代码位置：frameworks/base/services/core/java/com/android/server/SystemServiceManager.java
顾名思义，SystemServiceManager是管理SystemService的，这么说还不够严谨，应该说是管理继承了SystemService的类的，这个从注释中可以看到。
/**
* Manages creating, starting, and other lifecycle events of
* {@link com.android.server.SystemService system services}.
*
* {@hide}
*/
这个类在SystemServer中被大量用到，主要用于开机时的各种系统服务的初始化和配置工作。
接下来看下具体实现：
private static final String TAG = "SystemServiceManager";
// 这个是操作service的一个延时提醒，如果对一个service的某个操作超过了这个值，Log中就会有日志
// 输出，这个一会看warnIfTooLong函数时会看到
private static final int SERVICE_CALL_WARN_TIME_MS = 50;
private final Context mContext;
private boolean mSafeMode;
private boolean mRuntimeRestarted;
// 管理的所有继承SystemService的类的集合
// Services that should receive lifecycle events.
private final ArrayList mServices = new ArrayList();
private int mCurrentPhase = -1;
SystemServiceManager(Context context) {
mContext = context;
}
其中最重要的一个函数为startService，这里传入的是一个String类型的className，然后通过反射的方法得到具体的类（为什么要用反射呢？稍后有说明）
/**
* Starts a service by class name.
*
* @return The service instance.
*/
@SuppressWarnings("unchecked")
public SystemService startService(String className) {
final Class serviceClass;
try {
serviceClass = (Class)Class.forName(className);
} catch (ClassNotFoundException ex) {
Slog.i(TAG, "Starting " + className);
// 注意这里的异常提示信息，前面我们抛出一个疑问，为什么startService要用反射
// 构造Service呢？从这里的异常信息我们可以看出，如果当前设备有些featrue不支
// 持的话，对应的Service类很可能不存在，如果直接引用的话编译就会报错。这里
// 用反射的话，就可以避免因为编译报错而去修改代码，这样适配性也高。
throw new RuntimeException("Failed to create service " + className
+ ": service class not found, usually indicates that the caller should "
+ "have called PackageManager.hasSystemFeature() to check whether the "
+ "feature is available on this device before trying to start the "
+ "services that implement it", ex);
}
// 调用泛型函数
return startService(serviceClass);
}
/**
* 下面这句话说参数必须是SystemService类的子类
* Creates and starts a system service. The class must be a subclass of
* {@link com.android.server.SystemService}.
*
* @param serviceClass A Java class that implements the SystemService interface.
* @return The service instance, never null.
* @throws RuntimeException if the service fails to start.
*/
@SuppressWarnings("unchecked")
public T startService(Class serviceClass) {
try {
final String name = serviceClass.getName();
Slog.i(TAG, "Starting " + name);
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);
// Create the service.
// isAssignableFrom函数的作用是判断是否为子类
if (!SystemService.class.isAssignableFrom(serviceClass)) {
throw new RuntimeException("Failed to create " + name
+ ": service must extend " + SystemService.class.getName());
}
final T service;
try {
// 通过反射构造类的实例
Constructor constructor = serviceClass.getConstructor(Context.class);
service = constructor.newInstance(mContext);
} 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);
}
// 这里就是真正执行启动service了
startService(service);
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
public void startService(@NonNull final SystemService service) {
// Register it.
// 将要启动的service加到mService列表中
mServices.add(service);
// Start it.
long time = System.currentTimeMillis(); // 记录开始操作的时间
try {
service.onStart(); // 调用要启动的service的 onStart方法
} catch (RuntimeException ex) {
throw new RuntimeException("Failed to start service " + service.getClass().getName()
+ ": onStart threw an exception", ex);
}
// 记录上述操作执行的时间，如果超时，会有warning提示
warnIfTooLong(System.currentTimeMillis() - time, service, "onStart");
}
// 可以看到，如果对Service的某个操作超过了SERVICE_CALL_WARN_TIME_MS（50ms)，Log中就会有warning提示
private void warnIfTooLong(long duration, SystemService service, String operation) {
if (duration > SERVICE_CALL_WARN_TIME_MS) {
Slog.w(TAG, "Service " + service.getClass().getName() + " took " + duration + " ms in "
+ operation);
}
}
到此，SystemServiceManager的主要内容就介绍完了，还有剩下的一些对Service的操作都是类似的，循环遍历mService列表并回调相应的函数，比如：
/**
* Starts the specified boot phase for all system services that have been started up to
* this point.
*
* @param phase The boot phase to start.
*/
public void startBootPhase(final int phase) {
if (phase <= mCurrentPhase) {
throw new IllegalArgumentException("Next phase must be larger than previous");
}
mCurrentPhase = phase;
Slog.i(TAG, "Starting phase " + mCurrentPhase);
try {
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "OnBootPhase " + phase);
final int serviceLen = mServices.size();
// 遍历mService并调用service的onBootPhase函数
for (int i = 0; i < serviceLen; i++) {
final SystemService service = mServices.get(i);
long time = System.currentTimeMillis();
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, service.getClass().getName());
try {
service.onBootPhase(mCurrentPhase);
} catch (Exception ex) {
throw new RuntimeException("Failed to boot service "
+ service.getClass().getName()
+ ": onBootPhase threw an exception during phase "
+ mCurrentPhase, ex);
}
// 如果操作延时则打印warning日志
warnIfTooLong(System.currentTimeMillis() - time, service, "onBootPhase");
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}