刨根究底之Activity為什麼能用Service啟動

摘要： 今天開發過程中被測試提了個bug.原來我一不注意用Service的Context啟動Activity導致報錯(我用我自己的手機可以啟動). java.lang.RuntimeException: Error receiving broadcast Intent { act=com....}...

今天開發過程中被測試提了個bug.原來我一不注意用Service的Context啟動Activity導致報錯(我用我自己的手機可以啟動).

java.lang.RuntimeException: Error receiving broadcast Intent { act=com....}
...
Caused by: android.util.AndroidRuntimeException: 
Calling startActivity() from outside of an Activity context requires the FLAG_ACTIVITY_NEW_TASK flag. Is this really what you want?

最終解決辦法就是給Intent加個FLAG_ACTIVITY_NEW_TASK的flag就能順利啟動了

intent.addFlag(Intent.FLAG_ACTIVITY_NEW_TASK);

這裡先上一張圖

可以看出,除了Activity以外,其他元件都是不允許去啟動Activity的.但今天用低端三星機(Android4.4)啟動報錯.而用我的P20Pro徠卡三攝(Android8.0)卻能正常啟動.這不是跟這張圖說的不一樣嗎...

那就來看看原始碼是怎麼寫的吧

首先看下Activity是怎麼啟動的.在Activity啟動Activity會呼叫Activity.startActivity()

Activity
@Override
public void startActivity(Intent intent) {
this.startActivity(intent, null);
}
//最終會調
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this, intent, requestCode, options);
......//省略無關程式碼
} else {
......//省略無關程式碼
}
}

這裡的mParent是ActivityThread.scheduleLaunchActivity()裡的ActivityClientRecord傳給他的,預設為空.這裡的Instrumentation.startActivity()就表示開始啟動Activity了,具體啟動流程就不在本篇裡詳細說明了,之前寫過一篇縷清楚Activity啟動流程的文章,給個傳送門.這個啟動過程比較繁瑣,我是建議大家自己找個原始碼一步一步跟下去會學的比較紮實.

那麼其他元件啟動Activity的流程是怎樣呢?

像Service,BroadcastReceiver,Application等都是ContextWrapper的子類,呼叫startActivity()時會呼叫ContextWrapper的startActivity()

ContextWrapper
@Override
public void startActivity(Intent intent) {
mBase.startActivity(intent);
}

這個mBase就是ContextImpl.這個ContextImpl是在ActivityThread裡賦值的:

Activity-->ActivityThread::performLaunchActivity()

Service-->ActivityThread::handleCreateService()

Application-->LoadedApk::makeApplication()

那我們來看下ContextImpl的startActivity().先看低版本的

Android4.4 api-19
ContextImpl
@Override
public void startActivity(Intent intent, Bundle options) {
warnIfCallingFromSystemProcess();
if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
throw new AndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?");
}
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity)null, intent, -1, options);
}

很簡單,就是非Activity啟動Activity時如果不給intent設定FLAG_ACTIVITY_NEW_TASK就會報錯.

然後再來看高版本的:

Android8.0 api-27
ContextImpl
@Override
public void startActivity(Intent intent, Bundle options) {
warnIfCallingFromSystemProcess();

// Calling start activity from outside an activity without FLAG_ACTIVITY_NEW_TASK is
// generally not allowed, except if the caller specifies the task id the activity should
// be launched in.
if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0
&& options != null && ActivityOptions.fromBundle(options).getLaunchTaskId() == -1) {
throw new AndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?");
}
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity) null, intent, -1, options);
}

跟舊的程式碼相比,這裡多了個options的非空判斷(options != null),關鍵就在這裡.

由於這裡options傳的就是null,於是就跳過了這個異常.

ContextImpl
@Override
public void startActivity(Intent intent) {
warnIfCallingFromSystemProcess();
startActivity(intent, null);
}

那麼,這裡的邏輯是"&&",有一個不成立就算失敗,那麼我們不設定FLAG_ACTIVITY_NEW_TASK就能順利的啟動Activity了.

這裡的註釋翻譯下是:從Activity外部(Service,BroadcastReceiver,Application等)不設定FLAG_ACTIVITY_NEW_TASK是不能啟動Activity的,除非呼叫者指定Activity要啟動的task的Id.

可是我也並沒有指定task的id就能成功啟動了啊,這應該算Android系統的一個bug吧.看來谷歌開發人員也有犯渾的時候.

這段程式碼我查了下是Android7.0(api24)開始有的,也就是說Android6.0或以下系統還是不能啟動的,我測試了下確實是這樣.

以下是Android7.0的測試:

以下是Android6.0的測試:

邏輯就是MainActivity點選按鈕啟動TService,然後TService啟動SubActivity

MainActivity:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}

public void startService(View view) {
startService(new Intent(this, TService.class));
}

TService:
@Override
public void onCreate() {
super.onCreate();
Intent intent = new Intent(this, SubActivity.class);
intent.putExtra("data","從Service啟動的Activity");
startActivity(intent);
}

SubActivity:
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_sub);
TextView textView = findViewById(R.id.tv);
String data = getIntent().getStringExtra("data");
textView.setText(data);
}

我們最終得出結論是從Android7.0開始Service,BroadcastReceiver,Application等都是可以啟動Activity的.