Android中的sp和wp指针

经常会在android的framework代码中发现sp和wp这样的指针,平时看的时候都把他当成一个普通的指针封装过掉了,这几天终于忍不住了,想深入了解一下。

相关的代码：

frameworks/base/include/utils/RefBase.h

frameworks/base/libs/utils/RefBase.cpp

sp和wp都是一个模板类，看一下sp类的定义：

 

template <typename T>
class sp
{
public:
    typedef typename RefBase::weakref_type weakref_type;
    inline sp() : m_ptr(0) { }
    sp(T* other);
    sp(const sp& other);
    ~sp();
    ......
private:
    // Optimization for wp::promote().
    sp(T* p, weakref_type* refs);

    T*              m_ptr;
};

template class sp { public: typedef typename RefBase::weakref_type weakref_type; inline sp() : m_ptr(0) { } sp(T* other); sp(const sp& other); ~sp(); ...... private: // Optimization for wp::promote(). sp(T* p, weakref_type* refs); T* m_ptr; };

 

可以看到他确实封转了一个原生指针T* m_ptr. 再看一下其中一个构造函数和析构函数：

 

template
sp::sp(T* other)
    : m_ptr(other)
{
    if (other) other->incStrong(this);
}

template
sp::~sp()
{
    if (m_ptr) m_ptr->decStrong(this);
}

template sp::sp(T* other) : m_ptr(other) { if (other) other->incStrong(this); } template sp::~sp() { if (m_ptr) m_ptr->decStrong(this); }

 

咋一看好奇怪，因为在构造函数中调用了incStrong()，在析构函数中调用的decStrong()，显然是管理引用计数的函数，但是sp类的中并没有定义这两个函数，这两个函数是在RefBase类中定义的，由此可以得出结论：

要想使用sp或者wp, T必需要继承RefBase类才行。

RefBase的静态关系如下：

 

 其中weakref_type是RefBase的内嵌类，weakref_impl则是weakref_type的子类，RefBase的大部分工作都是交由weakref_impl类来完成，通过RefBase的成员变量weakref_impl* const mRefs。查看其中一个sp的构造函数：

 

template
sp::sp(T* other)
    : m_ptr(other)
{
    if (other) other->incStrong(this);
}

template sp::sp(T* other) : m_ptr(other) { if (other) other->incStrong(this); }

 

建立sp的动态关系如下：

sp

  --> RefBase : incStrong()

  -->weakref_impl : addStrongRef()

  -->android_atomic_inc(&refs->mStrong)

可见当一个普通指针变成一个sp指针后，将会由RefBase类维护该指针的引用计数，当引用为零时则自动释放该指针指向的内存：

 

void RefBase::decStrong(const void* id) const
{
    weakref_impl* const refs = mRefs;
    refs->removeStrongRef(id);
    const int32_t c = android_atomic_dec(&refs->mStrong);
    if (c == 1) {
        const_cast(this)->onLastStrongRef(id);
        if ((refs->mFlags&OBJECT_LIFETIME_WEAK) != OBJECT_LIFETIME_WEAK) {
            delete this;    //引用为0，销毁
        }
    }
    refs->removeWeakRef(id);
    refs->decWeak(id);
}

void RefBase::decStrong(const void* id) const { weakref_impl* const refs = mRefs; refs->removeStrongRef(id); const int32_t c = android_atomic_dec(&refs->mStrong); if (c == 1) { const_cast(this)->onLastStrongRef(id); if ((refs->mFlags&OBJECT_LIFETIME_WEAK) != OBJECT_LIFETIME_WEAK) { delete this; //引用为0，销毁 } } refs->removeWeakRef(id); refs->decWeak(id); }

 

 wp是怎么一回事？

wp其实是弱指针的意思，wp类型不能直接对类型T进行操作，要想对T进行某种操作，必需把wp升级为sp指针，使用promote()来实现升级：

        wp weakp= new T();

        sp t = weakp.promote();

wp可能会在弱引用计数不为0的情况下被销毁，执行如下代码：

 

class WPTest : public RefBase {
public:
    WPTest(){
        LOGD("WPTest constructor");
    }
    virtual ~WPTest() {
        LOGD("WPTest destructor");
    }

    virtual void onFirstRef() {
        LOGD("first weak ptr ref callback");
    }

    virtual void onLastStrongRef(const void* id) {
        LOGD("last strong ptr ref callback");
    }

    virtual void onLastWeakRef(const void* id) {
        LOGD("last weak ptr ref callback");
    }
};


int main()
{
    WPTest *T = new WPTest();
    {
        wp weakp(T);

        {
            LOGD("promote to strong ptr.../n");

            sp strongp = weakp.promote();

            LOGD("strong ptr's lifetime is just about to finish .../n");
        }

        LOGD("weak ptr's lifetime is just about to finish .../n");
    }

    LOGD("weak ptr is out of scope./n");

    return 0;
}

class WPTest : public RefBase { public: WPTest(){ LOGD("WPTest constructor"); } virtual ~WPTest() { LOGD("WPTest destructor"); } virtual void onFirstRef() { LOGD("first weak ptr ref callback"); } virtual void onLastStrongRef(const void* id) { LOGD("last strong ptr ref callback"); } virtual void onLastWeakRef(const void* id) { LOGD("last weak ptr ref callback"); } }; int main() { WPTest *T = new WPTest(); { wp weakp(T); { LOGD("promote to strong ptr.../n"); sp strongp = weakp.promote(); LOGD("strong ptr's lifetime is just about to finish .../n"); } LOGD("weak ptr's lifetime is just about to finish .../n"); } LOGD("weak ptr is out of scope./n"); return 0; }

 

程序打印的结果是：

D/sp-wp-sample(  225): WPTest constructor
D/sp-wp-sample(  225): promote to strong ptr...
D/sp-wp-sample(  225): first weak ptr ref callback
D/sp-wp-sample(  225): strong ptr's lifetime is just about to finish ...
D/sp-wp-sample(  225): last strong ptr ref callback
D/sp-wp-sample(  225): WPTest destructor
D/sp-wp-sample(  225): weak ptr's lifetime is just about to finish ...
D/sp-wp-sample(  225): weak ptr is out of scope.

由此可见虽然wp的生命周期还没有结束，但是因为升级为sp后，sp的强引用计数为0，导致WPTest 被销毁，当强引用为0而弱引用不为0时，WPTest 销毁时，基类RefBase的mRefs指向的weakref_impl类并没有释放，从而保证了弱引用可以继续起作用，这点可以从RefBase的析构函数中看出来：

 

RefBase::~RefBase()
{
//    LOGV("Destroying RefBase %p (refs %p)/n", this, mRefs);
    if (mRefs->mWeak == 0) {
//        LOGV("Freeing refs %p of old RefBase %p/n", mRefs, this);
        delete mRefs;
    }
}

RefBase::~RefBase() { // LOGV("Destroying RefBase %p (refs %p)/n", this, mRefs); if (mRefs->mWeak == 0) { // LOGV("Freeing refs %p of old RefBase %p/n", mRefs, this); delete mRefs; } }

 

不过也可以改变这一行为，我们修改一下WPTest的构造函数：

 

WPTest(){
        LOGD("WPTest constructor");
        extendObjectLifetime(OBJECT_LIFETIME_WEAK);
    }

WPTest(){ LOGD("WPTest constructor"); extendObjectLifetime(OBJECT_LIFETIME_WEAK); }

 

这时的打印结果是：

D/sp-wp-sample(  217): WPTest constructor
D/sp-wp-sample(  217): promote to strong ptr...
D/sp-wp-sample(  217): first weak ptr ref callbac
D/sp-wp-sample(  217): strong ptr's lifetime is j
D/sp-wp-sample(  217): last strong ptr ref callba
D/sp-wp-sample(  217): weak ptr's lifetime is j
D/sp-wp-sample(  217): last weak ptr ref callback
D/sp-wp-sample(  217): WPTest destructor
D/sp-wp-sample(  217): weak ptr is out of scope.

可以看出现在只有当强引用和弱引用的计数都为0时，WPTest对象才会被销毁。