Println In Production

ID

kotlin.println_in_production

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Best Practice

Language

Kotlin

Tags

best-practice, leftover-debug, logging

Description

Reports println(…​) / print(…​) calls left in production code. Two shapes are flagged:

  • The top-level kotlin.io stdlib functions (println("x"), print("x")).

  • The System.out.println(…​) / System.err.println(…​) family — calls whose receiver resolves to java.io.PrintStream.

Rationale

println and print are diagnostic-only helpers. They write to standard output/error bypassing the application’s structured logging pipeline. In a deployed service the output cannot be filtered by log level, lacks the usual metadata (timestamp, thread, MDC), and may be silently truncated by the container runtime.

// Bad
fun load() {
    println("loaded")
    System.err.println("oops")
}

// Good
private val log = LoggerFactory.getLogger(MyService::class.java)

fun load() {
    log.info("loaded")
    log.error("oops")
}

Exceptions

Member-style println on a caller-supplied PrintWriter or other appender is not flagged: passing the sink in is a deliberate routing decision.

Remediation

Use a logging facade — slf4j-api, kotlin-logging, or Android Log — and ship structured logs through the application’s existing pipeline.

References