HAL層作爲紐帶，把Framework層的APIs和底層驅動連接起來。簡言之，上層需要什麼操作接口，HAL層就負責實現之。
HAL在整個Camera框架中的位置如下圖紅框：
Android Camera框架及Camera服務HAL(紅框)
HAL層的代碼編譯爲so庫，通常位於/system/lib/hw/或/vender/lib/hw/目錄下。接下來看下so庫的加載實現。
廣而知之，C/C++的入口爲main()函數，Android應用的入口是主Activity的onCreate()，那so庫的入口呢？就是：
int hw_get_module(const char *id, const struct hw_module_t **module)參數module：通過她獲取指向實際模塊對象的指針。
對於Camera，啓動加載流程的函數：
/**
* The id of this module
*/
#define CAMERA_HARDWARE_MODULE_ID "camera"
CameraService.cpp (frameworks\av\services\camera\libcameraservice)
camera_module_t *mModule;
void CameraService::onFirstRef()
{
if (hw_get_module(CAMERA_HARDWARE_MODULE_ID,
(const hw_module_t **)&mModule) < 0) {
ALOGE("Could not load camera HAL module");
mNumberOfCameras = 0;
} else {
// so成功加載
ALOGI("Loaded \"%s\" camera module", mModule->common.name);
mNumberOfCameras = mModule->get_number_of_cameras();
....
}
}在定義自己模塊的時候，把hw_module_t成員放在起始位置，如這裏的camera_module_t：
typedef struct camera_module {
hw_module_t common;
int (*get_number_of_cameras)(void);
int (*get_camera_info)(int camera_id, struct camera_info *info);
int (*set_callbacks)(const camera_module_callbacks_t *callbacks);
void (*get_vendor_tag_ops)(vendor_tag_ops_t* ops);
} camera_module_t;整個模塊在實現HAL的時候已經分配好了空間且已經初始化：
camera_module_t HAL_MODULE_INFO_SYM = {
common: {
// init code
},
get_number_of_cameras: camera_get_number_of_cameras,
get_camera_info: camera_get_camera_info,
set_callbacks:NULL,
get_vendor_tag_ops:NULL
};這是對上，對下就是so庫的加載，看函數hw_get_module：
int hw_get_module(const char *id, const struct hw_module_t **module)
{
return hw_get_module_by_class(id, NULL, module);
}
int hw_get_module_by_class(const char *class_id, const char *inst,
const struct hw_module_t **module)
{
int status;
int i;
const struct hw_module_t *hmi = NULL;
char prop[PATH_MAX];
char path[PATH_MAX];
char name[PATH_MAX];
if (inst)
snprintf(name, PATH_MAX, "%s.%s", class_id, inst);
else
strlcpy(name, class_id, PATH_MAX);
/*
* Here we rely on the fact that calling dlopen multiple times on
* the same .so will simply increment a refcount (and not load
* a new copy of the library).
* We also assume that dlopen() is thread-safe.
*/
/* Loop through the configuration variants looking for a module */
for (i=0 ; i<HAL_VARIANT_KEYS_COUNT+1 ; i++) {
if (i < HAL_VARIANT_KEYS_COUNT) {
if (property_get(variant_keys[i], prop, NULL) == 0) {
continue;
}
snprintf(path, sizeof(path), "%s/%s.%s.so",
HAL_LIBRARY_PATH2, name, prop);
if (access(path, R_OK) == 0) break;
snprintf(path, sizeof(path), "%s/%s.%s.so",
HAL_LIBRARY_PATH1, name, prop);
if (access(path, R_OK) == 0) break;
} else {
snprintf(path, sizeof(path), "%s/%s.default.so",
HAL_LIBRARY_PATH2, name);
if (access(path, R_OK) == 0) break;
snprintf(path, sizeof(path), "%s/%s.default.so",
HAL_LIBRARY_PATH1, name);
if (access(path, R_OK) == 0) break;
}
}
status = -ENOENT;
if (i < HAL_VARIANT_KEYS_COUNT+1) {
/* load the module, if this fails, we're doomed, and we should not try
* to load a different variant. */
status = load(class_id, path, module);
}
return status;
}1、拼湊so庫的路徑：路徑+name+屬性+".so"
路徑之前提到過，/system/lib/hw、/vendor/lib/hw。
name就是穿進來的id，比如camera。
屬性，這裏指定的屬性有：
static const char *variant_keys[] = {
"ro.hardware", /* This goes first so that it can pick up a different
file on the emulator. */
"ro.product.board",
"ro.product.device",
"ro.board.platform",
"ro.arch"
};so庫的完整路徑比如/system/lib/hw/camera.default.so等等。
2、查找庫是否存在
我們看到，這些so是有優先級的，找到滿意的即刻break，正應了句老話：弱水三千，只取一瓢飲。有了so庫，調用load加載之。
擴展：onFirstRef()的來源及調用
C++裏面有智能指針的概念，用於對象內存的自動釋放；類似地，Android實現類似功能的是sp(Strong Pointer)、wp(Weak Pointer)，要使用它們需要在定義自己類的時候繼承RefBase類。RefBase概覽：
class RefBase
{
public:
void incStrong(const void* id) const;
void decStrong(const void* id) const;
protected:
RefBase();
virtual ~RefBase
virtual void onFirstRef();
};
void RefBase::incStrong(const void* id) const
{
weakref_impl* const refs = mRefs;
refs->incWeak(id);
refs->addStrongRef(id);
const int32_t c = android_atomic_inc(&refs->mStrong);
ALOG_ASSERT(c > 0, "incStrong() called on %p after last strong ref", refs);
#if PRINT_REFS
ALOGD("incStrong of %p from %p: cnt=%d\n", this, id, c);
#endif
if (c != INITIAL_STRONG_VALUE) {
return;
}
android_atomic_add(-INITIAL_STRONG_VALUE, &refs->mStrong);
refs->mBase->onFirstRef();
}我們寫一個小程序體驗下：
#include <utils/Errors.h>
#include <utils/RefBase.h>
#include <utils/String16.h>
#include <utils/Vector.h>
namespace android {
class ITest: public virtual RefBase
{
public:
ITest() {
printf("ITest create.\n");
}
private:
virtual void onFirstRef();
};
void ITest::onFirstRef()
{
printf("onFirstRef\n");
}
};
using namespace android;
int main(int argc, char **argv)
{
sp<ITest> spTest(new ITest());
return 0;
}
ITest create.
onFirstRef
