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 i = 0;
    char prop[PATH_MAX] = {0};
    char path[PATH_MAX] = {0};
    char name[PATH_MAX] = {0};
    char prop_name[PATH_MAX] = {0};
 
 
    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.
     */
 
    /* First try a property specific to the class and possibly instance */
    snprintf(prop_name, sizeof(prop_name), "ro.hardware.%s", name);
    if (property_get(prop_name, prop, NULL) > 0) {
        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
            goto found;
        }
    }
 
    /* Loop through the configuration variants looking for a module */
    for (i=0 ; i<HAL_VARIANT_KEYS_COUNT; i++) {
        if (property_get(variant_keys[i], prop, NULL) == 0) {
            continue;
        }
        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
            goto found;
        }
    }
 
    /* Nothing found, try the default */
    if (hw_module_exists(path, sizeof(path), name, "default") == 0) {
        goto found;
    }
 
    return -ENOENT;
 
found:
    /* load the module, if this fails, we're doomed, and we should not try
     * to load a different variant. */
    return load(class_id, path, module);
}

我们主要看hw_get_module_by_class，这里传入的参数分别是“sensors”，null，以及我们的mSensorModule结构体。

首先将字符串拷贝给name：

strlcpy(name, class_id, PATH_MAX);

接着拼接prop_name为ro.hardware.name，即prop_name=ro.hardware.sensors

通过property_get方法并没有得到这个值的定义(因为在系统中并没有对其定义)，所以接下来会进入下面的循环：

for (i=0 ; i<HAL_VARIANT_KEYS_COUNT; i++) {
        if (property_get(variant_keys[i], prop, NULL) == 0) {
            continue;
        }
        if (hw_module_exists(path, sizeof(path), name, prop) == 0) {
            goto found;
        }
    }

/**
 * There are a set of variant filename for modules. The form of the filename
 * is "<MODULE_ID>.variant.so" so for the led module the Dream variants 
 * of base "ro.product.board", "ro.board.platform" and "ro.arch" would be:
 *
 * led.trout.so
 * led.msm7k.so
 * led.ARMV6.so
 * led.default.so
 */
 
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.board.platform",
    "ro.arch"
};

根据上面的解析我门也可以看到，将会分别查找sensors.variant.so，sensors.product.so，sensors.platform.so，以及sensors.default.so，最终我们会在/system/lib/hw/路径下找到sensors.msm8909.so，然后将其通过load方法加载进内存中运行。由此也可知，我分析的是高通8909平台。

小细节：当我们实现了自己的HAL层module，并且写了一个应用程序测试module是否正常工作，那么在编译的时候，下面的参数应该要这样写：

LOCAL_MODULE := moduleName.default

或者

LOCAL_MODULE := moduleName.$(TARGET_BOARD_PLATFORM)

由于上面源码的原因，如果module名字对应不到，你的这个模块将不会被正常的load进去，因而也就无法正常工作了。

接着我们分析load的实现。