Brushing up on Lint (NYC Android Meetup, April 2017)

If you're an Android developer, then you're probably familiar with the Lint static analysis tool and the suite of checks it provides out of the box. But did you know you could write your own Lint checks? In this talk, we'll dive into some internals and learn how to write your own checks in no time...with tests to instill confidence!

Video: https://youtu.be/RQHYVj2k3NM?t=30m34s

1. Brushing up on Lint John Rodriguez
4. Examples • library developer • Butterknife • Timber • domain-specific rules
5. Dagger -> Dagger 2 goo.gl/rExt9t
6. @Provides2 @Inject2
 @Module2 Dagger -> Dagger 2 @Provides @Inject
 @Module
7. Dagger -> Dagger 2 @Module public static class PaymentModule { @Provides PaymentTypePresenter providePresenter() { return new PaymentTypePresenter(...); } }
8. Dagger -> Dagger 2 @Module public static class PaymentModule { @Provides PaymentTypePresenter providePresenter() { return new PaymentTypePresenter(...); } }
9. Dagger -> Dagger 2 @Module2 public static class PaymentModule { @Provides2 PaymentTypePresenter providePresenter() { return new PaymentTypePresenter(...); } }
10. What’s in a custom check? • Issues • Detectors • Implementations • Registries
12. Abstract Syntax Trees
13. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 }
14. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } File
15. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File
16. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class
17. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList
18. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier
19. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method
20. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method ModifierList
21. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList
22. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList Identifier
23. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList Parameter Identifier
24. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList Parameter TypeElement Identifier
25. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList Parameter TypeElement Identifier Identifier
26. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList Parameter Identifier CodeBlock TypeElement Identifier
27. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement PackageStatement File Class ModifierList Identifier Method TypeElement ModifierList Parameter Identifier CodeBlock TypeElement Identifier
28. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement
29. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BinaryExpression
30. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BlockStatement BinaryExpression
31. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BlockStatement BinaryExpression ExpressionStatement AssignmentExpression
32. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BlockStatement BinaryExpression ExpressionStatement AssignmentExpression Identifier
33. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BlockStatement BinaryExpression ExpressionStatement AssignmentExpression Identifier LiteralExpression
34. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BlockStatement BinaryExpression BlockStatement ExpressionStatement AssignmentExpression Identifier LiteralExpression
35. package com.example;
 
 public class Example { private void foo(int b) {
 if (b != 5) {
 b = 3;
 } else {
 throw new RuntimeException();
 }
 }
 } IfStatement BlockStatement BinaryExpression BlockStatement ExpressionStatement AssignmentExpression Identifier LiteralExpression ThrowStatement
36. Writing a custom check
37. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { } Writing a custom check
39. public interface JavaPsiScanner { /* required unless if return true from {@link #appliesToResourceRefs()} or return non null from {@link #getApplicableMethodNames()}. */
 JavaElementVisitor createPsiVisitor(JavaContext context);
 
 List<Class<? extends PsiElement>> getApplicablePsiTypes();
 
 List<String> getApplicableMethodNames();
 void visitMethod(JavaContext context, JavaElementVisitor visitor,
 PsiMethodCallExpression call, PsiMethod method);
 
 List<String> getApplicableConstructorTypes();
 void visitConstructor(JavaContext context, JavaElementVisitor visitor, PsiNewExpression node, PsiMethod constructor);
 
 List<String> getApplicableReferenceNames();
 void visitReference(JavaContext context, JavaElementVisitor visitor,
 PsiJavaCodeReferenceElement reference, PsiElement referenced);
 
 boolean appliesToResourceRefs();
 void visitResourceReference(JavaContext context, JavaElementVisitor visitor,
 PsiElement node, ResourceType type, String name, boolean isFramework);
 
 List<String> applicableSuperClasses();
 void checkClass(JavaContext context, PsiClass declaration);
 }
40. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { } Writing a custom check
41. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { public static final Issue NEW_DAGGER_1_MODULE = Issue.create("NewDagger1ModuleDetected", "New Dagger1 Module detected", "Please create a Dagger2 Module and/or Component instead.", Category.CORRECTNESS, 10, Severity.ERROR, new Implementation(DaggerUsageDetector.class, Scope.JAVA_FILE_SCOPE)); } Writing a custom check
42. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { public static final Issue NEW_DAGGER_1_MODULE = … } Writing a custom check
43. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { @Override public List<Class<? extends PsiElement>> getApplicablePsiTypes() { return Collections.singletonList(PsiAnnotation.class); } @Override public EnumSet<Scope> getApplicableFiles() { return Scope.JAVA_FILE_SCOPE; } public static final Issue NEW_DAGGER_1_MODULE = … } Writing a custom check
44. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { public static final Issue NEW_DAGGER_1_MODULE = … } Writing a custom check
45. class DaggerUsageDetector extends Detector implements Detector.JavaPsiScanner { @Override public JavaElementVisitor createPsiVisitor(JavaContext context) { return new JavaElementVisitor() { @Override public void visitAnnotation(PsiAnnotation annotation) { String type = annotation.getQualifiedName(); if (!"dagger.Module".equals(type)) { return; } PsiElement modifierList = LintUtils.skipParentheses(annotation.getParent()); PsiElement markedModule = LintUtils.skipParentheses(modifierList.getParent()); if (!(markedModule instanceof PsiClass)) { return; } PsiClass moduleClass = (PsiClass) markedModule; context.report(NEW_DAGGER_1_MODULE, context.getLocation(moduleClass), "Don't add new Dagger1 Modules; use Dagger2 Modules/Components instead"); } }; } public static final Issue NEW_DAGGER_1_MODULE = … } Writing a custom check
46. class CustomIssueRegistry extends IssueRegistry {
 @Override public List<Issue> getIssues() {
 return Arrays.asList(
 StringNamesDetector.OUT_OF_ORDER,
 DaggerUsageDetector.NEW_DAGGER_1_MODULE,
 RxObservableCreateDetector.NEW_INVOCATION
 );
 }
 } Writing a custom check
47. Setup
48. apply plugin: 'java'
 
 dependencies {
 compile "com.android.tools.lint:lint-api:${lintVersion}" testCompile "com.android.tools.lint:lint:${lintVersion}" testCompile "com.android.tools.lint:lint-tests:${lintVersion}" …
 }
 
 jar {
 manifest {
 attributes('Lint-Registry': 'com.example.lint.CustomIssueRegistry')
 }
 } Setup
49. apply plugin: 'com.android.library'
 apply plugin: ‘com.kageiit.lintrules' 
 android {
 lintOptions { … }
 }
 
 dependencies { … 
 lintRules project(‘:your-lint-project')
 } Setup https://github.com/kageiit/gradle-lintrules-plugin
52. Testing a custom check
53. public class DaggerUsageDetectorTest extends LintDetectorTest { } Testing a custom check
54. public class DaggerUsageDetectorTest extends LintDetectorTest { @Override protected Detector getDetector() { return new DaggerUsageDetector(); } @Override protected List<Issue> getIssues() { return Collections.singletonList(NEW_DAGGER_1_MODULE); } } Testing a custom check
55. public class DaggerUsageDetectorTest extends LintDetectorTest { … } Testing a custom check
56. public class DaggerUsageDetectorTest extends LintDetectorTest { public void testShouldNotTriggerNewDaggerModuleIssue() throws Exception { String noModuleSource = "" + "package foobar;\n" + "public class NoModule {}"; String lintOutput = lintProject(java(noModuleSource)); assertThat(lintOutput).isEqualTo("No warnings."); } } Testing a custom check
57. public class DaggerUsageDetectorTest extends LintDetectorTest { … } Testing a custom check
58. public class DaggerUsageDetectorTest extends LintDetectorTest { public void testShouldTriggerNewDaggerModuleIssue() throws Exception { String newModuleSource = "" + "package foobar;\n" + "import dagger.Module;\n" + "@Module\n" + "public class NewModule {}"; String fakeAnnotationSource = "" + "package dagger;\n" + "public @interface Module {}"; String lintOutput = lintProject(java(newModuleSource), java(fakeAnnotationSource)); … } } Testing a custom check
59. public class DaggerUsageDetectorTest extends LintDetectorTest { public void testShouldTriggerNewDaggerModuleIssue() throws Exception { … String format = "" + "src/foobar/NewModule.java:3: Error: %s [%s]\n" + "@Module\n" + "^\n" + "1 errors, 0 warnings\n"; String warningMessage = String.format(format, "Don't add new Dagger1 Modules; use Dagger2 Modules/Components instead", NEW_DAGGER_1_MODULE.getId()); assertThat(lintOutput).isEqualTo(warningMessage); } } Testing a custom check
60. Timber Example
61. public final class WrongTimberUsageDetector extends Detector implements JavaPsiScanner {
 @Override public List<String> getApplicableMethodNames() { return asList("tag", "format", …); }
 @Override public void visitMethod(JavaContext context, JavaElementVisitor visitor,
 PsiMethodCallExpression call, PsiMethod method) {
 PsiReferenceExpression methodExpression = call.getMethodExpression();
 String fullyQualifiedMethodName = methodExpression.getQualifiedName(); …
 if (fullyQualifiedMethodName.startsWith("timber.log.Timber.tag")) {
 checkTagLength(context, call);
 return;
 } … 
 } private static void checkTagLength(JavaContext context, PsiMethodCallExpression call) {
 PsiExpression argument = call.getArgumentList().getExpressions()[0];
 String tag = findLiteralValue(argument);
 if (tag != null && tag.length() > 23) {
 String message = String.format("Tag must be <= 23 characters, was %1$d (%2$s)”, tag.length(), tag);
 context.report(ISSUE_TAG_LENGTH, argument, context.getLocation(argument), message);
 }
 }
 
 Issue ISSUE_TAG_LENGTH = Issue.create("TimberTagLength", …, new Implementation(…));
63. public class WrongTimberUsageDetectorTest extends LintDetectorTest {
 private final TestFile timberStub = java(""
 + "package timber.log;\n"
 + "public class Timber {\n"
 + " public static Tree tag(String s) { return new Tree(); }\n"
 + "}");
 
 public void testTagTooLong() throws Exception {
 String source = ""
 + "package foo;\n"
 + "import timber.log.Timber;\n"
 + "public class Example {\n"
 + " public void log() {\n"
 + " Timber.tag(\"abcdefghijklmnopqrstuvwx\");\n"
 + " }\n"
 + “}"; 
 assertThat(lintProject(java(source), timberStub)).isEqualTo("src/foo/Example.java:5: "
 + "Error: Tag must be <= 23 characters, was 24 (abcdefghijklmnopqrstuvwx) [TimberTagLength]\n"
 + " Timber.tag(\"abcdefghijklmnopqrstuvwx\");\n"
 + " ~~~~~~~~~~~~~~~~~~~~~~~~~~\n"
 + "1 errors, 0 warnings\n");
 }
64. Lint internals
65. :buildEnvironment ------------------------------------------------------------ Root project ------------------------------------------------------------ classpath \--- com.android.tools.build:gradle:2.3.1 \--- com.android.tools.build:gradle-core:2.3.1 +--- com.android.tools.build:builder:2.3.1 | . . . +--- com.android.tools.lint:lint:25.3.1 | +--- com.android.tools.lint:lint-checks:25.3.1 | | +--- com.android.tools.lint:lint-api:25.3.1 Note the versions! v(Lint) = 23 + v(AGP) $ ./gradlew buildEnvironment
66. $ pwd /Users/jrod/Library/Android/sdk/tools/ /Users/jrod/Library/Android/sdk/tools/bin # as of SDK Tools v25.2.3 $ archquery avdmanager jobb lint monkeyrunner screenshot2 sdkmanager uiautomatorviewer ls $ lint: version 25.2.3 lint --version
67. $ file lint $ jarfile=lint.jar ... jarpath="$frameworkdir/$jarfile" exec "$javaCmd" \ ... -classpath "$jarpath" \ com.android.tools.lint.Main “$@“ lint: Bourne-Again shell script text executable, ASCII text tail lint
70. Learn more • Android Lint • https://android.googlesource.com/platform/tools/base/+/ studio-master-dev/lint • https://groups.google.com/forum/#!forum/lint-dev • OSS Examples • https://github.com/JakeWharton/butterknife • https://github.com/JakeWharton/timber
71. Brushing up on Lint @jrodbx