android boot 代码流程 1
之前这篇，从整体展示了 android 的整个启动流程，为了搞清楚 android 启动到底在代码层面上是如何调用的，将从源代码角度去分析，另所有代码基于 android 4.0 source tree
all story begin with the init process startup
故事从 init 进程启动开始
init 运行，代码：system/core/init ，入口：system/core/init/init.c main 函数：
1 int main(int argc, char **argv){ 2 3 ... 4 // 初始化文件系统 5 mkdir("/dev", 0755); 6 mkdir("/proc", 0755); 7 mkdir("/sys", 0755); 8 9 mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755"); 10 mkdir("/dev/pts", 0755); 11 mkdir("/dev/socket", 0755); 12 mount("devpts", "/dev/pts", "devpts", 0, NULL); 13 mount("proc", "/proc", "proc", 0, NULL); 14 mount("sysfs", "/sys", "sysfs", 0, NULL); 15 16 ...
17 // 解析 /init.rc 和 /init.$hardware.rc 脚本，其中 $hardware 参数从 /proc/cpuinfo 中读取，模拟器默认是 goldfish 18 INFO("reading config file\n"); 19 init_parse_config_file("/init.rc"); 20 21 /* pull the kernel commandline and ramdisk properties file in */ 22 import_kernel_cmdline(0, import_kernel_nv); 23 /* don't expose the raw commandline to nonpriv processes */ 24 chmod("/proc/cmdline", 0440); 25 get_hardware_name(hardware, &revision); 26 snprintf(tmp, sizeof(tmp), "/init.%s.rc", hardware); 27 init_parse_config_file(tmp); 28 29 ... 30 }
解析 init.rc 文件，主要是由 /system/core/init/init_parser.c 来完成，截取 init.rc 的部分内容如下：（具体 init.rc 文件规范，可参考：/system/core/init/readme.txt）
on early-init start ueventd # create mountpoints mkdir /mnt 0775 root system # setup the global environment export PATH /sbin:/vendor/bin:/system/sbin:/system/bin:/system/xbin # Create cgroup mount points for process groups mkdir /dev/cpuctl mount cgroup none /dev/cpuctl cpu chown system system /dev/cpuctl chown system system /dev/cpuctl/tasks chmod 0777 /dev/cpuctl/tasks write /dev/cpuctl/cpu.shares 1024 service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server class main socket zygote stream 666 onrestart write /sys/android_power/request_state wake onrestart write /sys/power/state on onrestart restart media onrestart restart netd
init.rc 使用的是 android init language 规范，它支持4种语句，Actions，Commands，Services，Options：
- Action 定义方式：
on <trigger> #以 on 开头，后面是触发器名字，触发器有3种方式：
<command> #1.只是一个名字，如： on early-init；
<command> #2.name=value 对，如：on property:vold.decrypt=trigger_reset_main；
<command> #3.系统自带的，如：device-added-<path>，device-removed-<path>，service-exited-<name>
<command> #在触发器下一行就是在触发器触发的时候需要执行的命令，如：start...，mkdir，...
- Command 就是系统支持的一系列命令，如：export，hostname，mkdir，mount，等等，其中一部分是 linux 命令，还有一些是 android 添加的，如：class_start <serviceclass>： 启动服务，
class_stop <serviceclass>：关闭服务，等等。
- Service 定义方式：
service <name> <pathname> [ <argument> ]* <option> <option> ... #如：启动 android 最重要的服务 zygote service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server class main socket zygote stream 666 onrestart write /sys/android_power/request_state wake onrestart write /sys/power/state on onrestart restart media onrestart restart netd
- Option 是针对 Service 的选项，如：
setenv <name> <value> 在启动服务时设置环境变量 user <username> 运行服务之前切换用户 oneshot 如果服务已经存在，将不再启动 class <classname> 为服务设置名字，具有相同名字的服务将一起启动或者关闭
socket <name> <type> <perm> [ <user> [ <group> ] ] 创建以<name>命名的 socket，并将该 socket 的文件描述符返回给启动的服务
onrestart <command> 在服务重新启动的时候执行<command>
在对 init.rc 和 init.$hardware.rc 2个文件按照 android init language 规范进行解析完成以后，会将所有将要执行的命令放到一个大的 action_list 当中，然后紧跟上面解析 init 文件下面：
// 寻找 early-init 触发器，加到 action_queue 中
action_for_each_trigger("early-init", action_add_queue_tail);
// 添加系统的一些触发器 queue_builtin_action(wait_for_coldboot_done_action, "wait_for_coldboot_done"); queue_builtin_action(property_init_action, "property_init"); queue_builtin_action(keychord_init_action, "keychord_init"); queue_builtin_action(console_init_action, "console_init"); queue_builtin_action(set_init_properties_action, "set_init_properties"); /* execute all the boot actions to get us started */ action_for_each_trigger("init", action_add_queue_tail); /* skip mounting filesystems in charger mode */ if (strcmp(bootmode, "charger") != 0) { action_for_each_trigger("early-fs", action_add_queue_tail); action_for_each_trigger("fs", action_add_queue_tail); action_for_each_trigger("post-fs", action_add_queue_tail); action_for_each_trigger("post-fs-data", action_add_queue_tail); } queue_builtin_action(property_service_init_action, "property_service_init"); queue_builtin_action(signal_init_action, "signal_init"); queue_builtin_action(check_startup_action, "check_startup"); if (!strcmp(bootmode, "charger")) { action_for_each_trigger("charger", action_add_queue_tail); } else { action_for_each_trigger("early-boot", action_add_queue_tail); action_for_each_trigger("boot", action_add_queue_tail); } /* run all property triggers based on current state of the properties */ queue_builtin_action(queue_property_triggers_action, "queue_propety_triggers"); ...
// 按顺序执行 action_queue 中的命令 for(;;) { int nr, i, timeout = -1; execute_one_command(); restart_processes(); ... }
从 action_list 中找到制定触发器，将触发器需要执行的命令添加到 action_queue 中，最后按顺序执行 action_queue 中的命令来完成初始化，初始化除了设置一些环境变量和创建文件夹以外，
更多的是关心 Service 的启动，init 文件里面的服务有2种，1种是 class core，还有1种是 class main，对 init.rc 的 service 按照 class <name> 分类如下：
class core: service ueventd /sbin/ueventd service console /system/bin/sh service adbd /sbin/adbd service servicemanager /system/bin/servicemanager service vold /system/bin/vold class main: service netd /system/bin/netd service debuggerd /system/bin/debuggerd service ril-daemon /system/bin/rild service surfaceflinger /system/bin/surfaceflinger service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-server service drm /system/bin/drmserver service media /system/bin/mediaserver service bootanim /system/bin/bootanimation service dbus /system/bin/dbus-daemon --system --nofork service bluetoothd /system/bin/bluetoothd -n service installd /system/bin/installd service flash_recovery /system/etc/install-recovery.sh service racoon /system/bin/racoon service mtpd /system/bin/mtpd service keystore /system/bin/keystore /data/misc/keystore service dumpstate /system/bin/dumpstate -s
service ueventd：会读取 /ueventd.rc 和 /ueventd.$hadrware.rc 文件，解析跟 init.rc 解析类似，主要是对文件系统的权限和用户进行设置：（代码：system/core/init/ueventd.c）
#目录 权限 user group
/dev/null 0666 root root /dev/zero 0666 root root
在 class core 服务启动以后， class main 开始启动，service zygote 是标志进入 android 最重要的一个服务，服务名字 zygote，实际上启动的是 app_process，（代码：frameworks/base/cmds/app_process/app_main.cpp）
int main(int argc, const char* const argv[]) { // These are global variables in ProcessState.cpp mArgC = argc; mArgV = argv; mArgLen = 0;
//读取参数，传递的参数就是 init.rc 中启动时传入的： -Xzygote /system/bin --zygote --start-system-server for (int i=0; i<argc; i++) { mArgLen += strlen(argv[i]) + 1; } mArgLen--; AppRuntime runtime; const char* argv0 = argv[0]; //-Xzygote // Process command line arguments // ignore argv[0] argc--; //之前是 4， 现在是 3 argv++; //argv 指向 argv[1] // Everything up to '--' or first non '-' arg goes to the vm
// i = 0，代码：frameworks/base/core/jni/AndroidRuntime.cpp int i = runtime.addVmArguments(argc, argv); // Parse runtime arguments. Stop at first unrecognized option. bool zygote = false; bool startSystemServer = false; bool application = false; const char* parentDir = NULL; const char* niceName = NULL; const char* className = NULL; while (i < argc) { const char* arg = argv[i++]; if (!parentDir) { parentDir = arg; //parentDir = /system/bin } else if (strcmp(arg, "--zygote") == 0) { //当 i = 2，arg = argv[1] 时，即：--zygote zygote = true; niceName = "zygote"; } else if (strcmp(arg, "--start-system-server") == 0) { startSystemServer = true; } else if (strcmp(arg, "--application") == 0) { application = true; } else if (strncmp(arg, "--nice-name=", 12) == 0) { niceName = arg + 12; } else { className = arg; break; } } if (niceName && *niceName) { setArgv0(argv0, niceName); set_process_name(niceName); } runtime.mParentDir = parentDir; if (zygote) {
// zygote = true，启动 com.android.internal.os.ZygoteInit，参数：startSystemServer runtime.start("com.android.internal.os.ZygoteInit",startSystemServer ? "start-system-server" : ""); } else if (className) { // Remainder of args get passed to startup class main() runtime.mClassName = className; runtime.mArgC = argc - i; runtime.mArgV = argv + i; runtime.start("com.android.internal.os.RuntimeInit", application ? "application" : "tool"); } else { fprintf(stderr, "Error: no class name or --zygote supplied.\n"); app_usage(); LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied."); return 10; } }
检测传入参数，将调用 ：
runtime.start("com.android.internal.os.ZygoteInit",startSystemServer ? "start-system-server" : "");
runtime 的代码：frameworks/base/core/jni/AndroidRuntime.cpp，start 函数会启动虚拟机， 执行 com.android.internal.os.ZygoteInit 该类的 main 函数，并传入参数： start-system-server：
void AndroidRuntime::start(const char* className, const char* options) { LOGD("\n>>>>>> AndroidRuntime START %s <<<<<<\n", className != NULL ? className : "(unknown)");
...
/* start the virtual machine */
//设置 dalvik 虚拟机参数，创建并启动虚拟机 JNIEnv* env; if (startVm(&mJavaVM, &env) != 0) { return; } onVmCreated(env); /* * Register android functions. */ if (startReg(env) < 0) { LOGE("Unable to register all android natives\n"); return; } ...
/* * Start VM. This thread becomes the main thread of the VM, and will * not return until the VM exits. */
//将类名 com.xxx.xxx 转换成 com/xxx/xxx char* slashClassName = toSlashClassName(className); jclass startClass = env->FindClass(slashClassName); if (startClass == NULL) { LOGE("JavaVM unable to locate class '%s'\n", slashClassName); /* keep going */ } else {
// jni 调用 java 方法，获取对应类名的 class，然后调用静态 main 方法 jmethodID startMeth = env->GetStaticMethodID(startClass, "main", "([Ljava/lang/String;)V"); if (startMeth == NULL) { LOGE("JavaVM unable to find main() in '%s'\n", className); /* keep going */ } else { env->CallStaticVoidMethod(startClass, startMeth, strArray); } } free(slashClassName); LOGD("Shutting down VM\n"); if (mJavaVM->DetachCurrentThread() != JNI_OK) LOGW("Warning: unable to detach main thread\n"); if (mJavaVM->DestroyJavaVM() != 0) LOGW("Warning: VM did not shut down cleanly\n"); }
ZygoteInit 类 main 函数：
public static void main(String argv[]) { try { // Start profiling the zygote initialization. SamplingProfilerIntegration.start();
//注册 socket server registerZygoteSocket(); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis());
//预加载资源，有 preloadClasses() 和 preloadResources()，加载的开始和结束会被记录在 /system/etc/event-log-tags 文件中 preload(); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis());
// Finish profiling the zygote initialization. SamplingProfilerIntegration.writeZygoteSnapshot(); // Do an initial gc to clean up after startup gc(); // If requested, start system server directly from Zygote if (argv.length != 2) { throw new RuntimeException(argv[0] + USAGE_STRING); } if (argv[1].equals("start-system-server")) {
//在调用 Zygote 的 main 函数时，已经传入 start-system-server，调用 startSystemServer() startSystemServer(); } else if (!argv[1].equals("")) { throw new RuntimeException(argv[0] + USAGE_STRING); } Log.i(TAG, "Accepting command socket connections"); if (ZYGOTE_FORK_MODE) { runForkMode(); } else {
//上面通过 registerZygoteSocket() 函数调用注册的 server scocket，会启动，开始监听 Zygote 连接 runSelectLoopMode(); } closeServerSocket(); } catch (MethodAndArgsCaller caller) { caller.run(); } catch (RuntimeException ex) { Log.e(TAG, "Zygote died with exception", ex); closeServerSocket(); throw ex; } }
那 startSystemServer 到底又做了什么东东呢，以及最后系统如何发出 ACTION_BOOT_COMPLETED 广播的呢，且听下回分解。 -):
