1 // Copyright 2009 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.
15 // An Encoder manages the transmission of type and data information to the
16 // other side of a connection.
18 mutex sync.Mutex // each item must be sent atomically
19 w []io.Writer // where to send the data
20 sent map[reflect.Type]typeId // which types we've already sent
21 countState *encoderState // stage for writing counts
22 freeList *encoderState // list of free encoderStates; avoids reallocation
23 byteBuf bytes.Buffer // buffer for top-level encoderState
27 // Before we encode a message, we reserve space at the head of the
28 // buffer in which to encode its length. This means we can use the
29 // buffer to assemble the message without another allocation.
30 const maxLength = 9 // Maximum size of an encoded length.
31 var spaceForLength = make([]byte, maxLength)
33 // NewEncoder returns a new encoder that will transmit on the io.Writer.
34 func NewEncoder(w io.Writer) *Encoder {
36 enc.w = []io.Writer{w}
37 enc.sent = make(map[reflect.Type]typeId)
38 enc.countState = enc.newEncoderState(new(bytes.Buffer))
42 // writer() returns the innermost writer the encoder is using
43 func (enc *Encoder) writer() io.Writer {
44 return enc.w[len(enc.w)-1]
47 // pushWriter adds a writer to the encoder.
48 func (enc *Encoder) pushWriter(w io.Writer) {
49 enc.w = append(enc.w, w)
52 // popWriter pops the innermost writer.
53 func (enc *Encoder) popWriter() {
54 enc.w = enc.w[0 : len(enc.w)-1]
57 func (enc *Encoder) badType(rt reflect.Type) {
58 enc.setError(errors.New("gob: can't encode type " + rt.String()))
61 func (enc *Encoder) setError(err error) {
62 if enc.err == nil { // remember the first.
67 // writeMessage sends the data item preceded by a unsigned count of its length.
68 func (enc *Encoder) writeMessage(w io.Writer, b *bytes.Buffer) {
69 // Space has been reserved for the length at the head of the message.
70 // This is a little dirty: we grab the slice from the bytes.Buffer and massage
73 messageLen := len(message) - maxLength
75 enc.countState.b.Reset()
76 enc.countState.encodeUint(uint64(messageLen))
77 // Copy the length to be a prefix of the message.
78 offset := maxLength - enc.countState.b.Len()
79 copy(message[offset:], enc.countState.b.Bytes())
81 _, err := w.Write(message[offset:])
82 // Drain the buffer and restore the space at the front for the count of the next message.
84 b.Write(spaceForLength)
90 // sendActualType sends the requested type, without further investigation, unless
91 // it's been sent before.
92 func (enc *Encoder) sendActualType(w io.Writer, state *encoderState, ut *userTypeInfo, actual reflect.Type) (sent bool) {
93 if _, alreadySent := enc.sent[actual]; alreadySent {
97 info, err := getTypeInfo(ut)
103 // Send the pair (-id, type)
105 state.encodeInt(-int64(info.id))
107 enc.encode(state.b, reflect.ValueOf(info.wire), wireTypeUserInfo)
108 enc.writeMessage(w, state.b)
113 // Remember we've sent this type, both what the user gave us and the base type.
114 enc.sent[ut.base] = info.id
115 if ut.user != ut.base {
116 enc.sent[ut.user] = info.id
118 // Now send the inner types
119 switch st := actual; st.Kind() {
121 for i := 0; i < st.NumField(); i++ {
122 enc.sendType(w, state, st.Field(i).Type)
124 case reflect.Array, reflect.Slice:
125 enc.sendType(w, state, st.Elem())
127 enc.sendType(w, state, st.Key())
128 enc.sendType(w, state, st.Elem())
133 // sendType sends the type info to the other side, if necessary.
134 func (enc *Encoder) sendType(w io.Writer, state *encoderState, origt reflect.Type) (sent bool) {
135 ut := userType(origt)
137 // The rules are different: regardless of the underlying type's representation,
138 // we need to tell the other side that this exact type is a GobEncoder.
139 return enc.sendActualType(w, state, ut, ut.user)
142 // It's a concrete value, so drill down to the base type.
143 switch rt := ut.base; rt.Kind() {
145 // Basic types and interfaces do not need to be described.
148 // If it's []uint8, don't send; it's considered basic.
149 if rt.Elem().Kind() == reflect.Uint8 {
152 // Otherwise we do send.
155 // arrays must be sent so we know their lengths and element types.
158 // maps must be sent so we know their lengths and key/value types.
161 // structs must be sent so we know their fields.
163 case reflect.Chan, reflect.Func:
164 // Probably a bad field in a struct.
169 return enc.sendActualType(w, state, ut, ut.base)
172 // Encode transmits the data item represented by the empty interface value,
173 // guaranteeing that all necessary type information has been transmitted first.
174 func (enc *Encoder) Encode(e interface{}) error {
175 return enc.EncodeValue(reflect.ValueOf(e))
178 // sendTypeDescriptor makes sure the remote side knows about this type.
179 // It will send a descriptor if this is the first time the type has been
181 func (enc *Encoder) sendTypeDescriptor(w io.Writer, state *encoderState, ut *userTypeInfo) {
182 // Make sure the type is known to the other side.
183 // First, have we already sent this type?
188 if _, alreadySent := enc.sent[rt]; !alreadySent {
190 sent := enc.sendType(w, state, rt)
194 // If the type info has still not been transmitted, it means we have
195 // a singleton basic type (int, []byte etc.) at top level. We don't
196 // need to send the type info but we do need to update enc.sent.
199 info, err := getTypeInfo(ut)
205 enc.sent[rt] = info.id
210 // sendTypeId sends the id, which must have already been defined.
211 func (enc *Encoder) sendTypeId(state *encoderState, ut *userTypeInfo) {
212 // Identify the type of this top-level value.
213 state.encodeInt(int64(enc.sent[ut.base]))
216 // EncodeValue transmits the data item represented by the reflection value,
217 // guaranteeing that all necessary type information has been transmitted first.
218 func (enc *Encoder) EncodeValue(value reflect.Value) error {
219 // Make sure we're single-threaded through here, so multiple
220 // goroutines can share an encoder.
222 defer enc.mutex.Unlock()
224 // Remove any nested writers remaining due to previous errors.
227 ut, err := validUserType(value.Type())
234 enc.byteBuf.Write(spaceForLength)
235 state := enc.newEncoderState(&enc.byteBuf)
237 enc.sendTypeDescriptor(enc.writer(), state, ut)
238 enc.sendTypeId(state, ut)
243 // Encode the object.
244 enc.encode(state.b, value, ut)
246 enc.writeMessage(enc.writer(), state.b)
249 enc.freeEncoderState(state)