一个app到底会创建多少个Application对象
问题背景
最近跟群友讨论一个技术问题：
一个应用开启了多进程，最终到底会创建几个application对象，执行几次onCreate()方法？
有的群友根据自己的想法给出了猜想
甚至有的群友直接咨询起了ChatGPT
但至始至终都没有一个最终的结论。于是乎，为了弄清这个问题，我决定先写个demo测试得出结论，然后从源码着手分析原因
Demo验证
首先创建了一个app项目，开启多进程
```xml
<application
android:name=".DemoApplication"
android:allowBackup="true"
android:dataExtractionRules="@xml/data_extraction_rules"
android:fullBackupContent="@xml/backup_rules"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/Theme.Demo0307"
tools:targetApi="31">
<!--android:process 开启多进程并设置进程名-->
<activity
android:name=".MainActivity"
android:exported="true"
android:process=":remote">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
```
然后在DemoApplication的onCreate()方法打印application对象的地址，当前进程名称
```java public class DemoApplication extends Application { private static final String TAG = "jasonwan";
@Override
public void onCreate() {
super.onCreate();
Log.d(TAG, "Demo application onCreate: " + this + ", processName=" + getProcessName(this));
}
private String getProcessName(Application app) {
int myPid = Process.myPid();
ActivityManager am = (ActivityManager) app.getApplicationContext().getSystemService(Context.ACTIVITY_SERVICE);
List<ActivityManager.RunningAppProcessInfo> runningAppProcesses = am.getRunningAppProcesses();
for (ActivityManager.RunningAppProcessInfo runningAppProcess : runningAppProcesses) {
if (runningAppProcess.pid == myPid) {
return runningAppProcess.processName;
}
}
return "null";
}
} ```
运行，得到的日志如下
2023-03-07 11:15:27.785 19563-19563/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote
查看当前应用所有进程
说明此时app只有一个进程，且只有一个application对象，对象地址为@fb06c2d
现在我们将进程增加到多个，看看情况如何
```xml
<application
android:name=".DemoApplication"
android:allowBackup="true"
android:dataExtractionRules="@xml/data_extraction_rules"
android:fullBackupContent="@xml/backup_rules"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/Theme.Demo0307"
tools:targetApi="31">
<!--android:process 开启多进程并设置进程名-->
<activity
android:name=".MainActivity"
android:exported="true"
android:process=":remote">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".TwoActivity"
android:process=":remote2" />
<activity
android:name=".ThreeActivity"
android:process=":remote3" />
<activity
android:name=".FourActivity"
android:process=":remote4" />
<activity
android:name=".FiveActivity"
android:process=":remote5" />
</application>
```
逻辑是点击MainActivity启动TwoActivity，点击TwoActivity启动ThreeActivity，以此类推。最后我们运行，启动所有Activity得到的日志如下
2023-03-07 11:25:35.433 19955-19955/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote
2023-03-07 11:25:43.795 20001-20001/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote2
2023-03-07 11:25:45.136 20046-20046/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote3
2023-03-07 11:25:45.993 20107-20107/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote4
2023-03-07 11:25:46.541 20148-20148/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote5
查看当前应用所有进程
此时app有5个进程，但application对象地址均为@fb06c2d，地址相同意味着它们是同一个对象。
那是不是就可以得出结论，无论启动多少个进程都只会创建一个application对象呢？并不能妄下此定论，我们将MainActivity的process属性去掉再运行，得到的日志如下
2023-03-07 11:32:10.156 20318-20318/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@5d49e29, processName=com.jason.demo0307
2023-03-07 11:32:15.143 20375-20375/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote2
2023-03-07 11:32:16.477 20417-20417/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote3
2023-03-07 11:32:17.582 20463-20463/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote4
2023-03-07 11:32:18.882 20506-20506/com.jason.demo0307 D/jasonwan: Demo application onCreate: com.jason.demo0307.DemoApplication@fb06c2d, processName=com.jason.demo0307:remote5
查看当前应用所有进程
此时app有5个进程，但有2个application对象，对象地址为@5d49e29和@fb06c2d，且子进程的application对象都相同。
上述所有进程的父进程ID为678，而此进程正是zygote进程
根据上面的测试结果我们目前能得出的结论：
- 结论1：单进程只创建一个
Application对象，执行一次onCreate()方法； - 结论2：多进程至少创建2个
Application对象，执行多次onCreate()方法，几个进程就执行几次；
结论2为什么说至少创建2个，因为我在集成了JPush的商业项目中测试发现，JPush创建的进程跟我自己创建的进程，Application地址是不同的。
这里三个进程，分别创建了三个Application对象，对象地址分别是@f31ba9d，@2c586f3，@fb06c2d
源码分析
这里需要先了解App的启动流程，具体可以参考《App启动流程》
Application的创建位于frameworks/base/core/java/android/app/ActivityThread.java的handleBindApplication()方法中
```java @UnsupportedAppUsage private void handleBindApplication(AppBindData data) { long st_bindApp = SystemClock.uptimeMillis(); //省略部分代码
// Note when this process has started.
//设置进程启动时间
Process.setStartTimes(SystemClock.elapsedRealtime(), SystemClock.uptimeMillis());
//省略部分代码
// send up app name; do this *before* waiting for debugger
//设置进程名称
Process.setArgV0(data.processName);
//省略部分代码
// Allow disk access during application and provider setup. This could
// block processing ordered broadcasts, but later processing would
// probably end up doing the same disk access.
Application app;
final StrictMode.ThreadPolicy savedPolicy = StrictMode.allowThreadDiskWrites();
final StrictMode.ThreadPolicy writesAllowedPolicy = StrictMode.getThreadPolicy();
try {
// If the app is being launched for full backup or restore, bring it up in
// a restricted environment with the base application class.
//此处开始创建application对象，注意参数2为null
app = data.info.makeApplication(data.restrictedBackupMode, null);
//省略部分代码
try {
if ("com.jason.demo0307".equals(app.getPackageName())){
Log.d("jasonwan", "execute app onCreate(), app=:"+app+", processName="+getProcessName(app)+", pid="+Process.myPid());
}
//执行application的onCreate方法()
mInstrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!mInstrumentation.onException(app, e)) {
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
} finally {
// If the app targets < O-MR1, or doesn't change the thread policy
// during startup, clobber the policy to maintain behavior of b/36951662
if (data.appInfo.targetSdkVersion < Build.VERSION_CODES.O_MR1
|| StrictMode.getThreadPolicy().equals(writesAllowedPolicy)) {
StrictMode.setThreadPolicy(savedPolicy);
}
}
//省略部分代码
}
```
实际创建过程在frameworks/base/core/java/android/app/LoadedApk.java中的makeApplication()方法中，LoadedApk顾名思义就是加载好的Apk文件，里面包含Apk所有信息，像包名、Application对象，app所在的目录等，这里直接看application的创建过程
```java @UnsupportedAppUsage public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { if ("com.jason.demo0307".equals(mApplicationInfo.packageName)) { Log.d("jasonwan", "makeApplication: mApplication="+mApplication+", pid="+Process.myPid()); } //如果已经创建过了就不再创建 if (mApplication != null) { return mApplication; }
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "makeApplication");
Application app = null;
String appClass = mApplicationInfo.className;
if (forceDefaultAppClass || (appClass == null)) {
appClass = "android.app.Application";
}
try {
java.lang.ClassLoader cl = getClassLoader();
if (!mPackageName.equals("android")) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
"initializeJavaContextClassLoader");
initializeJavaContextClassLoader();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
}
ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
//反射创建application对象
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
if ("com.jason.demo0307.DemoApplication".equals(appClass)){
Log.d("jasonwan", "create application, app="+app+", processName="+mActivityThread.getProcessName()+", pid="+Process.myPid());
}
appContext.setOuterContext(app);
} catch (Exception e) {
Log.d("jasonwan", "fail to create application, "+e.getMessage());
if (!mActivityThread.mInstrumentation.onException(app, e)) {
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
throw new RuntimeException(
"Unable to instantiate application " + appClass
+ ": " + e.toString(), e);
}
}
mActivityThread.mAllApplications.add(app);
mApplication = app;
if (instrumentation != null) {
try {
//第一次启动创建时，instrumentation为null，不会执行onCreate()方法
instrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!instrumentation.onException(app, e)) {
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
}
// 省略部分代码
return app;
}
```
为了看清application到底被创建了几次，我在关键地方埋下了log，TAG为jasonwan的log是我自己加的，编译验证，得到如下log
``` 启动app，进入MainActivity 03-08 17:20:29.965 4069 4069 D jasonwan: makeApplication: mApplication=null, pid=4069 //创建application对象，地址为@c2f8311，当前进程id为4069 03-08 17:20:29.967 4069 4069 D jasonwan: create application, app=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307, pid=4069 03-08 17:20:29.988 4069 4069 D jasonwan: execute app onCreate(), app=:com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307, pid=4069 03-08 17:20:29.989 4069 4069 D jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307, pid=4069 03-08 17:20:36.614 4069 4069 D jasonwan: makeApplication: mApplication=com.jason.demo0307.DemoApplication@c2f8311, pid=4069
点击MainActivity，跳转到TwoActivity 03-08 17:20:39.686 4116 4116 D jasonwan: makeApplication: mApplication=null, pid=4116 //创建application对象，地址为@c2f8311，当前进程id为4116 03-08 17:20:39.687 4116 4116 D jasonwan: create application, app=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote2, pid=4116 03-08 17:20:39.688 4116 4116 D jasonwan: execute app onCreate(), app=:com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote2, pid=4116 03-08 17:20:39.688 4116 4116 D jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote2, pid=4116 03-08 17:20:39.733 4116 4116 D jasonwan: makeApplication: mApplication=com.jason.demo0307.DemoApplication@c2f8311, pid=4116
点击TwoActivity，跳转到ThreeActivity 03-08 17:20:41.473 4147 4147 D jasonwan: makeApplication: mApplication=null, pid=4147 //创建application对象，地址为@c2f8311，当前进程id为4147 03-08 17:20:41.475 4147 4147 D jasonwan: create application, app=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote3, pid=4147 03-08 17:20:41.475 4147 4147 D jasonwan: execute app onCreate(), app=:com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote3, pid=4147 03-08 17:20:41.476 4147 4147 D jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote3, pid=4147 03-08 17:20:41.519 4147 4147 D jasonwan: makeApplication: mApplication=com.jason.demo0307.DemoApplication@c2f8311, pid=4147
点击ThreeActivity，跳转到FourActivity 03-08 17:20:42.966 4174 4174 D jasonwan: makeApplication: mApplication=null, pid=4174 //创建application对象，地址为@c2f8311，当前进程id为4174 03-08 17:20:42.968 4174 4174 D jasonwan: create application, app=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote4, pid=4174 03-08 17:20:42.969 4174 4174 D jasonwan: execute app onCreate(), app=:com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote4, pid=4174 03-08 17:20:42.969 4174 4174 D jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote4, pid=4174 03-08 17:20:43.015 4174 4174 D jasonwan: makeApplication: mApplication=com.jason.demo0307.DemoApplication@c2f8311, pid=4174
点击FourActivity，跳转到FiveActivity 03-08 17:20:44.426 4202 4202 D jasonwan: makeApplication: mApplication=null, pid=4202 //创建application对象，地址为@c2f8311，当前进程id为4202 03-08 17:20:44.428 4202 4202 D jasonwan: create application, app=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote5, pid=4202 03-08 17:20:44.429 4202 4202 D jasonwan: execute app onCreate(), app=:com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote5, pid=4202 03-08 17:20:44.430 4202 4202 D jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@c2f8311, processName=com.jason.demo0307:remote5, pid=4202 03-08 17:20:44.473 4202 4202 D jasonwan: makeApplication: mApplication=com.jason.demo0307.DemoApplication@c2f8311, pid=4202 ```
结果很震惊，我们在5个进程中创建的application对象，地址均为@c2f8311，也就是至始至终创建的都是同一个Application对象，那么上面的结论2显然并不成立，只是测试的偶然性导致的。
至此，我们可以得出最终结论：
- 单进程，创建1个application对象，执行一次
onCreate()方法 - 多进程(N)，至少创建1个application对象，执行N次
onCreate()方法
至于为什么上面5个进程创建的application对象地址都相同，这个暂时没搞清楚，可能跟linux中的进程组设计有关系，也就是说因为进程隔离的原因，这几个application对象在不同的进程中有着相同的内存地址，但他们并不属于同一个对象，如果这个猜想成立，那就意味着
- 多进程(N)，创建N个application对象，执行N次
onCreate()方法
这仅仅是我个人的猜想，如果你对这一块了解，可以在评论区谈谈你的见解！