android 重启app_[Boot]Android系统启动-zygote篇

本文转载自Gityuan的博客，原文地址为：http://gityuan.com/2016/02/13/android-zygote/
转载请注明作者及原文链接。

基于Android 6.0的源码剖析，分析Android启动过程的Zygote进程

/frameworks/base/cmds/app_process/App_main.cpp
/frameworks/base/core/jni/AndroidRuntime.cpp/frameworks/base/core/java/com/android/internal/os/- ZygoteInit.java- Zygote.java- ZygoteConnection.java/frameworks/base/core/java/android/net/LocalServerSocket.java
/system/core/libutils/Threads.cpp

一. 概述

Zygote是由init进程通过解析init.zygote.rc文件而创建的，zygote所对应的可执行程序app_process，所对应的源文件是App_main.cpp，进程名为zygote。

service zygote /system/bin/app_process -Xzygote /system/bin --zygote --start-system-serverclass mainsocket zygote stream 660 root systemonrestart write /sys/android_power/request_state wakeonrestart write /sys/power/state ononrestart restart mediaonrestart restart netd

Zygote进程能够重启的地方:

servicemanager进程被杀; (onresart)
surfaceflinger进程被杀; (onresart)
Zygote进程自己被杀; (oneshot=false)
system_server进程被杀; (waitpid)

从App_main()开始，Zygote启动过程的函数调用类大致流程如下：

二、Zygote启动过程

2.1 App_main.main

[-> App_main.cpp]

int main(int argc, char* const argv[])
{//传到的参数argv为“-Xzygote /system/bin --zygote --start-system-server”AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv));argc--; argv++; //忽略第一个参数int i;for (i = 0; i < argc; i++) {if (argv[i][0] != '-') {break;}if (argv[i][1] == '-' && argv[i][2] == 0) {++i;break;}runtime.addOption(strdup(argv[i]));}//参数解析bool zygote = false;bool startSystemServer = false;bool application = false;String8 niceName;String8 className;++i;while (i < argc) {const char* arg = argv[i++];if (strcmp(arg, "--zygote") == 0) {zygote = true;//对于64位系统nice_name为zygote64; 32位系统为zygoteniceName = ZYGOTE_NICE_NAME;} 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.setTo(arg + 12);} else if (strncmp(arg, "--", 2) != 0) {className.setTo(arg);break;} else {--i;break;}}Vector<String8> args;if (!className.isEmpty()) {// 运行application或tool程序args.add(application ? String8("application") : String8("tool"));runtime.setClassNameAndArgs(className, argc - i, argv + i);} else {//进入zygote模式，创建 /data/dalvik-cache路径maybeCreateDalvikCache();if (startSystemServer) {args.add(String8("start-system-server"));}char prop[PROP_VALUE_MAX];if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) {return 11;}String8 abiFlag("--abi-list=");abiFlag.append(prop);args.add(abiFlag);for (; i < argc; ++i) {args.add(String8(argv[i]));}}//设置进程名if (!niceName.isEmpty()) {runtime.setArgv0(niceName.string());set_process_name(niceName.string());}if (zygote) {// 启动AppRuntime 【见小节2.2】runtime.start("com.android.internal.os.ZygoteInit", args, zygote);} else if (className) {runtime.start("com.android.internal.os.RuntimeInit", args, zygote);} else {//没有指定类名或zygote，参数错误return 10;}
}

2.2 start

[-> AndroidRuntime.cpp]

void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote)
{static const String8 startSystemServer("start-system-server");for (size_t i = 0; i < options.size(); ++i) {if (options[i] == startSystemServer) {const int LOG_BOOT_PROGRESS_START = 3000;}}const char* rootDir = getenv("ANDROID_ROOT");if (rootDir == NULL) {rootDir = "/system";if (!hasDir("/system")) {return;}setenv("ANDROID_ROOT", rootDir, 1);}JniInvocation jni_invocation;jni_invocation.Init(NULL);JNIEnv* env;// 虚拟机创建【见小节2.3】if (startVm(&mJavaVM, &env, zygote) != 0) {return;}onVmCreated(env);// JNI方法注册【见小节2.4】if (startReg(env) < 0) {return;}jclass stringClass;jobjectArray strArray;jstring classNameStr;//等价 strArray= new String[options.size() + 1];stringClass = env->FindClass("java/lang/String");strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);//等价 strArray[0] = "com.android.internal.os.ZygoteInit"classNameStr = env->NewStringUTF(className);env->SetObjectArrayElement(strArray, 0, classNameStr);//等价 strArray[1] = "start-system-server"；// strArray[2] = "--abi-list=xxx"；//其中xxx为系统响应的cpu架构类型，比如arm64-v8a.for (size_t i = 0; i < options.size(); ++i) {jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());env->SetObjectArrayElement(strArray, i + 1, optionsStr);}//将"com.android.internal.os.ZygoteInit"转换为"com/android/internal/os/ZygoteInit"char* slashClassName = toSlashClassName(className);jclass startClass = env->FindClass(slashClassName);if (startClass == NULL) {...} else {jmethodID startMeth = env->GetStaticMethodID(startClass, "main","([Ljava/lang/String;)V");// 调用ZygoteInit.main()方法【见小节3.1】env->CallStaticVoidMethod(startClass, startMeth, strArray);}//释放相应对象的内存空间free(slashClassName);mJavaVM->DetachCurrentThread();mJavaVM->DestroyJavaVM();
}

2.3 startVm

[–> AndroidRuntime.cpp]

创建Java虚拟机方法的主要篇幅是关于虚拟机参数的设置，下面只列举部分在调试优化过程中常用参数。

int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv, bool zygote)
{// JNI检测功能，用于native层调用jni函数时进行常规检测，比较弱字符串格式是否符合要求，资源是否正确释放。该功能一般用于早期系统调试或手机Eng版，对于User版往往不会开启，引用该功能比较消耗系统CPU资源，降低系统性能。bool checkJni = false;property_get("dalvik.vm.checkjni", propBuf, "");if (strcmp(propBuf, "true") == 0) {checkJni = true;} else if (strcmp(propBuf, "false") != 0) {property_get("ro.kernel.android.checkjni", propBuf, "");if (propBuf[0] == '1') {checkJni = true;}}if (checkJni) {addOption("-Xcheck:jni");}//虚拟机产生的trace文件，主要用于分析系统问题，路径默认为/data/anr/traces.txtparseRuntimeOption("dalvik.vm.stack-trace-file", stackTraceFileBuf, "-Xstacktracefile:");//对于不同的软硬件环境，这些参数往往需要调整、优化，从而使系统达到最佳性能parseRuntimeOption("dalvik.vm.heapstartsize", heapstartsizeOptsBuf, "-Xms", "4m");parseRuntimeOption("dalvik.vm.heapsize", heapsizeOptsBuf, "-Xmx", "16m");parseRuntimeOption("dalvik.vm.heapgrowthlimit", heapgrowthlimitOptsBuf, "-XX:HeapGrowthLimit=");parseRuntimeOption("dalvik.vm.heapminfree", heapminfreeOptsBuf, "-XX:HeapMinFree=");parseRuntimeOption("dalvik.vm.heapmaxfree", heapmaxfreeOptsBuf, "-XX:HeapMaxFree=");parseRuntimeOption("dalvik.vm.heaptargetutilization",heaptargetutilizationOptsBuf, "-XX:HeapTargetUtilization=");...//preloaded-classes文件内容是由WritePreloadedClassFile.java生成的，//在ZygoteInit类中会预加载工作将其中的classes提前加载到内存，以提高系统性能if (!hasFile("/system/etc/preloaded-classes")) {return -1;}//初始化虚拟机if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {ALOGE("JNI_CreateJavaVM failedn");return -1;}
}

2.4 startReg

[–> AndroidRuntime.cpp]

int AndroidRuntime::startReg(JNIEnv* env)
{//设置线程创建方法为javaCreateThreadEtc 【见小节2.4.1】androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);env->PushLocalFrame(200);//进程NI方法的注册【见小节2.4.2】if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {env->PopLocalFrame(NULL);return -1;}env->PopLocalFrame(NULL);return 0;
}

2.4.1 androidSetCreateThreadFunc

[-> Threads.cpp]

void androidSetCreateThreadFunc(android_create_thread_fn func) {gCreateThreadFn = func;
}

虚拟机启动后startReg()过程，会设置线程创建函数指针gCreateThreadFn指向javaCreateThreadEtc.

2.4.2 register_jni_procs

static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env) {for (size_t i = 0; i < count; i++) {//【见小节2.4.3】if (array[i].mProc(env) < 0) {return -1;}}return 0;
}

2.4.3 gRegJNI.mProc

static const RegJNIRec gRegJNI[] = {REG_JNI(register_com_android_internal_os_RuntimeInit),REG_JNI(register_android_os_Binder)，...
}；

array[i]是指gRegJNI数组, 该数组有100多个成员。其中每一项成员都是通过REG_JNI宏定义的：

#define REG_JNI(name) { name }
struct RegJNIRec {int (*mProc)(JNIEnv*);
};

可见，调用mProc，就等价于调用其参数名所指向的函数。例如REG_JNI(register_com_android_internal_os_RuntimeInit).mProc也就是指进入register_com_android_internal_os_RuntimeInit方法，接下来就继续以此为例来说明：

int register_com_android_internal_os_RuntimeInit(JNIEnv* env) {return jniRegisterNativeMethods(env, "com/android/internal/os/RuntimeInit",gMethods, NELEM(gMethods));
}//gMethods：java层方法名与jni层的方法的一一映射关系
static JNINativeMethod gMethods[] = {{ "nativeFinishInit", "()V",(void*) com_android_internal_os_RuntimeInit_nativeFinishInit },{ "nativeZygoteInit", "()V",(void*) com_android_internal_os_RuntimeInit_nativeZygoteInit },{ "nativeSetExitWithoutCleanup", "(Z)V",(void*) com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup },
};

三. 进入Java层

前面[小节2.2]AndroidRuntime.start()执行到最后通过反射调用到ZygoteInit.main(),见下文:

3.1 ZygoteInit.main

[–>ZygoteInit.java]

public static void main(String argv[]) {try {RuntimeInit.enableDdms(); //开启DDMS功能SamplingProfilerIntegration.start();boolean startSystemServer = false;String socketName = "zygote";String abiList = null;for (int i = 1; i < argv.length; i++) {if ("start-system-server".equals(argv[i])) {startSystemServer = true;} else if (argv[i].startsWith(ABI_LIST_ARG)) {abiList = argv[i].substring(ABI_LIST_ARG.length());} else if (argv[i].startsWith(SOCKET_NAME_ARG)) {socketName = argv[i].substring(SOCKET_NAME_ARG.length());} else {throw new RuntimeException("Unknown command line argument: " + argv[i]);}}...registerZygoteSocket(socketName); //为Zygote注册socket【见小节3.2】preload(); // 预加载类和资源【见小节3.3】SamplingProfilerIntegration.writeZygoteSnapshot();gcAndFinalize(); //GC操作if (startSystemServer) {startSystemServer(abiList, socketName);//启动system_server【见小节3.4】}runSelectLoop(abiList); //进入循环模式【见小节3.5】closeServerSocket();} catch (MethodAndArgsCaller caller) {caller.run(); //启动system_server中会讲到。} catch (RuntimeException ex) {closeServerSocket();throw ex;}
}

在异常捕获后调用的方法caller.run()，会在后续的system_server文章会讲到。

3.2 registerZygoteSocket

[–>ZygoteInit.java]

private static void registerZygoteSocket(String socketName) {if (sServerSocket == null) {int fileDesc;final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName;try {String env = System.getenv(fullSocketName);fileDesc = Integer.parseInt(env);} catch (RuntimeException ex) {...}try {FileDescriptor fd = new FileDescriptor();fd.setInt$(fileDesc); //设置文件描述符sServerSocket = new LocalServerSocket(fd); //创建Socket的本地服务端} catch (IOException ex) {...}}
}

3.3 preload

[–>ZygoteInit.java]

static void preload() {//预加载位于/system/etc/preloaded-classes文件中的类preloadClasses();//预加载资源，包含drawable和color资源preloadResources();//预加载OpenGLpreloadOpenGL();//通过System.loadLibrary()方法，//预加载"android","compiler_rt","jnigraphics"这3个共享库preloadSharedLibraries();//预加载 文本连接符资源preloadTextResources();//仅用于zygote进程，用于内存共享的进程WebViewFactory.prepareWebViewInZygote();
}

执行Zygote进程的初始化,对于类加载，采用反射机制Class.forName()方法来加载。对于资源加载，主要是 com.android.internal.R.array.preloaded_drawables和com.android.internal.R.array.preloaded_color_state_lists，在应用程序中以http://com.android.internal.R.xxx开头的资源，便是此时由Zygote加载到内存的。

zygote进程内加载了preload()方法中的所有资源，当需要fork新进程时，采用copy on write技术，如下：

3.4 startSystemServer

[–>ZygoteInit.java]

private static boolean startSystemServer(String abiList, String socketName) throws MethodAndArgsCaller, RuntimeException {long capabilities = posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND,OsConstants.CAP_KILL,OsConstants.CAP_NET_ADMIN,OsConstants.CAP_NET_BIND_SERVICE,OsConstants.CAP_NET_BROADCAST,OsConstants.CAP_NET_RAW,OsConstants.CAP_SYS_MODULE,OsConstants.CAP_SYS_NICE,OsConstants.CAP_SYS_RESOURCE,OsConstants.CAP_SYS_TIME,OsConstants.CAP_SYS_TTY_CONFIG);//参数准备String args[] = {"--setuid=1000","--setgid=1000","--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007","--capabilities=" + capabilities + "," + capabilities,"--nice-name=system_server","--runtime-args","com.android.server.SystemServer",};ZygoteConnection.Arguments parsedArgs = null;int pid;try {//用于解析参数，生成目标格式parsedArgs = new ZygoteConnection.Arguments(args);ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);// fork子进程，用于运行system_serverpid = Zygote.forkSystemServer(parsedArgs.uid, parsedArgs.gid,parsedArgs.gids,parsedArgs.debugFlags,null,parsedArgs.permittedCapabilities,parsedArgs.effectiveCapabilities);} catch (IllegalArgumentException ex) {throw new RuntimeException(ex);}//进入子进程system_serverif (pid == 0) {if (hasSecondZygote(abiList)) {waitForSecondaryZygote(socketName);}// 完成system_server进程剩余的工作handleSystemServerProcess(parsedArgs);}return true;
}

准备参数并fork新进程，从上面可以看出system server进程参数信息为uid=1000,gid=1000,进程名为sytem_server，从zygote进程fork新进程后，需要关闭zygote原有的socket。另外，对于有两个zygote进程情况，需等待第2个zygote创建完成。更多详情见Android系统启动-systemServer上篇。

3.5 runSelectLoop

[–>ZygoteInit.java]

private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>();ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>();//sServerSocket是socket通信中的服务端，即zygote进程。保存到fds[0]fds.add(sServerSocket.getFileDescriptor());peers.add(null);while (true) {StructPollfd[] pollFds = new StructPollfd[fds.size()];for (int i = 0; i < pollFds.length; ++i) {pollFds[i] = new StructPollfd();pollFds[i].fd = fds.get(i);pollFds[i].events = (short) POLLIN;}try {//处理轮询状态，当pollFds有事件到来则往下执行，否则阻塞在这里Os.poll(pollFds, -1);} catch (ErrnoException ex) {...}for (int i = pollFds.length - 1; i >= 0; --i) {//采用I/O多路复用机制，当接收到客户端发出连接请求 或者数据处理请求到来，则往下执行；// 否则进入continue，跳出本次循环。if ((pollFds[i].revents & POLLIN) == 0) {continue;}if (i == 0) {//即fds[0]，代表的是sServerSocket，则意味着有客户端连接请求；// 则创建ZygoteConnection对象,并添加到fds。ZygoteConnection newPeer = acceptCommandPeer(abiList);peers.add(newPeer);fds.add(newPeer.getFileDesciptor()); //添加到fds.} else {//i>0，则代表通过socket接收来自对端的数据，并执行相应操作【见小节3.6】boolean done = peers.get(i).runOnce();if (done) {peers.remove(i);fds.remove(i); //处理完则从fds中移除该文件描述符}}}}
}

Zygote采用高效的I/O多路复用机制，保证在没有客户端连接请求或数据处理时休眠，否则响应客户端的请求。

3.6 runOnce

[-> ZygoteConnection.java]

boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {String args[];Arguments parsedArgs = null;FileDescriptor[] descriptors;try {//读取socket客户端发送过来的参数列表args = readArgumentList();descriptors = mSocket.getAncillaryFileDescriptors();} catch (IOException ex) {...return true;}...try {//将binder客户端传递过来的参数，解析成Arguments对象格式parsedArgs = new Arguments(args);...//【见小节7】pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,parsedArgs.appDataDir);} catch (Exception e) {...}try {if (pid == 0) {//子进程执行IoUtils.closeQuietly(serverPipeFd);serverPipeFd = null;//进入子进程流程handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);return true;} else {//父进程执行IoUtils.closeQuietly(childPipeFd);childPipeFd = null;return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);}} finally {IoUtils.closeQuietly(childPipeFd);IoUtils.closeQuietly(serverPipeFd);}
}

更多内容，见理解Android进程创建流程

四、总结

Zygote启动过程的调用流程图：

解析init.zygote.rc中的参数，创建AppRuntime并调用AppRuntime.start()方法；
调用AndroidRuntime的startVM()方法创建虚拟机，再调用startReg()注册JNI函数；
通过JNI方式调用ZygoteInit.main()，第一次进入Java世界；
registerZygoteSocket()建立socket通道，zygote作为通信的服务端，用于响应客户端请求；
preload()预加载通用类、drawable和color资源、openGL以及共享库以及WebView，用于提高app启动效率；
zygote完毕大部分工作，接下来再通过startSystemServer()，fork得力帮手system_server进程，也是上层framework的运行载体。
zygote功成身退，调用runSelectLoop()，随时待命，当接收到请求创建新进程请求时立即唤醒并执行相应工作。

最后，介绍给通过cmd命令，来fork新进程来执行类中main方法的方式：(启动后进入RuntimeInit.main)

app_process [可选参数] 命令所在路径 启动的类名 [可选参数]