Android 源码分析（五） Zygote 进程
一.前言:
init进程 –> Zygote进程 –> SystemServer进程 –> Launcher桌面程序 -> 我们的App应用
init进程：linux的根进程，android系统是基于linux系统的，因此可以算作是整个android操作系统的第一个进程；
Zygote进程：android系统的根进程，主要作用：可以作用Zygote进程fork出SystemServer进程和各种应用进程；
SystemService进程：主要是在这个进程中启动系统的各项服务，比如ActivityManagerService，PackageManagerService，WindowManagerService服务等等；
Launcher桌面程序:就是我们平时看到的桌面程序，它其实也是一个android应用程序，只不过这个应用程序是系统默认第一个启动的应用程序.
二. Zygote进程
Android系统是基于Linux内核的，而在Linux系统中，所有的进程都是init进程的子孙进程，也就是说，所有的进程都是直接或者间接地由init进程fork出来的。Zygote进程也不例外，它是在系统启动的过程，由init进程创建的.
init进程在启动Zygote进程时一般都会调用ZygoteInit类的main方法.
如果SystemServer崩溃了，Zygote也会kill自己，重新创建进程服务。
//ZygoteInit.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 { // Report Zygote start time to tron unless it is a runtime restart if (!"1".equals(SystemProperties.get("sys.boot_completed"))) { MetricsLogger.histogram(null, "boot_zygote_init", (int) SystemClock.elapsedRealtime()); } String bootTimeTag = Process.is64Bit() ? "Zygote64Timing" : "Zygote32Timing"; BootTimingsTraceLog bootTimingsTraceLog = new BootTimingsTraceLog(bootTimeTag, Trace.TRACE_TAG_DALVIK); bootTimingsTraceLog.traceBegin("ZygoteInit"); RuntimeInit.enableDdms(); // Start profiling the zygote initialization. SamplingProfilerIntegration.start(); boolean startSystemServer = false; String socketName = "zygote"; String abiList = null; boolean enableLazyPreload = false; for (int i = 1; i < argv.length; i++) { if ("start-system-server".equals(argv[i])) { startSystemServer = true; } else if ("--enable-lazy-preload".equals(argv[i])) { enableLazyPreload = 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."); } zygoteServer.registerServerSocket(socketName); // In some configurations, we avoid preloading resources and classes eagerly. // In such cases, we will preload things prior to our first fork. if (!enableLazyPreload) { bootTimingsTraceLog.traceBegin("ZygotePreload"); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis()); preload(bootTimingsTraceLog); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis()); bootTimingsTraceLog.traceEnd(); // ZygotePreload } else { Zygote.resetNicePriority(); } // Finish profiling the zygote initialization. SamplingProfilerIntegration.writeZygoteSnapshot(); // Do an initial gc to clean up after startup bootTimingsTraceLog.traceBegin("PostZygoteInitGC"); gcAndFinalize(); bootTimingsTraceLog.traceEnd(); // PostZygoteInitGC bootTimingsTraceLog.traceEnd(); // ZygoteInit // 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(); // Set seccomp policy Seccomp.setPolicy(); ZygoteHooks.stopZygoteNoThreadCreation(); if (startSystemServer) { startSystemServer(abiList, socketName, zygoteServer); } Log.i(TAG, "Accepting command socket connections"); 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; } } //1.registerServerSocket //2.调用preload加载资源， //3.利用gcAndFinalize初始化gc， //4.启动SystemServer，startSystemServer() 这里启动SystemServer服务. //5.调用runSelectLoop运行Zygote进程选择的looper， //6.关闭和清理zygote sockets
//ZygoteServer.java /** * Registers a server socket for zygote command connections * * @throws RuntimeException when open fails */ void registerServerSocket(String socketName) { if (mServerSocket == null) { int fileDesc; final String fullSocketName = ANDROID_SOCKET_PREFIX + socketName; try { String env = System.getenv(fullSocketName); fileDesc = Integer.parseInt(env); } catch (RuntimeException ex) { throw new RuntimeException(fullSocketName + " unset or invalid", ex); } try { FileDescriptor fd = new FileDescriptor(); fd.setInt$(fileDesc); mServerSocket = new LocalServerSocket(fd); } catch (IOException ex) { throw new RuntimeException( "Error binding to local socket '" + fileDesc + "'", ex); } } }
//ZygoteInit.java static void preload(BootTimingsTraceLog bootTimingsTraceLog) { Log.d(TAG, "begin preload"); bootTimingsTraceLog.traceBegin("BeginIcuCachePinning"); beginIcuCachePinning(); bootTimingsTraceLog.traceEnd(); // BeginIcuCachePinning bootTimingsTraceLog.traceBegin("PreloadClasses"); //初始化Zygote中需要的class类 preloadClasses(); bootTimingsTraceLog.traceEnd(); // PreloadClasses bootTimingsTraceLog.traceBegin("PreloadResources"); //初始化系统资源 preloadResources(); bootTimingsTraceLog.traceEnd(); // PreloadResources Trace.traceBegin(Trace.TRACE_TAG_DALVIK, "PreloadOpenGL"); //初始化OpenGL preloadOpenGL(); Trace.traceEnd(Trace.TRACE_TAG_DALVIK); //初始化系统libraries preloadSharedLibraries(); //初始化文字资源 preloadTextResources(); // Ask the WebViewFactory to do any initialization that must run in the zygote process, // for memory sharing purposes. //初始化webview WebViewFactory.prepareWebViewInZygote(); endIcuCachePinning(); warmUpJcaProviders(); Log.d(TAG, "end preload"); sPreloadComplete = true; }
//进去看看初始化webview方法 //WebViewFactory.java /** * Perform any WebView loading preparations that must happen in the zygote. * Currently, this means allocating address space to load the real JNI library later. */ public static void prepareWebViewInZygote() { try { System.loadLibrary("webviewchromium_loader"); long addressSpaceToReserve = SystemProperties.getLong(CHROMIUM_WEBVIEW_VMSIZE_SIZE_PROPERTY, CHROMIUM_WEBVIEW_DEFAULT_VMSIZE_BYTES); sAddressSpaceReserved = nativeReserveAddressSpace(addressSpaceToReserve); if (sAddressSpaceReserved) { if (DEBUG) { Log.v(LOGTAG, "address space reserved: " + addressSpaceToReserve + " bytes"); } } else { Log.e(LOGTAG, "reserving " + addressSpaceToReserve + " bytes of address space failed"); } } catch (Throwable t) { // Log and discard errors at this stage as we must not crash the zygote. Log.e(LOGTAG, "error preparing native loader", t); } }
