Source file src/encoding/json/stream.go

Documentation: encoding/json

     1  // Copyright 2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package json
     6  
     7  import (
     8  	"bytes"
     9  	"errors"
    10  	"io"
    11  )
    12  
    13  // A Decoder reads and decodes JSON values from an input stream.
    14  type Decoder struct {
    15  	r       io.Reader
    16  	buf     []byte
    17  	d       decodeState
    18  	scanp   int   // start of unread data in buf
    19  	scanned int64 // amount of data already scanned
    20  	scan    scanner
    21  	err     error
    22  
    23  	tokenState int
    24  	tokenStack []int
    25  }
    26  
    27  // NewDecoder returns a new decoder that reads from r.
    28  //
    29  // The decoder introduces its own buffering and may
    30  // read data from r beyond the JSON values requested.
    31  func NewDecoder(r io.Reader) *Decoder {
    32  	return &Decoder{r: r}
    33  }
    34  
    35  // UseNumber causes the Decoder to unmarshal a number into an interface{} as a
    36  // Number instead of as a float64.
    37  func (dec *Decoder) UseNumber() { dec.d.useNumber = true }
    38  
    39  // DisallowUnknownFields causes the Decoder to return an error when the destination
    40  // is a struct and the input contains object keys which do not match any
    41  // non-ignored, exported fields in the destination.
    42  func (dec *Decoder) DisallowUnknownFields() { dec.d.disallowUnknownFields = true }
    43  
    44  // Decode reads the next JSON-encoded value from its
    45  // input and stores it in the value pointed to by v.
    46  //
    47  // See the documentation for Unmarshal for details about
    48  // the conversion of JSON into a Go value.
    49  func (dec *Decoder) Decode(v interface{}) error {
    50  	if dec.err != nil {
    51  		return dec.err
    52  	}
    53  
    54  	if err := dec.tokenPrepareForDecode(); err != nil {
    55  		return err
    56  	}
    57  
    58  	if !dec.tokenValueAllowed() {
    59  		return &SyntaxError{msg: "not at beginning of value", Offset: dec.offset()}
    60  	}
    61  
    62  	// Read whole value into buffer.
    63  	n, err := dec.readValue()
    64  	if err != nil {
    65  		return err
    66  	}
    67  	dec.d.init(dec.buf[dec.scanp : dec.scanp+n])
    68  	dec.scanp += n
    69  
    70  	// Don't save err from unmarshal into dec.err:
    71  	// the connection is still usable since we read a complete JSON
    72  	// object from it before the error happened.
    73  	err = dec.d.unmarshal(v)
    74  
    75  	// fixup token streaming state
    76  	dec.tokenValueEnd()
    77  
    78  	return err
    79  }
    80  
    81  // Buffered returns a reader of the data remaining in the Decoder's
    82  // buffer. The reader is valid until the next call to Decode.
    83  func (dec *Decoder) Buffered() io.Reader {
    84  	return bytes.NewReader(dec.buf[dec.scanp:])
    85  }
    86  
    87  // readValue reads a JSON value into dec.buf.
    88  // It returns the length of the encoding.
    89  func (dec *Decoder) readValue() (int, error) {
    90  	dec.scan.reset()
    91  
    92  	scanp := dec.scanp
    93  	var err error
    94  Input:
    95  	// help the compiler see that scanp is never negative, so it can remove
    96  	// some bounds checks below.
    97  	for scanp >= 0 {
    98  
    99  		// Look in the buffer for a new value.
   100  		for ; scanp < len(dec.buf); scanp++ {
   101  			c := dec.buf[scanp]
   102  			dec.scan.bytes++
   103  			switch dec.scan.step(&dec.scan, c) {
   104  			case scanEnd:
   105  				break Input
   106  			case scanEndObject, scanEndArray:
   107  				// scanEnd is delayed one byte.
   108  				// We might block trying to get that byte from src,
   109  				// so instead invent a space byte.
   110  				if stateEndValue(&dec.scan, ' ') == scanEnd {
   111  					scanp++
   112  					break Input
   113  				}
   114  			case scanError:
   115  				dec.err = dec.scan.err
   116  				return 0, dec.scan.err
   117  			}
   118  		}
   119  
   120  		// Did the last read have an error?
   121  		// Delayed until now to allow buffer scan.
   122  		if err != nil {
   123  			if err == io.EOF {
   124  				if dec.scan.step(&dec.scan, ' ') == scanEnd {
   125  					break Input
   126  				}
   127  				if nonSpace(dec.buf) {
   128  					err = io.ErrUnexpectedEOF
   129  				}
   130  			}
   131  			dec.err = err
   132  			return 0, err
   133  		}
   134  
   135  		n := scanp - dec.scanp
   136  		err = dec.refill()
   137  		scanp = dec.scanp + n
   138  	}
   139  	return scanp - dec.scanp, nil
   140  }
   141  
   142  func (dec *Decoder) refill() error {
   143  	// Make room to read more into the buffer.
   144  	// First slide down data already consumed.
   145  	if dec.scanp > 0 {
   146  		dec.scanned += int64(dec.scanp)
   147  		n := copy(dec.buf, dec.buf[dec.scanp:])
   148  		dec.buf = dec.buf[:n]
   149  		dec.scanp = 0
   150  	}
   151  
   152  	// Grow buffer if not large enough.
   153  	const minRead = 512
   154  	if cap(dec.buf)-len(dec.buf) < minRead {
   155  		newBuf := make([]byte, len(dec.buf), 2*cap(dec.buf)+minRead)
   156  		copy(newBuf, dec.buf)
   157  		dec.buf = newBuf
   158  	}
   159  
   160  	// Read. Delay error for next iteration (after scan).
   161  	n, err := dec.r.Read(dec.buf[len(dec.buf):cap(dec.buf)])
   162  	dec.buf = dec.buf[0 : len(dec.buf)+n]
   163  
   164  	return err
   165  }
   166  
   167  func nonSpace(b []byte) bool {
   168  	for _, c := range b {
   169  		if !isSpace(c) {
   170  			return true
   171  		}
   172  	}
   173  	return false
   174  }
   175  
   176  // An Encoder writes JSON values to an output stream.
   177  type Encoder struct {
   178  	w          io.Writer
   179  	err        error
   180  	escapeHTML bool
   181  
   182  	indentBuf    *bytes.Buffer
   183  	indentPrefix string
   184  	indentValue  string
   185  }
   186  
   187  // NewEncoder returns a new encoder that writes to w.
   188  func NewEncoder(w io.Writer) *Encoder {
   189  	return &Encoder{w: w, escapeHTML: true}
   190  }
   191  
   192  // Encode writes the JSON encoding of v to the stream,
   193  // followed by a newline character.
   194  //
   195  // See the documentation for Marshal for details about the
   196  // conversion of Go values to JSON.
   197  func (enc *Encoder) Encode(v interface{}) error {
   198  	if enc.err != nil {
   199  		return enc.err
   200  	}
   201  	e := newEncodeState()
   202  	err := e.marshal(v, encOpts{escapeHTML: enc.escapeHTML})
   203  	if err != nil {
   204  		return err
   205  	}
   206  
   207  	// Terminate each value with a newline.
   208  	// This makes the output look a little nicer
   209  	// when debugging, and some kind of space
   210  	// is required if the encoded value was a number,
   211  	// so that the reader knows there aren't more
   212  	// digits coming.
   213  	e.WriteByte('\n')
   214  
   215  	b := e.Bytes()
   216  	if enc.indentPrefix != "" || enc.indentValue != "" {
   217  		if enc.indentBuf == nil {
   218  			enc.indentBuf = new(bytes.Buffer)
   219  		}
   220  		enc.indentBuf.Reset()
   221  		err = Indent(enc.indentBuf, b, enc.indentPrefix, enc.indentValue)
   222  		if err != nil {
   223  			return err
   224  		}
   225  		b = enc.indentBuf.Bytes()
   226  	}
   227  	if _, err = enc.w.Write(b); err != nil {
   228  		enc.err = err
   229  	}
   230  	encodeStatePool.Put(e)
   231  	return err
   232  }
   233  
   234  // SetIndent instructs the encoder to format each subsequent encoded
   235  // value as if indented by the package-level function Indent(dst, src, prefix, indent).
   236  // Calling SetIndent("", "") disables indentation.
   237  func (enc *Encoder) SetIndent(prefix, indent string) {
   238  	enc.indentPrefix = prefix
   239  	enc.indentValue = indent
   240  }
   241  
   242  // SetEscapeHTML specifies whether problematic HTML characters
   243  // should be escaped inside JSON quoted strings.
   244  // The default behavior is to escape &, <, and > to \u0026, \u003c, and \u003e
   245  // to avoid certain safety problems that can arise when embedding JSON in HTML.
   246  //
   247  // In non-HTML settings where the escaping interferes with the readability
   248  // of the output, SetEscapeHTML(false) disables this behavior.
   249  func (enc *Encoder) SetEscapeHTML(on bool) {
   250  	enc.escapeHTML = on
   251  }
   252  
   253  // RawMessage is a raw encoded JSON value.
   254  // It implements Marshaler and Unmarshaler and can
   255  // be used to delay JSON decoding or precompute a JSON encoding.
   256  type RawMessage []byte
   257  
   258  // MarshalJSON returns m as the JSON encoding of m.
   259  func (m RawMessage) MarshalJSON() ([]byte, error) {
   260  	if m == nil {
   261  		return []byte("null"), nil
   262  	}
   263  	return m, nil
   264  }
   265  
   266  // UnmarshalJSON sets *m to a copy of data.
   267  func (m *RawMessage) UnmarshalJSON(data []byte) error {
   268  	if m == nil {
   269  		return errors.New("json.RawMessage: UnmarshalJSON on nil pointer")
   270  	}
   271  	*m = append((*m)[0:0], data...)
   272  	return nil
   273  }
   274  
   275  var _ Marshaler = (*RawMessage)(nil)
   276  var _ Unmarshaler = (*RawMessage)(nil)
   277  
   278  // A Token holds a value of one of these types:
   279  //
   280  //	Delim, for the four JSON delimiters [ ] { }
   281  //	bool, for JSON booleans
   282  //	float64, for JSON numbers
   283  //	Number, for JSON numbers
   284  //	string, for JSON string literals
   285  //	nil, for JSON null
   286  //
   287  type Token interface{}
   288  
   289  const (
   290  	tokenTopValue = iota
   291  	tokenArrayStart
   292  	tokenArrayValue
   293  	tokenArrayComma
   294  	tokenObjectStart
   295  	tokenObjectKey
   296  	tokenObjectColon
   297  	tokenObjectValue
   298  	tokenObjectComma
   299  )
   300  
   301  // advance tokenstate from a separator state to a value state
   302  func (dec *Decoder) tokenPrepareForDecode() error {
   303  	// Note: Not calling peek before switch, to avoid
   304  	// putting peek into the standard Decode path.
   305  	// peek is only called when using the Token API.
   306  	switch dec.tokenState {
   307  	case tokenArrayComma:
   308  		c, err := dec.peek()
   309  		if err != nil {
   310  			return err
   311  		}
   312  		if c != ',' {
   313  			return &SyntaxError{"expected comma after array element", dec.offset()}
   314  		}
   315  		dec.scanp++
   316  		dec.tokenState = tokenArrayValue
   317  	case tokenObjectColon:
   318  		c, err := dec.peek()
   319  		if err != nil {
   320  			return err
   321  		}
   322  		if c != ':' {
   323  			return &SyntaxError{"expected colon after object key", dec.offset()}
   324  		}
   325  		dec.scanp++
   326  		dec.tokenState = tokenObjectValue
   327  	}
   328  	return nil
   329  }
   330  
   331  func (dec *Decoder) tokenValueAllowed() bool {
   332  	switch dec.tokenState {
   333  	case tokenTopValue, tokenArrayStart, tokenArrayValue, tokenObjectValue:
   334  		return true
   335  	}
   336  	return false
   337  }
   338  
   339  func (dec *Decoder) tokenValueEnd() {
   340  	switch dec.tokenState {
   341  	case tokenArrayStart, tokenArrayValue:
   342  		dec.tokenState = tokenArrayComma
   343  	case tokenObjectValue:
   344  		dec.tokenState = tokenObjectComma
   345  	}
   346  }
   347  
   348  // A Delim is a JSON array or object delimiter, one of [ ] { or }.
   349  type Delim rune
   350  
   351  func (d Delim) String() string {
   352  	return string(d)
   353  }
   354  
   355  // Token returns the next JSON token in the input stream.
   356  // At the end of the input stream, Token returns nil, io.EOF.
   357  //
   358  // Token guarantees that the delimiters [ ] { } it returns are
   359  // properly nested and matched: if Token encounters an unexpected
   360  // delimiter in the input, it will return an error.
   361  //
   362  // The input stream consists of basic JSON values—bool, string,
   363  // number, and null—along with delimiters [ ] { } of type Delim
   364  // to mark the start and end of arrays and objects.
   365  // Commas and colons are elided.
   366  func (dec *Decoder) Token() (Token, error) {
   367  	for {
   368  		c, err := dec.peek()
   369  		if err != nil {
   370  			return nil, err
   371  		}
   372  		switch c {
   373  		case '[':
   374  			if !dec.tokenValueAllowed() {
   375  				return dec.tokenError(c)
   376  			}
   377  			dec.scanp++
   378  			dec.tokenStack = append(dec.tokenStack, dec.tokenState)
   379  			dec.tokenState = tokenArrayStart
   380  			return Delim('['), nil
   381  
   382  		case ']':
   383  			if dec.tokenState != tokenArrayStart && dec.tokenState != tokenArrayComma {
   384  				return dec.tokenError(c)
   385  			}
   386  			dec.scanp++
   387  			dec.tokenState = dec.tokenStack[len(dec.tokenStack)-1]
   388  			dec.tokenStack = dec.tokenStack[:len(dec.tokenStack)-1]
   389  			dec.tokenValueEnd()
   390  			return Delim(']'), nil
   391  
   392  		case '{':
   393  			if !dec.tokenValueAllowed() {
   394  				return dec.tokenError(c)
   395  			}
   396  			dec.scanp++
   397  			dec.tokenStack = append(dec.tokenStack, dec.tokenState)
   398  			dec.tokenState = tokenObjectStart
   399  			return Delim('{'), nil
   400  
   401  		case '}':
   402  			if dec.tokenState != tokenObjectStart && dec.tokenState != tokenObjectComma {
   403  				return dec.tokenError(c)
   404  			}
   405  			dec.scanp++
   406  			dec.tokenState = dec.tokenStack[len(dec.tokenStack)-1]
   407  			dec.tokenStack = dec.tokenStack[:len(dec.tokenStack)-1]
   408  			dec.tokenValueEnd()
   409  			return Delim('}'), nil
   410  
   411  		case ':':
   412  			if dec.tokenState != tokenObjectColon {
   413  				return dec.tokenError(c)
   414  			}
   415  			dec.scanp++
   416  			dec.tokenState = tokenObjectValue
   417  			continue
   418  
   419  		case ',':
   420  			if dec.tokenState == tokenArrayComma {
   421  				dec.scanp++
   422  				dec.tokenState = tokenArrayValue
   423  				continue
   424  			}
   425  			if dec.tokenState == tokenObjectComma {
   426  				dec.scanp++
   427  				dec.tokenState = tokenObjectKey
   428  				continue
   429  			}
   430  			return dec.tokenError(c)
   431  
   432  		case '"':
   433  			if dec.tokenState == tokenObjectStart || dec.tokenState == tokenObjectKey {
   434  				var x string
   435  				old := dec.tokenState
   436  				dec.tokenState = tokenTopValue
   437  				err := dec.Decode(&x)
   438  				dec.tokenState = old
   439  				if err != nil {
   440  					return nil, err
   441  				}
   442  				dec.tokenState = tokenObjectColon
   443  				return x, nil
   444  			}
   445  			fallthrough
   446  
   447  		default:
   448  			if !dec.tokenValueAllowed() {
   449  				return dec.tokenError(c)
   450  			}
   451  			var x interface{}
   452  			if err := dec.Decode(&x); err != nil {
   453  				return nil, err
   454  			}
   455  			return x, nil
   456  		}
   457  	}
   458  }
   459  
   460  func (dec *Decoder) tokenError(c byte) (Token, error) {
   461  	var context string
   462  	switch dec.tokenState {
   463  	case tokenTopValue:
   464  		context = " looking for beginning of value"
   465  	case tokenArrayStart, tokenArrayValue, tokenObjectValue:
   466  		context = " looking for beginning of value"
   467  	case tokenArrayComma:
   468  		context = " after array element"
   469  	case tokenObjectKey:
   470  		context = " looking for beginning of object key string"
   471  	case tokenObjectColon:
   472  		context = " after object key"
   473  	case tokenObjectComma:
   474  		context = " after object key:value pair"
   475  	}
   476  	return nil, &SyntaxError{"invalid character " + quoteChar(c) + context, dec.offset()}
   477  }
   478  
   479  // More reports whether there is another element in the
   480  // current array or object being parsed.
   481  func (dec *Decoder) More() bool {
   482  	c, err := dec.peek()
   483  	return err == nil && c != ']' && c != '}'
   484  }
   485  
   486  func (dec *Decoder) peek() (byte, error) {
   487  	var err error
   488  	for {
   489  		for i := dec.scanp; i < len(dec.buf); i++ {
   490  			c := dec.buf[i]
   491  			if isSpace(c) {
   492  				continue
   493  			}
   494  			dec.scanp = i
   495  			return c, nil
   496  		}
   497  		// buffer has been scanned, now report any error
   498  		if err != nil {
   499  			return 0, err
   500  		}
   501  		err = dec.refill()
   502  	}
   503  }
   504  
   505  func (dec *Decoder) offset() int64 {
   506  	return dec.scanned + int64(dec.scanp)
   507  }
   508  

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