Add PMD and enable quickstart ruleset

PMD allows checking code for violation and quickstart provides a ruleset
which are "rules that are most likely to apply everywhere". Violated
rules are disabled to get started.

build.gradle

@@ -47,6 +47,16 @@ allprojects {
     }
 
     apply plugin: "checkstyle"
+    apply plugin: "pmd"
+
+    pmd {
+        consoleOutput = true
+        toolVersion = "6.51.0"
+        rulesMinimumPriority = 5
+        ruleSetFiles = files("${rootProject.rootDir}/config/pmd/pmd-ruleset.xml")
+        ruleSets = []
+        // ignoreFailures = true
+    }
 }
 
 task clean(type: Delete) {

config/pmd/pmd-ruleset.xml

@@ -0,0 +1,62 @@
+<?xml version="1.0"?>
+
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="Custom Rules"
+    xmlns="http://pmd.sourceforge.net/ruleset/2.0.0"
+    xsi:schemaLocation="http://pmd.sourceforge.net/ruleset/2.0.0 https://pmd.sourceforge.io/ruleset_2_0_0.xsd">
+
+    <description>
+        BRouter PMD Ruleset
+    </description>
+
+    <rule ref="rulesets/java/quickstart.xml">
+        <exclude name="AbstractClassWithoutAnyMethod" />
+        <exclude name="AssignmentInOperand" />
+        <exclude name="AssignmentToNonFinalStatic" />
+        <exclude name="AvoidBranchingStatementAsLastInLoop" />
+        <exclude name="AvoidCatchingThrowable" />
+        <exclude name="AvoidInstanceofChecksInCatchClause" />
+        <exclude name="AvoidProtectedFieldInFinalClass" />
+        <exclude name="AvoidUsingVolatile" />
+        <exclude name="CloseResource" />
+        <exclude name="CompareObjectsWithEquals" />
+        <exclude name="ControlStatementBraces" />
+        <exclude name="DoNotCallGarbageCollectionExplicitly" />
+        <exclude name="EmptyCatchBlock" />
+        <exclude name="EmptyControlStatement" />
+        <exclude name="ForLoopCanBeForeach" />
+        <exclude name="LiteralsFirstInComparisons" />
+        <exclude name="LogicInversion" />
+        <exclude name="LooseCoupling" />
+        <exclude name="MissingOverride" />
+        <exclude name="OneDeclarationPerLine" />
+        <exclude name="OverrideBothEqualsAndHashcode" />
+        <exclude name="PreserveStackTrace" />
+        <exclude name="PrimitiveWrapperInstantiation" />
+        <exclude name="ReturnEmptyCollectionRatherThanNull" />
+        <exclude name="SimplifiableTestAssertion" />
+        <exclude name="SimplifiedTernary" />
+        <exclude name="SimplifyBooleanReturns" />
+        <exclude name="SingularField" />
+        <exclude name="UncommentedEmptyConstructor" />
+        <exclude name="UncommentedEmptyMethodBody" />
+        <exclude name="UnnecessaryFullyQualifiedName" />
+        <exclude name="UnnecessaryImport" />
+        <exclude name="UnnecessaryModifier" />
+        <exclude name="UnnecessaryReturn" />
+        <exclude name="UnusedFormalParameter" />
+        <exclude name="UnusedLocalVariable" />
+        <exclude name="UnusedPrivateField" />
+        <exclude name="UnusedPrivateMethod" />
+        <exclude name="UseCollectionIsEmpty" />
+        <exclude name="UseLocaleWithCaseConversions" />
+        <exclude name="UseNotifyAllInsteadOfNotify" />
+        <exclude name="UseUtilityClass" />
+        <exclude name="UselessParentheses" />
+
+        <!-- Naming -->
+        <exclude name="FormalParameterNamingConventions" />
+        <exclude name="LocalVariableNamingConventions" />
+        <exclude name="MethodNamingConventions" />
+    </rule>
+
+</ruleset>