Android入门:监听ContentProvider数据改变

ServiceMananger是android中比较重要的一个进程,它是在init进程启动之后启动,从名字上就可以看出来它是用来管理系统中的service。比如:InputMethodService、ActivityManagerService等。在ServiceManager中有两个比较重要的方法:add_service、check_service。系统的service需要通过add_service把自己的信息注册到ServiceManager中,当需要使用时,通过check_service检查该service是否存在。

主函数

从它的主函数代码开始:

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    int main(int argc, char **argv)  
    {  
        struct binder_state *bs;  
        void *svcmgr = BINDER_SERVICE_MANAGER;  
        bs = binder_open(128*1024);  
        if (binder_become_context_manager(bs)) {  
            LOGE("cannot become context manager (%s)\n", strerror(errno));  
            return -1;  
        }  
        svcmgr_handle = svcmgr;  
        binder_loop(bs, svcmgr_handler);  
        return 0;  
    }  

从main函数中可以看出,它主要做了三件事情:
打开/dev/binder设备,并在内存中映射128K的空间。
通知Binder设备,把自己变成context_manager
进入循环,不停的去读Binder设备,看是否有对service的请求,如果有的话,就去调用svcmgr_handler函数回调处理请求。

服务注册

再来看看ServiceManager中是怎么样去注册服务的。先来看先,当有对service的请求时,调用的回调函数svcmgr_handler:

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    int svcmgr_handler(struct binder_state *bs,  
                       struct binder_txn *txn,  
                       struct binder_io *msg,  
                       struct binder_io *reply)  
    {  
        struct svcinfo *si;  
        uint16_t *s;  
        unsigned len;  
        void *ptr;  
        uint32_t strict_policy;  
    //  LOGI("target=%p code=%d pid=%d uid=%d\n",  
    //  txn->target, txn->code, txn->sender_pid, txn->sender_euid);  
        if (txn->target != svcmgr_handle)  
            return -1;  
        // Equivalent to Parcel::enforceInterface(), reading the RPC  
        // header with the strict mode policy mask and the interface name.  
        // Note that we ignore the strict_policy and don't propagate it  
        // further (since we do no outbound RPCs anyway).  
        strict_policy = bio_get_uint32(msg);  
        s = bio_get_string16(msg, &len);  
        if ((len != (sizeof(svcmgr_id) / 2)) ||  
            memcmp(svcmgr_id, s, sizeof(svcmgr_id))) {  
            fprintf(stderr,"invalid id %s\n", str8(s));  
            return -1;  
        }  
        switch(txn->code) {  
        case SVC_MGR_GET_SERVICE:  
        case SVC_MGR_CHECK_SERVICE:  
            s = bio_get_string16(msg, &len);  
            ptr = do_find_service(bs, s, len);  
            if (!ptr)  
                break;  
            bio_put_ref(reply, ptr);  
            return 0;  
        case SVC_MGR_ADD_SERVICE:  
            s = bio_get_string16(msg, &len);  
            ptr = bio_get_ref(msg);  
            if (do_add_service(bs, s, len, ptr, txn->sender_euid))  
                return -1;  
            breakcase SVC_MGR_LIST_SERVICES: {  
            unsigned n = bio_get_uint32(msg);  
            si = svclist;  
            while ((n-- > 0) && si)  
                si = si->next;  
            if (si) {  
                bio_put_string16(reply, si->name);  
                return 0;  
            }  
            return -1;  
        }  
        default:  
            LOGE("unknown code %d\n", txn->code);  
            return -1;  
        }  
        bio_put_uint32(reply, 0);  
        return 0;  
    }  

在该回调函数中会判断Service有什么需要,如果是请求注册service,那么久执行:
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    case SVC_MGR_ADD_SERVICE:  
            s = bio_get_string16(msg, &len);  
            ptr = bio_get_ref(msg);  
            if (do_add_service(bs, s, len, ptr, txn->sender_euid))  
                return -1;  
            break;  

我们再来看看do_add_service中做了什么事情:

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    int do_add_service(struct binder_state *bs,  
                       uint16_t *s, unsigned len,  
                       void *ptr, unsigned uid)  
    {  
        struct svcinfo *si;  
    //    LOGI("add_service('%s',%p) uid=%d\n", str8(s), ptr, uid);  
        if (!ptr || (len == 0) || (len > 127))  
            return -1if (!svc_can_register(uid, s)) {  
            LOGE("add_service('%s',%p) uid=%d - PERMISSION DENIED\n",  
                 str8(s), ptr, uid);  
            return -1;  
        }  
        si = find_svc(s, len);  
        if (si) {  
            if (si->ptr) {  
                LOGE("add_service('%s',%p) uid=%d - ALREADY REGISTERED\n",  
                     str8(s), ptr, uid);  
                return -1;  
            }  
            si->ptr = ptr;  
        } else {  
            si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));  
            if (!si) {  
                LOGE("add_service('%s',%p) uid=%d - OUT OF MEMORY\n",  
                     str8(s), ptr, uid);  
                return -1;  
            }  
            si->ptr = ptr;  
            si->len = len;  
            memcpy(si->name, s, (len + 1) * sizeof(uint16_t));  
            si->name[len] = '\0';  
            si->death.func = svcinfo_death;  
            si->death.ptr = si;  
            si->next = svclist;  
            svclist = si;  
        }  
        binder_acquire(bs, ptr);  
        binder_link_to_death(bs, ptr, &si->death);  
        return 0;  
    }  

在该函数中,首先会去检查是否有权限注册service,如果没有权限就直接返回,不能注册。
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    if (!svc_can_register(uid, s)) {  
            LOGE("add_service('%s',%p) uid=%d - PERMISSION DENIED\n",  
                 str8(s), ptr, uid);  
            return -1;  
        }  

然后会去检查该service是否已经注册过了,如果已经注册过,那么就不能再注册了:

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    si = find_svc(s, len);  
    if (si) {  
          if (si->ptr) {  
              LOGE("add_service('%s',%p) uid=%d - ALREADY REGISTERED\n",  
                   str8(s), ptr, uid);  
              return -1;  
          }  
          si->ptr = ptr;  
      }  

再判断内存是否足够:

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    si = malloc(sizeof(*si) + (len + 1) * sizeof(uint16_t));  
            if (!si) {  
                LOGE("add_service('%s',%p) uid=%d - OUT OF MEMORY\n",  
                     str8(s), ptr, uid);  
                return -1;  
            }  

如果都没什么问题,会注册该service,加入到svcList中来。注意,在ServiceManager中维护service信息的地方就是svclist。里面存了service的name和handler。

服务获取

通过以上几个步骤,service就算注册成功了。那么当要获得该service的时候又是怎么去处理的。还是来看下回调函数中的判断:
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    case SVC_MGR_CHECK_SERVICE:  
            s = bio_get_string16(msg, &len);  
            ptr = do_find_service(bs, s, len);  
            if (!ptr)  
                break;  
            bio_put_ref(reply, ptr);  
            return 0;  

如果是获取service,那么执行SVC_MGR_CHECK_SERVICE,并把返回的数据写入reply,返回给客户端。

do_find_service函数中主要执行service的查找。

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    void *do_find_service(struct binder_state *bs, uint16_t *s, unsigned len)  
    {  
        struct svcinfo *si;  
        si = find_svc(s, len);  
    //    LOGI("check_service('%s') ptr = %p\n", str8(s), si ? si->ptr : 0);  
        if (si && si->ptr) {  
            return si->ptr;  
        } else {  
            return 0;  
        }  
    }  

这样在ServiceManager中就完成了服务的注册和查找。来看下ServiceManager的功能图:

 

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