accesslog: SetFormatter called with nil Writer

Error message

accesslog: SetFormatter called with nil Writer

What it means

AccessLog.SetFormatter panics immediately if the AccessLog's Writer field is nil (middleware/accesslog/accesslog.go:672), because a formatter has nowhere to write output. The check runs before the formatter is even inspected.

Source

Thrown at middleware/accesslog/accesslog.go:672

				err = fmt.Errorf("%v, %v", err, tErr)
			}
		}
	}
	ac.mu.Unlock()

	return err
}

// SetFormatter sets a custom formatter to print the logs.
// Any custom output writers should be
// already registered before calling this method.
// Returns this AccessLog instance.
//
// Usage:
// ac.SetFormatter(&accesslog.JSON{Indent: "    "})
func (ac *AccessLog) SetFormatter(f Formatter) *AccessLog {
	if ac.Writer == nil {
		panic("accesslog: SetFormatter called with nil Writer")
	}

	if f == nil {
		return ac
	}

	if flusher, ok := ac.formatter.(Flusher); ok {
		// PREPEND formatter flushes, they should run before destination's ones.
		ac.Flushers = append([]Flusher{flusher}, ac.Flushers...)
	}

	// Inject the writer (AccessLog) here, the writer
	// is protected with mutex.
	f.SetOutput(ac)

	ac.formatter = f
	return ac
}

View on GitHub (pinned to 7bedaf55a0)

Solutions

Initialize the AccessLog with accesslog.New(io.Writer) so Writer is always set.
Set ac.Writer to a valid io.Writer (e.g. an *os.File) before calling SetFormatter.
Reorder code so SetFormatter is called after Writer assignment when building AccessLog manually.

Example fix

// before
ac := &accesslog.AccessLog{}
ac.SetFormatter(&accesslog.JSON{}) // panics: nil Writer

// after
f := accesslog.File("./access.log")
ac := accesslog.New(f)
ac.SetFormatter(&accesslog.JSON{})

Defensive patterns

Strategy: validation

Validate before calling

// Guard before calling SetFormatter
if ac.Writer == nil {
    return errors.New("accesslog writer not initialized")
}
ac.SetFormatter(&accesslog.JSON{})

Type guard

func writerSet(ac *accesslog.AccessLog) bool {
    return ac != nil && ac.Writer != nil
}

Try / catch

defer func() {
    if r := recover(); r != nil {
        log.Fatalf("SetFormatter: %v", r)
    }
}()

Prevention

Always build AccessLog via accesslog.New(writer), never as a bare struct literal.
Set Writer before any formatter configuration in initialization code.
Add an init-time assertion that ac.Writer != nil.

When it happens

Trigger: Creating an AccessLog struct manually (not via accesslog.New, which requires a writer) with Writer left nil and then calling ac.SetFormatter(&accesslog.JSON{...}).

Common situations: Constructing &accesslog.AccessLog{} directly and setting fields piecemeal; assigning a nil writer variable by mistake; reordering initialization so SetFormatter runs before Writer is set.

Related errors

AI-assisted analysis of kataras/iris@7bedaf55a0 (2026-08-30). Data as JSON: /api/errors/abc7917f6b8687d8. Report an issue: GitHub.