Android源码分析--MediaServer源码分析（一）

MediaServer中包括了系统中的许多重要的Server：

• AudioFlinger：音频系统中的核心服务
• AudioPolicyService：音频系统中关于音频策略的重要服务
• MediaPlayerService：多媒体系统中的重要服务
• CameraService：有关照相和摄像的重要服务

同时，分析MediaServer对于理解Android中的IPC机制能够提供较好的帮助。Android系统基本上可以看做是一个基于Binder机制的C/S架构，对于Binder机制的理解相对比较复杂，如果能够通过具体的例子入手会比较容易理解。

Android的通信体制架构

Android的通信机制基本上可以看做是Client、Server和ServiceManager三者之间的交互：

1. Server首先要注册一些Service到ServiceManager，在这里Server是ServiceManager的客户端；
2. 如果某个Client要使用Service，则首先到ServiceManager中获得该Service的相关信息，所有Client是ServiceManager的客户端；
3. Client得到Service信息，然后和该Service所在的Server进程建立通信之后使用Service，在这里Client是Server的客户端。 
   在这些交互的过程中，Android系统都是使用的Binder来进行通信。

MediaServer入口函数

MS是一个可执行程序，它的入口函数是main函数，所在文件位置：frameworks\base\media\mediaserver\main_mediaserver.cpp 
代码如下：

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">int main(int argc, char<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">**</span> argv)
{
    sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>ProcessState<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> proc(ProcessState<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::self</span>());
    sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IServiceManager<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> sm <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> defaultServiceManager();
    ALOGI(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ServiceManager: %p"</span>, sm<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>get());
    AudioFlinger<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::instantiate</span>();
    MediaPlayerService<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::instantiate</span>();
    CameraService<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::instantiate</span>();
    AudioPolicyService<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::instantiate</span>();
    ProcessState<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::self</span>()<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>startThreadPool();
    IPCThreadState<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::self</span>()<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>joinThreadPool();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

可以看到，在main函数中，

1. 我们首先获得了一个ProcessState的实例，由self方法我们可以猜得到该类使用了单例模式；
2. 接下来我们调用了defaultServiceManager方法获得IServiceManager实例；
3. 接下来进行了几个重要服务的初始化工作；
4. 调用startThreadPool方法和joinThreadPool方法。

ProcessState类的分析

文件位置：frameworks\base\libs\binder\ProcessState.cpp 
self方法：在main函数中，我们调用了self方法得到了一个ProcessState实例，下面我们来看看这个方法

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">sp<ProcessState> ProcessState::<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span>()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (gProcess != <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">NULL</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gProcess;
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//提供原子操作</span>
    AutoMutex _l(gProcessMutex);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (gProcess == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">NULL</span>) gProcess = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ProcessState;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gProcess;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

可以看到，不出所料，ProcessState使用的就是单例模式。 
接下来我们来看一看ProcessState的构造函数：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ProcessState::ProcessState()
    : mDriverFD(open_driver())
    , mVMStart(MAP_FAILED)
    , mManagesContexts(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>)
    , mBinderContextCheckFunc(NULL)
    , mBinderContextUserData(NULL)
    , mThreadPoolStarted(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">false</span>)
    , mThreadPoolSeq(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mDriverFD >= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// XXX Ideally, there should be a specific define for whether we</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// have mmap (or whether we could possibly have the kernel module</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// availabla).</span>
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">if</span> !defined(HAVE_WIN32_IPC)</span>
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// mmap the binder, 提供一个虚拟放入地址内存空间块去接收事务</span>
        mVMStart = mmap(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, BINDER_VM_SIZE, PROT_READ, MAP_PRIVATE | MAP_NORESERVE, mDriverFD, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (mVMStart == MAP_FAILED) {
            <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// *sigh*</span>
            ALOGE(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Using /dev/binder failed: unable to mmap transaction memory.\n"</span>);
            close(mDriverFD);
            mDriverFD = -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
        }
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">else</span></span>
        mDriverFD = -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#<span class="hljs-keyword" style="box-sizing: border-box;">endif</span></span>
    }
    LOG_ALWAYS_FATAL_IF(mDriverFD < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Binder driver could not be opened.  Terminating."</span>);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

可以看到，在构造函数中首先调用了open_driver函数并将返回值赋给了mDriverFD，让我们来看看这个函数：

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">static <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> open_driver()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> fd = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">open</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"/dev/binder"</span>, O_RDWR);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (fd >= <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">fcntl</span>(fd, F_SETFD, FD_CLOEXEC);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> vers;
        status_t result = ioctl(fd, BINDER_VERSION, &vers);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (result == -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) {
            ALOGE(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Binder ioctl to obtain version failed: <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%s</span>"</span>, strerror(errno));
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">close</span>(fd);
            fd = -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (result != <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> || vers != BINDER_CURRENT_PROTOCOL_VERSION) {
            ALOGE(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Binder driver protocol does not match user space protocol!"</span>);
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">close</span>(fd);
            fd = -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;
        }
        size_t maxThreads = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">15</span>;
        result = ioctl(fd, BINDER_SET_MAX_THREADS, &maxThreads);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (result == -<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>) {
            ALOGE(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Binder ioctl to set max threads failed: <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%s</span>"</span>, strerror(errno));
        }
    } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
        ALOGW(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Opening '/dev/binder' failed: <span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">%s</span>\n"</span>, strerror(errno));
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> fd;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>

可以看到open_driver函数主要是打开了/dev/binder这个设备并返回了这个设备的fd。

接下来我继续回到构造函数中，在mDriverFD中保存了这个设备的fd，接着我们又对其他的成员变量做了一些初始化，然后调用mmap函数为Binder设备开辟一块内存由于接收数据。

总结一下，我们的ProcessState类的任务：

1. 打开了Binder设备，并保存了设备的fd；
2. 利用保存的fd为Binder设备开辟一块内存用于接收数据；
3. 因为ProcessState采用了单例模式，因此每个进程只能打开一次Binder设备。

defaultServiceManager函数分析

文件位置：frameworks\base\libs\binder\IServiceManager.cpp

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IServiceManager<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> defaultServiceManager()
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (gDefaultServiceManager <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!=</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NULL</span>) <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gDefaultServiceManager;

    {
        AutoMutex _l(gDefaultServiceManagerLock);
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (gDefaultServiceManager <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NULL</span>) {
            gDefaultServiceManager <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> interface_cast<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IServiceManager<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span>(
                ProcessState<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::self</span>()<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>getContextObject(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NULL</span>));
        }
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> gDefaultServiceManager;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li></ul>

可以看到gDefaultServiceManager函数主要是对gDefaultServiceManager 进行赋值，首先我们来看看这个函数的传入参数： ProcessState::self()->getContextObject(NULL)

<code class="hljs lasso has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IBinder<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> ProcessState<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::getContextObject</span>(const sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IBinder<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">>&</span> caller)
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> getStrongProxyForHandle(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>);
}

sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IBinder<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> ProcessState<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;">::getStrongProxyForHandle</span>(int32_t <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">handle</span>)
{
    sp<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;"><</span>IBinder<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">></span> result;

    AutoMutex _l(mLock);

    handle_entry<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span> e <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> lookupHandleLocked(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">handle</span>);

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (e <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!=</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NULL</span>) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 如果现在不存在或者我们不能得到它的引用时，我们需要创建一个新的BpBinder, </span>
        IBinder<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">*</span> b <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> e<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>binder;
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (b <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">==</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NULL</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">||</span> <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">!</span>e<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>refs<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>attemptIncWeak(this)) {
            b <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">new</span> BpBinder(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">handle</span>); 
            e<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>binder <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> b;
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (b) e<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>refs <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> b<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>getWeakRefs();
            result <span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">=</span> b;
        } <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span> {
            result<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">.</span>force_set(b);
            e<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>refs<span class="hljs-subst" style="color: rgb(0, 0, 0); box-sizing: border-box;">-></span>decWeak(this);
        }
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> result;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

可以看到其实我们返回了一个BpBinder(handle)；handle的值为0。

interface_cast看起来像一个强制类型转换，其实是一个模板函数，下面我们来看看它的庐山真面目：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">template</span><<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">typename</span> INTERFACE>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">inline</span> sp<INTERFACE> interface_cast(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> sp<IBinder>& obj)
{
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> INTERFACE::asInterface(obj);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li></ul>

我们传入的模板是IServiceManager，则实际上调用的就是IServiceManager的asInterface方法。 
asInterface方法的声明和实现实际上是通过两个宏定义实现的，位于IInterface.h文件中：

<code class="hljs cpp has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define DECLARE_META_INTERFACE(INTERFACE)                               \</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> android::String16 descriptor;                          \
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> android::sp<I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE> asInterface(                       \</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> android::sp<android::IBinder>& obj);                  \
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> android::String16& getInterfaceDescriptor() <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span>;    \
    I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE();                                                     \</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">virtual</span> ~I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE();                                            \</span>


<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">#define IMPLEMENT_META_INTERFACE(INTERFACE, NAME)                       \</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> android::String16 I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE::descriptor(NAME);             \</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> android::String16&                                            \
            I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE::getInterfaceDescriptor() const {              \</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE::descriptor;                                \</span>
    }                                                                   \
    android::sp<I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE> I##INTERFACE::asInterface(                \</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">const</span> android::sp<android::IBinder>& obj)                   \
    {                                                                   \
        android::sp<I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE> intr;                                 \</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (obj != NULL) {                                              \
            intr = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static_cast</span><I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE*>(                          \</span>
                obj->queryLocalInterface(                               \
                        I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE::descriptor).get());               \</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (intr == NULL) {                                         \
                intr = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Bp<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE(obj);                          \</span>
            }                                                           \
        }                                                               \
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> intr;                                                    \
    }                                                                   \
    I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE::I##INTERFACE() { }                                    \</span>
    I<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">##INTERFACE::~I##INTERFACE() { }                                   \</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li></ul>

将INTERFACE替换为IServiceManager后可以得到：

<code class="hljs ruby has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">static const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:String16</span> descriptor;                          
    static <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sp<IServiceManager></span> asInterface(                       
            const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sp<android</span><span class="hljs-constant" style="box-sizing: border-box;">::IBinder</span>>& obj);                  
    virtual const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:String16&</span> getInterfaceDescriptor() const;    
    <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager</span>();                                                     
virtual ~<span class="hljs-constant" style="box-sizing: border-box;">IServiceManager</span>();                      

    const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:String16</span> <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::</span>descriptor(<span class="hljs-constant" style="box-sizing: border-box;">NAME</span>);            
    const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:String16&</span>                                            
            <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::</span>getInterfaceDescriptor() const {              
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::</span>descriptor;                                
    }                                                                   
    <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sp<IServiceManager></span> <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::</span>asInterface(                
            const <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sp<android</span><span class="hljs-constant" style="box-sizing: border-box;">::IBinder</span>>& obj)                   
    {                                                                   
        <span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">android:</span><span class="hljs-symbol" style="color: rgb(0, 102, 102); box-sizing: border-box;">:sp<IServiceManager></span> intr;                                 
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (obj != <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>) {                                              
            intr = static_cast<<span class="hljs-constant" style="box-sizing: border-box;">IServiceManager</span>*>(                          
                obj->queryLocalInterface(                               
                        <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::</span>descriptor).get());               
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (intr == <span class="hljs-constant" style="box-sizing: border-box;">NULL</span>) {                                         
                intr = new <span class="hljs-constant" style="box-sizing: border-box;">BpServiceManager</span>(obj);                          
            }                                                           
        }                                                               
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> intr;                                                    
    }                                                                   <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::IServiceManager</span>() { }                                    <span class="hljs-constant" style="box-sizing: border-box;">IServiceManager::</span>~<span class="hljs-constant" style="box-sizing: border-box;">IServiceManager</span>() { } </code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul>

可以看到asInterface方法最终实际上返回了一个BpServiceManager对象。 
总结一下defaultServiceManager方法的工作：

1. 创建了一个BpBinder对象，用来负责客户端Binder通信（之后会讲到），因为对于ServiceManager来说我们是客户端，所以在这里我们创建了Binder的客户端；
2. 创建了一个BpServiceManager对象，主要负责IServiceManager的业务函数，在他的内部持有一个BpBinder对象mRemote。

类关系总结

看到这里，已经有点眼花缭乱了，又是IBinder，又是IServiceManager，又是BpBinder，又是BpServiceManager，是时候来总结一下这些类的关系了，翻了一下这些类，下面用一个不标准的UML图来说明一下： 

需要注意的是：

1. RefBase是Android中所有类的祖先，相当于Java中Object；
2. BpBinder和BBinder都是Android中Binder通信的代表类，其中BpBinder是客户端用来与Server交互的代理类，p代表的就是proxy，而BBinder则是交互的目的端；
3. BpBinder和BBinder是相互对应的，Binder系统会通过handle来标识对应的BBinder；I
4. Interface为server端提供接口，它的子类声明了service能够实现的所有的方法；
5. client端在查询SM获得所需的的BpBinder后，BpRefBase负责管理当前获得的BpBinder实例；
6. BnInterface和BpInterface是两个接口类：BnInterface类型定义了一个onTransact函数，这个函数负责解包收到的数据l并执行client端的请求的方法；使用 BpInterface，client的应用能够想本地调用一样直接调用server端的方法，而不用去关心具体的binder IPC实现；

1. BnServiceManager中n代表的是native，与Bn相对应的应该是BpServiceManager，表示ServiceManager的业务代理类。

原文地址： http://blog.youkuaiyun.com/smbroe/article/details/44685651