Zygote启动及其作用

目录

1、Zygote简介

2、Zygote进程如何启动

　　2.1 init.zygote64_32.rc文件

　　2.2 查看ps信息

　　2.3 启动

3、Zygote作用

　　3.1 启动system_server

　　3.2 查看与验证Zygote启动systemserver

　　3.3 Zygote启动其他子进程

4、总结

1、Zygote简介

• 1.1 系统启动流程
  • 

  • 从按下电源到看到launcher，手机的启动是一个非常复杂的过程。

    bootloader是手机上电之后第一个运行的程序，其作用是硬件的初始化，其作用类似于PC机的bios。

    bootloader完成其工作后，将 Linux kernel镜像拷贝到内存中。完成剩余的与硬件平台相关的初始化工作，比如文件系统，驱动模块。最后启动第一个用户进程－init 进程并等待用户进程的执行。

    用户空间的第一个进程init

• 1.2 zygote理解
  • 在Android系统中，所有的应用程序进程以及系统服务进程SystemServer都是由Zygote进程孕育（fork）出来的。由于Zygote进程在Android系统中有着如此重要的地位，本文将详细分析它的启动过程。---老罗
  • android系统中创建java世界的盘古  创建新进程的女娲。 ---邓凡平

2、Zygote进程如何启动

• 2.1 init.zygote64_32.rc文件
  • system/core/rootdir/init.zygote64_32.rc文件内容
  • 

     1 service zygote /system/bin/app_process32 -Xzygote /system/bin --zygote --start-system-server --socket-name=zygote
     2     class main
     3     socket zygote stream 660 root system
     4     onrestart write /sys/android_power/request_state wake
     5     onrestart write /sys/power/state on
     6     onrestart restart media
     7     onrestart restart netd
     8
     9 service zygote_secondary /system/bin/app_process64 -Xzygote /system/bin --zygote --socket-name=zygote_secondary
    10     class main
    11     socket zygote_secondary stream 660 root system
    12     onrestart restart zygote

    
• 2.2 查看ps信息
  • 

  • Zygote64、Zygote是在init.rc中定义的服务进程，由init进程启动，PPID均为1。其中比较关键的vold、rild、surfaceflinger等关键进程也都是通过init进程启动的。

  • 

  • LOCAL_MULTILIB: Android.mk中用来设置编译为32位或者64位的apk，so等

• 2.3 启动
  • 
  • Zygote process main(frameworks/base/cmds/app_process/app_main.cpp)
  • 

      1 int main(int argc, char* const argv[])
      2 {
      3     ...
    121     if (zygote) {
    122         runtime.start("com.android.internal.os.ZygoteInit", args);
    123     } else if (className) {
    124         runtime.start("com.android.internal.os.RuntimeInit", args);
    125     } else {
    126         fprintf(stderr, "Error: no class name or --zygote supplied.\n");
    127         app_usage();
    128         LOG_ALWAYS_FATAL("app_process: no class name or --zygote supplied.");
    129         return 10;
    130     }
    131 }

    
  • 将init.rc中指定的-Xzygote参数传给JVM
    将进程的名字改为zygote（可以回答前面的问题）
    执行AppRuntime类的start()方法，runtime.start(“com.android.internal.os.ZygoteInit”, true);
    
  • 

      1 void AndroidRuntime::start(const char* className, const Vector<String8>& options)
      2 {
      3     ...
     30     //const char* kernelHack = getenv("LD_ASSUME_KERNEL");
     31     //ALOGD("Found LD_ASSUME_KERNEL='%s'\n", kernelHack);
     32
     33     /* start the virtual machine */
     34     JniInvocation jni_invocation;
     35     jni_invocation.Init(NULL);
     36     JNIEnv* env;
     37     if (startVm(&mJavaVM, &env) != 0) {
     38         return;
     39     }
     40     onVmCreated(env);
     41
     42     /*
     43      * Register android functions.
     44      */
     45     if (startReg(env) < 0) {
     46         ALOGE("Unable to register all android natives\n");
     47         return;
     48     }
     49
     50     /*
     51      * We want to call main() with a String array with arguments in it.
     52      * At present we have two arguments, the class name and an option string.
     53      * Create an array to hold them.
     54      */
     55     jclass stringClass;
     56     jobjectArray strArray;
     57     jstring classNameStr;
     58
     59     stringClass = env->FindClass("java/lang/String");
     60     assert(stringClass != NULL);
     61     strArray = env->NewObjectArray(options.size() + 1, stringClass, NULL);
     62     assert(strArray != NULL);
     63     classNameStr = env->NewStringUTF(className);
     64     assert(classNameStr != NULL);
     65     env->SetObjectArrayElement(strArray, 0, classNameStr);
     66
     67     for (size_t i = 0; i < options.size(); ++i) {
     68         jstring optionsStr = env->NewStringUTF(options.itemAt(i).string());
     69         assert(optionsStr != NULL);
     70         env->SetObjectArrayElement(strArray, i + 1, optionsStr);
     71     }
     72
     73     /*
     74      * Start VM.  This thread becomes the main thread of the VM, and will
     75      * not return until the VM exits.
     76      */
     77     char* slashClassName = toSlashClassName(className);
     78     jclass startClass = env->FindClass(slashClassName);
     79     if (startClass == NULL) {
     80         ALOGE("JavaVM unable to locate class '%s'\n", slashClassName);
     81         /* keep going */
     82     } else {
     83         jmethodID startMeth = env->GetStaticMethodID(startClass, "main",
     84             "([Ljava/lang/String;)V");
     85         if (startMeth == NULL) {
     86             ALOGE("JavaVM unable to find main() in '%s'\n", className);
     87             /* keep going */
     88         } else {
     89             env->CallStaticVoidMethod(startClass, startMeth, strArray);
     90
     91 #if 0
     92             if (env->ExceptionCheck())
     93                 threadExitUncaughtException(env);
     94 #endif
     95         }
     96     }
     97     free(slashClassName);
     98
     99     ALOGD("Shutting down VM\n");
    100     if (mJavaVM->DetachCurrentThread() != JNI_OK)
    101         ALOGW("Warning: unable to detach main thread\n");
    102     if (mJavaVM->DestroyJavaVM() != 0)
    103         ALOGW("Warning: VM did not shut down cleanly\n");
    104 }

    
  • runtime.start函数，即在frameworks\base\core\jni\AndroidRuntime.cpp文件中
    • AndroidRuntime::startVm()中，设置一些虚拟机的参数后，通过JNI_CreateJavaVM()启动虚拟机。
    • StartReg()注册JNI 函数
    • env->CallStaticVoidMethod，调用ZygoteInit类的main()方法，正式进入到Java世界.
  • frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
  • 

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

    

  • registerZygoteSocket，创建Socket服务端对象sServerSocket

    preload方法预加载类，资源等

    调用startSystemServer方法启动系统服务system_server

    runSelectLoopMode监听和处理sServerSocket的Socket请求

• 3、Zygote作用
  • 3.1 启动system_server
    • 
    • 

       1 /**
       2      * Prepare the arguments and fork for the system server process.
       3      */
       4     private static boolean startSystemServer(String abiList, String socketName)
       5             throws MethodAndArgsCaller, RuntimeException {
       6        ...
      31         int pid;
      32
      33         try {
      34             parsedArgs = new ZygoteConnection.Arguments(args);
      35             ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
      36             ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
      37
      38             /* Request to fork the system server process */
      39             pid = Zygote.forkSystemServer(
      40                     parsedArgs.uid, parsedArgs.gid,
      41                     parsedArgs.gids,
      42                     parsedArgs.debugFlags,
      43                     null,
      44                     parsedArgs.permittedCapabilities,
      45                     parsedArgs.effectiveCapabilities);
      46         } catch (IllegalArgumentException ex) {
      47             throw new RuntimeException(ex);
      48         }
      49
      50         /* For child process */
      51         if (pid == 0) {//子进程进入
      52             if (hasSecondZygote(abiList)) {
      53                 waitForSecondaryZygote(socketName);
      54             }
      55
      56             handleSystemServerProcess(parsedArgs);
      57         }
      58
      59         return true;
      60     }

      
    • forkSystemServer，调用Native方法fork子进程
      通过forkSystemServer方法返回的值，进入两个分支处理：父进程返回子进程pid值，进入到ZygoteInit类中的main方法继续处理；而子进程调用handleSystemServerProcess方法，最终会运行system_server。
    • 

       1 /**
       2      * Finish remaining work for the newly forked system server process.
       3      */
       4     private static void handleSystemServerProcess(
       5             ZygoteConnection.Arguments parsedArgs)
       6             throws ZygoteInit.MethodAndArgsCaller {
       7
       8         closeServerSocket();//子进程已经不是服务器了，所以关掉。
       9
      10         // set umask to 0077 so new files and directories will default to owner-only permissions.
      11         Os.umask(S_IRWXG | S_IRWXO);
      12
      13         if (parsedArgs.niceName != null) {
      14             Process.setArgV0(parsedArgs.niceName);
      15         }
      16
      17         final String systemServerClasspath = Os.getenv("SYSTEMSERVERCLASSPATH");
      18         if (systemServerClasspath != null) {
      19             performSystemServerDexOpt(systemServerClasspath);
      20         }
      21
      22         if (parsedArgs.invokeWith != null) {
      23             String[] args = parsedArgs.remainingArgs;
      24             // If we have a non-null system server class path, we'll have to duplicate the
      25             // existing arguments and append the classpath to it. ART will handle the classpath
      26             // correctly when we exec a new process.
      27             if (systemServerClasspath != null) {
      28                 String[] amendedArgs = new String[args.length + 2];
      29                 amendedArgs[0] = "-cp";
      30                 amendedArgs[1] = systemServerClasspath;
      31                 System.arraycopy(parsedArgs.remainingArgs, 0, amendedArgs, 2, parsedArgs.remainingArgs.length);
      32             }
      33
      34             WrapperInit.execApplication(parsedArgs.invokeWith,
      35                     parsedArgs.niceName, parsedArgs.targetSdkVersion,
      36                     null, args);
      37         } else {
      38             ClassLoader cl = null;
      39             if (systemServerClasspath != null) {
      40                 cl = new PathClassLoader(systemServerClasspath, ClassLoader.getSystemClassLoader());
      41                 Thread.currentThread().setContextClassLoader(cl);
      42             }
      43
      44             /*
      45              * Pass the remaining arguments to SystemServer.
      46              */
      47             RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
      48         }
      49
      50         /* should never reach here */
      51     }

      
    • frameworks/base/core/java/com/android/internal/os/RuntimeInit.java
    • 

      43 public class RuntimeInit {
       44     。。。
      197     private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader)
      198             throws ZygoteInit.MethodAndArgsCaller {
      199        。。。
      232        throw new ZygoteInit.MethodAndArgsCaller(m, argv);
      233     }
      267     public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
      268             throws ZygoteInit.MethodAndArgsCaller {
      269         if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
      270
      271         redirectLogStreams();
      272
      273         commonInit();
      274         nativeZygoteInit();
      275
      276         applicationInit(targetSdkVersion, argv, classLoader);
      277     }
      297     private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
      298             throws ZygoteInit.MethodAndArgsCaller {
      299         。。。
      308         VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
      309         VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
      310
      311         。。。
      320         // Remaining arguments are passed to the start class's static main
      321         invokeStaticMain(args.startClass, args.startArgs, classLoader);
      322     }
      363     public static final IBinder getApplicationObject() {
      364         return mApplicationObject;
      365     }
      425 }

      
    • 

       1 public static void main(String argv[]) {
       2         try {
       3             。。。
      42             if (startSystemServer) {
      43                 startSystemServer(abiList, socketName);
      44             }
      45             。。。//然而并没有机会进入循环
      50         } catch (MethodAndArgsCaller caller) {
      51             caller.run();
      52         } catch (RuntimeException ex) {
      53             Log.e(TAG, "Zygote died with exception", ex);
      54             closeServerSocket();
      55             throw ex;
      56         }
      57     }

      
    • 这个函数会执行两个操作，一个是调用zygoteInitNative函数来执行一个Binder进程间通信机制的初始化工作，这个工作完成之后，这个进 程中的Binder对象就可以方便地进行进程间通信了，另一个是调用上面传进来的com.android.server.SystemServer类的main函数。
  • 3.2 查看与验证Zygote启动systemserver
    • 
    • 
    • ps进程信息，验证system_server是Zygote的分裂出的第一个子进程.
  • 3.3 Zygote启动其他子进程
    • 
    • 注意：重复的不再涉及，我们只是分析一下7-12吧。
    • frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
    • 

       1 /**
       2      * Runs the zygote process's select loop. Accepts new connections as
       3      * they happen, and reads commands from connections one spawn-request's
       4      * worth at a time.
       5      *
       6      * @throws MethodAndArgsCaller in a child process when a main() should
       7      * be executed.
       8      */
       9     private static void runSelectLoop(String abiList) throws MethodAndArgsCaller {
      10         。。。
      18         while (true) {
      19             。。。
      38             try {
      39                 fdArray = fds.toArray(fdArray);
      40                 index = selectReadable(fdArray);
      41             } catch (IOException ex) {
      42                 throw new RuntimeException("Error in select()", ex);
      43             }
      44
      45             if (index < 0) {
      46                 throw new RuntimeException("Error in select()");
      47             } else if (index == 0) {
      48                 ZygoteConnection newPeer = acceptCommandPeer(abiList);
      49                 peers.add(newPeer);
      50                 fds.add(newPeer.getFileDescriptor());
      51             } else {
      52                 boolean done;
      53                 done = peers.get(index).runOnce();
      54
      55                 if (done) {
      56                     peers.remove(index);
      57                     fds.remove(index);
      58                 }
      59             }
      60         }
      61     }

      
    • runSelectLoopMode中while(true)循环，接收到Socket请求后，会fork出子进程，子进程调用handleChildProc方法，最终抛出RuntimeInit.invokeStaticMain异常，退出while(true)循环（与fork Systemserver不一样，后者没有进入循环），进入到android.app.ActivityThread类的main方法执行；父进程调用handleParentProc方法，再次进入runSelectLoopMode中while(true)循环，准备接收下一个的请求事件。
    • frameworks/base/core/java/com/android/internal/os/ZygoteConnection.java
    • 

        1 boolean runOnce() throws ZygoteInit.MethodAndArgsCaller {
        　　       ...
       37         try {
       38             ...
      105             pid = Zygote.forkAndSpecialize(parsedArgs.uid, parsedArgs.gid, parsedArgs.gids,
      106                     parsedArgs.debugFlags, rlimits, parsedArgs.mountExternal, parsedArgs.seInfo,
      107                     parsedArgs.niceName, fdsToClose, parsedArgs.instructionSet,
      108                     parsedArgs.appDataDir);
      109             checkTime(startTime, "zygoteConnection.runOnce: postForkAndSpecialize");
      110         } catch (IOException ex) {
      111             logAndPrintError(newStderr, "Exception creating pipe", ex);
      112         } catch (ErrnoException ex) {
      113             logAndPrintError(newStderr, "Exception creating pipe", ex);
      114         } catch (IllegalArgumentException ex) {
      115             logAndPrintError(newStderr, "Invalid zygote arguments", ex);
      116         } catch (ZygoteSecurityException ex) {
      117             logAndPrintError(newStderr,
      118                     "Zygote security policy prevents request: ", ex);
      119         }
      120
      121         try {
      122             if (pid == 0) {
      123                 // in child
      124                 IoUtils.closeQuietly(serverPipeFd);
      125                 serverPipeFd = null;
      126                 handleChildProc(parsedArgs, descriptors, childPipeFd, newStderr);
      127
      128                 // should never get here, the child is expected to either
      129                 // throw ZygoteInit.MethodAndArgsCaller or exec().
      130                 return true;
      131             } else {
      132                 // in parent...pid of < 0 means failure
      133                 IoUtils.closeQuietly(childPipeFd);
      134                 childPipeFd = null;
      135                 return handleParentProc(pid, descriptors, serverPipeFd, parsedArgs);
      136             }
      137         } finally {
      138             IoUtils.closeQuietly(childPipeFd);
      139             IoUtils.closeQuietly(serverPipeFd);
      140         }
      141     }

      
    • 10和11略过，只是返回而已。我们现在进入子进程的handleChildProc。
    • 

       1 private void handleChildProc(Arguments parsedArgs,
       2             FileDescriptor[] descriptors, FileDescriptor pipeFd, PrintStream newStderr)
       3             throws ZygoteInit.MethodAndArgsCaller {
       4
       5         /**
       6          * By the time we get here, the native code has closed the two actual Zygote
       7          * socket connections, and substituted /dev/null in their place.  The LocalSocket
       8          * objects still need to be closed properly.
       9          */
      10
      11         closeSocket();
      12         ZygoteInit.closeServerSocket();
      13
      14         。。。
      55             if (parsedArgs.invokeWith != null) {
      56                 WrapperInit.execStandalone(parsedArgs.invokeWith,
      57                         parsedArgs.classpath, className, mainArgs);
      58             } else {
      59                 ClassLoader cloader;
      60                 if (parsedArgs.classpath != null) {
      61                     cloader = new PathClassLoader(parsedArgs.classpath,
      62                             ClassLoader.getSystemClassLoader());
      63                 } else {
      64                     cloader = ClassLoader.getSystemClassLoader();
      65                 }
      66
      67                 try {
      68                     ZygoteInit.invokeStaticMain(cloader, className, mainArgs);
      69                 } catch (RuntimeException ex) {
      70                     logAndPrintError(newStderr, "Error starting.", ex);
      71                 }
      72             }
      73         。。。
      74     }

      
    • 剩余的就和上面雷同了，这里不再分析。

4、总结

• 系统启动时init进程会创建Zygote进程，Zygote进程负责后续Android应用程序框架层的其它进程的创建和启动工作。（可以使用ps查看）

  Zygote进程会首先创建一个SystemServer进程，SystemServer进程负责启动系统的关键服务，如包管理服务PackageManagerService和应用程序组件管理服务ActivityManagerService。

  当我们需要启动一个Android应用程序时，ActivityManagerService会通过Socket进程间通信机制，通知Zygote进程为这个应用程序创建一个新的进程。

•