深入Golang的接口interface
前言
go不要求類型顯示地聲明實現(xiàn)了哪個接口,只要實現(xiàn)了相關的方法即可,編譯器就能檢測到
空接口類型可以接收任意類型的數(shù)據(jù):
type eface struct { // _type 指向接口的動態(tài)類型元數(shù)據(jù) // 描述了實體類型、包括內存對齊方式、大小等 _type *_type // data 指向接口的動態(tài)值 data unsafe.Pointer }
空接口在賦值時,_type 和 data 都是nil。賦值后,_type 會指向賦值的數(shù)據(jù)元類型,data 會指向該值
非空接口是有方法列表的接口類型,一個變量要賦值給非空接口,就要實現(xiàn)該接口里的所有方法
type iface struct { // tab 接口表指針,指向一個itab實體,存儲方法列表和接口動態(tài)類型信息 tab *itab // data 指向接口的動態(tài)值(一般是指向堆內存的) data unsafe.Pointer } // layout of Itab known to compilers // allocated in non-garbage-collected memory // Needs to be in sync with // ../cmd/compile/internal/gc/reflect.go:/^func.dumptabs. type itab struct { // inter 指向interface的類型元數(shù)據(jù),描述了接口的類型 inter *interfacetype // _type 描述了實體類型、包括內存對齊方式、大小等 _type *_type // hash 從動態(tài)類型元數(shù)據(jù)中拷貝的hash值,用于快速判斷類型是否相等 hash uint32 // copy of _type.hash. Used for type switches. _ [4]byte // fun 記錄動態(tài)類型實現(xiàn)的那些接口方法的地址 // 存儲的是第一個方法的函數(shù)指針 // 這些方法是按照函數(shù)名稱的字典序進行排列的 fun [1]uintptr // variable sized. fun[0]==0 means _type does not implement inter. } // interfacetype 接口的類型元數(shù)據(jù) type interfacetype struct { typ _type // 記錄定義了接口的包名 pkgpath name // mhdr 標識接口所定義的函數(shù)列表 mhdr []imethod }
itab是可復用的,go會將itab緩存起來,構造一個hash表用于查出和查詢緩存信息<接口類型, 動態(tài)類型>
如果itab緩存中有,可以直接拿來使用,如果沒有,則新創(chuàng)建一個itab,并放入緩存中
一個Iface中的具體類型中實現(xiàn)的方法會被拷貝到Itab的fun數(shù)組中
// Note: change the formula in the mallocgc call in itabAdd if you change these fields. type itabTableType struct { size uintptr // length of entries array. Always a power of 2. count uintptr // current number of filled entries. entries [itabInitSize]*itab // really [size] large } func itabHashFunc(inter *interfacetype, typ *_type) uintptr { // compiler has provided some good hash codes for us. // 用接口類型hash和動態(tài)類型hash進行異或 return uintptr(inter.typ.hash ^ typ.hash) }
接口類型和nil作比較
接口值的零值指接口動態(tài)類型和動態(tài)值都為nil,當且僅當此時接口值==nil
如何打印出接口的動態(tài)類型和值?
定義一個iface結構體,用兩個指針來描述itab和data,然后將具體遍歷在內存中的內容強行解釋為我們定義的iface
type iface struct{ itab, data uintptr } func main() { var a interface{} = nil var b interface{} = (*int)(nil) x := 5 var c interface{} = (*int)(&x) ia := *(*iface)(unsafe.Pointer(&a)) ib := *(*iface)(unsafe.Pointer(&b)) ic := *(*iface)(unsafe.Pointer(&c)) fmt.Println(ia, ib, ic) fmt.Println(*(*int)(unsafe.Pointer(ic.data))) } // 輸出 // {0 0} {17426912 0} {17426912 842350714568} // 5
檢測類型是否實現(xiàn)了接口:
賦值語句會發(fā)生隱式的類型轉換,在轉換過程中,編譯器會檢測等號右邊的類型是否實現(xiàn)了等號左邊接口所規(guī)定的函數(shù)
// 檢查 *myWriter 類型是否實現(xiàn)了 io.Writer 接口 var _ io.Writer = (*myWriter)(nil) // 檢查 myWriter 類型是否實現(xiàn)了 io.Writer 接口 var _ io.Writer = myWriter{}
接口轉換的原理
將一個接口轉換為另一個接口:
- 實際上是調用了convI2I
- 如果目標是空接口或和目標接口的inter一樣就直接返回
- 否則去獲取itab,然后將值data賦入,再返回
// 接口間的賦值實際上是調用了runtime.convI2I // 實際上是要找到新interface的tab和data // convI2I returns the new itab to be used for the destination value // when converting a value with itab src to the dst interface. func convI2I(dst *interfacetype, src *itab) *itab { if src == nil { return nil } if src.inter == dst { return src } return getitab(dst, src._type, false) } // 關鍵函數(shù),獲取itab // getitab根據(jù)interfacetype和_type去全局的itab哈希表中查找,如果找到了直接返回 // 否則根據(jù)inter和typ新生成一個itab,插入到全局itab哈希表中 // // 查找了兩次,第二次上鎖了,目的是可能會寫入hash表,阻塞其余協(xié)程的第二次查找 func getitab(inter *interfacetype, typ *_type, canfail bool) *itab { // 函數(shù)列表為空 if len(inter.mhdr) == 0 { throw("internal error - misuse of itab") } // easy case if typ.tflag&tflagUncommon == 0 { if canfail { return nil } name := inter.typ.nameOff(inter.mhdr[0].name) panic(&TypeAssertionError{nil, typ, &inter.typ, name.name()}) } var m *itab // First, look in the existing table to see if we can find the itab we need. // This is by far the most common case, so do it without locks. // Use atomic to ensure we see any previous writes done by the thread // that updates the itabTable field (with atomic.Storep in itabAdd). // 使用原子性去保證我們能看見在該線程之前的任意寫操作 // 確保更新全局hash表的字段 t := (*itabTableType)(atomic.Loadp(unsafe.Pointer(&itabTable))) // 遍歷一次,找到了就返回 if m = t.find(inter, typ); m != nil { goto finish } // 沒找到就上鎖,再試一次 // Not found. Grab the lock and try again. lock(&itabLock) if m = itabTable.find(inter, typ); m != nil { unlock(&itabLock) goto finish } // hash表中沒找到itab就新生成一個itab // Entry doesn't exist yet. Make a new entry & add it. m = (*itab)(persistentalloc(unsafe.Sizeof(itab{})+uintptr(len(inter.mhdr)-1)*goarch.PtrSize, 0, &memstats.other_sys)) m.inter = inter m._type = typ // The hash is used in type switches. However, compiler statically generates itab's // for all interface/type pairs used in switches (which are added to itabTable // in itabsinit). The dynamically-generated itab's never participate in type switches, // and thus the hash is irrelevant. // Note: m.hash is _not_ the hash used for the runtime itabTable hash table. m.hash = 0 m.init() // 加到全局的hash表中 itabAdd(m) unlock(&itabLock) finish: if m.fun[0] != 0 { return m } if canfail { return nil } // this can only happen if the conversion // was already done once using the , ok form // and we have a cached negative result. // The cached result doesn't record which // interface function was missing, so initialize // the itab again to get the missing function name. panic(&TypeAssertionError{concrete: typ, asserted: &inter.typ, missingMethod: m.init()}) } // 查找全局的hash表,有沒有itab // find finds the given interface/type pair in t. // Returns nil if the given interface/type pair isn't present. func (t *itabTableType) find(inter *interfacetype, typ *_type) *itab { // Implemented using quadratic probing. // Probe sequence is h(i) = h0 + i*(i+1)/2 mod 2^k. // We're guaranteed to hit all table entries using this probe sequence. mask := t.size - 1 // 根據(jù)inter,typ算出hash值 h := itabHashFunc(inter, typ) & mask for i := uintptr(1); ; i++ { p := (**itab)(add(unsafe.Pointer(&t.entries), h*goarch.PtrSize)) // Use atomic read here so if we see m != nil, we also see // the initializations of the fields of m. // m := *p m := (*itab)(atomic.Loadp(unsafe.Pointer(p))) if m == nil { return nil } // inter和typ指針都相同 if m.inter == inter && m._type == typ { return m } h += i h &= mask } } // 核心函數(shù)。填充itab // 檢查_type是否符合interface_type并且創(chuàng)建對應的itab結構體將其放到hash表中 // init fills in the m.fun array with all the code pointers for // the m.inter/m._type pair. If the type does not implement the interface, // it sets m.fun[0] to 0 and returns the name of an interface function that is missing. // It is ok to call this multiple times on the same m, even concurrently. func (m *itab) init() string { inter := m.inter typ := m._type x := typ.uncommon() // both inter and typ have method sorted by name, // and interface names are unique, // so can iterate over both in lock step; // the loop is O(ni+nt) not O(ni*nt). // inter和typ的方法都按方法名稱進行排序 // 并且方法名是唯一的,因此循環(huán)次數(shù)的固定的 // 復雜度為O(ni+nt),而不是O(ni*nt) ni := len(inter.mhdr) nt := int(x.mcount) xmhdr := (*[1 << 16]method)(add(unsafe.Pointer(x), uintptr(x.moff)))[:nt:nt] j := 0 methods := (*[1 << 16]unsafe.Pointer)(unsafe.Pointer(&m.fun[0]))[:ni:ni] var fun0 unsafe.Pointer imethods: for k := 0; k < ni; k++ { i := &inter.mhdr[k] itype := inter.typ.typeOff(i.ityp) name := inter.typ.nameOff(i.name) iname := name.name() ipkg := name.pkgPath() if ipkg == "" { ipkg = inter.pkgpath.name() } // 第二層循環(huán)是從上一次遍歷到的位置開始的 for ; j < nt; j++ { t := &xmhdr[j] tname := typ.nameOff(t.name) // 檢查方法名字是否一致 if typ.typeOff(t.mtyp) == itype && tname.name() == iname { pkgPath := tname.pkgPath() if pkgPath == "" { pkgPath = typ.nameOff(x.pkgpath).name() } if tname.isExported() || pkgPath == ipkg { if m != nil { // 獲取函數(shù)地址,放入itab.func數(shù)組中 ifn := typ.textOff(t.ifn) if k == 0 { fun0 = ifn // we'll set m.fun[0] at the end } else { methods[k] = ifn } } continue imethods } } } // didn't find method m.fun[0] = 0 return iname } m.fun[0] = uintptr(fun0) return "" } // 檢查是否需要擴容,并調用方法將itab存入 // itabAdd adds the given itab to the itab hash table. // itabLock must be held. func itabAdd(m *itab) { // Bugs can lead to calling this while mallocing is set, // typically because this is called while panicing. // Crash reliably, rather than only when we need to grow // the hash table. if getg().m.mallocing != 0 { throw("malloc deadlock") } t := itabTable // 檢查是否需要擴容 if t.count >= 3*(t.size/4) { // 75% load factor // Grow hash table. // t2 = new(itabTableType) + some additional entries // We lie and tell malloc we want pointer-free memory because // all the pointed-to values are not in the heap. t2 := (*itabTableType)(mallocgc((2+2*t.size)*goarch.PtrSize, nil, true)) t2.size = t.size * 2 // Copy over entries. // Note: while copying, other threads may look for an itab and // fail to find it. That's ok, they will then try to get the itab lock // and as a consequence wait until this copying is complete. iterate_itabs(t2.add) if t2.count != t.count { throw("mismatched count during itab table copy") } // Publish new hash table. Use an atomic write: see comment in getitab. atomicstorep(unsafe.Pointer(&itabTable), unsafe.Pointer(t2)) // Adopt the new table as our own. t = itabTable // Note: the old table can be GC'ed here. } // 核心函數(shù) t.add(m) } // 核心函數(shù) // add adds the given itab to itab table t. // itabLock must be held. func (t *itabTableType) add(m *itab) { // See comment in find about the probe sequence. // Insert new itab in the first empty spot in the probe sequence. // 在探針序列第一個空白點插入itab mask := t.size - 1 // 計算hash值 h := itabHashFunc(m.inter, m._type) & mask for i := uintptr(1); ; i++ { p := (**itab)(add(unsafe.Pointer(&t.entries), h*goarch.PtrSize)) m2 := *p if m2 == m { // itab已被插入 // A given itab may be used in more than one module // and thanks to the way global symbol resolution works, the // pointed-to itab may already have been inserted into the // global 'hash'. return } if m2 == nil { // Use atomic write here so if a reader sees m, it also // sees the correctly initialized fields of m. // NoWB is ok because m is not in heap memory. // *p = m // 使用原子操作確保其余的goroutine下次查找的時候可以看到他 atomic.StorepNoWB(unsafe.Pointer(p), unsafe.Pointer(m)) t.count++ return } h += i h &= mask } } // hash函數(shù),編譯期間提供較好的hash codes func itabHashFunc(inter *interfacetype, typ *_type) uintptr { // compiler has provided some good hash codes for us. return uintptr(inter.typ.hash ^ typ.hash) }
具體類型轉空接口時,_type 字段直接復制源類型的 _type;調用 mallocgc 獲得一塊新內存,把值復制進去,data 再指向這塊新內存。
具體類型轉非空接口時,入?yún)?tab 是編譯器在編譯階段預先生成好的,新接口 tab 字段直接指向入?yún)?tab 指向的 itab;調用 mallocgc 獲得一塊新內存,把值復制進去,data 再指向這塊新內存。
而對于接口轉接口,itab 調用 getitab 函數(shù)獲取。只用生成一次,之后直接從 hash 表中獲取。
實現(xiàn)多態(tài)
- 多態(tài)的特點
- 一種類型具有多種類型的能力
- 允許不同的對象對同一消息做出靈活的反應
- 以一種通用的方式對待多個使用的對象
- 非動態(tài)語言必須通過繼承和接口的方式實現(xiàn)
實現(xiàn)函數(shù)的內部,接口綁定了實體類型,會直接調用fun里保存的函數(shù),類似于s.tab->fun[0],而fun數(shù)組中保存的是實體類型實現(xiàn)的函數(shù),當函數(shù)傳入不同實體類型時,實際上調用的是不同的函數(shù)實現(xiàn),從而實現(xiàn)了多態(tài)
到此這篇關于深入Golang的接口interface的文章就介紹到這了,更多相關Go 接口 interface內容請搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關文章希望大家以后多多支持腳本之家!
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