Source: typeinfo.go in package encoding/xml


package xml

import (
	"fmt"
	"reflect"
	"strings"
	"sync"
)

typeInfo holds details for the xml representation of a type.

type typeInfo struct {
	xmlname *fieldInfo
	fields  []fieldInfo
}

fieldInfo holds details for the xml representation of a single field.

type fieldInfo struct {
	idx     []int
	name    string
	xmlns   string
	flags   fieldFlags
	parents []string
}

type fieldFlags int

const (
	fElement fieldFlags = 1 << iota
	fAttr
	fCDATA
	fCharData
	fInnerXML
	fComment
	fAny

	fOmitEmpty

	fMode = fElement | fAttr | fCDATA | fCharData | fInnerXML | fComment | fAny

	xmlName = "XMLName"
)

var tinfoMap sync.Map // map[reflect.Type]*typeInfo

var nameType = reflect.TypeOf(Name{})

getTypeInfo returns the typeInfo structure with details necessary for marshaling and unmarshaling typ.

func getTypeInfo(typ reflect.Type) (*typeInfo, error) {
	if ti, ok := tinfoMap.Load(typ); ok {
		return ti.(*typeInfo), nil
	}

	tinfo := &typeInfo{}
	if typ.Kind() == reflect.Struct && typ != nameType {
		n := typ.NumField()
		for i := 0; i < n; i++ {
			f := typ.Field(i)
			if (f.PkgPath != "" && !f.Anonymous) || f.Tag.Get("xml") == "-" {
				continue // Private field
			}

For embedded structs, embed its fields.

			if f.Anonymous {
				t := f.Type
				if t.Kind() == reflect.Ptr {
					t = t.Elem()
				}
				if t.Kind() == reflect.Struct {
					inner, err := getTypeInfo(t)
					if err != nil {
						return nil, err
					}
					if tinfo.xmlname == nil {
						tinfo.xmlname = inner.xmlname
					}
					for _, finfo := range inner.fields {
						finfo.idx = append([]int{i}, finfo.idx...)
						if err := addFieldInfo(typ, tinfo, &finfo); err != nil {
							return nil, err
						}
					}
					continue
				}
			}

			finfo, err := structFieldInfo(typ, &f)
			if err != nil {
				return nil, err
			}

			if f.Name == xmlName {
				tinfo.xmlname = finfo
				continue
			}

Add the field if it doesn't conflict with other fields.

			if err := addFieldInfo(typ, tinfo, finfo); err != nil {
				return nil, err
			}
		}
	}

	ti, _ := tinfoMap.LoadOrStore(typ, tinfo)
	return ti.(*typeInfo), nil
}

structFieldInfo builds and returns a fieldInfo for f.

func structFieldInfo(typ reflect.Type, f *reflect.StructField) (*fieldInfo, error) {
	finfo := &fieldInfo{idx: f.Index}

Split the tag from the xml namespace if necessary.

	tag := f.Tag.Get("xml")
	if i := strings.Index(tag, " "); i >= 0 {
		finfo.xmlns, tag = tag[:i], tag[i+1:]
	}

Parse flags.

	tokens := strings.Split(tag, ",")
	if len(tokens) == 1 {
		finfo.flags = fElement
	} else {
		tag = tokens[0]
		for _, flag := range tokens[1:] {
			switch flag {
			case "attr":
				finfo.flags |= fAttr
			case "cdata":
				finfo.flags |= fCDATA
			case "chardata":
				finfo.flags |= fCharData
			case "innerxml":
				finfo.flags |= fInnerXML
			case "comment":
				finfo.flags |= fComment
			case "any":
				finfo.flags |= fAny
			case "omitempty":
				finfo.flags |= fOmitEmpty
			}
		}

Validate the flags used.

		valid := true
		switch mode := finfo.flags & fMode; mode {
		case 0:
			finfo.flags |= fElement
		case fAttr, fCDATA, fCharData, fInnerXML, fComment, fAny, fAny | fAttr:
			if f.Name == xmlName || tag != "" && mode != fAttr {
				valid = false
			}

This will also catch multiple modes in a single field.

			valid = false
		}
		if finfo.flags&fMode == fAny {
			finfo.flags |= fElement
		}
		if finfo.flags&fOmitEmpty != 0 && finfo.flags&(fElement|fAttr) == 0 {
			valid = false
		}
		if !valid {
			return nil, fmt.Errorf("xml: invalid tag in field %s of type %s: %q",
				f.Name, typ, f.Tag.Get("xml"))
		}
	}

Use of xmlns without a name is not allowed.

	if finfo.xmlns != "" && tag == "" {
		return nil, fmt.Errorf("xml: namespace without name in field %s of type %s: %q",
			f.Name, typ, f.Tag.Get("xml"))
	}

The XMLName field records the XML element name. Don't process it as usual because its name should default to empty rather than to the field name.

		finfo.name = tag
		return finfo, nil
	}

If the name part of the tag is completely empty, get default from XMLName of underlying struct if feasible, or field name otherwise.

		if xmlname := lookupXMLName(f.Type); xmlname != nil {
			finfo.xmlns, finfo.name = xmlname.xmlns, xmlname.name
		} else {
			finfo.name = f.Name
		}
		return finfo, nil
	}

Prepare field name and parents.

	parents := strings.Split(tag, ">")
	if parents[0] == "" {
		parents[0] = f.Name
	}
	if parents[len(parents)-1] == "" {
		return nil, fmt.Errorf("xml: trailing '>' in field %s of type %s", f.Name, typ)
	}
	finfo.name = parents[len(parents)-1]
	if len(parents) > 1 {
		if (finfo.flags & fElement) == 0 {
			return nil, fmt.Errorf("xml: %s chain not valid with %s flag", tag, strings.Join(tokens[1:], ","))
		}
		finfo.parents = parents[:len(parents)-1]
	}

If the field type has an XMLName field, the names must match so that the behavior of both marshaling and unmarshaling is straightforward and unambiguous.

	if finfo.flags&fElement != 0 {
		ftyp := f.Type
		xmlname := lookupXMLName(ftyp)
		if xmlname != nil && xmlname.name != finfo.name {
			return nil, fmt.Errorf("xml: name %q in tag of %s.%s conflicts with name %q in %s.XMLName",
				finfo.name, typ, f.Name, xmlname.name, ftyp)
		}
	}
	return finfo, nil
}

lookupXMLName returns the fieldInfo for typ's XMLName field in case it exists and has a valid xml field tag, otherwise it returns nil.

func lookupXMLName(typ reflect.Type) (xmlname *fieldInfo) {
	for typ.Kind() == reflect.Ptr {
		typ = typ.Elem()
	}
	if typ.Kind() != reflect.Struct {
		return nil
	}
	for i, n := 0, typ.NumField(); i < n; i++ {
		f := typ.Field(i)
		if f.Name != xmlName {
			continue
		}
		finfo, err := structFieldInfo(typ, &f)
		if err == nil && finfo.name != "" {
			return finfo

Also consider errors as a non-existent field tag and let getTypeInfo itself report the error.

		break
	}
	return nil
}

func min(a, b int) int {
	if a <= b {
		return a
	}
	return b
}

addFieldInfo adds finfo to tinfo.fields if there are no conflicts, or if conflicts arise from previous fields that were obtained from deeper embedded structures than finfo. In the latter case, the conflicting entries are dropped. A conflict occurs when the path (parent + name) to a field is itself a prefix of another path, or when two paths match exactly. It is okay for field paths to share a common, shorter prefix.

func addFieldInfo(typ reflect.Type, tinfo *typeInfo, newf *fieldInfo) error {
	var conflicts []int

First, figure all conflicts. Most working code will have none.

	for i := range tinfo.fields {
		oldf := &tinfo.fields[i]
		if oldf.flags&fMode != newf.flags&fMode {
			continue
		}
		if oldf.xmlns != "" && newf.xmlns != "" && oldf.xmlns != newf.xmlns {
			continue
		}
		minl := min(len(newf.parents), len(oldf.parents))
		for p := 0; p < minl; p++ {
			if oldf.parents[p] != newf.parents[p] {
				continue Loop
			}
		}
		if len(oldf.parents) > len(newf.parents) {
			if oldf.parents[len(newf.parents)] == newf.name {
				conflicts = append(conflicts, i)
			}
		} else if len(oldf.parents) < len(newf.parents) {
			if newf.parents[len(oldf.parents)] == oldf.name {
				conflicts = append(conflicts, i)
			}
		} else {
			if newf.name == oldf.name {
				conflicts = append(conflicts, i)
			}
		}

Without conflicts, add the new field and return.

	if conflicts == nil {
		tinfo.fields = append(tinfo.fields, *newf)
		return nil
	}

If any conflict is shallower, ignore the new field. This matches the Go field resolution on embedding.

	for _, i := range conflicts {
		if len(tinfo.fields[i].idx) < len(newf.idx) {
			return nil
		}
	}

Otherwise, if any of them is at the same depth level, it's an error.

	for _, i := range conflicts {
		oldf := &tinfo.fields[i]
		if len(oldf.idx) == len(newf.idx) {
			f1 := typ.FieldByIndex(oldf.idx)
			f2 := typ.FieldByIndex(newf.idx)
			return &TagPathError{typ, f1.Name, f1.Tag.Get("xml"), f2.Name, f2.Tag.Get("xml")}
		}
	}

Otherwise, the new field is shallower, and thus takes precedence, so drop the conflicting fields from tinfo and append the new one.

	for c := len(conflicts) - 1; c >= 0; c-- {
		i := conflicts[c]
		copy(tinfo.fields[i:], tinfo.fields[i+1:])
		tinfo.fields = tinfo.fields[:len(tinfo.fields)-1]
	}
	tinfo.fields = append(tinfo.fields, *newf)
	return nil
}

A TagPathError represents an error in the unmarshaling process caused by the use of field tags with conflicting paths.

type TagPathError struct {
	Struct       reflect.Type
	Field1, Tag1 string
	Field2, Tag2 string
}

func (e *TagPathError) Error() string {
	return fmt.Sprintf("%s field %q with tag %q conflicts with field %q with tag %q", e.Struct, e.Field1, e.Tag1, e.Field2, e.Tag2)
}

const (
	initNilPointers     = true
	dontInitNilPointers = false
)

value returns v's field value corresponding to finfo. It's equivalent to v.FieldByIndex(finfo.idx), but when passed initNilPointers, it initializes and dereferences pointers as necessary. When passed dontInitNilPointers and a nil pointer is reached, the function returns a zero reflect.Value.

func (finfo *fieldInfo) value(v reflect.Value, shouldInitNilPointers bool) reflect.Value {
	for i, x := range finfo.idx {
		if i > 0 {
			t := v.Type()
			if t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Struct {
				if v.IsNil() {
					if !shouldInitNilPointers {
						return reflect.Value{}
					}
					v.Set(reflect.New(v.Type().Elem()))
				}
				v = v.Elem()
			}
		}
		v = v.Field(x)
	}
	return v