一、sync Map 包整体结构

本文主要阐述：Load、Store、Delete，更加详细的阐述可以参考源码描述（建议先大体浏览一下Map源码）。

导言：

1. 空间换时间。 通过冗余的两个数据结构(read、dirty),实现加锁对性能的影响。
2. 使用只读数据(read)，避免读写冲突。
3. 动态调整，miss次数多了之后，将dirty数据提升为read。
4. double-checking。
5. 延迟删除。 删除一个键值只是打标记（会将key对应value的pointer置为nil，但read中仍然有这个key:key;value:nil的键值对），只有在提升dirty的时候才清理删除的数据。
6. 优先从read读取、更新、删除，因为对read的读取不需要锁。
7. 虽然read和dirty有冗余数据，但这些数据是通过指针指向同一个数据，所以尽管Map的value会很大，但是冗余的空间占用还是有限的。

二、基础数据结构

1、Map

// Map is a concurrent map with amortized-constant-time loads,stores,and deletes. // It is safe for multiple goroutines to call a Map's methods concurrently. // // It is optimized for use in concurrent loops with keys that are // stable over time,and either few steady-state stores,or stores // localized to one goroutine per key. // // For use cases that do not share these attributes,it will likely have // comparable or worse performance and worse type safety than an ordinary // map paired with a read-write mutex. // // The zero Map is valid and empty. // // A Map must not be copied after first use.
 //该 Map 是线程安全的，读取，插入，删除也都保持着常数级的时间复杂度。 //多个 goroutines 协程同时调用 Map 方法也是线程安全的。该 Map 的零值是有效的， //并且零值是一个空的 Map 。线程安全的 Map 在第一次使用之后，不允许被拷贝。
type Map struct {
    mu Mutex

    // read contains the portion of the map's contents that are safe for
    // concurrent access (with or without mu held).
    //
    // The read field itself is always safe to load,but must only be stored with
    // mu held.
    //
    // Entries stored in read may be updated concurrently without mu,but updating
    // a prevIoUsly-expunged entry requires that the entry be copied to the dirty
    // map and unexpunged with mu held.

     // 一个只读的数据结构，因为只读，所以不会有读写冲突。
    // 所以从这个数据中读取总是安全的。
    // 实际上，实际也会更新这个数据的entries,如果entry是未删除的(unexpunged),并不需要加锁。如果entry已经被删除了，需要加锁，以便更新dirty数据。
    read atomic.Value // readOnly

    // dirty contains the portion of the map's contents that require mu to be
    // held. To ensure that the dirty map can be promoted to the read map quickly,
    // it also includes all of the non-expunged entries in the read map.
    //
    // Expunged entries are not stored in the dirty map. An expunged entry in the
    // clean map must be unexpunged and added to the dirty map before a new value
    // can be stored to it.
    //
    // If the dirty map is nil,the next write to the map will initialize it by
    // making a shallow copy of the clean map,omitting stale entries.

    // dirty数据包含当前的map包含的entries,它包含最新的entries(包括read中未删除的数据,虽有冗余，但是提升dirty字段为read的时候非常快，不用一个一个的复制，而是直接将这个数据结构作为read字段的一部分),有些数据还可能没有移动到read字段中。
    // 对于dirty的操作需要加锁，因为对它的操作可能会有读写竞争。
    // 当dirty为空的时候， 比如初始化或者刚提升完，下一次的写操作会复制read字段中未删除的数据到这个数据中。
    dirty map[interface{}]*entry

    // misses counts the number of loads since the read map was last updated that
    // needed to lock mu to determine whether the key was present.
    //
    // Once enough misses have occurred to cover the cost of copying the dirty
    // map,the dirty map will be promoted to the read map (in the unamended
    // state) and the next store to the map will make a new dirty copy.
    // 当从Map中读取entry的时候，如果read中不包含这个entry,会尝试从dirty中读取，这个时候会将misses加一，
    // 当misses累积到 dirty的长度的时候， 就会将dirty提升为read,避免从dirty中miss太多次。因为操作dirty需要加锁。
    misses int
}

2、readOnly

// readOnly is an immutable struct stored atomically in the Map.read field.
type readOnly struct {
    m       map[interface{}]*entry
    // true if the dirty map contains some key not in m.
    // 如果Map.dirty有些数据不在中的时候，这个值为true
    amended bool 
}

3、entry

// An entry is a slot in the map corresponding to a particular key.
type entry struct {
    // p points to the interface{} value stored for the entry.
    //
    // If p == nil,the entry has been deleted and m.dirty == nil.
    //
    // If p == expunged,the entry has been deleted,m.dirty != nil,and the entry
    // is missing from m.dirty.
    //
    // Otherwise,the entry is valid and recorded in m.read.m[key] and,if m.dirty
    // != nil,in m.dirty[key].
    //
    // An entry can be deleted by atomic replacement with nil: when m.dirty is
    // next created,it will atomically replace nil with expunged and leave
    // m.dirty[key] unset.
    //
    // An entry's associated value can be updated by atomic replacement,provided
    // p != expunged. If p == expunged,an entry's associated value can be updated
    // only after first setting m.dirty[key] = e so that lookups using the dirty
    // map find the entry.

    //p有三种值：
    //nil: entry已被删除了，并且m.dirty为nil
    //expunged: entry已被删除了，并且m.dirty不为nil，而且这个entry不存在于m.dirty中
    //其它： entry是一个正常的值
    p unsafe.Pointer // *interface{}
}

4、Value

// A Value provides an atomic load and store of a consistently typed value. // Values can be created as part of other data structures. // The zero value for a Value returns nil from Load. // Once Store has been called,a Value must not be copied. // // A Value must not be copied after first use. type Value struct { noCopy noCopy v interface{} }

下图来自：http://www.jianshu.com/p/43e66dab535b

三、Load

根据指定的key,查找对应的值value,如果不存在，通过ok反映

func (m *Map) Load(key interface{}) (value interface{},ok bool) {
    read,_ := m.read.Load().(readOnly)
    e,ok := read.m[key]
    // 如果没找到，并且m.dirty中有新数据，需要从m.dirty查找，这个时候需要加锁
    if !ok && read.amended {
        m.mu.Lock()
        // Avoid reporting a spurIoUs miss if m.dirty got promoted while we were
        // blocked on m.mu. (If further loads of the same key will not miss,it's
        // not worth copying the dirty map for this key.)
        //double check,避免加锁的时候m.dirty提升为m.read,这个时候m.read可能被替换了。
        read,_ = m.read.Load().(readOnly)
        e,ok = read.m[key]
        if !ok && read.amended {
            e,ok = m.dirty[key]
            // Regardless of whether the entry was present,record a miss: this key
            // will take the slow path until the dirty map is promoted to the read
            // map.
            m.missLocked()
        }
        m.mu.Unlock()
    }
    if !ok {
        return nil,false
    }
    return e.load()
}

func (m *Map) missLocked() {
    m.misses++
    if m.misses < len(m.dirty) {
        return
    }
    m.read.Store(readOnly{m: m.dirty})
    m.dirty = nil
    m.misses = 0
}

四、Store

更新或者新增一个entry

// Store sets the value for a key.
func (m *Map) Store(key,value interface{}) {
    read,_ := m.read.Load().(readOnly)
    // 从 read map 中读取 key 成功并且取出的 entry 尝试存储 value 成功，直接返回
    if e,ok := read.m[key]; ok && e.tryStore(&value) {
        return
    }

    m.mu.Lock()
    read,_ = m.read.Load().(readOnly)
    if e,ok := read.m[key]; ok {
        if e.unexpungeLocked() {//确保未被标记成删除，即e 指向的是非 nil 的
            // The entry was prevIoUsly expunged,which implies that there is a
            // non-nil dirty map and this entry is not in it.
            //m.dirty中不存在这个键，所以加入m.dirty
            m.dirty[key] = e
        }
        e.storeLocked(&value)
    } else if e,ok := m.dirty[key]; ok {
        e.storeLocked(&value)
    } else {
        if !read.amended {
            // We're adding the first new key to the dirty map.
            // Make sure it is allocated and mark the read-only map as incomplete.
            m.dirtyLocked()
            m.read.Store(readOnly{m: read.m,amended: true})
        }
        m.dirty[key] = newEntry(value)
    }
    m.mu.Unlock()
}
 // tryStore stores a value if the entry has not been expunged. // // If the entry is expunged,tryStore returns false and leaves the entry // unchanged.
func (e *entry) tryStore(i *interface{}) bool {
    p := atomic.LoadPointer(&e.p)
    if p == expunged {
        return false
    }
    for {
        if atomic.CompareAndSwapPointer(&e.p,p,unsafe.Pointer(i)) {
            return true
        }
        p = atomic.LoadPointer(&e.p)
        if p == expunged {
            return false
        }
    }
}


func (m *Map) dirtyLocked() {
    if m.dirty != nil {
        return
    }

    read,_ := m.read.Load().(readOnly)
    m.dirty = make(map[interface{}]*entry,len(read.m))
    for k,e := range read.m {
        if !e.tryExpungeLocked() {
            m.dirty[k] = e
        }
    }
}

func (e *entry) tryExpungeLocked() (isExpunged bool) {
    p := atomic.LoadPointer(&e.p)
    for p == nil {
         // 将已经删除标记为nil的数据标记为expunged
        if atomic.CompareAndSwapPointer(&e.p,nil,expunged) {
            return true
        }
        p = atomic.LoadPointer(&e.p)
    }
    return p == expunged
}
 // unexpungeLocked ensures that the entry is not marked as expunged. // If the entry was prevIoUsly expunged,it must be added to the dirty map // before m.mu is unlocked.
 // unexpungeLocked 函数确保了 entry 没有被标记成已被清除。 // 如果 entry 先前被清除过了，那么在 mutex 解锁之前，它一定要被加入到 dirty map 中
 //如果 entry 的 unexpungeLocked 返回为 true，那么就说明 entry  //之前被标记成了 expunged，并经过 CAS 操作成功把它置为 nil。
func (e *entry) unexpungeLocked() (wasExpunged bool) {
    return atomic.CompareAndSwapPointer(&e.p,expunged,nil)
}

五、Delete

删除一个键值

// Delete deletes the value for a key. func (m *Map) Delete(key interface{}) { read,_ := m.read.Load().(readOnly) e,ok := read.m[key] if !ok && read.amended { m.mu.Lock() read,_ = m.read.Load().(readOnly) e,ok = read.m[key] if !ok && read.amended { delete(m.dirty,key) } m.mu.Unlock() } if ok { e.delete() } } func (e *entry) delete() (hadValue bool) { for { p := atomic.LoadPointer(&e.p) // 已标记为删除 if p == nil || p == expunged { return false } // 原子操作，e.p标记为nil if atomic.CompareAndSwapPointer(&e.p,nil) { return true } } }

六、疑问

1、已经删除的key,再次Load的时候，会怎么样？

func (e *entry) load() (value interface{},ok bool) {
    p := atomic.LoadPointer(&e.p)
    if p == nil || p == expunged {
        return nil,false
    }
    return *(*interface{})(p),true
}

在Map Load方法中调用e.load()时，load方法会识别该值是否已被删除

本文map结构描述部分参考：https://studygolang.com/articles/10511

Java 1.8 ConcurrentHashMap 源码注解部分：
https://github.com/jiankunking/backups/blob/master/ConcurrentHashMap.java

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作者：jiankunking 出处：http://blog.csdn.net/jiankunking