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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：


交流1


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


交流2


甚至有的群友直接咨询起了ChatGPT


chatgpt1.jpg


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 version="1.0" encoding="utf-8"?>
<manifest
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">

<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>

</manifest>

然后在DemoApplication的onCreate()方法打印application对象的地址，当前进程名称


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


查看进程1


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 version="1.0" encoding="utf-8"?>
<manifest
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">

<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>

</manifest>

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


查看进程2


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


查看进程3


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


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。


jpush进程


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


	@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


	@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。


可真的是这样子的吗，这也太颠覆我的三观了，为此我跟群友讨论了这个问题：


不同进程中的多个对象，内存地址相同，是否代表这些对象都是同一个对象？


群友的想法是，java中获取的都是虚拟内存地址，虚拟内存地址相同，不代表是同一个对象，必须物理内存地址相同，才表示是同一块内存空间，也就意味着是同一个对象，物理内存地址和虚拟内存地址存在一个映射关系，同时给出了java中获取物理内存地址的方法Android获取对象地址，主要是利用Unsafe这个类来操作，这个类有一个作用就是直接访问系统内存资源，具体描述见Java中的魔法类-Unsafe，因为这种操作是不安全的，所以被标为了私有，但我们可以通过反射去调用此API， 然后我又去请教了部门搞寄存器的大佬，大佬肯定了群友的想法，于是我添加代码，尝试获取对象的物理内存地址，看看是否相同


public class DemoApplication extends Application {
public static final String TAG = "jasonwan";

@Override
public void onCreate() {
super.onCreate();

        Log.d(TAG, "DemoApplication=" + this + ", address=" + addressOf(this) + ", pid=" + Process.myPid());

    }

//获取对象的真实物理地址
public static long addressOf(Object o) {

        Object[] array = new Object[]{o};
long objectAddress = -1;
try {
Class cls = Class.forName("sun.misc.Unsafe");
Field field = cls.getDeclaredField("theUnsafe");

            field.setAccessible(true);
Object unsafe = field.get(null);
Class unsafeCls = unsafe.getClass();
Method arrayBaseOffset = unsafeCls.getMethod("arrayBaseOffset", Object.class.getClass());
int baseOffset = (int) arrayBaseOffset.invoke(unsafe, Object[].class);
Method size = unsafeCls.getMethod("addressSize");
int addressSize = (int) size.invoke(unsafe);
switch (addressSize) {
case 4:
Method getInt = unsafeCls.getMethod("getInt", Object.class, long.class);

                    objectAddress = (int) getInt.invoke(unsafe, array, baseOffset);
break;
case 8:
Method getLong = unsafeCls.getMethod("getLong", Object.class, long.class);

                    objectAddress = (long) getLong.invoke(unsafe, array, baseOffset);
break;
default:
throw new Error("unsupported address size: " + addressSize);

            }

        } catch (Exception e) {

            e.printStackTrace();

        }
return objectAddress;

    }

}

运行后得到如下日志


2023-03-10 11:01:54.043 6535-6535/com.jason.demo0307 D/jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@930d275, address=8050489105119022792, pid=6535

2023-03-10 11:02:22.610 6579-6579/com.jason.demo0307 D/jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@331b3b9, address=8050489105119027136, pid=6579

2023-03-10 11:02:36.369 6617-6617/com.jason.demo0307 D/jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@331b3b9, address=8050489105119029912, pid=6617

2023-03-10 11:02:39.244 6654-6654/com.jason.demo0307 D/jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@331b3b9, address=8050489105119032760, pid=6654

2023-03-10 11:02:40.841 6692-6692/com.jason.demo0307 D/jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@331b3b9, address=8050489105119036016, pid=6692

2023-03-10 11:02:52.429 6729-6729/com.jason.demo0307 D/jasonwan: DemoApplication=com.jason.demo0307.DemoApplication@331b3b9, address=8050489105119038720, pid=6729

可以看到，虽然Application的虚拟内存地址相同，都是331b3b9，但它们的真实物理地址却不同，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)，创建N个application对象，执行N次onCreate()方法

作者：小迪vs同学
来源：juejin.cn/post/7208345469658415159
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2023-03-14

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