上一篇(http://www.linuxidc.com/Linux/2015-04/116466.htm)文章我們講了net/rpc中client部分的實現，我本機源碼安裝路徑在/usr/local/go，這net/rpc（golang 1.4版本）涉及到的相關代碼主要有：
server.go
方法注冊：
因為從client我們知道是復用的socket來實現並發調用rpc方法，我們先從方法注冊來看源碼部分：
// Server對象大都是保存方法存根，保證對象互斥的
type Serverstruct{
mu sync.RWMutex// protects the serviceMap
serviceMap map[string]*service
reqLock sync.Mutex// protects freeReq
freeReq *Request
respLock sync.Mutex// protects freeResp
freeResp *Response}
func NewServer()*Server{return&Server{serviceMap: make(map[string]*service)}}// rpc.Register默認使用了一個Server，只對serviceMap進行了初始化 varDefaultServer=NewServer()// rpc的service包括方法名、方法反射，類型等
type service struct{
name string// name of service
rcvr reflect.Value// receiver of methods for the service
typ reflect.Type// type of the receiver
method map[string]*methodType // registered methods }// 無論是RegisterName、Register最終都調用了register的內部方法
func (server *Server)register(rcvr interface{}, name string, useName bool) error {// 保證注冊服務安全，先加鎖
server.mu.Lock()
defer server.mu.Unlock()// 如果服務為空，默認注冊一個 if server.serviceMap ==nil{
server.serviceMap = make(map[string]*service)}// 獲取注冊服務的反射信息
s :=new(service)
s.typ = reflect.TypeOf(rcvr)
s.rcvr = reflect.ValueOf(rcvr)// 可以使用自定義名稱
sname := reflect.Indirect(s.rcvr).Type().Name()if useName {
sname = name
}if sname ==""{
s :="rpc.Register: no service name for type "+ s.typ.String()
log.Print(s)return errors.New(s)}// 方法必須是暴露的，既服務名首字符大寫 if!isExported(sname)&&!useName {
s :="rpc.Register: type "+ sname +" is not exported"
log.Print(s)return errors.New(s)}// 不允許重復注冊 if _, present := server.serviceMap[sname]; present {return errors.New("rpc: service already defined: "+ sname)}
s.name = sname
// 開始注冊rpc struct內部的方法存根
s.method = suitableMethods(s.typ,true)// 如果struct內部一個方法也沒，那麼直接報錯，錯誤信息還非常詳細 if len(s.method)==0{
str :=""// To help the user, see if a pointer receiver would work.
method := suitableMethods(reflect.PtrTo(s.typ),false)if len(method)!=0{
str ="rpc.Register: type "+ sname +" has no exported methods of suitable type (hint: pass a pointer to value of that type)"}else{
str ="rpc.Register: type "+ sname +" has no exported methods of suitable type"}
log.Print(str)return errors.New(str)}// 保存在server的serviceMap中
server.serviceMap[s.name]= s
returnnil}// 上文提到了服務還需要方法存根的注冊
func suitableMethods(typ reflect.Type, reportErr bool) map[string]*methodType {// 根據方法名創建保存內部方法map
methods := make(map[string]*methodType)// 獲取rpc struct內部的方法 for m :=0; m < typ.NumMethod(); m++{
method := typ.Method(m)
mtype := method.Type
mname := method.Name// 之前對這行代碼覺得比較奇葩，方法是否是暴露，是看是否有PkgPath的，如果是私有方法，PkgPath顯示包名 if method.PkgPath!=""{continue}// 判斷是否是三個參數：第一個是結構本身，第二個是參數，第三個是返回值 // Method needs three ins: receiver, *args, *reply. if mtype.NumIn()!=3{if reportErr {
log.Println("method", mname,"has wrong number of ins:", mtype.NumIn())}continue}// args是指針類型 // First arg need not be a pointer.
argType := mtype.In(1)if!isExportedOrBuiltinType(argType){if reportErr {
log.Println(mname,"argument type not exported:", argType)}continue}// reply是指針類型 // Second arg must be a pointer.
replyType := mtype.In(2)if replyType.Kind()!= reflect.Ptr{if reportErr {
log.Println("method", mname,"reply type not a pointer:", replyType)}continue}// Reply type must be exported. // reply必須是可暴露的 if!isExportedOrBuiltinType(replyType){if reportErr {
log.Println("method", mname,"reply type not exported:", replyType)}continue}// Method needs one out. // 必須有一個返回值，而且要是error if mtype.NumOut()!=1{if reportErr {
log.Println("method", mname,"has wrong number of outs:", mtype.NumOut())}continue}// The return type of the method must be error. if returnType := mtype.Out(0); returnType != typeOfError {if reportErr {
log.Println("method", mname,"returns", returnType.String(),"not error")}continue}
methods[mname]=&methodType{method: method,ArgType: argType,ReplyType: replyType}}return methods
}請求調用：
方法已經被注冊成功，接下來我們看看是如何客戶端發送請求調用的：
func (server *Server)Accept(lis net.Listener){for{
conn, err := lis.Accept()if err !=nil{
log.Fatal("rpc.Serve: accept:", err.Error())// TODO(r): exit? }// accept連接以後，打開一個goroutine處理請求
go server.ServeConn(conn)}}
func (server *Server)ServeConn(conn io.ReadWriteCloser){
buf := bufio.NewWriter(conn)
srv :=&gobServerCodec{
rwc: conn,
dec: gob.NewDecoder(conn),
enc: gob.NewEncoder(buf),
encBuf: buf,}// 根據指定的codec進行協議解析
server.ServeCodec(srv)}
func (server *Server)ServeCodec(codec ServerCodec){
sending :=new(sync.Mutex)for{// 解析請求
service, mtype, req, argv, replyv, keepReading, err := server.readRequest(codec)if err !=nil{if debugLog && err != io.EOF {
log.Println("rpc:", err)}if!keepReading {break}// send a response if we actually managed to read a header. // 如果當前請求錯誤了，我們應該返回信息，然後繼續處理 if req !=nil{
server.sendResponse(sending, req, invalidRequest, codec, err.Error())
server.freeRequest(req)}continue}// 因為需要繼續處理後續請求，所以開一個gorutine處理rpc方法
go service.call(server, sending, mtype, req, argv, replyv, codec)}// 如果連接關閉了需要釋放資源
codec.Close()}
func (server *Server) readRequestHeader(codec ServerCodec)(service *service, mtype *methodType, req *Request, keepReading bool, err error){// 解析頭部，如果失敗，直接返回了
req = server.getRequest()
err = codec.ReadRequestHeader(req)if err !=nil{
req =nilif err == io.EOF || err == io.ErrUnexpectedEOF{return}
err = errors.New("rpc: server cannot decode request: "+ err.Error())return}if debugLog {
log.Printf("rpc: [trace:%v]\n", req.Tracer)}// We read the header successfully. If we see an error now, // we can still recover and move on to the next request.
keepReading =true// 獲取請求中xxx.xxx中.的位置
dot := strings.LastIndex(req.ServiceMethod,".")if dot <0{
err = errors.New("rpc: service/method request ill-formed: "+ req.ServiceMethod)return}// 拿到struct名字和方法名字
serviceName := req.ServiceMethod[:dot]
methodName := req.ServiceMethod[dot+1:]// Look up the request.// 加讀鎖，獲取對象
server.mu.RLock()
service = server.serviceMap[serviceName]
server.mu.RUnlock()if service ==nil{
err = errors.New("rpc: can't find service "+ req.ServiceMethod)return}// 獲取反射類型，看見rpc中的發射其實是預先放入map中的
mtype = service.method[methodName]if mtype ==nil{
err = errors.New("rpc: can't find method "+ req.ServiceMethod)}return}
func (server *Server) readRequest(codec ServerCodec)(service *service, mtype *methodType, req *Request, argv, replyv reflect.Value, keepReading bool, err error){
service, mtype, req, keepReading, err = server.readRequestHeader(codec)if err !=nil{if!keepReading {return}// discard body
codec.ReadRequestBody(nil)return}// 解析請求中的args
argIsValue :=false// if true, need to indirect before calling. if mtype.ArgType.Kind()== reflect.Ptr{
argv = reflect.New(mtype.ArgType.Elem())}else{
argv = reflect.New(mtype.ArgType)
argIsValue =true}// argv guaranteed to be a pointer now. if err = codec.ReadRequestBody(argv.Interface()); err !=nil{return}if argIsValue {
argv = argv.Elem()}// 初始化reply類型
replyv = reflect.New(mtype.ReplyType.Elem())return}
func (s *service) call(server *Server, sending *sync.Mutex, mtype *methodType, req *Request, argv, replyv reflect.Value, codec ServerCodec){
mtype.Lock()
mtype.numCalls++
mtype.Unlock()function:= mtype.method.Func// Invoke the method, providing a new value for the reply. // 這裡是真正調用rpc方法的地方
returnValues :=function.Call([]reflect.Value{s.rcvr, argv, replyv})// The return value for the method is an error.
errInter := returnValues[0].Interface()
errmsg :=""if errInter !=nil{
errmsg = errInter.(error).Error()}// 處理返回請求了
server.sendResponse(sending, req, replyv.Interface(), codec, errmsg)
server.freeRequest(req)}
func (server *Server) sendResponse(sending *sync.Mutex, req *Request, reply interface{}, codec ServerCodec, errmsg string){
resp := server.getResponse()// Encode the response header
resp.ServiceMethod= req.ServiceMethodif errmsg !=""{
resp.Error= errmsg
reply = invalidRequest
}// 上一文提到，客戶端是根據序號來定位請求的，所以需要原樣返回
resp.Seq= req.Seq
sending.Lock()
err := codec.WriteResponse(resp, reply)if debugLog && err !=nil{
log.Println("rpc: writing response:", err)}
sending.Unlock()
server.freeResponse(resp)}資源重用：
上面把大致的rpc請求都說明了，server有一個技巧是重用對象，這裡使用的是鏈表方式處理的：
// 可以看出使用一個free list鏈表，來避免Request以及Response對象頻繁創建，導致GC壓力
func (server *Server) getRequest()*Request{
server.reqLock.Lock()
req := server.freeReq
if req ==nil{
req =new(Request)}else{
server.freeReq = req.next*req =Request{}}
server.reqLock.Unlock()return req
}
func (server *Server) freeRequest(req *Request){
server.reqLock.Lock()
req.next= server.freeReq
server.freeReq = req
server.reqLock.Unlock()}
func (server *Server) getResponse()*Response{
server.respLock.Lock()
resp := server.freeResp
if resp ==nil{
resp =new(Response)}else{
server.freeResp = resp.next*resp =Response{}}
server.respLock.Unlock()return resp
}
func (server *Server) freeResponse(resp *Response){
server.respLock.Lock()
resp.next= server.freeResp
server.freeResp = resp
server.respLock.Unlock()}最後，sending這把鎖的目的是避免同一個套接字快速請求中避免返回包寫入亂序，因此避免一個包完整寫入完畢才允許下一個返回寫入套接字。通過rpc包源碼解析，可以看到標准庫中的核心思想還是channel+mutex實現復用對象，以及各種方式的復用，避免GC壓力，在我們以後寫高性能服務端可以借鑒的地方。
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