net.sourceforge.pmd:pmd-ruby (maven):  PMD is a source code analyzer. It finds common programming flaws like unused variables, empty catch blocks, unnecessary object creation, and so forth. It supports many languages. It can be extended with custom rules. It uses JavaCC and Antlr to parse source files into abstract syntax trees (AST) and runs rules against them to find violations. Rules can be written in Java or using a XPath query. It supports Java, JavaScript, Salesforce.com Apex and Visualforce, Kotlin, Swift, Modelica, PLSQL, Apache Velocity, JSP, WSDL, HTML, XML and XSL. Scala is supported, but there are currently no Scala rules available. Additionally, it includes CPD, the copy-paste-detector. CPD finds duplicated code in Coco, C/C++, C#, Dart, Fortran, Gherkin, Go, Groovy, HTML, Java, JavaScript, JSP, Julia, Kotlin, Lua, Matlab, Modelica, Objective-C, Perl, PHP, PLSQL, Python, Ruby, Salesforce.com Apex and Visualforce, Scala, Swift, T-SQL, Typescript and XML. 
Keywords: apex|code-analysis|code-quality|java|linter|plsql|static-analysis|static-code-analysis|swift