Scala compiler plugin for static code analysis
Scala Java
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README.md

Scapegoat

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Scapegoat is a Scala static code analyzer, what is more colloquially known as a code lint tool or linter. Scapegoat works in a similar vein to Java's FindBugs or checkstyle, or Scala's Scalastyle.

A static code analyzer is a tool that flag suspicious language usage in code. This can include behavior likely to lead or bugs, non idiomatic usage of a language, or just code that doesn't conform to specified style guidelines.

What's the difference between this project and Scalastyle (or others)?

Scalastyle is a similar linting tool which focuses mostly on enforcing style/code standards. There's no problems running multiple analysis tools on the same codebase. In fact it could be beneficial as the total set of possible warnings is the union of the inspections of all the enabled tools. The worst case is that the same warnings might be generated by multiple tools.

Usage

Scapegoat is developed as a scala compiler plugin, which can then be used inside your build tool.

SBT

See: sbt-scapegoat for SBT integration.

Maven

Firstly you need to add scapegoat plugin as a dependency:

<dependency>
    <groupId>com.sksamuel.scapegoat</groupId>
    <artifactId>scalac-scapegoat-plugin_${scala.binary.version}</artifactId>
    <version>1.3.3</version>
</dependency>

Then configure scala-maven-plugin by adding compilerPlugin

<plugin>
    <groupId>net.alchim31.maven</groupId>
    <artifactId>scala-maven-plugin</artifactId>
    <configuration>
        <args>
            <arg>-P:scapegoat:dataDir:./target/scapegoat</arg>
        </args>
        <compilerPlugins>
            <compilerPlugin>
                <groupId>com.sksamuel.scapegoat</groupId>
                <artifactId>scalac-scapegoat-plugin_${scala.binary.version}</artifactId>
                <version>1.3.3</version>
            </compilerPlugin>
        </compilerPlugins>
    </configuration>
</plugin>

The only required parameter is dataDir (where report will be generated):

<arg>-P:scapegoat:dataDir:./target/scapegoat</arg>

You can pass other configuration flags same way, e.g.

<arg>-P:scapegoat:disabledInspections:FinalModifierOnCaseClass</arg>

Note: You may use a separate maven profile, so that the dependency doesn't go to you runtime classpath.

Reports

Here is sample output from the console during the build for a project with warnings/errors:

[warning] [scapegoat] Unused method parameter - org.ensime.util.ClassIterator.scala:46
[warning] [scapegoat] Unused method parameter - org.ensime.util.ClassIterator.scala:137
[warning] [scapegoat] Use of var - org.ensime.util.ClassIterator.scala:22
[warning] [scapegoat] Use of var - org.ensime.util.ClassIterator.scala:157
[   info] [scapegoat]: Inspecting compilation unit [FileUtil.scala]
[warning] [scapegoat] Empty if statement - org.ensime.util.FileUtil.scala:157
[warning] [scapegoat] Expression as statement - org.ensime.util.FileUtil.scala:180

And if you prefer a prettier report, here is a screen shot of the type of HTML report scapegoat generates:

screenshot

Configuration

For instructions on suppressing warnings by file, by inspection or by line see the sbt-scapegoat README.

False positives

Please note that scapegoat is a new project. While it's been tested on some common open source projects, there is still a good chance you'll find false positives. Please open up issues if you run into these so we can fix them.

Changelog

  • 0.94.0 - Fixed some more false positives. Added MethodNames inspection, StripMarginOnRegex inspection, and VariableShadowing inspection (the latter being a work in progress, please report feedback).

  • 0.93.2 - Fixed false positives.

  • 0.93.1 - #67 fixed var args in duplicate map check, #66 ignoring methods returning nothing when checking for unused params, #69 fixed extended classes false pos, #73 Removed incorrect inspection, #64 updated suppression to use tree.symbol.isSynthetic instead of mods.synth, Merge pull request #77 from paulp/psp, Give access to the inspection logic through the sbt console, #76 Improve the contains test.

  • 0.93.0 - Added ability to define multiple traversers that run in separate phases of the compiler, #58 Updated suppression to work on objects and classes, #60 handling case objects in suspicious match on class object, Allow all inspections to be disabled, other fixes

  • 0.92.2 - Added debug option, Made summary optional and disabled in tests, Improved var could be val #54, Split null inspections into assignment and invocation #53, Bumped count on operators to > 2, loads of fixes, loads of verboseness removed.

  • 0.92.1 - Fixed a load of false positives.

  • 0.92.0 - Added swallowed exception inspection, Added public finalizer inspection, Added use expm1(x) instead of exp(x) - 1 inspection, Added use log1p(x) instead of log(x + 1) inspection, Added use log10(x) instead of log(x)/log(10) inspection, Added use cbrt inspection

  • 0.91.0 - Updated logging format to include less [scapegoat] everywhere, Added scala.math and java.StrictMath to useSqrt, Added ignored files patterns option, Added wildcard import inspection, Added comparison to empty set inspection

  • 0.90.17 - Added looks like interpolated string inspection, Added SuspiciousMatchOnClassObject inspection, Updated varuse to not warn on vars in actors #46, Added comparison to empty list inspection, #37 Changed emptyinterpolated string to error, #37 Fixed warning on max parameters

  • 0.90.14 - Bunch of bug fixes for false positives. No new inspections.

  • 0.90.13 - Fixed NPE in VarClosure inspection, Added Object Names inspection, Added classnames inspection, Added avoid to minus one inspection.

  • 0.90.12 - New inspections: unnecessary override, duplicate import, pointless type bounds, max parameters, var closure, method returning any. Updated repeated case body to ignore bodies with two or less statements #28. Removed false positives on getter/setter #27.

  • 0.90.11 - Added empty for inspection, AnyUse inspection, Added ArrayEquals inspection, Added double negation inspection, Disabled expression as statement inspection by default, Added avoid operator overload inspection, #25 improving repeated case bodies, Added lonely sealed trait. Added postInspection call to inspections

  • 0.90.10 - Added type shadowing inspection, var could be val inspection, unreachable catch inspection and unnecessary toString inspection

  • 0.90.09 - Added new inspections: bounded by final type, empty while block, prefer vector empty, finalizer without super, impossible option size condition, filter dot head, repeated case body. Added infos to HTML output header

  • 0.90.8 - Fixed erroneous partial functions inspection. Added inspection for empty case class. Changed levels in output to lowercase. Added console output option. Fixed seq empty on non empty seq. Changed return usage to info. Fixed odd issue with empty tree. Changed unused parameter in override to be info. Ignoring all synthetic method added to case classes. Fixed while(true) being detected by ConstantIf

Inspections

There are currently 117 inspections. An overview list is given, followed by a more detailed description of each inspection after the list (todo: finish rest of detailed descriptions)

Name Brief Description Default Level
ArrayEquals Checks for comparison of arrays using == which will always return false Info
ArraysInFormat Checks for arrays passed to String.format Error
ArraysToString Checks for explicit toString calls on arrays Warning
AsInstanceOf Checks for use of asInstanceOf Warning
AvoidOperatorOverload Checks for mental symbolic method names Info
AvoidSizeEqualsZero Traversable.size can be slow for some data structure, prefer .isEmpty Warning
AvoidSizeNotEqualsZero Traversable.size can be slow for some data structure, prefer .nonEmpty Warning
AvoidToMinusOne Checks for loops that use x to n-1 instead of x until n Info
BigDecimalDoubleConstructor Checks for use of BigDecimal(double) which can be unsafe Warning
BigDecimalScaleWithoutRoundingMode setScale() on a BigDecimal without setting the rounding mode can throw an exception Warning
BoundedByFinalType Looks for types with upper bounds of a final type Warning
BrokenOddness checks for a % 2 == 1 for oddness because this fails on negative numbers Warning
CatchException Checks for try blocks that catch Exception Warning
CatchFatal Checks for try blocks that catch fatal exceptions: VirtualMachineError, ThreadDeath, InterruptedException, LinkageError, ControlThrowable Warning
CatchNpe Checks for try blocks that catch null pointer exceptions Error
CatchThrowable Checks for try blocks that catch Throwable Warning
ClassNames Ensures class names adhere to the style guidelines Info
CollectionIndexOnNonIndexedSeq Checks for indexing on a Seq which is not an IndexedSeq Warning
CollectionNamingConfusion Checks for variables that are confusingly named Info
CollectionNegativeIndex Checks for negative access on a sequence eg list.get(-1) Warning
CollectionPromotionToAny Checks for collection operations that promote the collection to Any Warning
ComparingFloatingPointTypes Checks for equality checks on floating point types Error
ComparingUnrelatedTypes Checks for equality comparisons that cannot succeed Error
ComparisonToEmptyList Checks for code like a == List() or a == Nil Info
ComparisonToEmptySet Checks for code like a == Set() or a == Set.empty Info
ComparisonWithSelf Checks for equality checks with itself Warning
ConstantIf Checks for code where the if condition compiles to a constant Warning
DivideByOne Checks for divide by one, which always returns the original value Warning
DoubleNegation Checks for code like !(!b) Info
DuplicateImport Checks for import statements that import the same selector Info
DuplicateMapKey Checks for duplicate key names in Map literals Warning
DuplicateSetValue Checks for duplicate values in set literals Warning
EitherGet Checks for use of .get on Left or Right Error
EmptyCaseClass Checks for case classes like case class Faceman() Info
EmptyFor Checks for empty for loops Warning
EmptyIfBlock Checks for empty if blocks Warning
EmptyInterpolatedString Looks for interpolated strings that have no arguments Error
EmptyMethod Looks for empty methods Warning
EmptySynchronizedBlock Looks for empty synchronized blocks Warning
EmptyTryBlock Looks for empty try blocks Warning
EmptyWhileBlock Looks for empty while loops Warning
ExistsSimplifiableToContains exists(x => x == b) replaceable with contains(b) Info
FilterDotHead .filter(x => ).head can be replaced with find(x => ) match { .. } Info
FilterDotHeadOption .filter(x =>).headOption can be replaced with find(x => ) Info
FilterDotIsEmpty .filter(x => ).isEmpty can be replaced with !exists(x => ) Info
FilterDotSize .filter(x => ).size can be replaced more concisely with with count(x => ) Info
FilterOptionAndGet .filter(_.isDefined).map(_.get) can be replaced with flatten Info
FinalModifierOnCaseClass Using Case classes without final modifier can lead to surprising breakage Info
FinalizerWithoutSuper Checks for overridden finalizers that do not call super Warning
FindAndNotEqualsNoneReplaceWithExists .find(x => ) != None can be replaced with exist(x => ) Info
FindDotIsDefined find(x => ).isDefined can be replaced with exist(x => ) Info
IllegalFormatString Looks for invalid format strings Error
ImpossibleOptionSizeCondition Checks for code like option.size > 2 which can never be true Error
IncorrectNumberOfArgsToFormat Checks for wrong number of arguments to String.format Error
IncorrectlyNamedExceptions Checks for exceptions that are not called *Exception and vice versa Error
InvalidRegex Checks for invalid regex literals Info
IsInstanceOf Checks for use of isInstanceOf Warning
JavaConversionsUse Checks for use of implicit Java conversions Warning
ListAppend Checks for List :+ which is O(n) Info
ListSize Checks for List.size which is O(n). Info
LonelySealedTrait Checks for sealed traits which have no implementation Error
LooksLikeInterpolatedString Finds strings that look like they should be interpolated but are not Warning
MapGetAndGetOrElse Map.get(key).getOrElse(value) can be replaced with Map.getOrElse(key, value) Error
MaxParameters Checks for methods that have over 10 parameters Info
MethodNames Warns on method names that don't adhere to the Scala style guidelines Info
MethodReturningAny Checks for defs that are defined or inferred to return Any Warning
ModOne Checks for x % 1 which will always return 0 Warning
NanComparison Checks for x == Double.NaN which will always fail Error
NegationIsEmpty !Traversable.isEmpty can be replaced with Traversable.nonEmpty Info
NegationNonEmpty !Traversable.nonEmpty can be replaced with Traversable.isEmpty Info
NoOpOverride Checks for code that overrides parent method but simply calls super Info
NullAssignment Checks for use of null in assignments Warning
NullParameter Checks for use of null in method invocation Warning
ObjectNames Ensures object names adhere to the Scala style guidelines Info
OptionGet Checks for Option.get Error
OptionSize Checks for Option.size Error
ParameterlessMethodReturnsUnit Checks for def foo : Unit Warning
PartialFunctionInsteadOfMatch Warns when you could use a partial function directly instead of a match block Info
PointlessTypeBounds Finds type bounds of the form [A <: Any] or [A >: Nothing] Warning
PreferSeqEmpty Checks for Seq() when could use Seq.empty Info
PreferSetEmpty Checks for Set() when could use Set.empty Info
ProductWithSerializableInferred Checks for vals that have Product with Serializable as their inferred type Warning
PublicFinalizer Checks for overridden finalizes that are public Info
RedundantFinalModifierOnMethod Redundant final modifier on method that cannot be overridden Info
RedundantFinalModifierOnVar Redundant final modifier on var that cannot be overridden Info
RedundantFinalizer Checks for empty finalizers. Warning
RepeatedCaseBody Checks for case statements which have the same body Warning
ReverseFunc reverse followed by head, headOption, iterator, ormap can be replaced, respectively, with last, lastOption, reverseIterator, or reverseMap Info
ReverseTailReverse .reverse.tail.reverse can be replaced with init Info
ReverseTakeReverse .reverse.take(...).reverse can be replaced with takeRight Info
SimplifyBooleanExpression b == false can be simplified to !b Info
StripMarginOnRegex Checks for .stripMargin on regex strings that contain a pipe Error
SubstringZero Checks for String.substring(0) Info
SuspiciousMatchOnClassObject Finds code where matching is taking place on class literals Warning
SwallowedException Finds catch blocks that don't handle caught exceptions Warning
SwapSortFilter sort.filter can be replaced with filter.sort for performance Info
TraversableHead Looks for unsafe usage of Traversable.head Error
TraversableLast Looks for unsafe usage of Traversable.last Error
TryGet Checks for use of Try.get Error
TypeShadowing Checks for shadowed type parameters in methods Warning
UnnecessaryIf Checks for code like if (expr) true else false Info
UnnecessaryReturnUse Checks for use of return keyword in blocks Info
UnnecessaryToInt Checks for unnecessary toInt on instances of Int Warning
UnnecessaryToString Checks for unnecessary toString on instances of String Warning
UnreachableCatch Checks for catch clauses that cannot be reached Warning
UnsafeContains Checks for List.contains(value) for invalid types Error
UnsafeStringContains Checks for String.contains(value) for invalid types Error
UnusedMethodParameter Checks for unused method parameters Warning
UseCbrt Checks for use of math.pow for calculating math.cbrt Info
UseExpM1 Checks for use of math.exp(x) - 1 instead of math.expm1(x) Info
UseLog10 Checks for use of math.log(x)/math.log(10) instead of math.log10(x) Info
UseLog1P Checks for use of math.log(x + 1) instead of math.log1p(x) Info
UseSqrt Checks for use of math.pow for calculating math.sqrt Info
VarClosure Finds closures that reference var Warning
VarCouldBeVal Checks for vars that could be declared as vals Warning
WhileTrue Checks for code that uses a while(true) or do { } while(true) block. Warning
ZeroNumerator Checks for dividing by 0 by a number, eg 0 / x which will always return 0 Warning
Arrays to string

Checks for explicit toString calls on arrays. Since toString on an array does not perform a deep toString, like say scala's List, this is usually a mistake.

CollectionIndexOnNonIndexedSeq

Checks for calls of .apply(idx) on a Seq where the index is not a literal and the Seq is not an IndexedSeq.

Rationale If code which expects O(1) positional access to a Seq is given a non-IndexedSeq (such as a List, where indexing is O(n)) then this may cause poor performance.

ComparingUnrelatedTypes

Checks for equality comparisons that cannot succeed because the types are unrelated. Eg "string" == BigDecimal(1.0). The scala compiler has a less strict version of this inspection.

ConstantIf

Checks for if statements where the condition is always true or false. Not only checks for the boolean literals, but also any expression that the compiler is able to turn into a constant value. Eg, if (0 < 1) then else that

IllegalFormatString

Checks for a format string that is not invalid, such as invalid conversions, invalid flags, etc. Eg, "% s", "%qs", %.-4f"

IncorrectNumberOfArgsToFormat

Checks for an incorrect number of arguments to String.format. Eg, "%s %s %f".format("need", "three") flags an error because the format string specifies 3 parameters but the call only provides 2.

InvalidRegex

Checks for invalid regex literals that would fail at compile time. Either dangling metacharacters, or unclosed escape characters, etc that kind of thing.

List size

Checks for .size on an instance of List. Eg, val a = List(1,2,3); a.size

Rationale List.size is O(n) so for performance reasons if .size is needed on a list that could be large, consider using an alternative with O(1), eg Array, Vector or ListBuffer.

Redundant finalizer

Checks for empty finalizers. This is redundant code and should be removed. Eg, override def finalize : Unit = { }

PreferSetEmpty

Indicates where code using Set() could be replaced with Set.empty. Set() instantiates a new instance each time it is invoked, whereas Set.empty returns a pre-instantiated instance.

UnnecessaryReturnUse

Checks for use of return in a function or method. Since the final expression of a block is always the return value, using return is unnecessary. Eg, def foo = { println("hello"); return 12; }

UnreachableCatch

Checks for catch clauses that cannot be reached. This means the exception is dead and if you want that exception to take precedence you should move up further up the case list.

UnsafeContains

Checks for List.contains(value) for invalid types. The method for contains accepts any types. This inspection finds situations when you have a list of type A and you are checking for contains on type B which cannot hold.

While true

Checks for code that uses a while(true) or do { } while(true) block.

Rationale: This type of code is usually not meant for production as it will not return normally. If you need to loop until interrupted then consider using a flag.

Other static analysis tools: