本文是笔记博客，主要参考了https://blog.csdn.net/u012124438/article/details/76382894
的内容，感觉很有意思，记录一下
Service的启动过程
Service是四大组件之一，主要运行在后台，其生命周期受AMS的控制，
下面主要是讲解binderService的启动过程。
binderService最终在ContextImpl中的binderService中进行的，binderService直接调用了bindServiceCommon，在这个方法中调用了ActivityManagerNative借助AMS进而完成Service的绑定过程。这里需要提及一下sd变量，这个变量的类型是IServiceConnection，这个IServiceConnection与IApplicationThread是相同的，都是ActivityThread给AMS提供的用来与之进行通信的Binder对象，在ActivityManagerNative的bindService方法中调用了ActivityServices类的bindServiceLocked方法。
bindServiceLocked方法
这是在AMS的一个方法主要是完成三件事：
1.根据intent找到要启动的service
2.对IServiceConnection使用ConnectionRecord进行了包装，方便接下来使用
3.调用bringUpServiceLocked开始创建Service。
bringUpServiceLocked是是启动service的关键方法
bringUpServiceLocked方法
如果Service所在的进程已经启动，那么直接调用realStartServiceLocked方法来真正启动Service组件；如果Service所在的进程还没有启动，那么先在AMS中记下这个要启动的Service组件，然后通过startProcessLocked启动新的进程。只看第一种情况
如果service所在的进程存在了就利用IApplicationThread进行跨进程通信调用app的scheduleCreateService方法，这里发送一个消息给ActivityThread的H这个Handler，H类收到这个消息之后，直接调用了ActivityThread类的handleCreateService方法，在这里进行classloader加载service:
1.使用ClassLoader加载并通过反射创建Service 对象
2.创建ContextImpl ，并绑定Service
3.makeApplication创建Application
4.service.attach方法将创建的Application和ContextImpl 和service进行绑定
5.service.onCreate()方法进行service的创建
代码如下：
private void handleCreateService(CreateServiceData data) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
LoadedApk packageInfo = getPackageInfoNoCheck(
data.info.applicationInfo, data.compatInfo);
Service service = null;
try {
java.lang.ClassLoader cl = packageInfo.getClassLoader();
service = (Service) cl.loadClass(data.info.name).newInstance();
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to instantiate service " + data.info.name
+ ": " + e.toString(), e);
}
}
try {
if (localLOGV) Slog.v(TAG, "Creating service " + data.info.name);
ContextImpl context = ContextImpl.createAppContext(this, packageInfo);
context.setOuterContext(service);
Application app = packageInfo.makeApplication(false, mInstrumentation);
service.attach(context, this, data.info.name, data.token, app,
ActivityManagerNative.getDefault());
service.onCreate();
mServices.put(data.token, service);
try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (RemoteException e) {
// nothing to do.
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}
onBinder方法的启动
scheduleCreateService这个binder调用之后，执行了一个requestServiceBindingsLocked方法又通过IApplicationThread这个Binder进行了一次IPC调用，我们跟踪ActivityThread类里面的ApplicationThread的scheduleBindService方法，发现这个方法不过通过Handler转发了一次消息，真正的处理代码在handleBindService里面，在这个方法中会调用onBinder方法，
onServiceConnected方法
要Bind的Service在这里完成了绑定，绑定之后又通过ActivityManagerNative这个Binder进行一次IPC调用，我们查看AMS的publishService方法，这个方法又调用了publishServiceLocked方法
void publishServiceLocked(ServiceRecord r, Intent intent, IBinder service) {
final long origId = Binder.clearCallingIdentity();
try {
if (DEBUG_SERVICE) Slog.v(TAG, "PUBLISHING " + r
+ " " + intent + ": " + service);
if (r != null) {
Intent.FilterComparison filter
= new Intent.FilterComparison(intent);
IntentBindRecord b = r.bindings.get(filter);
if (b != null && !b.received) {
b.binder = service;
b.requested = true;
b.received = true;
for (int conni=r.connections.size()-1; conni>=0; conni--) {
ArrayList<ConnectionRecord> clist = r.connections.valueAt(conni);
for (int i=0; i<clist.size(); i++) {
ConnectionRecord c = clist.get(i);
if (!filter.equals(c.binding.intent.intent)) {
if (DEBUG_SERVICE) Slog.v(
TAG, "Not publishing to: " + c);
if (DEBUG_SERVICE) Slog.v(
TAG, "Bound intent: " + c.binding.intent.intent);
if (DEBUG_SERVICE) Slog.v(
TAG, "Published intent: " + intent);
continue;
}
if (DEBUG_SERVICE) Slog.v(TAG, "Publishing to: " + c);
try {
c.conn.connected(r.name, service);
} catch (Exception e) {
Slog.w(TAG, "Failure sending service " + r.name +
" to connection " + c.conn.asBinder() +
" (in " + c.binding.client.processName + ")", e);
}
}
}
}
serviceDoneExecutingLocked(r, mDestroyingServices.contains(r), false);
}
} finally {
Binder.restoreCallingIdentity(origId);
}
}这里终于到IServiceConnection的用处了，在bindServiceLocked方法里面，这个IServiceConnection放到了一个ConnectionRecord的List中存放在ServiceRecord里面，这里所做的就是取出已经被Bind的这个Service对应的IServiceConnection对象，然后调用它的connected方法；我们说过，这个IServiceConnection也是一个Binder对象，它的Server端在LoadedApk.ServiceDispatcher里面。接下来是LoadedApk.ServiceDispatcher的connected方法。在这个方法中最重会调用doConnected方法：
public void doConnected(ComponentName name, IBinder service) {
ServiceDispatcher.ConnectionInfo old;
ServiceDispatcher.ConnectionInfo info;
synchronized (this) {
if (mForgotten) {
// We unbound before receiving the connection; ignore
// any connection received.
return;
}
old = mActiveConnections.get(name);
if (old != null && old.binder == service) {
// Huh, already have this one. Oh well!
return;
}
if (service != null) {
// A new service is being connected... set it all up.
mDied = false;
info = new ConnectionInfo();
info.binder = service;
info.deathMonitor = new DeathMonitor(name, service);
try {
service.linkToDeath(info.deathMonitor, 0);
mActiveConnections.put(name, info);
} catch (RemoteException e) {
// This service was dead before we got it... just
// don't do anything with it.
mActiveConnections.remove(name);
return;
}
} else {
// The named service is being disconnected... clean up.
mActiveConnections.remove(name);
}
if (old != null) {
old.binder.unlinkToDeath(old.deathMonitor, 0);
}
}
// If there was an old service, it is not disconnected.
if (old != null) {
mConnection.onServiceDisconnected(name);
}
// If there is a new service, it is now connected.
if (service != null) {
mConnection.onServiceConnected(name, service);
}
}
这样service的启动过程就分析完了，可以看到，service的每一个周期方法都是在AMS发起的。可以知道，AMS是在管理service的生命周期。