Source: expvar.go in package expvar

Package expvar provides a standardized interface to public variables, such as operation counters in servers. It exposes these variables via HTTP at /debug/vars in JSON format. Operations to set or modify these public variables are atomic. In addition to adding the HTTP handler, this package registers the following variables: cmdline os.Args memstats runtime.Memstats The package is sometimes only imported for the side effect of registering its HTTP handler and the above variables. To use it this way, link this package into your program: import _ "expvar"

package expvar

import (
	"encoding/json"
	"fmt"
	"log"
	"math"
	"net/http"
	"os"
	"runtime"
	"sort"
	"strconv"
	"strings"
	"sync"
	"sync/atomic"
)

Var is an abstract type for all exported variables.

String returns a valid JSON value for the variable. Types with String methods that do not return valid JSON (such as time.Time) must not be used as a Var.

	String() string
}

Int is a 64-bit integer variable that satisfies the Var interface.

type Int struct {
	i int64
}

func (v *Int) Value() int64 {
	return atomic.LoadInt64(&v.i)
}

func (v *Int) String() string {
	return strconv.FormatInt(atomic.LoadInt64(&v.i), 10)
}

func (v *Int) Add(delta int64) {
	atomic.AddInt64(&v.i, delta)
}

func (v *Int) Set(value int64) {
	atomic.StoreInt64(&v.i, value)
}

Float is a 64-bit float variable that satisfies the Var interface.

type Float struct {
	f uint64
}

func (v *Float) Value() float64 {
	return math.Float64frombits(atomic.LoadUint64(&v.f))
}

func (v *Float) String() string {
	return strconv.FormatFloat(
		math.Float64frombits(atomic.LoadUint64(&v.f)), 'g', -1, 64)
}

Add adds delta to v.

func (v *Float) Add(delta float64) {
	for {
		cur := atomic.LoadUint64(&v.f)
		curVal := math.Float64frombits(cur)
		nxtVal := curVal + delta
		nxt := math.Float64bits(nxtVal)
		if atomic.CompareAndSwapUint64(&v.f, cur, nxt) {
			return
		}
	}
}

Set sets v to value.

func (v *Float) Set(value float64) {
	atomic.StoreUint64(&v.f, math.Float64bits(value))
}

Map is a string-to-Var map variable that satisfies the Var interface.

type Map struct {
	m      sync.Map // map[string]Var
	keysMu sync.RWMutex
	keys   []string // sorted
}

KeyValue represents a single entry in a Map.

type KeyValue struct {
	Key   string
	Value Var
}

func (v *Map) String() string {
	var b strings.Builder
	fmt.Fprintf(&b, "{")
	first := true
	v.Do(func(kv KeyValue) {
		if !first {
			fmt.Fprintf(&b, ", ")
		}
		fmt.Fprintf(&b, "%q: %v", kv.Key, kv.Value)
		first = false
	})
	fmt.Fprintf(&b, "}")
	return b.String()
}

Init removes all keys from the map.

func (v *Map) Init() *Map {
	v.keysMu.Lock()
	defer v.keysMu.Unlock()
	v.keys = v.keys[:0]
	v.m.Range(func(k, _ interface{}) bool {
		v.m.Delete(k)
		return true
	})
	return v
}

addKey updates the sorted list of keys in v.keys.

func (v *Map) addKey(key string) {
	v.keysMu.Lock()

Using insertion sort to place key into the already-sorted v.keys.

	if i := sort.SearchStrings(v.keys, key); i >= len(v.keys) {
		v.keys = append(v.keys, key)
	} else if v.keys[i] != key {
		v.keys = append(v.keys, "")
		copy(v.keys[i+1:], v.keys[i:])
		v.keys[i] = key
	}
}

func (v *Map) Get(key string) Var {
	i, _ := v.m.Load(key)
	av, _ := i.(Var)
	return av
}

Before we store the value, check to see whether the key is new. Try a Load before LoadOrStore: LoadOrStore causes the key interface to escape even on the Load path.

	if _, ok := v.m.Load(key); !ok {
		if _, dup := v.m.LoadOrStore(key, av); !dup {
			v.addKey(key)
			return
		}
	}

	v.m.Store(key, av)
}

Add adds delta to the *Int value stored under the given map key.

func (v *Map) Add(key string, delta int64) {
	i, ok := v.m.Load(key)
	if !ok {
		var dup bool
		i, dup = v.m.LoadOrStore(key, new(Int))
		if !dup {
			v.addKey(key)
		}
	}

Add to Int; ignore otherwise.

	if iv, ok := i.(*Int); ok {
		iv.Add(delta)
	}
}

AddFloat adds delta to the *Float value stored under the given map key.

func (v *Map) AddFloat(key string, delta float64) {
	i, ok := v.m.Load(key)
	if !ok {
		var dup bool
		i, dup = v.m.LoadOrStore(key, new(Float))
		if !dup {
			v.addKey(key)
		}
	}

Add to Float; ignore otherwise.

	if iv, ok := i.(*Float); ok {
		iv.Add(delta)
	}
}

Delete deletes the given key from the map.

func (v *Map) Delete(key string) {
	v.keysMu.Lock()
	defer v.keysMu.Unlock()
	i := sort.SearchStrings(v.keys, key)
	if i < len(v.keys) && key == v.keys[i] {
		v.keys = append(v.keys[:i], v.keys[i+1:]...)
		v.m.Delete(key)
	}
}

Do calls f for each entry in the map. The map is locked during the iteration, but existing entries may be concurrently updated.

func (v *Map) Do(f func(KeyValue)) {
	v.keysMu.RLock()
	defer v.keysMu.RUnlock()
	for _, k := range v.keys {
		i, _ := v.m.Load(k)
		f(KeyValue{k, i.(Var)})
	}
}

String is a string variable, and satisfies the Var interface.

type String struct {
	s atomic.Value // string
}

func (v *String) Value() string {
	p, _ := v.s.Load().(string)
	return p
}

String implements the Var interface. To get the unquoted string use Value.

func (v *String) String() string {
	s := v.Value()
	b, _ := json.Marshal(s)
	return string(b)
}

func (v *String) Set(value string) {
	v.s.Store(value)
}

Func implements Var by calling the function and formatting the returned value using JSON.

type Func func() interface{}

func (f Func) Value() interface{} {
	return f()
}

func (f Func) String() string {
	v, _ := json.Marshal(f())
	return string(v)
}

All published variables.

var (
	vars      sync.Map // map[string]Var
	varKeysMu sync.RWMutex
	varKeys   []string // sorted
)

Publish declares a named exported variable. This should be called from a package's init function when it creates its Vars. If the name is already registered then this will log.Panic.

func Publish(name string, v Var) {
	if _, dup := vars.LoadOrStore(name, v); dup {
		log.Panicln("Reuse of exported var name:", name)
	}
	varKeysMu.Lock()
	defer varKeysMu.Unlock()
	varKeys = append(varKeys, name)
	sort.Strings(varKeys)
}

Get retrieves a named exported variable. It returns nil if the name has not been registered.

func Get(name string) Var {
	i, _ := vars.Load(name)
	v, _ := i.(Var)
	return v
}

Convenience functions for creating new exported variables.


func NewInt(name string) *Int {
	v := new(Int)
	Publish(name, v)
	return v
}

func NewFloat(name string) *Float {
	v := new(Float)
	Publish(name, v)
	return v
}

func NewMap(name string) *Map {
	v := new(Map).Init()
	Publish(name, v)
	return v
}

func NewString(name string) *String {
	v := new(String)
	Publish(name, v)
	return v
}

Do calls f for each exported variable. The global variable map is locked during the iteration, but existing entries may be concurrently updated.

func Do(f func(KeyValue)) {
	varKeysMu.RLock()
	defer varKeysMu.RUnlock()
	for _, k := range varKeys {
		val, _ := vars.Load(k)
		f(KeyValue{k, val.(Var)})
	}
}

func expvarHandler(w http.ResponseWriter, r *http.Request) {
	w.Header().Set("Content-Type", "application/json; charset=utf-8")
	fmt.Fprintf(w, "{\n")
	first := true
	Do(func(kv KeyValue) {
		if !first {
			fmt.Fprintf(w, ",\n")
		}
		first = false
		fmt.Fprintf(w, "%q: %s", kv.Key, kv.Value)
	})
	fmt.Fprintf(w, "\n}\n")
}

Handler returns the expvar HTTP Handler. This is only needed to install the handler in a non-standard location.

func Handler() http.Handler {
	return http.HandlerFunc(expvarHandler)
}

func cmdline() interface{} {
	return os.Args
}

func memstats() interface{} {
	stats := new(runtime.MemStats)
	runtime.ReadMemStats(stats)
	return *stats
}

func init() {
	http.HandleFunc("/debug/vars", expvarHandler)
	Publish("cmdline", Func(cmdline))
	Publish("memstats", Func(memstats))