在上一篇文章Frameworkbi学习(二)Zygote进程启动过程中,我们已经知道Zygote进程会启动SystemServer进程,但具体启动流程还没有涉及,本文我们就来看看SystemServer进程具体启动过程。
首先回顾下ZygoteInit#startSystemServer()函数:
frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
ZygoteInit#startSystemServer()
private static boolean startSystemServer(String abiList, String socketName) throws MethodAndArgsCaller, RuntimeException {
...
int pid;
try {
parsedArgs = new ZygoteConnection.Arguments(args);
ZygoteConnection.applyDebuggerSystemProperty(parsedArgs);
ZygoteConnection.applyInvokeWithSystemProperty(parsedArgs);
/* 1 */
// 请求fork SystemServer进程
pid = Zygote.forkSystemServer(
parsedArgs.uid, parsedArgs.gid,
parsedArgs.gids,
parsedArgs.debugFlags,
null,
parsedArgs.permittedCapabilities,
parsedArgs.effectiveCapabilities);
} catch (IllegalArgumentException ex) {
throw new RuntimeException(ex);
}
// pid为0表示子进程,即SystemServer进程,从此SystemServer进程与Zygote进程分道扬镳
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
handleSystemServerProcess(parsedArgs); //2
}
return true;
}
注释1处调用Zygote的forkSystemServer,主要通过fork函数在当前进程创建一个子进程(也就是SystemServer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2处的handleSystemServerProcess,来看看handleSystemServerProcess是如何启动SystemServer进程的。
ZygoteInit#handleSystemServerProcess()
private static void handleSystemServerProcess(
ZygoteConnection.Arguments parsedArgs)
throws ZygoteInit.MethodAndArgsCaller {
closeServerSocket(); //1
if (parsedArgs.niceName != null) {
Process.setArgV0(parsedArgs.niceName); //2
}
...
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 = createSystemServerClassLoader(systemServerClasspath,
parsedArgs.targetSdkVersion);
Thread.currentThread().setContextClassLoader(cl);
}
/*
* Pass the remaining arguments to SystemServer.
*/
RuntimeInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl); //3
}
/* should never reach here */
}
注释1处SyetemServer进程是复制了Zygote进程的地址空间,因此也会得到Zygote进程创建的Socket,这个Socket对于SyetemServer进程没有用处,所以调用closeServerSocket()关闭它。
注释2如果你看Arguments封装函数,会发现parsedArgs.niceName=system_server,在这里调用Process.setArgV0()设置进程名为:system_server。
注释3由于parsedArgs.invokeWith属性默认为null,最后调用RuntimeInit.zygoteInit来进一步启动SystemServer。
frameworks/base/core/java/com/android/internal/os/RuntimeInit.java
RuntimeInit#zygoteInit()
public static final void zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
if (DEBUG) Slog.d(TAG, "RuntimeInit: Starting application from zygote");
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "RuntimeInit");
// 重定向Log输出
redirectLogStreams();
//初始化运行环境
commonInit();
//启动Binder线程池
nativeZygoteInit(); //1
//调用程序入口函数
applicationInit(targetSdkVersion, argv, classLoader); //2
}
注释1处调用nativeZygoteInit函数,一看函数的名称就知道调用Native层的代码,它是用来启动Binder线程池的。
注释2处调用了applicationInit函数启动应用程序。
Binder线程池启动过程
先看看启动Binder线程池。来查看nativeZygoteInit函数对应的JNI文件,如下所示。
frameworks/base/core/jni/AndroidRuntime.cpp
static const JNINativeMethod gMethods[] = {
{ "nativeFinishInit", "()V",
(void*) com_android_internal_os_RuntimeInit_nativeFinishInit },
{ "nativeZygoteInit", "()V",
(void*) com_android_internal_os_RuntimeInit_nativeZygoteInit },
{ "nativeSetExitWithoutCleanup", "(Z)V",
(void*) com_android_internal_os_RuntimeInit_nativeSetExitWithoutCleanup },
};
通过JNI的gMethods数组,可以看出nativeZygoteInit函数对应的是JNI文件AndroidRuntime.cpp的com_android_internal_os_RuntimeInit_nativeZygoteInit函数:
AndroidRuntime#com_android_internal_os_RuntimeInit_nativeZygoteInit()
...
static AndroidRuntime* gCurRuntime = NULL;
AndroidRuntime::AndroidRuntime(char* argBlockStart, const size_t argBlockLength) :
mExitWithoutCleanup(false),
mArgBlockStart(argBlockStart),
mArgBlockLength(argBlockLength)
{
...
gCurRuntime = this;
}
...
static void com_android_internal_os_RuntimeInit_nativeZygoteInit(JNIEnv* env, jobject clazz)
{
gCurRuntime->onZygoteInit();
}
这里gCurRuntime是AndroidRuntime类型的指针,AndroidRuntime的子类AppRuntime在app_main.cpp中定义,我们来查看AppRuntime的onZygoteInit函数
frameworks/base/cmds/app_process/app_main.cpp
AppRuntime#onZygoteInit()
virtual void onZygoteInit()
{
sp proc = ProcessState::self();
ALOGV("App process: starting thread pool.\n");
proc->startThreadPool();//1
}
注释1处会调用ProcessState的startThreadPool函数。
frameworks/native/libs/binder/ProcessState.cpp
ProcessState#startThreadPool()
void ProcessState::startThreadPool()
{
AutoMutex _l(mLock);
//1
if (!mThreadPoolStarted) {
mThreadPoolStarted = true;
spawnPooledThread(true);
}
}
支持Binder通信的进程中都有一个ProcessState类,它里面有一个mThreadPoolStarted 变量,来表示Binder线程池是否已经被启动过,默认值为false。在每次调用这个函数时都会先去检查这个标记,从而确保Binder线程池只会被启动一次。
注释1如果Binder线程池未被启动则设置mThreadPoolStarted为true,最后调用spawnPooledThread函数来创建线程池中的第一个线程,也就是线程池的main线程。
ProcessState#spawnPooledThread()
void ProcessState::spawnPooledThread(bool isMain)
{
if (mThreadPoolStarted) {
String8 name = makeBinderThreadName();
ALOGV("Spawning new pooled thread, name=%s\n", name.string());
sp t = new PoolThread(isMain);
t->run(name.string());//1
}
}
可以看到Binder线程为一个PoolThread。注释1调用PoolThread的run函数来启动一个新的线程。
PoolThread
class PoolThread : public Thread
{
..
protected:
virtual bool threadLoop()
{
IPCThreadState::self()->joinThreadPool(mIsMain);//1
return false;
}
const bool mIsMain;
};
PoolThread类继承了Thread类。注释1处会将调用IPCThreadState的joinThreadPool函数,将当前线程注册到Binder驱动程序中,这样我们创建的线程就加入了Binder线程池中,这样新创建的SyetemServer进程就支持Binder进程间通信了。
回到RuntimeInit#zygoteInit()的注释2。
RuntimeInit#applicationInit()
private static void applicationInit(int targetSdkVersion, String[] argv, ClassLoader classLoader)
throws ZygoteInit.MethodAndArgsCaller {
...
// 初始化虚拟机环境
VMRuntime.getRuntime().setTargetHeapUtilization(0.75f);
VMRuntime.getRuntime().setTargetSdkVersion(targetSdkVersion);
final Arguments args;
try {
args = new Arguments(argv);
} catch (IllegalArgumentException ex) {
Slog.e(TAG, ex.getMessage());
// let the process exit
return;
}
// Remaining arguments are passed to the start class's static main
invokeStaticMain(args.startClass, args.startArgs, classLoader); //1
}
注释1处applicationInit函数中主要调用了invokeStaticMain函数。
RuntimeInit#invokeStaticMain()
private static void invokeStaticMain(String className, String[] argv, ClassLoader classLoader) throws ZygoteInit.MethodAndArgsCaller {
Class cl;
try {
cl = Class.forName(className, true, classLoader); //1
} catch (ClassNotFoundException ex) {
throw new RuntimeException("Missing class when invoking static main " + className, ex);
}
Method m;
try {
// 获取main方法
m = cl.getMethod("main", new Class[] { String[].class }); //2
} catch (NoSuchMethodException ex) {
throw new RuntimeException("Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException("Problem getting static main on " + className, ex);
}
// 判断修饰符
int modifiers = m.getModifiers(); //3
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException("Main method is not public and static on " + className);
}
/*
* This throw gets caught in ZygoteInit.main(), which responds
* by invoking the exception's run() method. This arrangement
* clears up all the stack frames that were required in setting
* up the process.
*/
throw new ZygoteInit.MethodAndArgsCaller(m, argv); //4
}
注释1处传入的className就是com.android.server.SystemServer ,因此通过反射返回的cl为SystemServer类。
注释2获取main方法 。
注释3判断修饰符,必须是static而且必须是public类型。
注释4有点意思,做完这一切之后,将找到的main函数传入到MethodAndArgsCaller异常中并抛出该异常。辛苦辛苦各种初始化,各种变着法的调用,最后你居然给我抛个异常!先别急,这个异常在ZygoteInit#main()方法中捕获。这么做的作用是清除应用程序进程创建过程的调用栈。
ZygoteInit#main()函数见上一篇文章,我们这里再看一下框架:
public static void main(String argv[]) {
try {
...
startSystemServer(abiList, socketName);
...
} catch (MethodAndArgsCaller caller) {
caller.run(); //1
}
}
在注释1处调用了MethodAndArgsCaller的run函数。
MethodAndArgsCaller
public static class MethodAndArgsCaller extends Exception
implements Runnable {
/** method to call */
private final Method mMethod;
/** argument array */
private final String[] mArgs;
public MethodAndArgsCaller(Method method, String[] args) {
mMethod = method;
mArgs = args;
}
public void run() {
try {
mMethod.invoke(null, new Object[] { mArgs }); //1
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}
注释1处通过反射调用了com.android.server.SystemServer#main(String[] args)。至此,Zygote进程fork出SystemServer进程,并成功调用SystemServer#main()。
解析SyetemServer进程
我们先来查看SystemServer的main函数:
frameworks/base/services/java/com/android/server/SystemServer.java
SystemServer#main()
public static void main(String[] args) {
new SystemServer().run(); //1
}
注释1处main函数中只调用了SystemServer的run函数。
SystemServer#run()
private void run() {
...
System.loadLibrary("android_servers");//1
...
mSystemServiceManager = new SystemServiceManager(mSystemContext);//2
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
...
try {
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartServices");
startBootstrapServices();//3
startCoreServices();//4
startOtherServices();//5
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
...
}
注释1处加载了libandroid_servers.so。
注释2处创建SystemServiceManager,它会对系统的服务进行创建、启动和生命周期管理。启动系统的各种服务。
注释3中的startBootstrapServices函数中用SystemServiceManager启动了ActivityManagerService、PowerManagerService、PackageManagerService等服务。
注释4处的函数中则启动了BatteryService、UsageStatsService和WebViewUpdateService。
注释5处的startOtherServices函数中则启动了CameraService、AlarmManagerService、VrManagerService等服务,这些服务的父类为SystemService。
从注释3、4、5的函数可以看出,官方把系统服务分为了三种类型,分别是引导服务、核心服务和其他服务,其中其他服务为一些非紧要和一些不需要立即启动的服务。系统服务大约有80多个,这里列出部分系统服务以及它们的作用:
image
image
image
继续往下看,我们这里只看看注释3如何启动引导服务,注释4、5类似。
SystemServer#startBootstrapServices()
private void startBootstrapServices() {
...
mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
...
mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
...
}
这里面要启动的引导服务很多,我们来看看PowerManagerService和PackageManagerService吧。
首先是PowerManagerService,它是通过SystemServiceManager.startService()来启动的:
frameworks/base/services/core/java/com/android/server/SystemServiceManager.java
SystemServiceManager#startService()
public T startService(Class serviceClass) {
try {
final String name = serviceClass.getName();
...
final T service;
try {
Constructor constructor = serviceClass.getConstructor(Context.class);
service = constructor.newInstance(mContext); //1
} catch (InstantiationException ex) {
throw new RuntimeException("Failed to create service " + name + ": service could not be instantiated", ex);
} catch (IllegalAccessException ex) {
throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex);
} catch (NoSuchMethodException ex) {
throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex);
} catch (InvocationTargetException ex) {
throw new RuntimeException("Failed to create service " + name + ": service constructor threw an exception", ex);
}
// Register it.
mServices.add(service); //2
// Start it.
try {
service.onStart(); //3
} catch (RuntimeException ex) {
throw new RuntimeException("Failed to start service " + name + ": onStart threw an exception", ex);
}
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
注释1处通过构造器创建SystemService,这里的SystemService就是PowerManagerService。
注释2处将PowerManagerService添加到mServices中,这里mServices是一个存储SystemService类型的ArrayList。
注释3处接着调用PowerManagerService的onStart函数启动PowerManagerService并返回,这样就完成了PowerManagerService启动的过程。
接下来再来看看PackageManagerService,它是通过另外一种方式启动的,直接调用了PackageManagerService的main函数:
frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java
PackageManagerService#main()
public static PackageManagerService main(Context context, Installer installer, boolean factoryTest, boolean onlyCore) {
// Self-check for initial settings.
PackageManagerServiceCompilerMapping.checkProperties();
PackageManagerService m = new PackageManagerService(context, installer, factoryTest, onlyCore); //1
m.enableSystemUserPackages();
// Disable any carrier apps. We do this very early in boot to prevent the apps from being
// disabled after already being started.
CarrierAppUtils.disableCarrierAppsUntilPrivileged(context.getOpPackageName(), m, UserHandle.USER_SYSTEM);
ServiceManager.addService("package", m); //2
return m;
}
注释1创建PackageManagerService实例。
注释2将PackageManagerService实例注册到ServiceManager中。ServiceManager用来管理系统中的各种Service,用系统C/S架构中的Binder机制通信,Client端要使用某个Service,则需要先到ServiceManager查询Service的相关信息,然后根据Service的相关信息与Service所在的Server进程建立通讯通路,这样Client端就可以使用Service了。
SyetemServer在启动时做了如下工作:
1.启动Binder线程池,这样就可以与其他进程进行通信。
2.创建SystemServiceManager用于对系统的服务进行创建、启动和生命周期管理。
3.启动各种系统服务。
作者:码农小陈
链接:https://www.jianshu.com/p/e1d0ee1b8134
来源:简书
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