1.Zygote简介
在Android系统中，DVM(Dalvik虚拟机)、应用程序进程以及运行系统关键服务的SystemServer进程都是由Zygote进程来创建的，我们也将它称为孵化器。它通过fork(复制进程)的形式来创建应用程序进程和SystemServer进程，由于Zygote进程在启动时会创建DVM，因此通过fork而创建的应用程序进程和SystemServer进程可以在内部获取一个DVM的实例拷贝。
2.源码分析
frameworks\base\cmds\app_process\app_main.cpp
runtime.start("com.android.internal.os.ZygoteInit", args, zygote);//2 runtime为AppRuntime 类型 class AppRuntime : public AndroidRuntime
AndroidRuntime的start方法位于 frameworks\base\core\jni\AndroidRuntime.cpp
start方法的关键代码为：
启动Dvm JNIEnv* env; if (startVm(&mJavaVM, &env, zygote, primary_zygote) != 0) { return; } 为DVM注册JNI /* * Register android functions. */ if (startReg(env) < 0) { ALOGE("Unable to register all android natives\n"); return; } //找到ZygoteInit的main函数 jmethodID startMeth = env->GetStaticMethodID(startClass, "main","([Ljava/lang/String;)V"); //通过JNI调用ZygoteInit的main函数 env->CallStaticVoidMethod(startClass, startMeth, strArray);
进入Java层---Zygote开创了Java框架层。
frameworks\base\core\java\com\android\internal\os\ZygoteInit.java
ZygoteInit的main方法的关键代码如下：
if (!enableLazyPreload) { bootTimingsTraceLog.traceBegin("ZygotePreload"); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_START, SystemClock.uptimeMillis()); //预加载类和资源 preload(bootTimingsTraceLog); EventLog.writeEvent(LOG_BOOT_PROGRESS_PRELOAD_END, SystemClock.uptimeMillis()); bootTimingsTraceLog.traceEnd(); // ZygotePreload } //启动SystemServer进程 if (startSystemServer) { Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer); // {@code r == null} in the parent (zygote) process, and {@code r != null} in the // child (system_server) process. if (r != null) { r.run(); return; } } //等待客户端请求 caller = zygoteServer.runSelectLoop(abiList);
创建一个Server端的Socket，这个name为”zygote”的Socket用来等待ActivityManagerService来请求Zygote来创建新的应用程序进程.
启动systemServer
private static Runnable forkSystemServer(String abiList, String socketName, ZygoteServer zygoteServer) { 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_PTRACE, OsConstants.CAP_SYS_TIME, OsConstants.CAP_SYS_TTY_CONFIG, OsConstants.CAP_WAKE_ALARM, OsConstants.CAP_BLOCK_SUSPEND ); /* Containers run without some capabilities, so drop any caps that are not available. */ StructCapUserHeader header = new StructCapUserHeader( OsConstants._LINUX_CAPABILITY_VERSION_3, 0); StructCapUserData[] data; try { data = Os.capget(header); } catch (ErrnoException ex) { throw new RuntimeException("Failed to capget()", ex); } capabilities &= ((long) data[0].effective) | (((long) data[1].effective) << 32); ／／SystemServer进程的用户id和用户组id被设置为1000；进程名为system_server；启动的类名为com.android.server.SystemServer /* Hardcoded command line to start the system server */ String args[] = { "--setuid=1000", "--setgid=1000", "--setgroups=1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1018,1021,1023," + "1024,1032,1065,3001,3002,3003,3006,3007,3009,3010,3011", "--capabilities=" + capabilities + "," + capabilities, "--nice-name=system_server", "--runtime-args", "--target-sdk-version=" + VMRuntime.SDK_VERSION_CUR_DEVELOPMENT, "com.android.server.SystemServer", }; ZygoteArguments parsedArgs = null; int pid; try { parsedArgs = new ZygoteArguments(args); Zygote.applyDebuggerSystemProperty(parsedArgs); Zygote.applyInvokeWithSystemProperty(parsedArgs); if (Zygote.nativeSupportsTaggedPointers()) { /* Enable pointer tagging in the system server. Hardware support for this is present * in all ARMv8 CPUs. */ parsedArgs.mRuntimeFlags |= Zygote.MEMORY_TAG_LEVEL_TBI; } /* Enable gwp-asan on the system server with a small probability. This is the same * policy as applied to native processes and system apps. */ parsedArgs.mRuntimeFlags |= Zygote.GWP_ASAN_LEVEL_LOTTERY; if (shouldProfileSystemServer()) { parsedArgs.mRuntimeFlags |= Zygote.PROFILE_SYSTEM_SERVER; } /* Request to fork the system server process */ 调用Zygote的forkSystemServer，主要通过fork函数在当前进程创建一个子进程，如果返回的pid 为0，也就是表示在新创建的子进程中执行的，则执行注释4处的handleSystemServerProcess来启动SystemServer进程。 pid = Zygote.forkSystemServer( parsedArgs.mUid, parsedArgs.mGid, parsedArgs.mGids, parsedArgs.mRuntimeFlags, null, parsedArgs.mPermittedCapabilities, parsedArgs.mEffectiveCapabilities); } catch (IllegalArgumentException ex) { throw new RuntimeException(ex); } /* For child process */ if (pid == 0) { if (hasSecondZygote(abiList)) { waitForSecondaryZygote(socketName); } zygoteServer.closeServerSocket(); return handleSystemServerProcess(parsedArgs); } return null; }
进入 frameworks\base\core\java\com\android\internal\os\ZygoteServer.java的runSelectLoop
socketFDs.add(mZygoteSocket.getFileDescriptor());／／１ int pollIndex = 0; for (FileDescriptor socketFD : socketFDs) {／／２ pollFDs[pollIndex] = new StructPollfd(); pollFDs[pollIndex].fd = socketFD; pollFDs[pollIndex].events = (short) POLLIN; ++pollIndex; } while (--pollIndex >= 0) { if ((pollFDs[pollIndex].revents & POLLIN) == 0) {／／３ continue; } if (pollIndex == 0) { // Zygote server socket ZygoteConnection newPeer = acceptCommandPeer(abiList);／／４ peers.add(newPeer); socketFDs.add(newPeer.getFileDescriptor()); } else if (pollIndex < usapPoolEventFDIndex) { // Session socket accepted from the Zygote server socket try { ／／５ ZygoteConnection connection = peers.get(pollIndex); final Runnable command = connection.processOneCommand(this); // TODO (chriswailes): Is this extra check necessary? if (mIsForkChild) { // We're in the child. We should always have a command to run at // this stage if processOneCommand hasn't called "exec". if (command == null) { throw new IllegalStateException("command == null"); } return command; } else { // We're in the server - we should never have any commands to run. if (command != null) { throw new IllegalStateException("command != null"); } // We don't know whether the remote side of the socket was closed or // not until we attempt to read from it from processOneCommand. This // shows up as a regular POLLIN event in our regular processing // loop. if (connection.isClosedByPeer()) { connection.closeSocket(); peers.remove(pollIndex); socketFDs.remove(pollIndex); } }
注释1处中的sServerSocket就是我们在registerZygoteSocket函数中创建的服务端Socket，调用sServerSocket.getFileDescriptor()用来获得该Socket的fd字段的值并添加到fd列表fds中。接下来无限循环用来等待ActivityManagerService请求Zygote进程创建新的应用程序进程。注释2处通过遍历将fds存储的信息转移到pollFds数组中。最后在注释3处对pollFds进行遍历，如果i==0则说明服务端Socket与客户端连接上，也就是当前Zygote进程与ActivityManagerService建立了连接。则在注释4处通过acceptCommandPeer函数得到ZygoteConnection类并添加到Socket连接列表peers中，接着将该ZygoteConnection的fd添加到fd列表fds中，以便可以接收到ActivityManagerService发送过来的请求。如果i的值大于0，则说明ActivityManagerService向Zygote进程发送了一个创建应用进程的请求，则在注释5处调用ZygoteConnection的runOnce函数来创建一个新的应用程序进程。并在成功创建后将这个连接从Socket连接列表peers和fd列表fds中清除。
３.Zygote进程总结
Zygote进程共做了如下几件事：
1.创建AppRuntime并调用其start方法，启动Zygote进程。
2.创建DVM并为DVM注册JNI.
3.通过JNI调用ZygoteInit的main函数进入Zygote的Java框架层。
4.通过registerZygoteSocket函数创建服务端Socket，并通过runSelectLoop函数等待ActivityManagerService的请求来创建新的应用程序进程。
5.启动SystemServer进程。
