xml

package
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 Go to latest Published: May 10, 2022 License: BSD-3-Clause Imports: 12 Imported by: 0

Documentation

Overview

Package xml implements a simple XML 1.0 parser that understands XML name spaces.

Index

Examples

Constants

View Source 
const (
	// A generic XML header suitable for use with the output of Marshal.
	// This is not automatically added to any output of this package,
	// it is provided as a convenience.
	Header = `<?xml version="1.0" encoding="UTF-8"?>` + "\n"
)

Variables

View Source 
var HTMLAutoClose = htmlAutoClose

HTMLAutoClose is the set of HTML elements that should be considered to close automatically.

View Source 
var HTMLEntity = htmlEntity

HTMLEntity is an entity map containing translations for the standard HTML entity characters.

Functions

func Escape 

func Escape(w io.Writer, s []byte)

Escape is like EscapeText but omits the error return value. It is provided for backwards compatibility with Go 1.0. Code targeting Go 1.1 or later should use EscapeText.

func EscapeText 

func EscapeText(w io.Writer, s []byte) error

EscapeText writes to w the properly escaped XML equivalent of the plain text data s.

func Marshal 

func Marshal(v interface{}) ([]byte, error)

Marshal returns the XML encoding of v.

Marshal handles an array or slice by marshalling each of the elements. Marshal handles a pointer by marshalling the value it points at or, if the pointer is nil, by writing nothing. Marshal handles an interface value by marshalling the value it contains or, if the interface value is nil, by writing nothing. Marshal handles all other data by writing one or more XML elements containing the data.

The name for the XML elements is taken from, in order of preference:

  • the tag on the XMLName field, if the data is a struct
  • the value of the XMLName field of type xml.Name
  • the tag of the struct field used to obtain the data
  • the name of the struct field used to obtain the data
  • the name of the marshalled type

The XML element for a struct contains marshalled elements for each of the exported fields of the struct, with these exceptions:

  • the XMLName field, described above, is omitted.
  • a field with tag "-" is omitted.
  • a field with tag "name,attr" becomes an attribute with the given name in the XML element.
  • a field with tag ",attr" becomes an attribute with the field name in the XML element.
  • a field with tag ",chardata" is written as character data, not as an XML element.
  • a field with tag ",innerxml" is written verbatim, not subject to the usual marshalling procedure.
  • a field with tag ",comment" is written as an XML comment, not subject to the usual marshalling procedure. It must not contain the "--" string within it.
  • a field with a tag including the "omitempty" option is omitted if the field value is empty. The empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero.
  • an anonymous struct field is handled as if the fields of its value were part of the outer struct.

If a field uses a tag "a>b>c", then the element c will be nested inside parent elements a and b. Fields that appear next to each other that name the same parent will be enclosed in one XML element.

See MarshalIndent for an example.

Marshal will return an error if asked to marshal a channel, function, or map.

func MarshalIndent 

func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error)

MarshalIndent works like Marshal, but each XML element begins on a new indented line that starts with prefix and is followed by one or more copies of indent according to the nesting depth.

Example 
package main

import (
	"encoding/xml"
	"fmt"
	"os"
)

func main() {
	type Address struct {
		City, State string
	}
	type Person struct {
		XMLName   xml.Name `xml:"person"`
		Id        int      `xml:"id,attr"`
		FirstName string   `xml:"name>first"`
		LastName  string   `xml:"name>last"`
		Age       int      `xml:"age"`
		Height    float32  `xml:"height,omitempty"`
		Married   bool
		Address
		Comment string `xml:",comment"`
	}

	v := &Person{Id: 13, FirstName: "John", LastName: "Doe", Age: 42}
	v.Comment = " Need more details. "
	v.Address = Address{"Hanga Roa", "Easter Island"}

	output, err := xml.MarshalIndent(v, "  ", "    ")
	if err != nil {
		fmt.Printf("error: %v\n", err)
	}

	os.Stdout.Write(output)
}

Output:

  <person id="13">
      <name>
          <first>John</first>
          <last>Doe</last>
      </name>
      <age>42</age>
      <Married>false</Married>
      <City>Hanga Roa</City>
      <State>Easter Island</State>
      <!-- Need more details. -->
  </person>

func Unmarshal 

func Unmarshal(data []byte, v interface{}) error

Unmarshal parses the XML-encoded data and stores the result in the value pointed to by v, which must be an arbitrary struct, slice, or string. Well-formed data that does not fit into v is discarded.

Because Unmarshal uses the reflect package, it can only assign to exported (upper case) fields. Unmarshal uses a case-sensitive comparison to match XML element names to tag values and struct field names.

Unmarshal maps an XML element to a struct using the following rules. In the rules, the tag of a field refers to the value associated with the key 'xml' in the struct field's tag (see the example above).

  • If the struct has a field of type []byte or string with tag ",innerxml", Unmarshal accumulates the raw XML nested inside the element in that field. The rest of the rules still apply.

  • If the struct has a field named XMLName of type xml.Name, Unmarshal records the element name in that field.

  • If the XMLName field has an associated tag of the form "name" or "namespace-URL name", the XML element must have the given name (and, optionally, name space) or else Unmarshal returns an error.

  • If the XML element has an attribute whose name matches a struct field name with an associated tag containing ",attr" or the explicit name in a struct field tag of the form "name,attr", Unmarshal records the attribute value in that field.

  • If the XML element contains character data, that data is accumulated in the first struct field that has tag ",chardata". The struct field may have type []byte or string. If there is no such field, the character data is discarded.

  • If the XML element contains comments, they are accumulated in the first struct field that has tag ",comment". The struct field may have type []byte or string. If there is no such field, the comments are discarded.

  • If the XML element contains a sub-element whose name matches the prefix of a tag formatted as "a" or "a>b>c", unmarshal will descend into the XML structure looking for elements with the given names, and will map the innermost elements to that struct field. A tag starting with ">" is equivalent to one starting with the field name followed by ">".

  • If the XML element contains a sub-element whose name matches a struct field's XMLName tag and the struct field has no explicit name tag as per the previous rule, unmarshal maps the sub-element to that struct field.

  • If the XML element contains a sub-element whose name matches a field without any mode flags (",attr", ",chardata", etc), Unmarshal maps the sub-element to that struct field.

  • If the XML element contains a sub-element that hasn't matched any of the above rules and the struct has a field with tag ",any", unmarshal maps the sub-element to that struct field.

  • An anonymous struct field is handled as if the fields of its value were part of the outer struct.

  • A struct field with tag "-" is never unmarshalled into.

Unmarshal maps an XML element to a string or []byte by saving the concatenation of that element's character data in the string or []byte. The saved []byte is never nil.

Unmarshal maps an attribute value to a string or []byte by saving the value in the string or slice.

Unmarshal maps an XML element to a slice by extending the length of the slice and mapping the element to the newly created value.

Unmarshal maps an XML element or attribute value to a bool by setting it to the boolean value represented by the string.

Unmarshal maps an XML element or attribute value to an integer or floating-point field by setting the field to the result of interpreting the string value in decimal. There is no check for overflow.

Unmarshal maps an XML element to an xml.Name by recording the element name.

Unmarshal maps an XML element to a pointer by setting the pointer to a freshly allocated value and then mapping the element to that value.

Example

This example demonstrates unmarshaling an XML excerpt into a value with some preset fields. Note that the Phone field isn't modified and that the XML <Company> element is ignored. Also, the Groups field is assigned considering the element path provided in its tag. 

package main

import (
	"encoding/xml"
	"fmt"
)

func main() {
	type Email struct {
		Where string `xml:"where,attr"`
		Addr  string
	}
	type Address struct {
		City, State string
	}
	type Result struct {
		XMLName xml.Name `xml:"Person"`
		Name    string   `xml:"FullName"`
		Phone   string
		Email   []Email
		Groups  []string `xml:"Group>Value"`
		Address
	}
	v := Result{Name: "none", Phone: "none"}

	data := `
		<Person>
			<FullName>Grace R. Emlin</FullName>
			<Company>Example Inc.</Company>
			<Email where="home">
				<Addr>gre@example.com</Addr>
			</Email>
			<Email where='work'>
				<Addr>gre@work.com</Addr>
			</Email>
			<Group>
				<Value>Friends</Value>
				<Value>Squash</Value>
			</Group>
			<City>Hanga Roa</City>
			<State>Easter Island</State>
		</Person>
	`
	err := xml.Unmarshal([]byte(data), &v)
	if err != nil {
		fmt.Printf("error: %v", err)
		return
	}
	fmt.Printf("XMLName: %#v\n", v.XMLName)
	fmt.Printf("Name: %q\n", v.Name)
	fmt.Printf("Phone: %q\n", v.Phone)
	fmt.Printf("Email: %v\n", v.Email)
	fmt.Printf("Groups: %v\n", v.Groups)
	fmt.Printf("Address: %v\n", v.Address)
}

Output:

XMLName: xml.Name{Space:"", Local:"Person"}
Name: "Grace R. Emlin"
Phone: "none"
Email: [{home gre@example.com} {work gre@work.com}]
Groups: [Friends Squash]
Address: {Hanga Roa Easter Island}

Types

type Attr 

type Attr struct {
	Name  Name
	Value string
}

An Attr represents an attribute in an XML element (Name=Value).

type CharData 

type CharData []byte

A CharData represents XML character data (raw text), in which XML escape sequences have been replaced by the characters they represent.

func (CharData) Copy 

func (c CharData) Copy() CharData

type Comment 

type Comment []byte

A Comment represents an XML comment of the form <!--comment-->. The bytes do not include the <!-- and --> comment markers.

func (Comment) Copy 

func (c Comment) Copy() Comment

type Decoder 

type Decoder struct {
	// Strict defaults to true, enforcing the requirements
	// of the XML specification.
	// If set to false, the parser allows input containing common
	// mistakes:
	//	* If an element is missing an end tag, the parser invents
	//	  end tags as necessary to keep the return values from Token
	//	  properly balanced.
	//	* In attribute values and character data, unknown or malformed
	//	  character entities (sequences beginning with &) are left alone.
	//
	// Setting:
	//
	//	d.Strict = false;
	//	d.AutoClose = HTMLAutoClose;
	//	d.Entity = HTMLEntity
	//
	// creates a parser that can handle typical HTML.
	//
	// Strict mode does not enforce the requirements of the XML name spaces TR.
	// In particular it does not reject name space tags using undefined prefixes.
	// Such tags are recorded with the unknown prefix as the name space URL.
	Strict bool

	// When Strict == false, AutoClose indicates a set of elements to
	// consider closed immediately after they are opened, regardless
	// of whether an end element is present.
	AutoClose []string

	// Entity can be used to map non-standard entity names to string replacements.
	// The parser behaves as if these standard mappings are present in the map,
	// regardless of the actual map content:
	//
	//	"lt": "<",
	//	"gt": ">",
	//	"amp": "&",
	//	"apos": "'",
	//	"quot": `"`,
	Entity map[string]string

	// CharsetReader, if non-nil, defines a function to generate
	// charset-conversion readers, converting from the provided
	// non-UTF-8 charset into UTF-8. If CharsetReader is nil or
	// returns an error, parsing stops with an error. One of the
	// the CharsetReader's result values must be non-nil.
	CharsetReader func(charset string, input io.Reader) (io.Reader, error)

	// DefaultSpace sets the default name space used for unadorned tags,
	// as if the entire XML stream were wrapped in an element containing
	// the attribute xmlns="DefaultSpace".
	DefaultSpace string
	// contains filtered or unexported fields
}

A Decoder represents an XML parser reading a particular input stream. The parser assumes that its input is encoded in UTF-8.

func NewDecoder 

func NewDecoder(r io.Reader) *Decoder

NewDecoder creates a new XML parser reading from r. If r does not implement io.ByteReader, NewDecoder will do its own buffering.

func (*Decoder) Decode 

func (d *Decoder) Decode(v interface{}) error

Decode works like xml.Unmarshal, except it reads the decoder stream to find the start element.

func (*Decoder) DecodeElement 

func (d *Decoder) DecodeElement(v interface{}, start *StartElement) error

DecodeElement works like xml.Unmarshal except that it takes a pointer to the start XML element to decode into v. It is useful when a client reads some raw XML tokens itself but also wants to defer to Unmarshal for some elements.

func (*Decoder) InputOffset 

func (d *Decoder) InputOffset() int64

InputOffset returns the input stream byte offset of the current decoder position. The offset gives the location of the end of the most recently returned token and the beginning of the next token.

func (*Decoder) RawToken 

func (d *Decoder) RawToken() (Token, error)

RawToken is like Token but does not verify that start and end elements match and does not translate name space prefixes to their corresponding URLs.

func (*Decoder) Skip 

func (d *Decoder) Skip() error

Skip reads tokens until it has consumed the end element matching the most recent start element already consumed. It recurs if it encounters a start element, so it can be used to skip nested structures. It returns nil if it finds an end element matching the start element; otherwise it returns an error describing the problem.

func (*Decoder) Token 

func (d *Decoder) Token() (t Token, err error)

Token returns the next XML token in the input stream. At the end of the input stream, Token returns nil, io.EOF.

Slices of bytes in the returned token data refer to the parser's internal buffer and remain valid only until the next call to Token. To acquire a copy of the bytes, call CopyToken or the token's Copy method.

Token expands self-closing elements such as <br/> into separate start and end elements returned by successive calls.

Token guarantees that the StartElement and EndElement tokens it returns are properly nested and matched: if Token encounters an unexpected end element, it will return an error.

Token implements XML name spaces as described by http://www.w3.org/TR/REC-xml-names/. Each of the Name structures contained in the Token has the Space set to the URL identifying its name space when known. If Token encounters an unrecognized name space prefix, it uses the prefix as the Space rather than report an error.

type Directive 

type Directive []byte

A Directive represents an XML directive of the form <!text>. The bytes do not include the <! and > markers.

func (Directive) Copy 

func (d Directive) Copy() Directive

type Encoder 

type Encoder struct {
	// contains filtered or unexported fields
}

An Encoder writes XML data to an output stream.

Example 
package main

import (
	"encoding/xml"
	"fmt"
	"os"
)

func main() {
	type Address struct {
		City, State string
	}
	type Person struct {
		XMLName   xml.Name `xml:"person"`
		Id        int      `xml:"id,attr"`
		FirstName string   `xml:"name>first"`
		LastName  string   `xml:"name>last"`
		Age       int      `xml:"age"`
		Height    float32  `xml:"height,omitempty"`
		Married   bool
		Address
		Comment string `xml:",comment"`
	}

	v := &Person{Id: 13, FirstName: "John", LastName: "Doe", Age: 42}
	v.Comment = " Need more details. "
	v.Address = Address{"Hanga Roa", "Easter Island"}

	enc := xml.NewEncoder(os.Stdout)
	enc.Indent("  ", "    ")
	if err := enc.Encode(v); err != nil {
		fmt.Printf("error: %v\n", err)
	}

}

Output:

  <person id="13">
      <name>
          <first>John</first>
          <last>Doe</last>
      </name>
      <age>42</age>
      <Married>false</Married>
      <City>Hanga Roa</City>
      <State>Easter Island</State>
      <!-- Need more details. -->
  </person>

func NewEncoder 

func NewEncoder(w io.Writer) *Encoder

NewEncoder returns a new encoder that writes to w.

func (*Encoder) Encode 

func (enc *Encoder) Encode(v interface{}) error

Encode writes the XML encoding of v to the stream.

See the documentation for Marshal for details about the conversion of Go values to XML.

Encode calls Flush before returning.

func (*Encoder) EncodeElement 

func (enc *Encoder) EncodeElement(v interface{}, start StartElement) error

EncodeElement writes the XML encoding of v to the stream, using start as the outermost tag in the encoding.

See the documentation for Marshal for details about the conversion of Go values to XML.

EncodeElement calls Flush before returning.

func (*Encoder) EncodeToken 

func (enc *Encoder) EncodeToken(t Token) error

EncodeToken writes the given XML token to the stream. It returns an error if StartElement and EndElement tokens are not properly matched.

EncodeToken does not call Flush, because usually it is part of a larger operation such as Encode or EncodeElement (or a custom Marshaler's MarshalXML invoked during those), and those will call Flush when finished. Callers that create an Encoder and then invoke EncodeToken directly, without using Encode or EncodeElement, need to call Flush when finished to ensure that the XML is written to the underlying writer.

EncodeToken allows writing a ProcInst with Target set to "xml" only as the first token in the stream.

func (*Encoder) Flush 

func (enc *Encoder) Flush() error

Flush flushes any buffered XML to the underlying writer. See the EncodeToken documentation for details about when it is necessary.

func (*Encoder) Indent 

func (enc *Encoder) Indent(prefix, indent string)

Indent sets the encoder to generate XML in which each element begins on a new indented line that starts with prefix and is followed by one or more copies of indent according to the nesting depth.

type EndElement 

type EndElement struct {
	Name Name
}

An EndElement represents an XML end element.

type Marshaler 

type Marshaler interface {
	MarshalXML(e *Encoder, start StartElement) error
}

Marshaler is the interface implemented by objects that can marshal themselves into valid XML elements.

MarshalXML encodes the receiver as zero or more XML elements. By convention, arrays or slices are typically encoded as a sequence of elements, one per entry. Using start as the element tag is not required, but doing so will enable Unmarshal to match the XML elements to the correct struct field. One common implementation strategy is to construct a separate value with a layout corresponding to the desired XML and then to encode it using e.EncodeElement. Another common strategy is to use repeated calls to e.EncodeToken to generate the XML output one token at a time. The sequence of encoded tokens must make up zero or more valid XML elements.

type MarshalerAttr 

type MarshalerAttr interface {
	MarshalXMLAttr(name Name) (Attr, error)
}

MarshalerAttr is the interface implemented by objects that can marshal themselves into valid XML attributes.

MarshalXMLAttr returns an XML attribute with the encoded value of the receiver. Using name as the attribute name is not required, but doing so will enable Unmarshal to match the attribute to the correct struct field. If MarshalXMLAttr returns the zero attribute Attr{}, no attribute will be generated in the output. MarshalXMLAttr is used only for struct fields with the "attr" option in the field tag.

type Name 

type Name struct {
	Space, Local string
}

A Name represents an XML name (Local) annotated with a name space identifier (Space). In tokens returned by Decoder.Token, the Space identifier is given as a canonical URL, not the short prefix used in the document being parsed.

type ProcInst 

type ProcInst struct {
	Target string
	Inst   []byte
}

A ProcInst represents an XML processing instruction of the form <?target inst?>

func (ProcInst) Copy 

func (p ProcInst) Copy() ProcInst

type StartElement 

type StartElement struct {
	Name Name
	Attr []Attr
}

A StartElement represents an XML start element.

func (StartElement) Copy 

func (e StartElement) Copy() StartElement

func (StartElement) End 

func (e StartElement) End() EndElement

End returns the corresponding XML end element.

type SyntaxError 

type SyntaxError struct {
	Msg  string
	Line int
}

A SyntaxError represents a syntax error in the XML input stream.

func (*SyntaxError) Error 

func (e *SyntaxError) Error() string

type TagPathError 

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

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

func (*TagPathError) Error 

func (e *TagPathError) Error() string

type Token 

type Token interface{}

A Token is an interface holding one of the token types: StartElement, EndElement, CharData, Comment, ProcInst, or Directive.

func CopyToken 

func CopyToken(t Token) Token

CopyToken returns a copy of a Token.

type UnmarshalError 

type UnmarshalError string

An UnmarshalError represents an error in the unmarshalling process.

func (UnmarshalError) Error 

func (e UnmarshalError) Error() string

type Unmarshaler 

type Unmarshaler interface {
	UnmarshalXML(d *Decoder, start StartElement) error
}

Unmarshaler is the interface implemented by objects that can unmarshal an XML element description of themselves.

UnmarshalXML decodes a single XML element beginning with the given start element. If it returns an error, the outer call to Unmarshal stops and returns that error. UnmarshalXML must consume exactly one XML element. One common implementation strategy is to unmarshal into a separate value with a layout matching the expected XML using d.DecodeElement, and then to copy the data from that value into the receiver. Another common strategy is to use d.Token to process the XML object one token at a time. UnmarshalXML may not use d.RawToken.

type UnmarshalerAttr 

type UnmarshalerAttr interface {
	UnmarshalXMLAttr(attr Attr) error
}

UnmarshalerAttr is the interface implemented by objects that can unmarshal an XML attribute description of themselves.

UnmarshalXMLAttr decodes a single XML attribute. If it returns an error, the outer call to Unmarshal stops and returns that error. UnmarshalXMLAttr is used only for struct fields with the "attr" option in the field tag.

type UnsupportedTypeError 

type UnsupportedTypeError struct {
	Type reflect.Type
}

A MarshalXMLError is returned when Marshal encounters a type that cannot be converted into XML.

func (*UnsupportedTypeError) Error 

func (e *UnsupportedTypeError) Error() string

Notes

Bugs

  • Mapping between XML elements and data structures is inherently flawed: an XML element is an order-dependent collection of anonymous values, while a data structure is an order-independent collection of named values. See package json for a textual representation more suitable to data structures.

Source Files

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