Zygote进程启动流程分析

大家都知道android系统的Zygote进程是所有的android进程的父进程,包括SystemServer和各种应用进程都是通过Zygote进程fork出来的。Zygote(孵化)进程相当于是android系统的根进程,后面所有的进程都是通过这个进程fork出来的,而Zygote进程则是通过linux系统的init进程启动的,也就是说,android系统中各种进程的启动方式

init进程 –> Zygote进程 –> SystemServer进程 –>各种应用进程

  • 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;

  • 各种应用进程:启动自己编写的客户端应用时,一般都是重新启动一个应用进程,有自己的虚拟机与运行环境;

本文主要介绍一下Zygote进程的启动流程,关于SystenServer进程和各种应用进程的启动方式会在以后的文章中介绍。

init进程在启动Zygote进程时一般都会调用ZygoteInit类的main方法,因此我们这里看一下该方法的具体实现(基于android23源码);

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;
        }
    }

 

  • 第一行主要是调用enableDdms(),设置DDMS可用,可以发现DDMS启动的时机还是比较早的,在整个Zygote进程刚刚开始要启动额时候就设置可用了。

  • 下面的循环主要是解析main方法的参数获取是否需要启动SystemService进程,获取abi列表,获取scoket连接名称
    (这里需要注意的是:android系统中进程之间通讯的方式是Binder,但是有一个例外是SystemService进程与Zygote进程之间是通过Socket的方式进行通讯的)

  • 然后调用registerZygoteSocket(String socketName)为Zygote进程注册socket:

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);
            }
        }
    }接着调用系统方法preLoad()
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");
    }

 

这其中:
preloadClasses()用于初始化Zygote中需要的class类;
preloadResources()用于初始化系统资源;
preloadOpenGL()用于初始化OpenGL;
preloadSharedLibraries()用于初始化系统libraries;
preloadTextResources()用于初始化文字资源;
prepareWebViewInZygote()用于初始化webview;

然后调用startSystemServer(abiList, 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;
    }

 

可以看到这段逻辑的执行逻辑就是通过Zygote fork出SystemServer进程。

总结:
Zygote进程mian方法主要执行逻辑:

  • 初始化DDMS;

  • 注册Zygote进程的socket通讯;

  • 初始化Zygote中的各种类,资源文件,OpenGL,类库,Text资源等等;

  • 初始化完成之后fork出SystemServer进程;

  • fork出SystemServer进程之后,关闭socket连接;

    原文作者:水火379
    原文地址: https://www.cnblogs.com/zhangming-blog/articles/5571152.html
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
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