前言：
当我们在一个Activity里面startService的时候，具体的执行逻辑是怎么样的？需要我们一步步根据源码阅读。
在阅读源码的时候，要关注思路，不要陷在具体的实现细节中，一步步整理代码的思路。
注：源码使用Api 25的
1.起点：Activity-> startService
在Activity中没有找到startService的方法，
查看Activity的继承关系：
Context->ContextWrapper->ContextThemeWrapper->Activity
回溯到ContextWrapper中，找到了startService()的方法:
@Override
public ComponentName startService(Intent service) {
return mBase.startService(service);
}从ContextWrapper的命名和方法的实现上，我们可以猜到其实ContextWrapper只是一个包装
真正实现startService逻辑的是调用mBase.startService(service);
其中
Context mBase;是一个成员变量
由于Context是一个抽象类，startService是它的抽象方法，所以一定存在一个Context的子类实现了Context的抽象方法。
其实就是ContextImpl类，所以Context的抽象方法的具体实现由ContextImpl负责，其他Context的子类只需要在自己的方法里面调用
ContextImpl的即可。
2.ContextImpl ->startService(Intent service)
@Override
public ComponentName startService(Intent service) {
warnIfCallingFromSystemProcess();
return startServiceCommon(service, mUser);
}调用到startServiceCommon(service, mUser)
private ComponentName startServiceCommon(Intent service, UserHandle user) {
try {
validateServiceIntent(service);
service.prepareToLeaveProcess(this);
ComponentName cn = ActivityManagerNative.getDefault().startService(
mMainThread.getApplicationThread(), service, service.resolveTypeIfNeeded(
getContentResolver()), getOpPackageName(), user.getIdentifier());
//省略部分代码
}
return cn;
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}可以看到是调用ActivityManagerNative的 ActivityManagerNative.getDefault().startService方法
3.ActivityManagerNative-> ActivityManagerNative.getDefault().startService
观察public abstract class ActivityManagerNative extends Binder implements IActivityManager的定义
可以猜测这是一个可以Binder进程IPC操作的类。
PS：如果你对Binder还很陌生推荐你先了解Binder，不需要深入到底层实现，不过Java层的基本用法需要了解
下面的博客都可以参考:
weishu的博客
Gityuan的博客
引用一个对Binder很精彩的描述：
Binder使得一个进程A有一个真身，它实现了具体的逻辑，然后其他进程获得的是Binder的影子(或者说是引用？句柄？)
然后其他进程通过Binder的影子调用就仿佛调用了A的真身的操作。
其实很多资料都告诉我们启动四大组件是IPC的操作，app进程必须通过IPC与另外的进程通信，完成四大组件的启动。
结合上面的描述，我们知道，那么app进程的是影子。我们找到它：
在ActivityManagerNative它有一个子类：
class ActivityManagerProxy implements IActivityManager{
//省略代码
public ComponentName startService(IApplicationThread caller, Intent service,
String resolvedType, String callingPackage, int userId) throws RemoteException
{
Parcel data = Parcel.obtain();//写入参数值
Parcel reply = Parcel.obtain();//保持返回值
data.writeInterfaceToken(IActivityManager.descriptor);
data.writeStrongBinder(caller != null ? caller.asBinder() : null);
service.writeToParcel(data, 0);
data.writeString(resolvedType);
data.writeString(callingPackage);
data.writeInt(userId);
mRemote.transact(START_SERVICE_TRANSACTION, data, reply, 0);//挂起，通过Binder驱动IPC
reply.readException();
ComponentName res = ComponentName.readFromParcel(reply);
data.recycle();
reply.recycle();
return res;
}
//省略代码
}这就是影子，我们在App进程调用的startService最终会到达这里，然后经过Binder驱动的一系列操作到达另外一个进程的真身
所以：ActivityManagerNative.getDefault()这里获取的应该就是ActivityManagerProxy 的实例，我们看一下它的实现：
static public IActivityManager getDefault() {
return gDefault.get();
}
….
** gDefault **这是一个单例对象
private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");//注意这里，通过**ServiceManager**获取的字符串"activity"的IBinder
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
};
}
我们可以看到最终获取的是：IActivityManager am = asInterface(b);
asInterface的代码：
/**
* Cast a Binder object into an activity manager interface, generating
* a proxy if needed.
*/
static public IActivityManager asInterface(IBinder obj) {
if (obj == null) {
return null;
}
//如果是不是IPC调用执行下面的代码：
IActivityManager in =
(IActivityManager)obj.queryLocalInterface(descriptor);
if (in != null) {
return in;
}
//如果是IPC调用执行下面的代码：
return new ActivityManagerProxy(obj);
}可以看到我们通过：return new ActivityManagerProxy(obj);
获取到了影子的实例。
一句话总结：ActivityManagerNative.getDefault().startService()
最终调用的是ActivityManagerProxy 的startService()方法
这里将有一个IPC的逻辑：
mRemote.transact(START_SERVICE_TRANSACTION, data, reply, 0);
经过Binder驱动的各种操作
最终是运行到ActivityManagerNative的
onTransact方法里面的。
ActivityManagerNative->onTransact()
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
switch (code) {
//省略代码
case START_SERVICE_TRANSACTION: {
data.enforceInterface(IActivityManager.descriptor);
IBinder b = data.readStrongBinder();
IApplicationThread app = ApplicationThreadNative.asInterface(b);
Intent service = Intent.CREATOR.createFromParcel(data);
String resolvedType = data.readString();
String callingPackage = data.readString();
int userId = data.readInt();
ComponentName cn = startService(app, service, resolvedType, callingPackage, userId);
reply.writeNoException();
ComponentName.writeToParcel(cn, reply);
return true;
}
//省略代码
}
最终调用的是：
ComponentName cn = startService(app, service, resolvedType, callingPackage, userId);（注意这里已经不是运行在App的进程的，而是在system_Server进程中，system_Server是由Zygote进程孵化出来的。）
在ActivityManagerNative中找不到 startService(app, service, resolvedType, callingPackage, userId);这个方法，由于ActivityManagerNative是一个抽象类，自然想到方法的实现在子类中，其实就是ActivityManagerService
4.ActivityManagerService->startService()
@Override
public ComponentName startService(IApplicationThread caller, Intent service,
String resolvedType, String callingPackage, int userId)
throws TransactionTooLargeException {
enforceNotIsolatedCaller("startService");
// Refuse possible leaked file descriptors
if (service != null && service.hasFileDescriptors() == true) {
throw new IllegalArgumentException("File descriptors passed in Intent");
}
if (callingPackage == null) {
throw new IllegalArgumentException("callingPackage cannot be null");
}
if (DEBUG_SERVICE) Slog.v(TAG_SERVICE,
"startService: " + service + " type=" + resolvedType);
synchronized(this) {
final int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
ComponentName res = mServices.startServiceLocked(caller, service,
resolvedType, callingPid, callingUid, callingPackage, userId);
Binder.restoreCallingIdentity(origId);
return res;
}
}调用的是mServices.startServiceLocked()方法
mService是：
final ActiveServices mServices;
5.ActiveServices ->startServiceLocked
最终调用的是
ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ServiceRecord r,
boolean callerFg, boolean addToStarting) throws TransactionTooLargeException {
ServiceState stracker = r.getTracker();
if (stracker != null) {
stracker.setStarted(true, mAm.mProcessStats.getMemFactorLocked(), r.lastActivity);
}
r.callStart = false;
synchronized (r.stats.getBatteryStats()) {
r.stats.startRunningLocked();
}
//调用这个方法：
String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
if (error != null) {
return new ComponentName("!!", error);
}
if (r.startRequested && addToStarting) {
boolean first = smap.mStartingBackground.size() == 0;
smap.mStartingBackground.add(r);
r.startingBgTimeout = SystemClock.uptimeMillis() + BG_START_TIMEOUT;
if (DEBUG_DELAYED_SERVICE) {
RuntimeException here = new RuntimeException("here");
here.fillInStackTrace();
Slog.v(TAG_SERVICE, "Starting background (first=" + first + "): " + r, here);
} else if (DEBUG_DELAYED_STARTS) {
Slog.v(TAG_SERVICE, "Starting background (first=" + first + "): " + r);
}
if (first) {
smap.rescheduleDelayedStarts();
}
} else if (callerFg) {
smap.ensureNotStartingBackground(r);
}
return r.name;
}startServiceInnerLocked()调用的是 String error = bringUpServiceLocked(r, service.getFlags(), callerFg, false, false);
bringUpServiceLocked(r, service.getFlags(), callerFg, false, false)方法：
private String bringUpServiceLocked(ServiceRecord r, int intentFlags, boolean execInFg,
boolean whileRestarting, boolean permissionsReviewRequired)
throws TransactionTooLargeException {
//省略代码
if (!isolated) {
//注意这里获取的需要启动Service的进程的信息
app = mAm.getProcessRecordLocked(procName, r.appInfo.uid, false);
if (DEBUG_MU) Slog.v(TAG_MU, "bringUpServiceLocked: appInfo.uid=" + r.appInfo.uid
+ " app=" + app);
if (app != null && app.thread != null) {
try {
app.addPackage(r.appInfo.packageName, r.appInfo.versionCode, mAm.mProcessStats);
//调用到这里：
realStartServiceLocked(r, app, execInFg);
return null;
} catch (TransactionTooLargeException e) {
throw e;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting service " + r.shortName, e);
}
// If a dead object exception was thrown -- fall through to
// restart the application.
}
} else {
// If this service runs in an isolated process, then each time
// we call startProcessLocked() we will get a new isolated
// process, starting another process if we are currently waiting
// for a previous process to come up. To deal with this, we store
// in the service any current isolated process it is running in or
// waiting to have come up.
app = r.isolatedProc;
}
//省略代码
}
调用到了realStartServiceLocked()
private final void realStartServiceLocked(ServiceRecord r,ProcessRecord app, boolean execInFg) throws RemoteException {
//省略代码
这里就回到的app的进程
app.thread.scheduleCreateService(r,r.serviceInfo,mAm.compatibilityInfoForPackageLocked(r.serviceInfo.applicationInfo),app.repProcState);
//省略代码
}最终调用ActivityThread里面的ApplicationThread->scheduleCreateService()
public final void scheduleCreateService(IBinder token,
ServiceInfo info, CompatibilityInfo compatInfo, int processState) {
updateProcessState(processState, false);
CreateServiceData s = new CreateServiceData();
s.token = token;
s.info = info;
s.compatInfo = compatInfo;
sendMessage(H.CREATE_SERVICE, s);
}6.由ActivityThread的内部的Handler处理 sendMessage(H.CREATE_SERVICE, s);发送的消息
private class H extends Handler {
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
//略
case CREATE_SERVICE:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, ("serviceCreate: " + String.valueOf(msg.obj)));
handleCreateService((CreateServiceData)msg.obj);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
//略
}
}
最终是通过反射创建一个新的Service，然后调用它的onCreate()方法
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();//反射创建一个新的Service
} 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();//调用Service的onCreate()方法
mServices.put(data.token, service);
try {
ActivityManagerNative.getDefault().serviceDoneExecuting(
data.token, SERVICE_DONE_EXECUTING_ANON, 0, 0);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
} catch (Exception e) {
if (!mInstrumentation.onException(service, e)) {
throw new RuntimeException(
"Unable to create service " + data.info.name
+ ": " + e.toString(), e);
}
}
}
总结：
我们可以看到其实启动一个Service的过程也是挺复杂的，由于能力所限，仅跟踪了Java Framework这一层的代码，深入到底层的就不分析了。可以参考Gityuan大神的博客。