add Service 启动原理

blogs/Android/Framework/Interview/其他应用组件相关/Service 的启动原理.md

@@ -0,0 +1,136 @@
+---
+Service 的启动原理
+---
+
+1. Service 启动有哪几种方式？
+2. Service 启动过程中主要流程有哪些？
+3. Service 启动过程涉及哪些参与者，通信过程是怎样的？
+
+```java
+@Override
+public ComponentName startService(Intent service) {
+	return startServiceCommon(service, mUser);
+}
+```
+
+```java
+ComponentName startServiceCommon(Intent service, ...) {
+	ComponentName cn = ActivityManagerNative.getDefault().startService(mMainThread.getApplicationThread(), service, ...);
+	return cn;
+}
+```
+
+```java
+// ASM#startService
+ComponentName startService(IApplicationThread caller, Intent service, ...) {
+    ComponentName res = mService.startServiceLocked(caller, service, ...);
+    return res;
+}
+ComponentName startServiceLocked(Intent service, ...){
+    ServiceLookupResult res = retrieveServiceLocked(service, ...);
+    ServiceRecord r = res.record;
+    //...
+    r.pendingStarts.add(new ServiceRecord.StartItem(r, ...));
+    return startServiceInnerLocked(smap, service, r, ...);
+}
+ComponentName startServiceInnerLocked(ServiceMap smap, Intent service, ...){
+    bringUpServiceLocked(r, service.getFlags(), callerFg, false);
+}
+final String bringUpServiceLocked(ServiceRecord r, ...) {
+    if(r.app != null && r.app.thread != null){
+        // 如果 Service 已经启动，就执行 Service#onStartCommand
+        sendServiceArgsLocked(r, ...);
+        return null;
+    }
+    ProcessRecord app = mAm.getProcessRecordLocked(procName, ...);
+    if(app != null && app.thread != null) {
+        realStartServiceLocked(r, app, execlnFg);
+        return null;
+    }
+    if(app == null){
+        app = mAm.startProcessLocked(procName, ...);
+    }
+    if(!mPendingService.contains(r)){
+        mPendingService.add(r);
+    }
+}
+
+```
+
+```java
+//ASM#attachApplicationLocked
+boolean attachApplicationLocked(IApplicationThread thread, ...) {
+	// 处理 Pending 的 Service
+    mService.attachApplicationLocked(app, processName);
+    return true;
+}
+boolean attachApplicationLocked(ProcessRecord proc, ...) {
+    for(int i=0;i<mPendingService.size();i++) {
+        sr = mPendingService.get(i);
+        //...
+        mPendingServices.remove(i--);
+        realStartServiceLocked(sr, proc, ...);
+    }
+}
+void realStartServiceLocked(ServiceRecord r, ProcessRecord app, ...) {
+    r.app = app;
+    // scheduleCreateService: 向应用端发起 IPC 调用
+    // 应用端收到之后就会去创建 Service，并执行 onCreate 回调
+    // r: class ServiceRecord extends Binder{}
+    app.thread.scheduleCreateService(r, r.serviceInfo, ...);
+    // 触发 Service 的 onStartCommand 回调
+    sendServiceArgsLocked(r, ...);
+}
+
+// 应用端处理 CreateService 请求，运行在主线程
+private void handleCreateService(CreateServiceData data) {
+    LoadedApk = packageInfo = getPackageInfoNoCheck(...);
+    Service service = (Service)cl.loadClass(data.info.name).newInstance();
+    
+    ContextImpl context = ContextImpl.createAppContext(this, ...);
+    
+    Application app = packageInfo.makeApplication(false, ...);
+    service.attach(context, this, ...);
+    service.onCreate();
+    // mService: ArrayMap<IBinder, Service>,key: ServiceRecord
+    mService.put(data.token, service);
+}
+
+private final void sendServiceArgsLocked(ServiceRecord r, ){
+    while(r.pendingStarts.size() > 0){
+        StartItem si = r.pendingStarts.remove(0);
+        // 调到应用端
+        r.app.thread.scheduleServiceArgs(r, ...);
+    }
+}
+public final void scheduleServiceArgs(IBinder token, ...) {
+    ServiceArgsData s = new ServiceArgsData();
+    //...
+    sendMessage(H.SERVICE_ARGS, s);
+}
+private void handleServiceArgs(ServiceArgsData data) {
+    Service s = mService.get(data, token);
+    if(s != null){
+        s.onStartCommand(data.args, data.flags, data.startId);
+    }
+}
+```
+
+**总结**
+
+![](https://i.loli.net/2020/03/24/A6pSmv2zoZMGPnq.png)
+
+![](https://i.loli.net/2020/03/24/SGLOXC9IHlzBcop.png)
+
+绑定服务：
+
+```java
+int bindServiceLocked(IApplicationThread caller, ...) {
+	//...
+	if((flags & Context.BIND_AUTO_CREATE)!=0){
+		bringUpServiceLocked(s, ...);
+	}
+}
+```
+
+BindService 并不会触发 onStartCommand，因为 bind service 它没有给 ServiceRecord 加到 pendingStarts 队列里面。