Android笔记 从init到SystemServer
1.SystemServer介绍
android中PackageManagerService,WindowManagerService,ActivityManagerService等服务在这里被启动,android中各种大大小小的系统服务都源于SystemServer,(13、14、15行就是启动这些系统服务的方法)
frameworks/base/services/java/com/android/server/SystemServer.java
public static void main(String[] args) { new SystemServer().run(); } ... private void run() {... // Start services. try { traceBeginAndSlog("StartServices"); //123三个方法分别启动了系统中各种服务 startBootstrapServices();//1 startCoreServices();//2 startOtherServices();//3 SystemServerInitThreadPool.shutdown(); } catch (Throwable ex) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting system services", ex); throw ex; } finally { traceEnd(); } ...}
看看到底是哪里调到SystemServer的main方法的,带着这个目的开始出发
2.关于计算机开机方面的一些知识
计算机的启动流程:计算器上所有的软件包括系统都是放在硬盘上的,程序执行时需要将程序从硬盘读取到内存再加载到CPU来运行,当我们按下开机键的时候内存中什么都没有,需要将操作系统读取到内存中进而在CPU加载起来,完成这个工作的就是BIOS
BIOS:它是一组固化到计算机内主板上一个ROM芯片上的程序;理解:电脑的主板上有一个BIOS芯片,而BIOS程序就在其中
Linux:BIOS工作完以后,控制权交给操作系统,先加载kernel然后在加载操作系统第一个进程init进程
android:没有BIOS,替代BIOS的是Bootloader
总结android的开机流程应该是:上电->Bootloader加载kernel->init
3.代码分析
init进程的源码路径在/system/core/init/,而该进程的入口在
system/core/init/init.cpp
int main(int argc, char** argv) {... if (bootscript.empty()) { parser.ParseConfig("/init.rc");//解析init.rc parser.set_is_system_etc_init_loaded( parser.ParseConfig("/system/etc/init")); parser.set_is_vendor_etc_init_loaded( parser.ParseConfig("/vendor/etc/init")); parser.set_is_odm_etc_init_loaded(parser.ParseConfig("/odm/etc/init")); } else { parser.ParseConfig(bootscript); parser.set_is_system_etc_init_loaded(true); parser.set_is_vendor_etc_init_loaded(true); parser.set_is_odm_etc_init_loaded(true); }...}
这里mian函数中解析了init.rc,当然除此之外,还做了很多重要的工作。因为我的目的是找到哪里调到SystemServer的main方法的,对于一些其他不作分析,后面也是一样,这里会直奔主题
system/core/rootdir/init.rc
# Copyright (C) 2012 The Android Open Source Project## IMPORTANT: Do not create world writable files or directories.# This is a common source of Android security bugs.#import /init.environ.rcimport /init.usb.rcimport /init.${ro.hardware}.rcimport /vendor/etc/init/hw/init.${ro.hardware}.rcimport /init.usb.configfs.rcimport /init.${ro.zygote}.rc //注意这里...on post-fs # Load properties from # /system/build.prop, # /odm/build.prop, # /vendor/build.prop and # /factory/factory.prop load_system_props # start essential services start logd start servicemanager //启动servicemanager start hwservicemanager start vndservicemanager...
在init.rc中,需要注意的是第12行代码,这个ro.zygote会决定后面分析中读取的是那一个init.rc,而后启动了servicemanager,他是Binder机制中的"DNS服务器",它的源码在
frameworks/native/cmds/servicemanager/servicemanager.rc
service servicemanager /system/bin/servicemanager class core animation user system group system readproc critical onrestart restart healthd onrestart restart zygote//servicemanager每次重启zygote都会被restart onrestart restart audioserver onrestart restart media onrestart restart surfaceflinger onrestart restart inputflinger onrestart restart drm onrestart restart cameraserver writepid /dev/cpuset/system-background/tasks shutdown critical
原本这段代码是写在init.rc中的,后面改到这里去了,可以看到在servicemanager 会去重启zygote,前面又说到注意ro.zygote,通过他可以知道读取的是那个zygote.rc,如果ro.zygote的值为zygote32的话,service zygote就会被定义在init.zygote32.rc,再来看看init.zygote32.rc
system/core/rootdir/init.zygote32.rc
service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server class main priority -20 user root group root readproc socket zygote stream 660 root system onrestart write /sys/android_power/request_state wake onrestart write /sys/power/state on onrestart restart audioserver onrestart restart cameraserver onrestart restart media onrestart restart netd onrestart restart wificond writepid /dev/cpuset/foreground/tasks
可以看到启动service zygote实际上就是启动/system/bin/app_process模块,然后我们就去看看app_process做了什么事情,补充一点/system/bin/app_process指的是手机中的路径,并不是源码中的路径,就像前面的init.rc,解析的时候是/init.rc,这是手机中的路径,system/core/rootdir/init.rc的代码经过Android.mk模块编译后打包成软件进到手机里面就会存在根目录上了
system/core/rootdir/Android.mk
######################################## init.rcinclude $(CLEAR_VARS)LOCAL_MODULE := init.rc //生成模块的名字LOCAL_SRC_FILES := $(LOCAL_MODULE) //代码路径,表示在当前路径LOCAL_MODULE_CLASS := ETCLOCAL_MODULE_PATH := $(TARGET_ROOT_OUT) //生成路径在根目录下include $(BUILD_PREBUILT)
接着继续分析app_process,先找到app_process在哪里编译出来的
frameworks/base/cmds/app_process/Android.mk
...LOCAL_MODULE:= app_process...
那么app_process的实现就在frameworks/base/cmds/app_process同路径的代码上
frameworks/base/cmds/app_process/app_main.cpp
int main(int argc, char* const argv[]){... if (zygote) { runtime.start("com.android.internal.os.ZygoteInit", args, zygote); } else if (className) { runtime.start("com.android.internal.os.RuntimeInit", args, zygote); } else { fprintf(stderr, "Error: no class name or --zygote supplied.\n"); app_usage(); LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied."); }}
调用runtime.start函数(6行),参数有ZygoteInit的类名,看看start怎么实现的,runtime是AppRuntime类,但是AppRuntime没有实现start函数,看看AppRuntime的父类AndroidRuntime
frameworks/base/core/jni/AndroidRuntime.cpp
void AndroidRuntime::start(const char* className, const Vector& options, bool zygote){ ... /* start the virtual machine */ JniInvocation jni_invocation; jni_invocation.Init(NULL); JNIEnv* env; if (startVm(&mJavaVM, &env, zygote) != 0) { return; } onVmCreated(env);//启动虚拟机 ... char* slashClassName = toSlashClassName(className != NULL ? className : ""); //1 jclass startClass = env->FindClass(slashClassName); if (startClass == NULL) { ALOGE("JavaVM unable to locate class '%s'\n", slashClassName); /* keep going */ } else { jmethodID startMeth = env->GetStaticMethodID(startClass, "main", "([Ljava/lang/String;)V"); //2 if (startMeth == NULL) { ALOGE("JavaVM unable to find main() in '%s'\n", className); /* keep going */ } else { env->CallStaticVoidMethod(startClass, startMeth, strArray); //3 ...}
5到12行代码,启动虚拟机;28行env->CallStaticVoidMethod就是典型的native framework调java framework的方法,其中参数startClass是在app_main.cpp传进来的"com.android.internal.os.ZygoteInit",startMeth是第22、23行所定义的main,也就是说,来到这里,下一步就是ZygoteInit的main方法了
frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
public static void main(String argv[]) { ... try { ... if (startSystemServer) { Runnable r = forkSystemServer(abiList, socketName, zygoteServer); ... } ... } private static Runnable forkSystemServer(String abiList, String socketName, ZygoteServer zygoteServer) { ... String args[] = { //<------------------跟踪它 "--setuid=1000", "--setgid=1000", "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023,1032,3001,3002,3003,3006,3007,3009,3010", "--capabilities=" + capabilities + "," + capabilities, "--nice-name=system_server", "--runtime-args", "com.android.server.SystemServer",//这个不正是我们梦寐以求的SystemServer吗 }; ZygoteConnection.Arguments parsedArgs = null; int pid; try { parsedArgs = new ZygoteConnection.Arguments(args);//<-------1 ... } catch (IllegalArgumentException ex) { throw new RuntimeException(ex); } /* For child process */ if (pid == 0) { if (hasSecondZygote(abiList)) { waitForSecondaryZygote(socketName); } zygoteServer.closeServerSocket(); return handleSystemServerProcess(parsedArgs); //<-------2 } return null; } private static Runnable handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs) { ... return ZygoteInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl); //<---------3 ... } public static final Runnable zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) { .. return RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader); //<----4 }
从main方法的第6行forkSystemServer中发现了一个String类型的数组args(第16行),为什么关注他,因为它里面(23行),有文章开头的目标SystemServer,跟踪他传递到哪里去了,在30行他转化成了parsedArgs,在42行传到handleSystemServerProcess在51他继续以parsedArgs.remainingArgs往下传到了zygoteInit方法(57行),这里继续传到RuntimeInit.applicationInit。
frameworks/base/core/java/com/android/internal/os/RuntimeInit.java
protected static Runnable applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) { ... return findStaticMain(args.startClass, args.startArgs, classLoader);//<---- } private static Runnable findStaticMain(String className, String[] argv, ClassLoader classLoader) { Class<?> cl; try { cl = Class.forName(className, true, classLoader); } catch (ClassNotFoundException ex) { throw new RuntimeException( "Missing class when invoking static main " + className, ex); } Method m; try { //加载mian函数 m = cl.getMethod("main", new Class[] { String[].class }); } catch (NoSuchMethodException ex) { throw new RuntimeException( "Missing static main on " + className, ex); } catch (SecurityException ex) { throw new RuntimeException( "Problem getting static main on " + className, ex); } ... }
继续跟踪RuntimeInit的applicationInit方法,他把前面穿进来的参数分解成了所需的类名args.startClass,继续往下传递给findStaticMain方法(第4行),第22行,通过反射加载了SystemServer的main方法
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