Answer
Custom Java deployment writes directly into specific database tables in the configured environment: FND_JAVA_IMPL_LIBS_TAB stores third-party library JARs and utility JARs, while FND_JAVA_IMPLEMENTATIONS_TAB stores fragment and projection implementation JARs. To verify a deployment actually took effect, check these tables directly in the database for updated JAR content after deploying, rather than assuming success just because the deploy step itself completed without an error.