Android系统启动流程分析
ykul8261
8年前
<p>随着Android版本的升级,aosp项目中的代码也有了些变化,本文基于Android 7.0分析Android系统启动流程.</p> <p>简单来说Android系统启动大体如下:</p> <p><img src="https://simg.open-open.com/show/bc8759c2d0ec10675c220ec765c34a24.png"></p> <h2>init进程</h2> <p>和所有Linux系统一样,Android系统的启动同样是从init进程启动.init进程会解析init.rc文件(关于init.rc中的语法,可以参见我之前写的 <a href="/misc/goto?guid=4959739031190179698" rel="nofollow,noindex">深入分析AIL语言及init.rc文件</a> ),加载相关目录,并启动相关服务</p> <p>init进程在/system/core/init/init.c</p> <p>init.rc文件在/system/core/rootdir下</p> <p>在init.rc中,Zygote进程被启动.Zygote进程是Android所有进程的孵化器进程.init.rc通过include引入init.zygote.rc,这里以init.zygote64.rc为例,来看一下其中的代码:</p> <pre> <code class="language-java">service zygote /system/bin/app_process64 -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 writepid /dev/cpuset/foreground/tasks</code></pre> <p>我们队这个脚本简单分析:</p> <p>1. service zygote /system/bin/app_process64 :service命令告诉init进程要创建一个名字为zygote的进程,这个zygote进程执行的程序是/system/bin/app_process64,后面是传给app_process64程序的参数.</p> <p>3. socket zygote stream 660 root system :表示zygote进程需要一个名为”zygote”的socket,该socket用来实现进程间的通信.当新启动一个应用时,ActivityManagerService想向该Socket发起请求,请求zygote进程fork出一个新的进程.</p> <p>4. 后面的onretart表示zygote重启时需要执行的动作.</p> <h2>Zygote进程启动</h2> <p>现在我们知道Zygote进程要执行的程序是/system/bin/app_process64,</p> <p>它位于/frameworks/base/cmds/app_process/app_main.cpp当中,其入口函数是main():</p> <pre> <code class="language-java">int main(int argc, char* const argv[]) { if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) < 0) { LOG_ALWAYS_FATAL("PR_SET_NO_NEW_PRIVS failed: %s", strerror(errno)); } if (!LOG_NDEBUG) { String8 argv_String; for (int i = 0; i < argc; ++i) { argv_String.append("\""); argv_String.append(argv[i]); argv_String.append("\" "); } ALOGV("app_process main with argv: %s", argv_String.string()); } AppRuntime runtime(argv[0], computeArgBlockSize(argc, argv)); // Process command line arguments // ignore argv[0] argc--; argv++; const char* spaced_commands[] = { "-cp", "-classpath" }; bool known_command = false; int i; for (i = 0; i < argc; i++) { if (known_command == true) { runtime.addOption(strdup(argv[i])); ALOGV("app_process main add known option '%s'", argv[i]); known_command = false; continue; } for (int j = 0; j < static_cast<int>(sizeof(spaced_commands) / sizeof(spaced_commands[0])); ++j) { if (strcmp(argv[i], spaced_commands[j]) == 0) { known_command = true; ALOGV("app_process main found known command '%s'", argv[i]); } } if (argv[i][0] != '-') { break; } if (argv[i][1] == '-' && argv[i][2] == 0) { ++i; // Skip --. break; } runtime.addOption(strdup(argv[i])); ALOGV("app_process main add option '%s'", argv[i]); } // Parse runtime arguments. Stop at first unrecognized option. bool zygote = false; bool startSystemServer = false; bool application = false; String8 niceName; String8 className; ++i; // Skip unused "parent dir" argument. while (i < argc) { const char* arg = argv[i++]; if (strcmp(arg, "--zygote") == 0) { zygote = true; niceName = ZYGOTE_NICE_NAME; } else if (strcmp(arg, "--start-system-server") == 0) { //init.zygote64.rc中接受的参数,表示启动SystemServer组件 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()) { args.add(application ? String8("application") : String8("tool")); runtime.setClassNameAndArgs(className, argc - i, argv + i); if (!LOG_NDEBUG) { String8 restOfArgs; char* const* argv_new = argv + i; int argc_new = argc - i; for (int k = 0; k < argc_new; ++k) { restOfArgs.append("\""); restOfArgs.append(argv_new[k]); restOfArgs.append("\" "); } ALOGV("Class name = %s, args = %s", className.string(), restOfArgs.string()); } } else { // We're in zygote mode. maybeCreateDalvikCache(); if (startSystemServer) { args.add(String8("start-system-server")); } char prop[PROP_VALUE_MAX]; if (property_get(ABI_LIST_PROPERTY, prop, NULL) == 0) { LOG_ALWAYS_FATAL("app_process: Unable to determine ABI list from property %s.", ABI_LIST_PROPERTY); return 11; } String8 abiFlag("--abi-list="); abiFlag.append(prop); args.add(abiFlag); // In zygote mode, pass all remaining arguments to the zygote // main() method. for (; i < argc; ++i) { args.add(String8(argv[i])); } } if (!niceName.isEmpty()) { runtime.setArgv0(niceName.string(), true /* setProcName */); } if (zygote) { //此处见到了我们熟悉的ZygoteInit,但该方法的具体实现在AndroidRuntime.start() 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."); } }</code></pre> <p>该函数最主要的作用就是创建AppRuntime,该类同样是在该文件中定义,具体的定义如下:</p> <pre> <code class="language-java">class AppRuntime : public AndroidRuntime { public: AppRuntime(char* argBlockStart, const size_t argBlockLength) : AndroidRuntime(argBlockStart, argBlockLength) , mClass(NULL) { } void setClassNameAndArgs(const String8& className, int argc, char * const *argv) { mClassName = className; for (int i = 0; i < argc; ++i) { mArgs.add(String8(argv[i])); } } virtual void onVmCreated(JNIEnv* env) { if (mClassName.isEmpty()) { return; // Zygote. Nothing to do here. } char* slashClassName = toSlashClassName(mClassName.string()); mClass = env->FindClass(slashClassName); if (mClass == NULL) { ALOGE("ERROR: could not find class '%s'\n", mClassName.string()); } free(slashClassName); mClass = reinterpret_cast<jclass>(env->NewGlobalRef(mClass)); } virtual void onStarted() { sp<ProcessState> proc = ProcessState::self(); ALOGV("App process: starting thread pool.\n"); proc->startThreadPool(); AndroidRuntime* ar = AndroidRuntime::getRuntime(); ar->callMain(mClassName, mClass, mArgs); IPCThreadState::self()->stopProcess(); } virtual void onZygoteInit() { sp<ProcessState> proc = ProcessState::self(); ALOGV("App process: starting thread pool.\n"); proc->startThreadPool(); } virtual void onExit(int code) { if (mClassName.isEmpty()) { // if zygote IPCThreadState::self()->stopProcess(); } AndroidRuntime::onExit(code); } String8 mClassName; Vector<String8> mArgs; jclass mClass; };</code></pre> <p>AppRuntime继承了AndroidRuntime,来继续看看AndroidRuntime.start()方法:</p> <p>它在/frameworks/base/core/jni/AndroidRuntime.cpp,来看下源码:</p> <pre> <code class="language-java">/* * Start the Android runtime. This involves starting the virtual machine * and calling the "static void main(String[] args)" method in the class * named by "className". * * Passes the main function two arguments, the class name and the specified * options string. */ void AndroidRuntime::start(const char* className, const Vector<String8>& options, bool zygote) { ALOGD(">>>>>> START %s uid %d <<<<<<\n", className != NULL ? className : "(unknown)", getuid()); static const String8 startSystemServer("start-system-server"); /* * 'startSystemServer == true' means runtime is obsolete and not run from * init.rc anymore, so we print out the boot start event here. */ for (size_t i = 0; i < options.size(); ++i) { if (options[i] == startSystemServer) { /* track our progress through the boot sequence */ const int LOG_BOOT_PROGRESS_START = 3000; LOG_EVENT_LONG(LOG_BOOT_PROGRESS_START, ns2ms(systemTime(SYSTEM_TIME_MONOTONIC))); } } const char* rootDir = getenv("ANDROID_ROOT"); if (rootDir == NULL) { rootDir = "/system"; if (!hasDir("/system")) { LOG_FATAL("No root directory specified, and /android does not exist."); return; } setenv("ANDROID_ROOT", rootDir, 1); } //const char* kernelHack = getenv("LD_ASSUME_KERNEL"); //ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack); //1. 启动虚拟机 if (startVm(&mJavaVM, &env, zygote) != 0) { return; } onVmCreated(env); //2. 调用startReg()注册JNI方法 if (startReg(env) < 0) { ALOGE("Unable to register all android natives\n"); return; } /* * We want to call main() with a String array with arguments in it. * At present we have two arguments, the class name and an option string. * Create an array to hold them. */ jclass stringClass; jobjectArray strArray; jstring classNameStr; stringClass = env->FindClass("java/lang/String"); assert(stringClass != NULL); strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL); assert(strArray != NULL); classNameStr = env->NewStringUTF(className); assert(classNameStr != NULL); env->SetObjectArrayElement(strArray, 0, classNameStr); for (size_t i = 0; i < options.size(); ++i) { jstring optionsStr = env->NewStringUTF(options.itemAt(i).string()); assert(optionsStr != NULL); env->SetObjectArrayElement(strArray, i + 1, optionsStr); } /* * Start VM. This thread becomes the main thread of the VM, and will * not return until the VM exits. */ char* slashClassName = toSlashClassName(className); jclass startClass = env->FindClass(slashClassName); if (startClass == NULL) { ALOGE("JavaVM unable to locate class '%s'\n", slashClassName); /* keep going */ } else { //3. 本质就是调用com.android.intrnal.os.ZygoteInit类的main函数 jmethodID startMeth = env->GetStaticMethodID(startClass, "main", "([Ljava/lang/String;)V"); if (startMeth == NULL) { ALOGE("JavaVM unable to find main() in '%s'\n", className); /* keep going */ } else { env->CallStaticVoidMethod(startClass, startMeth, strArray); #if 0 if (env->ExceptionCheck()) threadExitUncaughtException(env); #endif } } free(slashClassName); ALOGD("Shutting down VM\n"); if (mJavaVM->DetachCurrentThread() != JNI_OK) ALOGW("Warning: unable to detach main thread\n"); if (mJavaVM->DestroyJavaVM() != 0) ALOGW("Warning: VM did not shut down cleanly\n"); }</code></pre> <p>在start()中主要做三件事情:</p> <p>1. 调用startVM函数启动虚拟机</p> <p>2. 调用startReg注册JNI方法</p> <p>3. 调用com.android.internal.os.ZygoteInit.java类的main函数.</p> <p>这里我们重点来ZygoteInit.java.它在</p> <p>rameworks/base/core/Java/com/android/internal/os/ZygoteInit.java,我们直接来看他的main方法:</p> <pre> <code class="language-java">public static void main(String argv[]) { ZygoteServer zygoteServer = new ZygoteServer(); // Mark zygote start. This ensures that thread creation will throw // an error. ZygoteHooks.startZygoteNoThreadCreation(); // Zygote goes into its own process group. try { Os.setpgid(0, 0); } catch (ErrnoException ex) { throw new RuntimeException("Failed to setpgid(0,0)", ex); } try { Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygoteInit"); RuntimeInit.enableDdms(); // Start profiling the zygote initialization. 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]); } } if (abiList == null) { throw new RuntimeException("No ABI list supplied."); } //创建名为zygote的socket zygoteServer.registerServerSocket(socketName); Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "ZygotePreload"); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis()); preload(); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis()); Trace.traceEnd(Trace.TRACE_TAG_DALVIK); // Finish profiling the zygote initialization. SamplingProfilerIntegration.writeZygoteSnapshot(); // Do an initial gc to clean up after startup Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PostZygoteInitGC"); gcAndFinalize(); Trace.traceEnd(Trace.TRACE_TAG_DALVIK); // Disable tracing so that forked processes do not inherit stale tracing tags from // Zygote. Trace.setTracingEnabled(false); // Zygote process unmounts root storage spaces. Zygote.nativeUnmountStorageOnInit(); ZygoteHooks.stopZygoteNoThreadCreation(); if (startSystemServer) { //启动SystemServer组件 startSystemServer(abiList, socketName, zygoteServer); } Log.i(TAG, "Accepting command socket connections"); //等待ActivityManagerService请求 zygoteServer.runSelectLoop(abiList); zygoteServer.closeServerSocket(); } catch (Zygote.MethodAndArgsCaller caller) { caller.run(); } catch (Throwable ex) { Log.e(TAG, "System zygote died with exception", ex); zygoteServer.closeServerSocket(); throw ex; } }</code></pre> <p>这里的main()方法中主要做了三件事情</p> <p>1. 通过registerServerSocket来创建Socket,用来和ActivityManagerService通信</p> <p>2. 通过startSystemServer方法来启动SystemServer</p> <p>3. 通过runSelectLoop方法来等待来自ActivityManagerService请求</p> <h2>Socket创建</h2> <p>下面我们分别来看看对应着几个方法的具体实现:</p> <p>首先来看ZygoteServer.resiterServerSocket(),它在</p> <p>/frameworks/base/core/java/com/android/internal/os/ZygoteServer.java当中:</p> <pre> <code class="language-java">void registerServerSocket(String socketName) { if (mServerSocket == null) { int fileDesc; final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName; try { String env = System.getenv(fullSocketName); //从环境变量env中获取文件描述符 fileDesc = Integer.parseInt(env); } catch (RuntimeException ex) { throw new RuntimeException(fullSocketName + " unset or invalid", ex); } try { //通过文件描述符创建socket,该描述符代表/dev/socket/zygote文件. FileDescriptor fd = new FileDescriptor(); fd.setInt$(fileDesc); mServerSocket = new LocalServerSocket(fd); } catch (IOException ex) { throw new RuntimeException( "Error binding to local socket '" + fileDesc + "'", ex); } } }</code></pre> <h2>SystemServer启动</h2> <p>再回到ZygoteInit中看startSystemServer()方法:</p> <pre> <code class="language-java">/** * Prepare the arguments and fork for the system server process. */ private static boolean startSystemServer(String abiList, String socketName, ZygoteServer zygoteServer) throws Zygote.MethodAndArgsCaller, RuntimeException { long capabilities = posixCapabilitiesAsBits( OsConstants.CAP_IPC_LOCK, 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, OsConstants.CAP_WAKE_ALARM ); /* Containers run without this capability, so avoid setting it in that case */ if (!SystemProperties.getBoolean(PROPERTY_RUNNING_IN_CONTAINER, false)) { capabilities |= posixCapabilitiesAsBits(OsConstants.CAP_BLOCK_SUSPEND); } /* Hardcoded command line to start the system server */ String args[] = { "--setuid=1000", "--setgid=1000", "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1032,3001,3002,3003,3006,3007,3009,3010", "--capabilities=" + capabilities + "," + capabilities, "--nice-name=system_server", "--runtime-args", "com.android.server.SystemServer", }; ZygoteConnection.Arguments parsedArgs = null; int pid; try { parsedArgs = new ZygoteConnection.Arguments(args); ZygoteConnection.applyDebuggerSystemProperty(parsedArgs); ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs); //通过forSystemServer()方法创建第一个子进程,此处即SystemServer进程 pid = Zygote.forkSystemServer( parsedArgs.uid, parsedArgs.gid, parsedArgs.gids, parsedArgs.debugFlags, null, parsedArgs.permittedCapabilities, parsedArgs.effectiveCapabilities); } catch (IllegalArgumentException ex) { throw new RuntimeException(ex); } //pid=0表示子进程,此处就是SystemServer进程 if (pid == 0) { if (hasSecondZygote(abiList)) { waitForSecondaryZygote(socketName); } //Zygote创建的子进程(此处就是SystemServer)不需要使用Zygote中创建的Socket文件描述符,因此通过closeServerSocket()关闭它. zygoteServer.closeServerSocket(); handleSystemServerProcess(parsedArgs); } return true; }</code></pre> <p>接着我们看一下handleSystemServerProcess()方法:</p> <pre> <code class="language-java">/** * Finish remaining work for the newly forked system server process. */ private static void handleSystemServerProcess( ZygoteConnection.Arguments parsedArgs) throws Zygote.MethodAndArgsCaller { // set umask to 0077 so new files and directories will default to owner-only permissions. Os.umask(S_IRWXG | S_IRWXO); if (parsedArgs.niceName != null) { Process.setArgV0(parsedArgs.niceName); } final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH"); if (systemServerClasspath != null) { performSystemServerDexOpt(systemServerClasspath); } if (parsedArgs.invokeWith != null) { String[] args = parsedArgs.remainingArgs; // If we have a non-null system server class path, we'll have to duplicate the // existing arguments and append the classpath to it. ART will handle the classpath // correctly when we exec a new process. if (systemServerClasspath != null) { String[] amendedArgs = new String[args.length + 2]; amendedArgs[0] = "-cp"; amendedArgs[1] = systemServerClasspath; System.arraycopy(parsedArgs.remainingArgs, 0, amendedArgs, 2, parsedArgs.remainingArgs.length); } WrapperInit.execApplication(parsedArgs.invokeWith, parsedArgs.niceName, parsedArgs.targetSdkVersion, VMRuntime.getCurrentInstructionSet(), null, args); } else { ClassLoader cl = null; if (systemServerClasspath != null) { cl = createSystemServerClassLoader(systemServerClasspath, parsedArgs.targetSdkVersion); Thread.currentThread().setContextClassLoader(cl); } //调用zygoteInit()方法进一步执行启动SystemServer组件的操作 RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl); } }</code></pre> <p>继续来看 RuntimeInit.zygoteInit()函数的具体代码,它在</p> <p>/frameworks/base/core/java/com/android/internal/os/RuntimeInit.java文件中:</p> <pre> <code class="language-java">public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) throws Zygote.MethodAndArgsCaller { if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote"); Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit"); redirectLogStreams(); commonInit(); //native方法,用来执行一个Binder进程间同步新机制的初始化工作.之后,这个进程中的Binder对象就可以方便的进行进程间通信了 nativeZygoteInit(); //执行com.android.server.SystemServer类的main函数 applicationInit(targetSdkVersion, argv, classLoader); } private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) throws Zygote.MethodAndArgsCaller { //...省略众多代码 invokeStaticMain(args.startClass, args.startArgs, classLoader); } //真正反射执行SystemServer的main方法 private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader) throws Zygote.MethodAndArgsCaller { 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 { // 通过反射执行SystemServer的main()方法 m = cl.getMethod("main", new Class[] { String[].class }); } catch (NoSuchMethodException ex) { //... } catch (SecurityException ex) { //... } //... }</code></pre> <p>接上文来看一下SystemServer.main(),</p> <p>它在/frameworks/base/services/java/com/android/server/SystemServer.java</p> <pre> <code class="language-java">public class SystemServer{ public static void main(String[] args) { new SystemServer().run(); } private void run() { try { //...省略一些初始化操作 android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_FOREGROUND); android.os.Process.setCanSelfBackground(false); //初始化主线程Looper Looper.prepareMainLooper(); //创建SystemServiceManager对象 mSystemServiceManager = new SystemServiceManager(mSystemContext); LocalServices.addService(SystemServiceManager.class, mSystemServiceManager); } finally { Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER); } // 启动关键服务 startBootstrapServices(); //启动核心服务 startCoreServices(); //启动其他服务 startOtherServices(); //...省略多行代码 //启动消息循环 Looper.loop(); } }</code></pre> <h2>Socket监听</h2> <p>到现在关于ZygoteInit.main()放中两个主要的方法</p> <p>ZygoteServer.registerServerSocket() 和 startSystemServer()</p> <p>我们已经分析完了,他们分别完成Socket的创建以及SystemServer的启动工作,接下来回到 ZygoteServer.runSelectLoop() 方法:</p> <pre> <code class="language-java">oid runSelectLoop(String abiList) throws Zygote.MethodAndArgsCaller { ArrayList<FileDescriptor> fds = new ArrayList<FileDescriptor>(); ArrayList<ZygoteConnection> peers = new ArrayList<ZygoteConnection>(); fds.add(mServerSocket.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 { Os.poll(pollFds, -1); } catch (ErrnoException ex) { throw new RuntimeException("poll failed", ex); } for (int i = pollFds.length - 1; i >= 0; --i) { if ((pollFds[i].revents & POLLIN) == 0) { continue; } if (i == 0) { //监听Socket链接,如果你做过Socket编程就发现此处充当了服务端Socket ZygoteConnection newPeer = acceptCommandPeer(abiList); peers.add(newPeer); fds.add(newPeer.getFileDesciptor()); } else { //重点关注runOnce()方法 boolean done = peers.get(i).runOnce(this); if (done) { peers.remove(i); fds.remove(i); } } } } }</code></pre> <p>该方法非常简单就是不断的从已经创建的Socket中取出ZygoteConnection请求,继续来:ZygoteConnection.runOnce():</p> <pre> <code class="language-java">boolean runOnce(ZygoteServer zygoteServer) throws Zygote.MethodAndArgsCaller { String args[]; Arguments parsedArgs = null; FileDescriptor[] descriptors; //...省略一堆参数解析和判断逻辑 try { //...省略多行 fd = zygoteServer.getServerSocketFileDescriptor(); if (fd != null) { fdsToClose[1] = fd.getInt$(); } fd = null; //关键:通过forkAndSpecialize()方法创建非Zygote进程,即普通应用 pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids, parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo, parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet, parsedArgs.appDataDir); } catch (ErrnoException ex) { //... } catch (IllegalArgumentException ex) { //... } catch (ZygoteSecurityException ex) { //... } try { if (pid == 0) {//此处就是普通应用的进程 // in child //此处创建的进程同样用不到Zygote进程中创建的Sockt,所以关闭它 zygoteServer.closeServerSocket(); IoUtils.closeQuietly(serverPipeFd); serverPipeFd = null; //该方法最终调用程序的入口main方法 handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr); return true; } else { // in parent...pid of < 0 means failure IoUtils.closeQuietly(childPipeFd); childPipeFd = null; return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs); } } finally { IoUtils.closeQuietly(childPipeFd); IoUtils.closeQuietly(serverPipeFd); } }</code></pre> <p>在runOnce()方法中,Zygote进程根据ActivityManagerService的请求,通过forkAndSpecialize()方法来为应用创建新进程,并执行应用的入口main()方法.</p> <p>到现在我们可以用一张顺序图大体的表示上述的整个流程:</p> <p><img src="https://simg.open-open.com/show/4d41793e54aa474f5803dfce19ca7709.png"></p> <p>总体而言整个流程还是非常清晰的.另外关于Zygote进程及其他进程的关系,可以看如下图:</p> <h2>总结</h2> <p>系统启动时init进程会创建Zygote进程,Zygote进程负责后续Android应用框架层的其他进程的创建和启动.Zygote进程会首先创建一个SystemSever进程,然后由SystemServer负责启动系统关键服务,如ActivityManagerService或者PackageManagerService等.当需要启动一个新的应用时,ActivityManagerService会通过Socket进行通信,Zygote的Socket收到请求之后会为这个应用fork一个新进程,fork成功之后,执行的工作就交给虚拟机.</p> <p> </p> <p>来自：http://blog.csdn.net/dd864140130/article/details/57624948</p> <p> </p>