How can I force the 'this' keyword on local fields and methods with the Spotless Java code formatter?

I want to enforce the use of the 'this' keyword using Spotless. For example: getTowerData().recordMap would be this.getTowerData().recordMap.

I'm running Spotless with Gradle with the current configuration:

spotless {java {importOrder('emu.grasscutter', '', 'java', 'javax', '\\#java', '\\#')googleJavaFormat('1.15.0')formatAnnotations()endWithNewline()indentWithTabs(2); indentWithSpaces(4)toggleOffOn()}}

Currently, I've tried using:

custom('this', {// Force use of 'this.' keywordit.replaceAll('^\\s*(?!public|private|protected)(?!class|interface|enum|@interface)\\b(?!return)\\w+\\b', 'this.$0')})

but this results in:

Step 'google-java-format' found problem in 'src\main\java\emu\grasscutter\auth\AuthenticationSystem.java':nulljava.lang.reflect.InvocationTargetExceptionCaused by: com.google.googlejavaformat.java.FormatterException: 1:2: error: class, interface, enum, or record expected