<p>
If you'd like to learn more about the Visitor pattern you should
grab a copy of the Gof
<a href="http://c2.com/cgi/wiki?DesignPatternsBook" class="externalLink">Design
Patterns</a> book.
</p>
</div>
<div class="section"><h2><a name="Visitor_in_action"></a>Visitor in action</h2>
<p>
Let's get back to our example and start writing code - that's why
you came here, right?
When you fire up the Checkstyle GUI and look at a few source
files you'll figure out pretty quickly that you are mainly
interested in the number of tree nodes of type METHOD_DEF. The
number of such tokens should be counted separately for each
CLASS_DEF / INTERFACE_DEF.
</p>
<p>
Hence we need to register the Check for the token types
CLASS_DEF and INTERFACE_DEF. The TreeWalker will only call
visitToken for these token types. Because the requirements of
our tasks are so simple, there is no need to implement the other
fancy methods, like <code>finishTree()</code>, etc., so here is our first
shot at our Check implementation:
</p>
<div class="source"><pre>
package com.mycompany.checks;
import com.puppycrawl.tools.checkstyle.api.*;
public class MethodLimitCheck extends Check
{
private static final int DEFAULT_MAX = 30;
private int max = DEFAULT_MAX;
@Override
public int[] getDefaultTokens()
{
return new int[]{TokenTypes.CLASS_DEF, TokenTypes.INTERFACE_DEF};
}
@Override
public void visitToken(DetailAST ast)
{
// find the OBJBLOCK node below the CLASS_DEF/INTERFACE_DEF
DetailAST objBlock = ast.findFirstToken(TokenTypes.OBJBLOCK);
// count the number of direct children of the OBJBLOCK
// that are METHOD_DEFS
int methodDefs = objBlock.getChildCount(TokenTypes.METHOD_DEF);
// report error if limit is reached
if (methodDefs > this.max) {
log(ast.getLineNo(),
"too many methods, only " + this.max + " are allowed");
}
}
}
</pre>
</div>
</div>
<div class="section"><h2><a name="Navigating_the_AST"></a>Navigating the AST</h2>
<p>
In the example above you already saw that the DetailsAST class
provides utility methods to extract information from the tree,
like <code>getChildCount()</code>. By now you have
probably consulted the api documentation and found that
DetailsAST additionally provides methods for navigating around
in the syntax tree, like finding the next sibling of a node, the
children of a node, the parent of a node, etc.
</p>
<p>
These methods provide great power for developing complex
Checks. Most of the Checks that Checkstyle provides by default
use these methods to analyze the environment of the ASTs that
are visited by the TreeWalker. Don't abuse that feature for
exploring the whole tree, though. Let the TreeWalker drive the
tree traversal and limit the visitor to the neighbours of a
single AST.
</p>
</div>
<div class="section"><h2><a name="Defining_Check_Properties"></a>Defining Check Properties</h2>
<p>
Ok Mr. Checkstyle, that's all very nice but in my company we
have several projects, and each has another number of allowed
methods. I need to control my Check through properties, so where
is the API to do that?
</p>
<p>
Well, the short answer is, there is no API. It's magic. Really!
</p>
<p>
If you need to make something configurable, just add a setter method
to the Check:
</p>
<div class="source"><pre>
public class MethodLimitCheck extends Check
{
// code from above omitted for brevity
public void setMax(int limit)
{
max = limit;
}
}
</pre>
</div>
<p>
With this code added, you can set the property <code>max</code> for the MethodLimitCheck module in the
configuration file. It doesn't get any simpler than that. The secret is
that Checkstyle uses JavaBean introspection to set the JavaBean
properties. That works for all primitive types like boolean,
int, long, etc., plus Strings, plus arrays of these types.
</p>
</div>
<div class="section"><h2><a name="Logging_errors"></a>Logging errors</h2>
<p>
Detecting errors is one thing, presenting them to the user is
another. To do that, the Check base class provides several log
methods, the simplest of them being <code>Check.log(String)</code>. In your
Check you can simply use a verbatim error string like in <code>log("Too many methods, only " + mMax +
" are allowed");</code> as the argument. That will
work, but it's not the best possible solution if your Check is
intended for a wider audience.
</p>
<p>
If you are not living in a country where people speak English,
you may have noticed that Checkstyle writes internationalized
error messages, for example if you live in Germany the error
messages are German. The individual Checks don't have to do
anything fancy to achieve this, it's actually quite easy and the
Checkstyle framework does most of the work.
</p>
<p>
To support internationalized error messages, you need to create
a messages.properties file alongside your Check class, i.e. the
Java file and the properties files should be in the same
directory. Add a symbolic error code and an English
representation to the messages.properties. The file should
contain the following line: <code>too.many.methods=Too many methods, only {0} are
allowed</code>. Then replace the verbatim error message with
the symbolic representation and use one of the log helper
methods to provide the dynamic part of the message (mMax in this
case): <code>log("too.many.methods",
mMax);</code>. Please consult the documentation of Java's <a href="http://java.sun.com/j2se/1.4.1/docs/api/java/text/MessageFormat.html" class="externalLink">MessageFormat</a>
to learn about the syntax of format strings (especially about
those funny numbers in the translated text).
</p>
<p>
Supporting a new language is very easy now, simply create a new
messages file for the language, e.g. messages_fr.properties to
provide french error messages. The correct file will be chosen
automatically, based on the language settings of the user's
operating system.
</p>
</div>
<div class="section"><h2><a name="Integrate_your_Check"></a>Integrate your Check</h2>
<p>
The great final moment has arrived, you are about to run your
Check. To integrate your Check, add a new subentry under the
TreeWalker module of your configuration file. Use the full
classname of your Check class as the name of the module.
Your configuration file should look something like this:
</p>
<div class="source"><pre>
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
"-//Puppy Crawl//DTD Check Configuration 1.3//EN"
"http://www.puppycrawl.com/dtds/configuration_1_3.dtd">
<module name="Checker">
<module name="TreeWalker">
<!-- your standard Checks that come with Checkstyle -->
<module name="UpperEll"/>
<module name="MethodLength"/>
<!-- your Check goes here -->
<module name="com.mycompany.checks.MethodLimitCheck">
<property name="max" value="45"/>
</module>
</module>
</module>
</pre>
</div>
<p>
To run the new Check on the command line compile your Check,
create a jar that contains the classes and property files,
e.g. <code>mycompanychecks.jar</code>. Then run
(with the path separator adjusted to your platform):
</p>
<div class="source"><pre>
java -classpath mycompanychecks.jar:checkstyle-5.3-all.jar \
com.puppycrawl.tools.checkstyle.Main \
-c config.xml -r .
</pre>
</div>
<p>
Did you see all those errors about "too many methods"
flying over your screen? Congratulations. You can now consider
yourself a Checkstyle expert. Go to your fridge. Have a beer.
</p>
<p>
Please consult the <a href="config.html#Packages">Checkstyle
configuration manual</a> to learn how to integrate your Checks
into the package configuration so that you can use <code>MethodLimit</code> instead of the full class name.
</p>
</div>
<div class="section"><h2><a name="Limitations"></a>Limitations</h2>
<p>
OK, so you have written your first Check, and you have found
several flaws in many of your programs. You now know that your
boss does not follow the coding conventions he wrote. And you
know that you are the king of the world. To become a programming
god, you want to write your second Check - now wait, first you
should know what your limits are.
</p>
<p>
There are basically only two of them:
</p>
<ul><li>You cannot determine the type of an expression.</li>
<li>You cannot see the content of other files. (although you can save processed files for use later)</li>
</ul>
<p>
This means that you cannot implement some of the code inspection
features that are available in advanced IDEs like <a href="http://www.intellij.com/idea/" class="externalLink">IntelliJ IDEA</a>. For
example you will not be able to implement a Check that finds
redundant type casts or unused public methods.
</p>
</div>
<div class="section"><h2><a name="Writing_FileSetChecks"></a>Writing FileSetChecks</h2>
<p> Writing a FileSetCheck is pretty straightforward: Just
inherit from <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractFileSetCheck.html">AbstractFileSetCheck</a>
and override the abstract <a href="apidocs/com/puppycrawl/tools/checkstyle/api/AbstractFileSetCheck.html#processFiltered(java.io.File,%20java.util.List)"><code>processFiltered(java.io.File, java.util.List)</code></a> method and you're
done. A very simple example could fire an error if the number of files
exceeds a certain limit. Here is a FileSetCheck that does just that:</p>
<div class="source"><pre>
package com.mycompany.checks;
import java.io.File;
import java.util.List;
import com.puppycrawl.tools.checkstyle.api.*;
public class LimitImplementationFiles extends AbstractFileSetCheck
{
private static final int DEFAULT_MAX = 100;
private int fileCount;
private int max = DEFAULT_MAX;
public void setMax(int aMax)
{
this.max = aMax;
}
@Override
public void beginProcessing(String aCharset)
{
super.beginProcessing(aCharset);
//reset the file count
this.fileCount = 0;
}
@Override
public void processFiltered(File file, List<String> aLines)
{
this.fileCount++;
if (this.fileCount > this.max) {
// log the message
log(0, "max.files.exceeded", Integer.valueOf(this.max));
// you can call log() multiple times to flag multiple
// errors in the same file
}
}
}
</pre>
</div>
<p>
Note that the configuration via bean introspection also applies
here. By implementing the <code>setMax()</code>
method the FileSetCheck automatically makes "max" a
legal configuration parameter that you can use in the Checkstyle
configuration file.
</p>
<p>
There are virtually no limits what you can do in
FileSetChecks. The craziest ideas we've had so far are:
</p>
<ul><li>to find global code problems like unused public methods.</li>
<li>to find duplicate code.</li>
<li>to port the TreeWalker solution to check C#
instead of Java.</li>
</ul>
</div>
<div class="section"><h2><a name="Huh_I_cant_figure_it_out"></a>Huh? I can't figure it out!</h2>
<p>
That's probably our fault, and it means that we have to provide
better documentation. Please do not hesitate to ask questions on
the user mailing list, this will help us to improve this
document. Please ask your questions as precisely as possible.
We will not be able to answer questions like "I want to
write a Check but I don't know how, can you help me?". Tell
us what you are trying to do (the purpose of the Check), what
you have understood so far, and what exactly you are getting stuck
on.
</p>
</div>
<div class="section"><h2><a name="Contributing"></a>Contributing</h2>
<p>
We need <em>your</em> help to keep improving Checkstyle.
Whenever you write a Check or FileSetCheck that you think is
generally useful, please consider
<a href="contributing.html">contributing</a> it to the
Checkstyle community and submit it for inclusion in the next
release of Checkstyle.
</p>
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