android binder 讲解（三）

o IBinder接口

IBinder接口是对跨进程的对象的抽象。普通对象在当前进程可以访问，如果希望对象能被其它进程访问，那就必须实现IBinder接口。IBinder接口可以指向本地对象，也可以指向远程对象，调用者不需要关心指向的对象是本地的还是远程。

transact是IBinder接口中一个比较重要的函数，它的函数原型如下：

virtual status_t transact(uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags = 0) = 0;

android中的IPC的基本模型是基于客户/服务器(C/S)架构的。

客户端请求通过内核模块中转服务端

如果IBinder指向的是一个客户端代理，那transact只是把请求发送给服务器。服务端的IBinder的transact则提供了实际的服务。

o 客户端

BpBinder是远程对象在当前进程的代理，它实现了IBinder接口。它的transact函数实现如下：

status_t BpBinder::transact(    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){    // Once a binder has died, it will never come back to life.    if (mAlive) {        status_t status = IPCThreadState::self()->transact(            mHandle, code, data, reply, flags);        if (status == DEAD_OBJECT) mAlive = 0;        return status;    }      return DEAD_OBJECT;}

参数说明：

• code 是请求的ID号。
• data 是请求的参数。
• reply 是返回的结果。
• flags 一些额外的标识，如FLAG_ONEWAY。通常为0。

transact只是简单的调用了IPCThreadState::self()的transact，在IPCThreadState::transact中：

status_t IPCThreadState::transact(int32_t handle,                                  uint32_t code, const Parcel& data,                                  Parcel* reply, uint32_t flags){    status_t err = data.errorCheck();     flags |= TF_ACCEPT_FDS;     IF_LOG_TRANSACTIONS() {        TextOutput::Bundle _b(alog);        alog << "BC_TRANSACTION thr " << (void*)pthread_self() << " / hand "            << handle << " / code " << TypeCode(code) << ": "            << indent << data << dedent << endl;    }     if (err == NO_ERROR) {        LOG_ONEWAY(">>>> SEND from pid %d uid %d %s", getpid(), getuid(),            (flags & TF_ONE_WAY) == 0 ? "READ REPLY" : "ONE WAY");        err = writeTransactionData(BC_TRANSACTION, flags, handle, code, data, NULL);    }     if (err != NO_ERROR) {        if (reply) reply->setError(err);        return (mLastError = err);    }     if ((flags & TF_ONE_WAY) == 0) {        if (reply) {            err = waitForResponse(reply);        } else {            Parcel fakeReply;            err = waitForResponse(&fakeReply);        }         IF_LOG_TRANSACTIONS() {            TextOutput::Bundle _b(alog);            alog << "BR_REPLY thr " << (void*)pthread_self() << " / hand "                << handle << ": ";            if (reply) alog << indent << *reply << dedent << endl;            else alog << "(none requested)" << endl;        }    } else {        err = waitForResponse(NULL, NULL);    }     return err;} status_t IPCThreadState::waitForResponse(Parcel *reply, status_t *acquireResult){    int32_t cmd;    int32_t err;     while (1) {        if ((err=talkWithDriver()) < NO_ERROR) break;        err = mIn.errorCheck();        if (err < NO_ERROR) break;        if (mIn.dataAvail() == 0) continue;         cmd = mIn.readInt32();         IF_LOG_COMMANDS() {            alog << "Processing waitForResponse Command: "                << getReturnString(cmd) << endl;        }         switch (cmd) {        case BR_TRANSACTION_COMPLETE:            if (!reply && !acquireResult) goto finish;            break;         case BR_DEAD_REPLY:            err = DEAD_OBJECT;            goto finish;         case BR_FAILED_REPLY:            err = FAILED_TRANSACTION;            goto finish;         case BR_ACQUIRE_RESULT:            {                LOG_ASSERT(acquireResult != NULL, "Unexpected brACQUIRE_RESULT");                const int32_t result = mIn.readInt32();                if (!acquireResult) continue;                *acquireResult = result ? NO_ERROR : INVALID_OPERATION;            }            goto finish;         case BR_REPLY:            {                binder_transaction_data tr;                err = mIn.read(&tr, sizeof(tr));                LOG_ASSERT(err == NO_ERROR, "Not enough command data for brREPLY");                if (err != NO_ERROR) goto finish;                 if (reply) {                    if ((tr.flags & TF_STATUS_CODE) == 0) {                        reply->ipcSetDataReference(                            reinterpret_cast(tr.data.ptr.buffer),                            tr.data_size,                            reinterpret_cast(tr.data.ptr.offsets),                            tr.offsets_size/sizeof(size_t),                            freeBuffer, this);                    } else {                        err = *static_cast(tr.data.ptr.buffer);                        freeBuffer(NULL,                            reinterpret_cast(tr.data.ptr.buffer),                            tr.data_size,                            reinterpret_cast(tr.data.ptr.offsets),                            tr.offsets_size/sizeof(size_t), this);                    }                } else {                    freeBuffer(NULL,                        reinterpret_cast(tr.data.ptr.buffer),                        tr.data_size,                        reinterpret_cast(tr.data.ptr.offsets),                        tr.offsets_size/sizeof(size_t), this);                    continue;                }            }            goto finish;         default:            err = executeCommand(cmd);            if (err != NO_ERROR) goto finish;            break;        }    } finish:    if (err != NO_ERROR) {        if (acquireResult) *acquireResult = err;        if (reply) reply->setError(err);        mLastError = err;    }     return err;}

这里transact把请求经内核模块发送了给服务端，服务端处理完请求之后，沿原路返回结果给调用者。这里也可以看出请求是同步操作，它会等待直到结果返回为止。

在BpBinder之上进行简单包装，我们可以得到与服务对象相同的接口，调用者无需要关心调用的对象是远程的还是本地的。拿ServiceManager来说：
(frameworks/base/libs/utils/IServiceManager.cpp)

class BpServiceManager : public BpInterface{public:    BpServiceManager(const sp& impl)        : BpInterface(impl)    {    }...    virtual status_t addService(const String16& name, const sp& service)    {        Parcel data, reply;        data.writeInterfaceToken(IServiceManager::getInterfaceDescriptor());        data.writeString16(name);        data.writeStrongBinder(service);        status_t err = remote()->transact(ADD_SERVICE_TRANSACTION, data, &reply);        return err == NO_ERROR ? reply.readInt32() : err;    }...};

BpServiceManager实现了 IServiceManager和IBinder两个接口，调用者可以把BpServiceManager的对象看作是一个 IServiceManager对象或者IBinder对象。当调用者把BpServiceManager对象当作IServiceManager对象使 用时，所有的请求只是对BpBinder::transact的封装。这样的封装使得调用者不需要关心IServiceManager对象是本地的还是远 程的了。

客户通过defaultServiceManager函数来创建BpServiceManager对象：
(frameworks/base/libs/utils/IServiceManager.cpp)

sp<IServiceManager> defaultServiceManager(){    if (gDefaultServiceManager != NULL) return gDefaultServiceManager;      {        AutoMutex _l(gDefaultServiceManagerLock);        if (gDefaultServiceManager == NULL) {            gDefaultServiceManager = interface_cast<IServiceManager>(                ProcessState::self()->getContextObject(NULL));        }    }      return gDefaultServiceManager;}

先通过ProcessState::self()->getContextObject(NULL)创建一个Binder对象，然后通过 interface_cast和IMPLEMENT_META_INTERFACE(ServiceManager, “android.os.IServiceManager”)把Binder对象包装成 IServiceManager对象。原理上等同于创建了一个BpServiceManager对象。

ProcessState::self()->getContextObject调用ProcessState::getStrongProxyForHandle创建代理对象：

sp<IBinder> ProcessState::getStrongProxyForHandle(int32_t handle){    sp<IBinder> result;      AutoMutex _l(mLock);      handle_entry* e = lookupHandleLocked(handle);      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)) {            b = new BpBinder(handle);            e->binder = b;            if (b) e->refs = b->getWeakRefs();            result = b;        } 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;}

如果handle为空，默认为context_manager对象，context_manager实际上就是ServiceManager。
o 服务端
服务端也要实现IBinder接口，BBinder类对IBinder接口提供了部分默认实现，其中transact的实现如下：

status_t BBinder::transact(    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){    data.setDataPosition(0);      status_t err = NO_ERROR;    switch (code) {        case PING_TRANSACTION:            reply->writeInt32(pingBinder());            break;        default:            err = onTransact(code, data, reply, flags);            break;    }      if (reply != NULL) {        reply->setDataPosition(0);    }      return err;}

PING_TRANSACTION请求用来检查对象是否还存在，这里简单的把 pingBinder的返回值返回给调用者。其它的请求交给onTransact处理。onTransact是BBinder里声明的一个 protected类型的虚函数，这个要求它的子类去实现。比如CameraService里的实现如下：

status_t CameraService::onTransact(    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){    // permission checks...    switch (code) {        case BnCameraService::CONNECT:            IPCThreadState* ipc = IPCThreadState::self();            const int pid = ipc->getCallingPid();            const int self_pid = getpid();            if (pid != self_pid) {                // we're called from a different process, do the real check                if (!checkCallingPermission(                        String16("android.permission.CAMERA")))                {                    const int uid = ipc->getCallingUid();                    LOGE("Permission Denial: "                            "can't use the camera pid=%d, uid=%d", pid, uid);                    return PERMISSION_DENIED;                }            }            break;    }     status_t err = BnCameraService::onTransact(code, data, reply, flags);     LOGD("+++ onTransact err %d code %d", err, code);     if (err == UNKNOWN_TRANSACTION || err == PERMISSION_DENIED) {        // the 'service' command interrogates this binder for its name, and then supplies it        // even for the debugging commands.  that means we need to check for it here, using        // ISurfaceComposer (since we delegated the INTERFACE_TRANSACTION handling to        // BnSurfaceComposer before falling through to this code).         LOGD("+++ onTransact code %d", code);         CHECK_INTERFACE(ICameraService, data, reply);         switch(code) {        case 1000:        {            if (gWeakHeap != 0) {                sp h = gWeakHeap.promote();                IMemoryHeap *p = gWeakHeap.unsafe_get();                LOGD("CHECKING WEAK REFERENCE %p (%p)", h.get(), p);                if (h != 0)                    h->printRefs();                bool attempt_to_delete = data.readInt32() == 1;                if (attempt_to_delete) {                    // NOT SAFE!                    LOGD("DELETING WEAK REFERENCE %p (%p)", h.get(), p);                    if (p) delete p;                }                return NO_ERROR;            }        }        break;        default:            break;        }    }    return err;}

由此可见，服务端的onTransact是一个请求分发函数，它根据请求码(code)做相应的处理。

o 消息循环

服务端(任何进程都可以作为服务端)有一个线程监听来自客户端的请求，并循环处理这些请求。

如果在主线程中处理请求，可以直接调用下面的函数：

IPCThreadState::self()->joinThreadPool(mIsMain);

如果想在非主线程中处理请求，可以按下列方式：

        sp proc = ProcessState::self();        if (proc->supportsProcesses()) {            LOGV("App process: starting thread pool./n");            proc->startThreadPool();        }

startThreadPool的实现原理：

void ProcessState::startThreadPool(){    AutoMutex _l(mLock);    if (!mThreadPoolStarted) {        mThreadPoolStarted = true;        spawnPooledThread(true);    }}  void ProcessState::spawnPooledThread(bool isMain){    if (mThreadPoolStarted) {        int32_t s = android_atomic_add(1, &mThreadPoolSeq);        char buf[32];        sprintf(buf, "Binder Thread #%d", s);        LOGV("Spawning new pooled thread, name=%s/n", buf);        sp t = new PoolThread(isMain);        t->run(buf);    }}

这里创建了PoolThread的对象，实现上就是创建了一个线程。所有的线程类都要实现threadLoop虚函数。PoolThread的threadLoop的实现如下：

    virtual bool threadLoop()    {        IPCThreadState::self()->joinThreadPool(mIsMain);        return false;    }

上述代码，简而言之就是创建了一个线程，然后在线程里调用 IPCThreadState::self()->joinThreadPool函数。

下面再看joinThreadPool的实现：

do{...        result = talkWithDriver();        if (result >= NO_ERROR) {            size_t IN = mIn.dataAvail();            if (IN < sizeof(int32_t)) continue;            cmd = mIn.readInt32();            IF_LOG_COMMANDS() {                alog << "Processing top-level Command: "                    << getReturnString(cmd) << endl;            }            result = executeCommand(cmd);        }...while(...);

这个函数在循环中重复执行下列动作：

1. talkWithDriver 通过ioctl(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr)读取请求和写回结果。
2. executeCommand 执行相应的请求

在IPCThreadState::executeCommand(int32_t cmd)函数中：

1. 对于控制对象生命周期的请求，像BR_ACQUIRE/BR_RELEASE直接做了处理。
2. 对于BR_TRANSACTION请求，它调用被请求对象的transact函数。

按下列方式调用实际的对象：

if (tr.target.ptr) {    sp<BBinder> b((BBinder*)tr.cookie);    const status_t error = b->transact(tr.code, buffer, &reply, 0);    if (error < NO_ERROR) reply.setError(error);  } else {    const status_t error = the_context_object->transact(tr.code, buffer, &reply, 0);    if (error < NO_ERROR) reply.setError(error);}

如果tr.target.ptr不为空，就把tr.cookie转换成一个Binder对象，并调用它的transact函数。如果没有目标对象， 就调用 the_context_object对象的transact函数。奇怪的是，根本没有谁对the_context_object进行初始 化，the_context_object是空指针。原因是context_mgr的请求发给了ServiceManager，所以根本不会走到else 语句里来。

o 内核模块

android使用了一个内核模块binder来中转各个进程之间的消息。模块源代码放在binder.c里，它是一个字符驱动程序，主要通过 binder_ioctl与用户空间的进程交换数据。其中BINDER_WRITE_READ用来读写数据，数据包中有一个cmd域用于区分不同的请求：

1. binder_thread_write用于发送请求或返回结果。
2. binder_thread_read用于读取结果。

从binder_thread_write中调用binder_transaction中转请求和返回结果，binder_transaction的实现如下：

对请求的处理：

1. 通过对象的handle找到对象所在的进程，如果handle为空就认为对象是context_mgr，把请求发给context_mgr所在的进程。
2. 把请求中所有的binder对象全部放到一个RB树中。
3. 把请求放到目标进程的队列中，等待目标进程读取。

如何成为context_mgr呢？内核模块提供了BINDER_SET_CONTEXT_MGR调用:

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg){...case BINDER_SET_CONTEXT_MGR:if (binder_context_mgr_node != NULL) {printk(KERN_ERR "binder: BINDER_SET_CONTEXT_MGR already set/n");ret = -EBUSY;goto err;}if (binder_context_mgr_uid != -1) {if (binder_context_mgr_uid != current->euid) {printk(KERN_ERR "binder: BINDER_SET_"       "CONTEXT_MGR bad uid %d != %d/n",       current->euid,       binder_context_mgr_uid);ret = -EPERM;goto err;}} elsebinder_context_mgr_uid = current->euid;binder_context_mgr_node = binder_new_node(proc, NULL, NULL);if (binder_context_mgr_node == NULL) {ret = -ENOMEM;goto err;}binder_context_mgr_node->local_weak_refs++;binder_context_mgr_node->local_strong_refs++;binder_context_mgr_node->has_strong_ref = 1;binder_context_mgr_node->has_weak_ref = 1;break;

ServiceManager（frameworks/base/cmds/servicemanager)通过下列方式成为了context_mgr进程：

int binder_become_context_manager(struct binder_state *bs){    return ioctl(bs->fd, BINDER_SET_CONTEXT_MGR, 0);} 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;}

o 如何得到服务对象的handle

1. 服务提供者通过defaultServiceManager得到ServiceManager对象，然后调用addService向服务管理器注册。
2. 服务使用者通过defaultServiceManager得到ServiceManager对象，然后调用getService通过服务名称查找到服务对象的handle。

o 如何通过服务对象的handle找到服务所在的进程

0表示服务管理器的handle，getService可以查找到系统服务的handle。这个handle只是代表了服务对象，内核模块是如何通过handle找到服务所在的进程的呢？

1. 对于ServiceManager: ServiceManager调用了binder_become_context_manager使用自己成为context_mgr，所有handle为0的请求都会被转发给ServiceManager。
2. 对于系统服务和应用程序的Listener，在第一次请求内核模块时(比如调用addService)，内核模块在一个RB树中建立了服务对象和进程的对应关系。

   off_end = (void *)offp + tr->offsets_size;for (; offp < off_end; offp++) {struct flat_binder_object *fp;if (*offp > t->buffer->data_size - sizeof(*fp)) {binder_user_error("binder: %d:%d got transaction with ""invalid offset, %d/n",proc->pid, thread->pid, *offp);return_error = BR_FAILED_REPLY;goto err_bad_offset;}fp = (struct flat_binder_object *)(t->buffer->data + *offp);switch (fp->type) {case BINDER_TYPE_BINDER:case BINDER_TYPE_WEAK_BINDER: {struct binder_ref *ref;struct binder_node *node = binder_get_node(proc, fp->binder);if (node == NULL) {node = binder_new_node(proc, fp->binder, fp->cookie);if (node == NULL) {return_error = BR_FAILED_REPLY;goto err_binder_new_node_failed;}node->min_priority = fp->flags & FLAT_BINDER_FLAG_PRIORITY_MASK;node->accept_fds = !!(fp->flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);}if (fp->cookie != node->cookie) {binder_user_error("binder: %d:%d sending u%p ""node %d, cookie mismatch %p != %p/n",proc->pid, thread->pid,fp->binder, node->debug_id,fp->cookie, node->cookie);goto err_binder_get_ref_for_node_failed;}ref = binder_get_ref_for_node(target_proc, node);if (ref == NULL) {return_error = BR_FAILED_REPLY;goto err_binder_get_ref_for_node_failed;}if (fp->type == BINDER_TYPE_BINDER)fp->type = BINDER_TYPE_HANDLE;elsefp->type = BINDER_TYPE_WEAK_HANDLE;fp->handle = ref->desc;binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE, &thread->todo);if (binder_debug_mask & BINDER_DEBUG_TRANSACTION)printk(KERN_INFO "        node %d u%p -> ref %d desc %d/n",       node->debug_id, node->ptr, ref->debug_id, ref->desc);} break;

3. 请求服务时，内核先通过handle找到对应的进程，然后把请求放到服务进程的队列中。

o C调用JAVA

前面我们分析的是C代码的处理。对于JAVA代码，JAVA调用C的函数通过JNI调用即可。从内核时读取请求是在C代码(executeCommand)里进行了，那如何在C代码中调用那些用JAVA实现的服务呢？

android_os_Binder_init里的JavaBBinder对Java里的Binder对象进行包装。

JavaBBinder::onTransact调用Java里的execTransact函数：

        jboolean res = env->CallBooleanMethod(mObject, gBinderOffsets.mExecTransact,            code, (int32_t)&data, (int32_t)reply, flags);        jthrowable excep = env->ExceptionOccurred();        if (excep) {            report_exception(env, excep,                "*** Uncaught remote exception!  "                "(Exceptions are not yet supported across processes.)");            res = JNI_FALSE;              /* clean up JNI local ref -- we don't return to Java code */            env->DeleteLocalRef(excep);        }

o 广播消息

binder不提供广播消息，不过可以ActivityManagerService服务来实现广播。
(frameworks/base/core/java/android/app/ActivityManagerNative.java)

接收广播消息需要实现接口BroadcastReceiver，然后调用ActivityManagerProxy::registerReceiver注册。

触发广播调用ActivityManagerProxy::broadcastIntent。(应用程序并不直接调用它，而是调用Context对它的包装)