How to test logging of a zap Logger built from custom Config?

Question

I have a Zap logger that is generated from a custom Config (i.e. config.Build()). I would like to test the logger by calling, for example, logger.Info() in the test method and assert the result to see if it is according to the config set. How can I achieve this?

Code example:

func GetLogger() *zap.Logger{
 config := &zap.Config{
  Encoding: "json",
  Level: zap.NewAtomicLevelAt(zapcore.InfoLevel),
  OutputPaths: []string{"stdout"},
  ErrorOutputPaths: []string{"stdout"},
  EncoderConfig: zapcore.EncoderConfig{
   MessageKey: "@m",
   LevelKey:    "@l",
   EncodeLevel: zapcore.CapitalLevelEncoder,
   TimeKey:    "@t",
   EncodeTime: zapcore.EpochMillisTimeEncoder,
   CallerKey:     "@c",
   EncodeCaller:  zapcore.ShortCallerEncoder,
   StacktraceKey: "@x",
  },
 }
 return config.Build()
}

Answer 1

Zap has a concept of sinks, destinations for log messages. For testing, implement a sink that simply remembers messages (for instance in a bytes.Buffer):

package main

import (
    "bytes"
    "net/url"
    "strings"
    "testing"
    "time"

    "go.uber.org/zap"
)

// MemorySink implements zap.Sink by writing all messages to a buffer.
type MemorySink struct {
    *bytes.Buffer
}

// Implement Close and Sync as no-ops to satisfy the interface. The Write 
// method is provided by the embedded buffer.

func (s *MemorySink) Close() error { return nil }
func (s *MemorySink) Sync() error  { return nil }


func TestLogger(t *testing.T) {
    // Create a sink instance, and register it with zap for the "memory" 
    // protocol.
    sink := &MemorySink{new(bytes.Buffer)}
    zap.RegisterSink("memory", func(*url.URL) (zap.Sink, error) {
        return sink, nil
    })

    conf := zap.NewProductionConfig() // TODO: replace with real config

    // Redirect all messages to the MemorySink.    
    conf.OutputPaths = []string{"memory://"}

    l, err := conf.Build()
    if err != nil {
        t.Fatal(err)
    }

    l.Info("failed to fetch URL",
        zap.String("url", "http://example.com"),
        zap.Int("attempt", 3),
        zap.Duration("backoff", time.Second),
    )

    // Assert sink contents

    output := sink.String()
    t.Logf("output = %s", output)

    if !strings.Contains(output, `"url":"http://example.com"`) {
        t.Error("output missing: url=http://example.com")
    }
}