Android源码(1) --- Zygote进程启动流程

Zygote进程简介

什么是Zygote进程？ Zygote进程 是整个Android系统的根进程，包括SystemServer进程和所有应用进程在内都是通过Zygote进程 fork 出来的。Zygote进程则是通过Linux系统init进程启动。

• 启动顺序： Linux系统init进程 --> Zygote进程 --> SystemServer进程 --> Application 进程
• init进程：Android系统第一个进程，也是linux的根进程，主要用于初始化各种文件系统，输入输出系统，log系统等等设备相关联的初始化
• Zygote进程：Android系统的根进城，用于fork除SystemServer进程和各种应用进程
• SystemServer进程 --> 启动ActivityManagerService,WindowManagerService,PowerManagerService等等各项服务
• Application 进程: App 应用进程

源码分析（Android 6.0）

• 1.从ZygoteInit main()方法中开始看。

  public static void main(String argv[]) {
        try {
            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.");
            }

            registerZygoteSocket(socketName);
            EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START,
                SystemClock.uptimeMillis());
            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
            gcAndFinalize();

            // Disable tracing so that forked processes do not inherit stale tracing tags from
            // Zygote.
            Trace.setTracingEnabled(false);

            if (startSystemServer) {
                startSystemServer(abiList, socketName);
            }

            Log.i(TAG, "Accepting command socket connections");
            runSelectLoop(abiList);

            closeServerSocket();
        } catch (MethodAndArgsCaller caller) {
            caller.run();
        } catch (RuntimeException ex) {
            Log.e(TAG, "Zygote died with exception", ex);
            closeServerSocket();
            throw ex;
        }
    }

• 从上面可以看到主要做了哪几件事

  • 1.1 enableDdms() 设置DDMS可用

  • 1.2 for 循环，解析是否需要启动SystemService进程；获取abi列表；获取socket连接名称

  • 1.3 registerZygoteSocket(String socketName) 为Zygote 进程注册socket;(PS：Android中进程间通都是用Binder，但是有一个例外，SystemService进程与Zygote进程之间是通过Socket的方式进行通讯的)

     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) {
                throw new RuntimeException(fullSocketName + " unset or invalid", ex);
            }
    
            try {
                FileDescriptor fd = new FileDescriptor();
                fd.setInt$(fileDesc);
                sServerSocket = new LocalServerSocket(fd);
            } catch (IOException ex) {
                throw new RuntimeException(
                        "Error binding to local socket '" + fileDesc + "'", ex);
            }
        }
    }
    

  • 1.4 preload(),可以看到
    preloadClasses 初始化Zygote所需的类
    preloadResources 初始化通用系统资源
    preloadOpenGL 初始化OpenGL
    preloadSharedLibraries 初始化 shared libraries
    preloadTextResources 初始化文字资源
    prepareWebViewInZygote 初始化WebView(必须是Zygote进程)

    static void preload() {
            Log.d(TAG, "begin preload");
            preloadClasses();
            preloadResources();
            preloadOpenGL();
            preloadSharedLibraries();
            preloadTextResources();
            // Ask the WebViewFactory to do any initialization that must run in the zygote process,
            // for memory sharing purposes.
            WebViewFactory.prepareWebViewInZygote();
            Log.d(TAG, "end preload");
        }
    

  • 1.5 SamplingProfilerIntegration.writeZygoteSnapshot() 存储一下zygote进程快照
    gcAndFinalize() fork之前调用下系统GC

  • 1.6 startSystemServer(abiList, socketName)，接下来就是fork SystemServer进程了
    通过Zygote.forkSystemServe() fork 出SystemServer进程

  • 1.7 关闭Socket
    handleSystemServerProcess 当fork出SystemServer后关闭socket

  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
                      );
                      /* 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",
                          "--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);
              
                          /* Request to fork the system server process */
                          pid = Zygote.forkSystemServer(
                                  parsedArgs.uid, parsedArgs.gid,
                                  parsedArgs.gids,
                                  parsedArgs.debugFlags,
                                  null,
                                  parsedArgs.permittedCapabilities,
                                  parsedArgs.effectiveCapabilities);
                      } catch (IllegalArgumentException ex) {
                          throw new RuntimeException(ex);
                      }
              
                      /* For child process */
                      if (pid == 0) {
                          if (hasSecondZygote(abiList)) {
                              waitForSecondaryZygote(socketName);
                          }
              
                          handleSystemServerProcess(parsedArgs);
                      }
              
                      return true;
                  }
  
  
      /**
       * Finish remaining work for the newly forked system server process.
       */
      private static void handleSystemServerProcess(
              ZygoteConnection.Arguments parsedArgs)
              throws ZygoteInit.MethodAndArgsCaller {
  
          closeServerSocket();
  
          // 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 = new PathClassLoader(systemServerClasspath, ClassLoader.getSystemClassLoader());
                  Thread.currentThread().setContextClassLoader(cl);
              }
  
              /*
               * Pass the remaining arguments to SystemServer.
               */
              RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
          }
  
          /* should never reach here */
      }
  

### 梳理一下流程
![流程图](https://github.com/jfson/ImgResource/blob/master/13.png?raw=true)