Android源码学习之浅析SystemServer脉络

http://www.cnblogs.com/yemeishu/archive/2012/12/26/2833415.html

    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nativepublic staticvoid 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 staticfinal 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~~~