Android源码学习之浅析SystemServer脉络

    在之前的博文中《Android源码学习之如何创建使用JNI》和《Android源码学习之如何使用eclipse+NDK》中，浅谈了如何创建使用JNI和如何利用NDK工具开发创建和lib**.so（Windows下）库和调用Naive函数，做了这些工作只有一个目的，就是因为Android源码中“大量”的使用到Native，所以了解一些Native语言和Java如何与Native互通，对分析Android源码还是有很大的帮助的（好像废话很多~~~）。

    本人通过对SystemServer源码进行浅析，看它是如何设计的。

    首先利用Source Insight工具搜索到SystemServer.java代码，找到入口函数main()，代码如下：

 native public static void init1(String[] args);

    public static void main(String[] args) {
        if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) {
            // If a device's clock is before 1970 (before 0), a lot of
            // APIs crash dealing with negative numbers, notably
            // java.io.File#setLastModified, so instead we fake it and
            // hope that time from cell towers or NTP fixes it
            // shortly.
            Slog.w(TAG, "System clock is before 1970; setting to 1970.");
            SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME);
        }

        if (SamplingProfilerIntegration.isEnabled()) {
            SamplingProfilerIntegration.start();
            timer = new Timer();
            timer.schedule(new TimerTask() {
                @Override
                public void run() {
                    SamplingProfilerIntegration.writeSnapshot("system_server", null);
                }
            }, SNAPSHOT_INTERVAL, SNAPSHOT_INTERVAL);
        }

        // Mmmmmm... more memory!
        dalvik.system.VMRuntime.getRuntime().clearGrowthLimit();

        // The system server has to run all of the time, so it needs to be
        // as efficient as possible with its memory usage.
        VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);

        System.loadLibrary("android_servers");
        init1(args);
    }

    public static final void init2() {
        Slog.i(TAG, "Entered the Android system server!");
        Thread thr = new ServerThread();
        thr.setName("android.server.ServerThread");
        thr.start();
    }

在main函数最后两行代码System.loadLibrary("android_servers");和init1(args);还有第一行代码native public static void init1(String[] args);要是理解了本人之前的两篇博文的，对这三行代码也太熟悉了，就是首先加载动态库libandroid_servers.so，然后调用native的init1()函数。接着往下走，看看native函数init1()是什么样子的（注：记得这里的init2()函数，一会儿在native层函数中会调用到这个函数）。

     首先如何找到该init1()函数？通过之前的博文，我们知道在调用native函数所在的类与native编写的类进行沟通，是通过“类名”的相似性或者是在Android.mk文件中定义，所以根据这条思路我们知道该java类的包名为package com.android.server;以及类名为“SystemServer”，所以将它们的"."改成"_"组合为“com_android_server_SystemServer”，这样通过source insight搜索有没有这个文件，终于找到了，并且可以找到调用的nitive函数init()。代码如下：

/*
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include <utils/Log.h>
#include <utils/misc.h>

#include "jni.h"
#include "JNIHelp.h"

namespace android {

extern "C" int system_init();

static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz)
{
    system_init();
}

/*
 * JNI registration.
 */
static JNINativeMethod gMethods[] = {
    /* name, signature, funcPtr */
    { "init1", "([Ljava/lang/String;)V", (void*) android_server_SystemServer_init1 },
};

int register_android_server_SystemServer(JNIEnv* env)
{
    return jniRegisterNativeMethods(env, "com/android/server/SystemServer",
            gMethods, NELEM(gMethods));
}

}; // namespace android

整个文件就这么几行代码，比较简单。目前只关心Native函数android_server_SystemServer_init1(JNIEnv* env, jobject clazz)，它是调用上面那个用extern声明的一个外部system_init函数。接着搜索这个函数所在的文件，这个函数是在另外一个库libsystem_server.so中实现，找到类System_init.cpp后，可以看到函数system_init()，代码如下：

extern "C" status_t system_init()
{
    LOGI("Entered system_init()");

    sp<ProcessState> proc(ProcessState::self());

    sp<IServiceManager> sm = defaultServiceManager();
    LOGI("ServiceManager: %p\n", sm.get());

    sp<GrimReaper> grim = new GrimReaper();
    sm->asBinder()->linkToDeath(grim, grim.get(), 0);

    char propBuf[PROPERTY_VALUE_MAX];
    property_get("system_init.startsurfaceflinger", propBuf, "1");
    if (strcmp(propBuf, "1") == 0) {
        // Start the SurfaceFlinger
        SurfaceFlinger::instantiate();
    }

    property_get("system_init.startsensorservice", propBuf, "1");
    if (strcmp(propBuf, "1") == 0) {
        // Start the sensor service
        SensorService::instantiate();
    }

    // And now start the Android runtime.  We have to do this bit
    // of nastiness because the Android runtime initialization requires
    // some of the core system services to already be started.
    // All other servers should just start the Android runtime at
    // the beginning of their processes's main(), before calling
    // the init function.
    LOGI("System server: starting Android runtime.\n");
    AndroidRuntime* runtime = AndroidRuntime::getRuntime();

    LOGI("System server: starting Android services.\n");
    JNIEnv* env = runtime->getJNIEnv();
    if (env == NULL) {
        return UNKNOWN_ERROR;
    }
    jclass clazz = env->FindClass("com/android/server/SystemServer");
    if (clazz == NULL) {
        return UNKNOWN_ERROR;
    }
    jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
    if (methodId == NULL) {
        return UNKNOWN_ERROR;
    }
    env->CallStaticVoidMethod(clazz, methodId);

    LOGI("System server: entering thread pool.\n");
    ProcessState::self()->startThreadPool();
    IPCThreadState::self()->joinThreadPool();
    LOGI("System server: exiting thread pool.\n");

    return NO_ERROR;
}

函数具体要做什么，有什么功能我们暂且不考虑，我们找到我们关系的几行代码：

jclass clazz = env->FindClass("com/android/server/SystemServer");
    if (clazz == NULL) {
        return UNKNOWN_ERROR;
    }
    jmethodID methodId = env->GetStaticMethodID(clazz, "init2", "()V");
    if (methodId == NULL) {
        return UNKNOWN_ERROR;
    }
    env->CallStaticVoidMethod(clazz, methodId);
这几行代码我们熟~~~，是通过“com/android/server/SystemServer”找到类，这个类不就是我们本博文开篇的SystemServer.java的类吗。找到类对象后，再调用该对象的init2函数，然后执行init2()函数，我们在温习下init2()函数，代码如下：

public static final void init2() {
        Slog.i(TAG, "Entered the Android system server!");
        Thread thr = new ServerThread(); thr.setName("android.server.ServerThread");
        thr.start();
    }

总结：我们放开代码的功能不说，单纯的从java和nitive的交互和相互调用上说，通过之前的学习，对于理解Android源码的脉络是有帮助的，只有在了解了整个Android源码的脉络和一些Android源码组织的框架架构，对理解整个源码是更加轻松的。

    记录下自己的学习过程，为己为人，内容肤浅，请高手多多指导~~~