本系列主要介紹Android8.0系統啟動過程中涉及到的init、Zygote、SystemServer和Launcher。
SystemServer 程序主要用于建立系統服務,像AMS、WMS和PMS等,都是由其建立。
一、啟動SyetmServer程序
在Android 8.0系統啟動流程_Zygote(二)中講解到在ZygoteInit中主要作用是啟動SystemServer程序,源碼如下:
frameworks\base\core\java\com\android\internal\os\ZygoteInit.java
public static void main(String argv[]) {
...
if (startSystemServer) {
Runnable r = forkSystemServer(abiList, socketName, 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;
}
}
...
private static Runnable forkSystemServer(String abiList, String socketName,
ZygoteServer zygoteServer) {
...
if (pid == 0) {
if (hasSecondZygote(abiList)) {
waitForSecondaryZygote(socketName);
}
zygoteServer.closeServerSocket();
return handleSystemServerProcess(parsedArgs);
}
return null;
}
}
由于SystemServer是複制Zygote的程序,是以也會包含Zygote的socket,該socket是服務端socket,對于SystemServer沒有其他作用,需要先将其關閉;通過handleSystemServerProcess開啟SystemServer程序。
frameworks\base\core\java\com\android\internal\os\ZygoteInit.java
private static Runnable handleSystemServerProcess(ZygoteConnection.Arguments parsedArgs) {
...
ClassLoader cl = null;
if (systemServerClasspath != null) {
//建立PathClassLoader
cl = createPathClassLoader(systemServerClasspath, parsedArgs.targetSdkVersion);
Thread.currentThread().setContextClassLoader(cl);
}
//初始化zygoteInit
return ZygoteInit.zygoteInit(parsedArgs.targetSdkVersion, parsedArgs.remainingArgs, cl);
...
}
public static final Runnable zygoteInit(int targetSdkVersion, String[] argv, ClassLoader classLoader) {
...
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ZygoteInit");
RuntimeInit.redirectLogStreams();
RuntimeInit.commonInit();
//啟動Binder線程池
ZygoteInit.nativeZygoteInit();
//執行SystemServer的main方法
return RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
}
啟動Binder線程池是便于SystemServer與其他程序執行通信操作;
在調用SystemServer的main方法,相對比較複雜,之前版本是通過RuntimeInit通過抛出MethodAndArgsCaller方式跳轉至ZygoteInit的main方法中,但在8.0中發現是通過建立MethodAndArgsCaller方式,但最終是執行至MethodAndArgsCaller的invoke中,實作SystemServer運作。流程如下:
frameworks\base\core\java\com\android\internal\os\RuntimeInit.java
protected static Runnable applicationInit(int targetSdkVersion, String[] argv,
ClassLoader classLoader) {
...
return findStaticMain(args.startClass, args.startArgs, classLoader);
}
private static Runnable findStaticMain(String className, String[] argv,
ClassLoader classLoader) {
try {
//通過反射,擷取SystemServer類,
cl = Class.forName(className, true, classLoader);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
//通過反射,擷取SystemServer類的main方法
m = cl.getMethod("main", new Class[] { String[].class });
} 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);
}
...
/*
* 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.
*/
//本版本中,是直接調制至RuntimeInit的靜态内部類中。
return new MethodAndArgsCaller(m, argv);
}
在通過反射擷取SystemServer類時,是如何确定該傳入的className就是SystemServer?這是由于在上面的ZygoteInit的mian方法時,對于啟動的程序做了判斷(Zyogte、SystemServer還是application?),上面部分已明确描述加載的是SystemServer類。
cl = createPathClassLoader(systemServerClasspath, parsedArgs.targetSdkVersion);
static class MethodAndArgsCaller implements Runnable {
...
public void run() {
try {
mMethod.invoke(null, new Object[] { mArgs });
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
...
}
以上過程完成了從Zygote經過RuntimeInit最後完成SystemServer的main方法的運作。下面重點分析SystemServer的main方法。
二、解析SystemServer的main方法
frameworks\base\services\java\com\android\server\SystemServer.java
public static void main(String[] args) {
new SystemServer().run();
}
private void run() {
try {
//建立Looper對象
Looper.prepareMainLooper();
// 加載系統聲明周期管理的servers的庫
System.loadLibrary("android_servers");
performPendingShutdown();
//建立系統的Context
createSystemContext()
mSystemServiceManager = new SystemServiceManager(mSystemContext);
mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
SystemServerInitThreadPool.get();
} finally {
traceEnd(); // InitBeforeStartServices
}
try {
traceBeginAndSlog("StartServices");
//啟動引導服務
startBootstrapServices();
//啟動核心服務
startCoreServices();
//啟動其他服務
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
throw ex;
} finally {
traceEnd();
}
}
該過程主要的作用是啟動三種服務(引導、核心和其他服務),分别是指:
- 引導:AMS(ActivityManagerService)、PMS(PackageManagerService)和UMS(UserManagerService)等;
- 核心:BatterService、UsageStateService和WebViewService等;
- 其他: CameraService、InputManagerService和AudioService等。
這些系統服務均來自于ServeryService,但是需要注意在應用層建立的Service不直接屬于ServeryService,因為其不是系統服務,其是通過AMS來 管理控制的。