Better Code through Lint and Checkstyle

Better Code through Lint and Checkstyle
droidcon New York City | 28.08.2015 | Marc Prengemann

About me
Marc Prengemann
Android Software Engineer
Mail: marc@wire.com
Wire: marcprengemann@gmail.com
Github: winterDroid
Google+: Marc Prengemann
Twitter: @winterDroid89

What is Lint and Checkstyle?
App Source
Files
Configuration
Analysis Tool Report

When should I use it?
• to ensure code quality
• focus in reviews on real code
• prevent people from misusing internal
libraries
… but what are the challenges?
• Lint API is @Beta
• getting familiar with the API
• integrating within your Gradle Build
• debugging / testing

Getting started with own Checks

Test ideas
• Fragments and Activities should extend your
BaseClass
• use ViewUtils instead of finding and casting a
View
• don’t check floats for equality - use
Float.equals instead
• find leaking resources
• enforce Naming conventions
• find hardcoded values in XMLs

Library misuse detection
with Lint

A real life example
• Timber
• logger by Jake Wharton
• https://github.com/JakeWharton/
timber
• want to create a detector that detects
misuse of android.util.Log instead of
Timber

public class WrongTimberUsageDetector extends Detector 
implements Detector.JavaScanner { 
 
public static final Issue ISSUE = ...; 
 
@Override 
public List<String> getApplicableMethodNames() { 
return Arrays.asList("v", "d", "i", "w", "e", "wtf"); 
} 
 
@Override 
public void visitMethod(@NonNull JavaContext context, 
AstVisitor visitor, 
@NonNull MethodInvocation node) { 
if (!(node.astOperand() instanceof VariableReference)) { 
return; 
} 
VariableReference ref = (VariableReference) node.astOperand(); 
String refName = ref.astIdentifier().astValue(); 
if (!"Timber".equals(refName)) { 
context.report(ISSUE, 
node, 
context.getLocation(node), 
String.format("Use 'Timber' instead of '%s'", 
refName)); 
} 
} 
}
Detector
• responsible for scanning through code
and to ﬁnd issues and report them

Detector
and to ﬁnd issues and report them
• most detectors implement one or more
scanner interfaces that depend on the
speciﬁed scope
• Detector.XmlScanner
• Detector.JavaScanner
• Detector.ClassScanner
 
 
@Override 
} 
 
@Override 
return; 
} 
node, 
refName)); 
} 
} 
}

Detector
and finding issue instances and reporting
them
scanner interfaces
• can detect multiple different issues
• allows you to have different severities for
different types of issues
 
 
@Override 
} 
 
@Override 
return; 
} 
node, 
refName)); 
} 
} 
}

Detector
them
scanner interfaces
• deﬁne the calls that should be analyzed
• overwritten method depends on
implemented scanner interface
• depends on the goal of the detector
 
 
@Override 
} 
 
@Override 
return; 
} 
node, 
refName)); 
} 
} 
}

Detector
them
scanner interfaces
• analyze the found calls
• overwritten method depends on
implemented scanner interface
• depends on the goal of the detector
 
 
@Override 
} 
 
@Override 
return; 
} 
node, 
refName)); 
} 
} 
}

Detector
them
scanner interfaces
• analyze the found calls
• report the found issue
• specify the location
• report() will handle to suppress warnings
• add a message for the warning
 
 
@Override 
} 
 
@Override 
return; 
} 
node, 
refName)); 
} 
} 
}

Issue
• potential bug in an Android application
• is discovered by a Detector
• are exposed to the user
public static final Issue ISSUE =
Issue.create("LogNotTimber",
"Used Log instead of Timber", 
"Since Timber is included in the project, "
+ "it is likely that calls to Log should " 
+ "instead be going to Timber.", 
Category.MESSAGES, 
5, 
Severity.WARNING, 
new Implementation(WrongTimberUsageDetector.class,
Scope.JAVA_FILE_SCOPE));

Issue
• the id of the issue
• should be unique
• recommended to add the package name as
a preﬁx like com.wire.LogNotTimber
5, 

Issue
• short summary
• typically 5-6 words or less
• describe the problem rather than the ﬁx public static final Issue ISSUE =
5, 

Issue
• short summary
• full explanation of the issue
• should include a suggestion how to ﬁx it public static final Issue ISSUE =
5, 

Issue
• short summary
• the associated category, if any
• Lint
• Correctness (incl. Messages)
• Security
• Performance
• Usability (incl. Icons, Typography)
• Accessibility
• Internationalization
• Bi-directional text
5, 

Issue
• short summary
• the priority
• a number from 1 to 10
• 10 being most important/severe
5, 

Issue
• short summary
• the priority
• the default severity of the issue
• Fatal
• Error
• Warning
• Informational
• Ignore
5, 

Issue
• short summary
• the priority
• the default implementation for this issue
• maps to the Detector class
• speciﬁes the scope of the implementation
5, 

Issue
• short summary
• the priority
• the default implementation for this issue
• the scope of the implementation
• describes set of files a detector must
consider when performing its analysis
• include:
• Resource files / folder
• Java files
• Class files
5, 

Issue Registry
• provide list of checks to be performed
• return a list of Issues in getIssues()
public class CustomIssueRegistry extends IssueRegistry { 
@Override 
public List<Issue> getIssues() { 
return Arrays.asList(WrongTimberUsageDetector.ISSUE); 
} 
}

Naming Convention enforcement
with Checkstyle

A real life example
• Naming Conventions for variables
• improve Readability
public final static int FirstNumber = 0;
public final static int SECOND_NUMBER = 0; 
private static int FirstNumber = 0; 
private static int sSecondNumber = 0;
private int firstNumber = 0; 
private int mSecondNumber = 0;
int FirstNumber = 0; 
int secondNumber = 0;

A real life example
• diﬀerent styles for
• constant variables public final static int FirstNumber = 0;

A real life example
• constant variables
• static variables

A real life example
• member variables
public final static int SECOND_NUMBER = 0;

A real life example
• member variables
• local variables

Check
• Visitor Pattern on the traversed Abstract
Syntax Tree (AST)
• Base class for checks
• beginTree()
• visitToken()
• leaveToken()
• finishTree()
public class NamingConventionCheck extends Check { 
 
private static final String MSG_CONSTANT = "Constant '%s' " + 
"should be named in all uppercase with underscores."; 
 
@Override 
public int[] getDefaultTokens() { 
return new int[] {VARIABLE_DEF}; 
} 
 
@Override 
public void visitToken(DetailAST ast) { 
final DetailAST modifier = ast.findFirstToken(MODIFIERS); 
final String name = ast.findFirstToken(IDENT) 
.getText(); 
if (ScopeUtils.inInterfaceOrAnnotationBlock(ast) || 
"serialVersionUID".equals(name)) { 
return; 
} 
 
if (modifier.branchContains(LITERAL_STATIC) && 
modifier.branchContains(FINAL)) { 
if (NamingConventions.isWrongConstantNaming(name)) { 
log(ast.getLineNo(), 
ast.getColumnNo(), 
String.format(MSG_CONSTANT, name)); 
} 
} 
} 
}
public class NamingConventions { 
 
private static final Pattern WRONG_CONSTANT_NAME_PATTERN = 
Pattern.compile(".*[a-z].*"); 
 
public static boolean isWrongConstantNaming(String variableName) { 
return WRONG_CONSTANT_NAME_PATTERN.matcher(variableName) 
.matches(); 
} 
}

Check
Syntax Tree (AST)
• register for token types
• see TokenTypes class
• popular examples
• CLASS_DEF
• VARIABLE_DEF
• METHOD_DEF
 
 
@Override 
} 
 
@Override 
.getText(); 
return; 
} 
 
} 
} 
} 
}
 
 
.matches(); 
} 
}

Check
Syntax Tree (AST)
• analyze the found tokens
• DetailAST provides utility methods
• possible to navigate around
• don’t abuse that feature! Just look at the
neighbours if necessary
 
 
@Override 
} 
 
@Override 
.getText(); 
return; 
} 
 
} 
} 
} 
}
 
 
.matches(); 
} 
}

Check
Syntax Tree (AST)
• DetailAST provides location
• supports internationalized error messages
 
 
@Override 
} 
 
@Override 
.getText(); 
return; 
} 
 
} 
} 
} 
}
 
 
.matches(); 
} 
}

Check
Syntax Tree (AST)
• (optional) deﬁne check properties
• uses JavaBean reﬂection
• works for all primitive types
• just use a setter
 
 
@Override 
} 
 
@Override 
.getText(); 
return; 
} 
 
} 
} 
} 
}
 
 
.matches(); 
} 
}

Include Lint within your Project

Project structure
• lintrules
• Java module
• source of all custom detectors and our
IssueRegistry
• specify reference in build.gradle as
attribute Lint-Registry
• will export a lint.jar
$ ./gradlew projects
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
Root project 'awsm_app'
+--- Project ':app'
+--- Project ':lintlib'
--- Project ':lintrules'
jar {
manifest {
attributes 'Manifest-Version': 1.0
attributes 'Lint-Registry':
'com.checks.CustomIssueRegistry'
}
}

Project structure
• lintrules
• lintlib
• Android library module
• has a dependency to lintrules $ ./gradlew projects
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
+--- Project ':app'

Project structure
• lintrules
• lintlib
• app
• has dependency to lintlib module
• since lintlib is an Android library module, we
can use the generated lint.jar
• circumvent bug introduced in build tools
version 1.2.0
https://goo.gl/1uCNSR
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
+--- Project ':app'

Project structure
• lintrules
• lintlib
• app
• check your project using with
./gradlew :app:lintDebug
• conﬁgure Lint as described here:
http://goo.gl/xABHhy
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
+--- Project ':app'

Include Checkstyle within your Project

Project structure
• custom-checkstyle
• Java module
• source of all custom checks
• dependency to selected Checkstyle version $ ./gradlew projects
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
+--- Project ':app'
--- Project ':custom-checkstyle'

Project structure
• app
• uses Checkstyle plugin
• holds configuration
• module must be added with its fully
qualified classname
com.wire.NamingConventionCheck
for example to the TreeWalker
• has dependency to custom-checkstyle
module in the Checkstyle configuration
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
+--- Project ':app'

Project structure
• app
• check your project using with
./gradlew :app:checkstyle
• conﬁgure Checkstyle as described here:
http://goo.gl/aDBeU9
:projects
------------------------------------------------------------
Root project
------------------------------------------------------------
+--- Project ':app'

Testing Lint Checks
• part of lintrules module
• register and execute tests as usual in
build.gradle
public class WrongTimberUsageTest extends LintDetectorTest { 
 
@Override 
protected Detector getDetector() { 
return new WrongTimberUsageDetector(); 
} 
 
@Override 
protected List<Issue> getIssues() { 
} 
 
public void testLogTest() throws Exception { 
final String expected = ...; 
final TestFile testFile = 
java("src/test/pkg/WrongTimberTest.java", 
...); 
assertEquals(expected, lintProject(testFile)); 
} 
}

build.gradle
• every test should extend
LintDetectorTest
• part of lint-tests dependency by
com.android.tools.lint
• tested with version 24.3.1
 
@Override 
} 
 
@Override 
} 
 
...); 
} 
}
Testing Lint Checks

build.gradle
LintDetectorTest
• need to deﬁne the to be used detector
and the selected issue(s)
 
@Override 
} 
 
@Override 
} 
 
...); 
} 
}
Testing Lint Checks

build.gradle
LintDetectorTest
• need to deﬁne the to be used detector
and the selected issue(s)
• perform usual unit tests with the helper
methods including:
• java - Create TestFile from source string
• lintProject - Lint on given ﬁles when
constructed as separate project
 
@Override 
} 
 
@Override 
} 
 
...); 
} 
}
Testing Lint Checks

Testing Lint Checks
• part of custom-checkstyle module
build.gradle
public class NamingConventionTest extends BaseCheckTestSupport { 
 
@Test 
public void testConstantNaming() throws Exception { 
final DefaultConfiguration checkConfig = 
createCheckConfig(NamingConventionCheck.class); 
final String[] expected = { 
"7:5: Constant test4 should be " + 
"named in all uppercase with underscores." 
}; 
verify(checkConfig, getPath("ConstantName.java"), expected); 
} 
}

Testing Lint Checks
build.gradle
BaseCheckTestSupport
• part of sevntu checks
• contains some nice helper methods
• createCheckConfig
• getPath
• getMessage
 
@Test 
}; 
} 
}

Testing Lint Checks
build.gradle
• need to create conﬁguration for check
 
@Test 
}; 
} 
}

Testing Lint Checks
build.gradle
• tests are performed on real Java ﬁles
• part of test resources
• should be in same package as test class
 
@Test 
}; 
} 
}

Testing Lint Checks
build.gradle
• tests are performed on real Java ﬁles
• verify for ﬁnal test
 
@Test 
}; 
} 
}

Debugging
• same for Checkstyle and Lint
• prepare and run Gradle daemon
./gradlew --daemon
$ ./gradle.properties
## Project-wide Gradle settings. 
# 
# For more details on how to configure your build environment visit 
# http://www.gradle.org/docs/current/userguide/build_environment.html 
# 
# Specifies the JVM arguments used for the daemon process. 
# The setting is particularly useful for tweaking memory settings. 
# Default value: -Xmx10248m -XX:MaxPermSize=256m 
# org.gradle.jvmargs=-Xmx2048m -XX:MaxPermSize=512m -XX:
+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8 
# 
# When configured, Gradle will run in incubating parallel mode. 
# This option should only be used with decoupled projects. More details,
visit 
# http://www.gradle.org/docs/current/userguide/
multi_project_builds.html#sec:decoupled_projects 
# org.gradle.parallel=true 
org.gradle.jvmargs=-XX:MaxPermSize=4g  
-XX:+HeapDumpOnOutOfMemoryError  
-Xmx4g  
-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000

Debugging
• create new debug conﬁguration in IntelliJ

Debugging
• create new debug conﬁguration in IntelliJ
• debug newly created conﬁguration

Conclusion
• Lint and Checkstyle do not have a well
documented API
• sample project:
https://goo.gl/QBfyf8
• Checkstyle is easier to use and to integrate
• Lint seems to be more ﬂexible
➡ to improve code quality
➡ help new team members
• integration of Lint Checks may become
easier with
https://goo.gl/d7U1qS

Better Code through Lint and Checkstyle

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