不管是客户端还是服务端，头部都要先调用
sp < IServiceManager > sm = defaultServiceManager();
defaultServiceManager()都干了什么，它返回的是什么实例呢？
该函数定义在frameworks/native/libs/binder/IserviceManager.cpp:33
sp<IServiceManager> defaultServiceManager()
{
if (gDefaultServiceManager != NULL) return gDefaultServiceManager;
{
AutoMutex _l(gDefaultServiceManagerLock);
while (gDefaultServiceManager == NULL) {
gDefaultServiceManager = interface_cast<IServiceManager>(
ProcessState::self()->getContextObject(NULL)); // 这里才是关键步骤
if (gDefaultServiceManager == NULL)
sleep(1);
}
}
return gDefaultServiceManager;
}
关键步骤可以分解为几步：1、ProcessState::self()，2、ProcessState::getContextObject(…)，3、interface_cast<IserviceManager>(…)
1 ProcessState::self()
frameworks/native/libs/binder/ProcessState.cpp:70
sp<ProcessState> ProcessState::self() // 这又是一个进程单体
{
Mutex::Autolock _l(gProcessMutex);
if (gProcess != NULL) {
return gProcess;
}
gProcess = new ProcessState; // 首次创建在这里
return gProcess;
}
ProcessState的构造函数很简单，frameworks/native/libs/binder/ProcessState.cpp:339
ProcessState::ProcessState()
: mDriverFD(open_driver()) // 这里打开了/dev/binder文件，并返回文件描述符
, mVMStart(MAP_FAILED)
, mThreadCountLock(PTHREAD_MUTEX_INITIALIZER)
, mThreadCountDecrement(PTHREAD_COND_INITIALIZER)
, mExecutingThreadsCount(0)
, mMaxThreads(DEFAULT_MAX_BINDER_THREADS)
, mManagesContexts(false)
, mBinderContextCheckFunc(NULL)
, mBinderContextUserData(NULL)
, mThreadPoolStarted(false)
, mThreadPoolSeq(1)
{
if (mDriverFD >= 0) {
// XXX Ideally, there should be a specific define for whether we
// have mmap (or whether we could possibly have the kernel module
// availabla).
#if !defined(HAVE_WIN32_IPC)
// mmap the binder, providing a chunk of virtual address space to receive transactions.
mVMStart = mmap(0, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, 0);
if (mVMStart == MAP_FAILED) {
// *sigh*
ALOGE("Using /dev/binder failed: unable to mmap transaction memory.\n");
close(mDriverFD);
mDriverFD = -1;
}
#else
mDriverFD = -1;
#endif
}
LOG_ALWAYS_FATAL_IF(mDriverFD < 0, "Binder driver could not be opened. Terminating.");
}
ProcessState的构造函数主要完成两件事：1、初始化列表里调用opern_driver()，打开了文件/dev/binder；2、将文件映射到内存。ProcessState::self()返回单体实例。
2 ProcessState::getContextObject(…)
该函数定义在frameworks/native/libs/binder/ProcessState.cpp:85
sp<IBinder> ProcessState::getContextObject(const sp<IBinder>& /*caller*/)
{
return getStrongProxyForHandle(0);
}
继续深入，frameworks/native/libs/binder/ProcessState/cpp:179
sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle)
{
sp<IBinder> result;
AutoMutex _l(mLock);
handle_entry* e = lookupHandleLocked(handle); //正常情况下总会返回一个非空实例
if (e != NULL) {
// We need to create a new BpBinder if there isn‘t currently one, OR we
// are unable to acquire a weak reference on this current one. See comment
// in getWeakProxyForHandle() for more info about this.
IBinder* b = e->binder;
if (b == NULL || !e->refs->attemptIncWeak(this)) {
if (handle == 0) { // 首次创建b为NULL，handle为0
// Special case for context manager...
// The context manager is the only object for which we create
// a BpBinder proxy without already holding a reference.
// Perform a dummy transaction to ensure the context manager
// is registered before we create the first local reference
// to it (which will occur when creating the BpBinder).
// If a local reference is created for the BpBinder when the
// context manager is not present, the driver will fail to
// provide a reference to the context manager, but the
// driver API does not return status.
//
// Note that this is not race-free if the context manager
// dies while this code runs.
//
// TODO: add a driver API to wait for context manager, or
// stop special casing handle 0 for context manager and add
// a driver API to get a handle to the context manager with
// proper reference counting.
Parcel data;
status_t status = IPCThreadState::self()->transact(
0, IBinder::PING_TRANSACTION, data, NULL, 0);
if (status == DEAD_OBJECT)
return NULL;
}
b = new BpBinder(handle);
e->binder = b;
if (b) e->refs = b->getWeakRefs();
result = b; // 返回的是BpBinder(0)
} else {
// This little bit of nastyness is to allow us to add a primary
// reference to the remote proxy when this team doesn‘t have one
// but another team is sending the handle to us.
result.force_set(b);
e->refs->decWeak(this);
}
}
return result;
}
因此getStrongProxyForHandle(0)返回的就是new BpBinder(0)。有几处细节可以再回头关注一下：
2.1 ProcessState::lookupHandleLocked(int32_t handle)
该函数定义在frameworks/native/libs/binder/ProcessState.cpp:166
ProcessState::handle_entry* ProcessState::lookupHandleLocked(int32_t handle)
{
const size_t N=mHandleToObject.size();
if (N <= (size_t)handle) {
handle_entry e;
e.binder = NULL;
e.refs = NULL;
status_t err = mHandleToObject.insertAt(e, N, handle+1-N);
if (err < NO_ERROR) return NULL;
}
return &mHandleToObject.editItemAt(handle);
}
成员变量mHandleToObject是一个数组
Vector<handle_entry>mHandleToObject;
该函数遍历数组查找handle，如果没找到则会向该数组中插入一个新元素，handle是数组下标。新元素的binder、refs成员默认均为NULL，在getStrongProxyForHandle(…)中会被赋值。
3 interface_cast<IserviceManager>(…)
interface_cast(…)函数在binder体系中非常常用，后面还会不断遇见。该函数定义在frameworks/native/include/binder/IInterface.h:41
template<typename INTERFACE>
inline sp<INTERFACE> interface_cast(const sp<IBinder>& obj)
{
return INTERFACE::asInterface(obj);
}
代入模板参数及实参后为：
IServiceManager::asInterface(new BpBinder(0));
该函数藏在宏IMPLEMENT_META_INTERFACE中，frameworks/native/libs/binder/IServiceManager.cpp:185
IMPLEMENT_META_INTERFACE(ServiceManager, "android.os.IServiceManager");
展开后为：
android::sp< IServiceManager > IServiceManager::asInterface(
const android::sp<android::IBinder>& obj)
{
android::sp< IServiceManager > intr;
if (obj != NULL) {
intr = static_cast< IServiceManager *>(
obj->queryLocalInterface(IServiceManager::descriptor).get());
if (intr == NULL) { // 首次会走这里
intr = new BpServiceManager(obj);
}
}
return intr;
}
因此它返回的就是new BpServiceManager(new BpBinder(0))。经过层层抽丝剥茧之后，defaultServiceManager()的返回值即为new BpServiceManager(new BpBinder(0))。
我们再顺道看一下BpServiceManager的继承关系以及构造函数，frameworks/native/libs/binder/IServiceManager.cpp:126
class BpServiceManager : public BpInterface<IServiceManager>
frameworks/native/libs/binder/IInterface.h:62
template<typename INTERFACE> class BpInterface : public INTERFACE, public BpRefBase
BpServiceManager继承自BpInterface，后者继承自BpRefBase。
frameworks/native/libs/binder/IServiceManager.cpp:129
BpServiceManager继承自BpInterface，后者继承自BpRefBase。 frameworks/native/libs/binder/IServiceManager.cpp:129
frameworks/native/include/binder/IInterface.h:134
template<typename INTERFACE>
inline BpInterface<INTERFACE>::BpInterface(const sp<IBinder>& remote)
: BpRefBase(remote)
{
}
frameworks/native/libs/binder/Binder.cpp:241
BpRefBase::BpRefBase(const sp<IBinder>& o)
: mRemote(o.get()), mRefs(NULL), mState(0)
{
extendObjectLifetime(OBJECT_LIFETIME_WEAK);
if (mRemote) {
mRemote->incStrong(this); // Removed on first IncStrong().
mRefs = mRemote->createWeak(this); // Held for our entire lifetime.
}
}
BpServiceManager通过构造函数，沿着继承关系一路将impl参数传递给基类BpRefBase，基类将它赋给数据成员mRemote。在defaultServiceManager()中传给BpServiceManager构造函数的参数是new BpBinder(0)。