go反射
什么是反射？使用反射可以实现什么功能？
反射提供了一种可以操作任意类型数据结构的能力。通过反射你可以实现对任意类型的深度复制，可以对任意类型执行自定义的操作。另外由于golang可以给结构体定义tag熟悉，结合反射，于是就可以实现结构体与数据库表、结构体与json对象之间的相互转换。
使用反射需要注意什么事情？
使用反射时需要先确定要操作的值是否是期望的类型，是否是可以进行“赋值”操作的，否则reflect包将会毫不留情的产生一个panic。
struct initializer示例
go-struct-initializer 是我在学习golang反射过程中实现的一个可以根据struct tag初始化结构体的go library，这里对其中用到的反射技术做一下说明
package structinitializer
import (
"reflect"
"fmt"
"strconv"
"strings"
)
const (
ERROR_NOT_A_POINTER = "NotAPointer"
ERROR_NOT_IMPLEMENT = "NotImplement"
ERROR_POINTE_TO_NON_STRUCT = "PointToNonStruct"
ERROR_TYPE_CONVERSION_FAILED = "TypeConversionFailed"
ERROR_MULTI_ERRORS_FOUND = "MultiErrorsFound"
)
type Error struct {
Reason string
Message string
Stack string
}
func NewError(reason, message string, stack string) error {
return &Error{reason, message, stack}
}
func mergedError(errors []error, stackName string) error {
if len(errors) == 0 {
return nil
}
if len(errors) == 1 {
return errors[0]
}
points := make([]string, len(errors))
for i, err := range errors {
points[i] = fmt.Sprintf("* %s", err)
}
return &Error{
ERROR_MULTI_ERRORS_FOUND,
strings.Join(points, "\n"),
stackName,
}
}
func (e *Error) Error() string {
if e.Stack == "" {
return fmt.Sprintf("%s: %s", e.Reason, e.Message)
}
return fmt.Sprintf("%s:(%s) %s", e.Reason, e.Stack, e.Message)
}
type InitialiserConfig struct {
TagName string
}
type Initialiser struct {
config *InitialiserConfig
}
func NewStructInitialiser() Initialiser {
config := InitialiserConfig{
TagName: "default",
}
initialiser := Initialiser{
config: &config,
}
return initialiser
}
func (self *Initialiser) initialiseInt(stackName, tag string, val reflect.Value) error {
if tag != "" {
i, err := strconv.ParseInt(tag, 10, 64)
if err != nil {
return NewError(
ERROR_TYPE_CONVERSION_FAILED,
fmt.Sprintf(`"%s" can't convert to int64`, tag),
stackName,
)
}
if val.Int() == 0 {
val.SetInt(int64(i))
}
}
return nil
}
func (self *Initialiser) initialiseUint(stackName, tag string, val reflect.Value) error {
if tag != "" {
i, err := strconv.ParseUint(tag, 10, 64)
if err != nil {
return NewError(
ERROR_TYPE_CONVERSION_FAILED,
fmt.Sprintf(`"%s" can't convert to uint64`, tag),
stackName,
)
}
if val.Uint() == 0 {
val.SetUint(uint64(i)) // 设置value
}
}
return nil
}
func (self *Initialiser) initialiseString(stackName, tag string, val reflect.Value) error {
if tag != "" && val.String() == ""{
val.SetString(tag)
}
return nil
}
// 检查某个结构体是否存在某个域设置了default标签
func (self *Initialiser) checkStructHasDefaultValue(typ reflect.Type) bool {
for i:=0; i<typ.NumField(); i++ {
field := typ.Field(i)
if field.Tag.Get(self.config.TagName) != "" { // 获取struct field的指定tag
return true
}
if field.Type.Kind() == reflect.Struct && self.checkStructHasDefaultValue(field.Type) { //
return true
}
}
return false
}
func (self *Initialiser) initialisePtr(stackName, tag string, val reflect.Value) error {
if tag == "-" {
return nil
}
typ := val.Type().Elem()
if tag == "" {
if typ.Kind() != reflect.Struct {
return nil
} else if !self.checkStructHasDefaultValue(typ) {
return nil
}
}
// 结构体存在至少一个域设置了default标签才有初始化的必要
realVal := reflect.New(typ) // 根据结构体类型创建一个新的结构体
// Indirect可以获取到value对应的真实value（指针指向的value）
if err := self.initialise(stackName, tag, reflect.Indirect(realVal)); err != nil {
return err
}
val.Set(realVal)
return nil
}
func (self *Initialiser) initialise(stackName string, tag string, val reflect.Value) error {
typ := val.Type()
kind := typ.Kind()
if kind == reflect.Struct {
return self.initialiseStruct(stackName, val)
}
switch kind {
case reflect.Int:
fallthrough
case reflect.Int8:
fallthrough
case reflect.Int16:
fallthrough
case reflect.Int32:
fallthrough
case reflect.Int64:
return self.initialiseInt(stackName, tag, val)
case reflect.Uint:
fallthrough
case reflect.Uint8:
fallthrough
case reflect.Uint16:
fallthrough
case reflect.Uint32:
fallthrough
case reflect.Uint64:
fallthrough
case reflect.Uintptr:
return self.initialiseUint(stackName, tag, val)
case reflect.String:
return self.initialiseString(stackName, tag, val)
case reflect.Ptr:
return self.initialisePtr(stackName, tag, val)
default:
return NewError(ERROR_NOT_IMPLEMENT, "not implement", stackName)
}
return NewError(ERROR_NOT_IMPLEMENT, "not implement", stackName)
}
func (self *Initialiser) initialiseStruct(stackName string, structVal reflect.Value) error {
structs := make([]reflect.Value, 1, 5)
structs[0] = structVal
errors := make([]error, 0)
for len(structs) > 0 {
structVal := structs[0]
structs = structs[1:]
structTyp := structVal.Type() // 结构体类型信息
for i:=0; i<structTyp.NumField(); i++ { // 循环结构体的所有域
structField := structTyp.Field(i) // 获取结构体域信息
tag := structField.Tag // 获取结构体域的标签
defaultTag := tag.Get(self.config.TagName);
if structField.Anonymous { // 这个域是一个结构体组合
structs = append(structs, structVal.Field(i))
continue
}
fieldName := structField.Name // 获取结构体域的名字
if stackName != "" {
fieldName = fmt.Sprintf("%s.%s", stackName, fieldName)
}
err := self.initialise(
fieldName,
defaultTag,
structVal.Field(i), // 获取结构体域对应的值信息
)
if err != nil {
errors = append(errors, err)
}
}
}
return mergedError(errors, stackName)
}
func (self *Initialiser) Initialise(inf interface{}) error {
typ := reflect.TypeOf(inf)
kind := typ.Kind() // kind与type的区别在于：kind无法区分基本类型的别名
if kind != reflect.Ptr { // 只有指针才能执行初始化操作
return NewError(
ERROR_NOT_A_POINTER,
fmt.Sprintf("%s not a pointer", typ),
"",
)
}
val := reflect.ValueOf(inf).Elem() // 获取指针对应的值信息
kind = val.Kind()
if kind != reflect.Struct {
return NewError(
ERROR_POINTE_TO_NON_STRUCT,
fmt.Sprintf("%s point to non struct", typ),
"",
)
}
return self.initialiseStruct(val.Type().Name(), val)
}
func InitializeStruct(struct_ interface{}) error {
initialiser := NewStructInitialiser()
return initialiser.Initialise(struct_)
}package structinitializer
import (
"testing"
"fmt"
"reflect"
)
type Validator interface {
Valid() bool
}
type TNormStruct struct {
AInt int
AStr string
}
type TStruct struct {
AInt int `default:"10"`
AStr string `default:"hello"`
}
func (self *TStruct) Valid() bool {
return self.AStr == "hello" && self.AInt == 10
}
type TWrongStruct struct {
AInt int `default:"hhhhh"`
AStr string `default:"hello"`
}
func (self *TWrongStruct) Valid() bool {
return false
}
type TEmbeddStruct struct {
AStruct TStruct
AInt int `default:"101"`
}
func (self *TEmbeddStruct) Valid() bool {
return self.AStruct.Valid() && self.AInt == 101
}
type TEmbeddStruct2 struct {
AStruct TStruct
AInt int
}
type TAnonymousStruct struct {
TStruct
AUint uint16 `default:"20"`
}
func (self *TAnonymousStruct) Valid() bool {
return self.TStruct.Valid() && self.AUint == 20
}
type TPointerStruct struct {
AStructPtr *TStruct
AIntPtr *int32 `default:"20"`
AStrPtr *string `default:"-"`
AUintPtr *uint
ANormStruct *TNormStruct
}
func (self *TPointerStruct) Valid() bool {
if self.AStructPtr != nil && self.AStructPtr.Valid() && self.AIntPtr != nil && *self.AIntPtr == 20 && self.AStrPtr == nil && self.AUintPtr == nil && self.ANormStruct == nil {
return true
}
return false
}
func AssertTrue(t *testing.T, a bool, msg string) {
if !a {
t.Errorf(msg)
}
}
func TestCheckStructHasDefaultValue(t *testing.T) {
self := NewStructInitialiser()
AssertTrue(t, self.checkStructHasDefaultValue(reflect.TypeOf(TStruct{})), "TStruct check failed!")
AssertTrue(t, !self.checkStructHasDefaultValue(reflect.TypeOf(TNormStruct{})), "TNormStrcut check succeed!")
AssertTrue(t, self.checkStructHasDefaultValue(reflect.TypeOf(TEmbeddStruct2{})), "TEmbeddStruct2 check failed!")
}
func checkError(t *testing.T, err error, wantReason string) {
if err == nil {
t.Errorf("except non pointer err!")
return
} else {
if err1 := err.(*Error); err1.Reason != wantReason {
t.Errorf("want '%s' but '%s': %s", wantReason, err1.Reason, err1.Message)
return
}
}
fmt.Printf("%#v\n", err)
}
func TestNonPointer(t *testing.T) {
aStruct := TStruct{}
err := InitializeStruct(aStruct)
checkError(t, err, ERROR_NOT_A_POINTER)
}
func TestPointToNonStruct(t *testing.T) {
aInt := 100
err := InitializeStruct(&aInt)
checkError(t, err, ERROR_POINTE_TO_NON_STRUCT)
}
func TestWrongTag(t *testing.T) {
aStruct := TWrongStruct{}
err := InitializeStruct(&aStruct)
checkError(t, err, ERROR_TYPE_CONVERSION_FAILED)
}
func TestDefaultSetted(t *testing.T) {
aStruct := TStruct{
AInt: 103,
}
err := InitializeStruct(&aStruct)
if err != nil || aStruct.AInt != 103 {
t.Errorf("BOOM!!! rewrite a user setted value? err: %s", err)
}
fmt.Printf("%#v\n", aStruct)
}
func checkStruct(t *testing.T, v Validator) {
err := InitializeStruct(v)
if err != nil {
t.Errorf("unexcept err: %#v, aStruct: %#v\n", err, v)
return
}
if !v.Valid() {
t.Errorf("init incorret: %#v\n", v)
return
}
fmt.Printf("%#v\n", v)
}
func TestEmbeddStruct(t *testing.T) {
aStruct := TEmbeddStruct{}
checkStruct(t, &aStruct)
}
func TestAnonymousStrct(t *testing.T) {
aStruct := TAnonymousStruct{}
checkStruct(t, &aStruct)
}
func TestNormalStruct(t *testing.T) {
aStruct := TStruct{}
checkStruct(t, &aStruct)
}
func TestPointerStruct(t *testing.T) {
aStruct := TPointerStruct{}
checkStruct(t, &aStruct)
}
