Android基础#14: Android问题解决--Unable to find explicit activity class，ActivityNotFoundException,未声明Act

    错误信息，通常直指问题根源。-----箴言-----

内容简介：

本节来看一个常见的崩溃问题。

问题描述：
在进行Android应用程序开发时，经常遇到“AndroidRuntime: FATAL EXCEPTION: main”，“android.content.ActivityNotFoundException: Unable to find explicit activity class ”问题，程序直接崩溃。

分析：

直接看log错误信息，很明显，是没有声明对应的Activity导致的。尤其是刚开始进行Android开发时，经常忘记声明Activity。参考关键log如下：

android.content.ActivityNotFoundException: Unable to find explicit activity class {com.test.myapplication5/com.test.myapplication5.activity.SubFrameActivity}; have you declared this activity in your AndroidManifest.xml?

“在AndroidManifest.xml中，你声明了这个activity么？”

再一检查代码，确实是没有声明。 

解决方式：
只要在AndroidMenifest.xml文件中进行Activity的声明即可。

举例：
下面是一个AndroidMenifest.xml，如果注释掉SubFrameActivity，就会出现“Unable to find explicit activity class”这种问题。

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.test.myapplication5">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.MyApplication5">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/Theme.MyApplication5.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
<!--        <activity android:name=".activity.SubFrameActivity"></activity>-->
    </application>

</manifest>

具体的错误信息参考如下：
 

10-16 20:32:50.481 564-929/system_process D/Interception: result: allow, reason: callerPkg == calleePkg, rule: runType===activity|||callerPkg===com.test.myapplication5|||calleePkg===com.test.myapplication5|||calleeClass===com.test.myapplication5.activity.SubFrameActivity|||action===null
10-16 20:32:50.481 564-929/system_process I/ActivityManager: START u0 {cmp=com.test.myapplication5/.activity.SubFrameActivity} from uid 10042 on display 0
10-16 20:32:50.482 2699-2699/com.test.myapplication5 D/AndroidRuntime: Shutting down VM
    
    
    --------- beginning of crash
10-16 20:32:50.483 2699-2699/com.test.myapplication5 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.test.myapplication5, PID: 2699
    android.content.ActivityNotFoundException: Unable to find explicit activity class {com.test.myapplication5/com.test.myapplication5.activity.SubFrameActivity}; have you declared this activity in your AndroidManifest.xml?
        at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1794)
        at android.app.Instrumentation.execStartActivity(Instrumentation.java:1512)
        at android.app.Activity.startActivityForResult(Activity.java:3942)
        at androidx.fragment.app.FragmentActivity.startActivityForResult(FragmentActivity.java:675)
        at android.app.Activity.startActivityForResult(Activity.java:3902)
        at androidx.fragment.app.FragmentActivity.startActivityForResult(FragmentActivity.java:662)
        at android.app.Activity.startActivity(Activity.java:4231)
        at android.app.Activity.startActivity(Activity.java:4199)
        at com.test.myapplication5.MainActivity$1.onClick(MainActivity.java:34)
        at android.view.View.performClick(View.java:5204)
        at android.view.View$PerformClick.run(View.java:21153)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5539)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:745)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:635)
10-16 20:32:50.489 564-1639/system_process W/ActivityManager:   Force finishing activity com.test.myapplication5/.MainActivity
10-16 20:32:50.492 564-596/system_process D/ActivityManager: Nemu don't show crash dialog of ProcessRecord{2592e89 2699:com.test.myapplication5/u0a42}
10-16 20:32:50.493 564-594/system_process D/Interception: result: allow, reason: calleePkg is system app, rule: runType===broadcast|||callerPkg===android|||calleePkg===com.mumu.acc|||calleeClass===com.mumu.acc.AccMsgReceiver|||action===uu.intent.action.STOP_ACCELERATE
10-16 20:32:50.493 906-1068/com.netease.nemu_vapi_android.nemu D/com.netease.nemu_android_watchdog_lib.core.Watchdog: upload event: {"architecture":"x86_64","channel":"mac-nochannel","engine":"MAC","language":"zh-Hans","package":"mac-nopackage","usage":"0","uuid":"2909ab3a6d7656d98d4abc40b42ebc06","version":"1.9.21","x":{"app_elapse":29,"app_name":"MyApplication5","app_package":"com.test.myapplication5","error":{"code":0,"msg":"ok"},"token":""}}	type: ExitAPP	sign: I2Zjzeiz7/+eE95c77VVf/5xrqA=	uid: 1000
10-16 20:32:50.495 1082-1082/com.mumu.acc D/AccService: AccMsgReceiver: uu.intent.action.STOP_ACCELERATE
10-16 20:32:50.497 564-1639/system_process I/com.android.server.tabs.TabManagerService: notifyClosing:741: notify tab closed in android as it's closed task com.test.myapplication5, taskId: 742, elapseTime: 29578
10-16 20:32:50.502 2699-2699/com.test.myapplication5 I/Process: Sending signal. PID: 2699 SIG: 9

扩展：

从log信息中，看可以看到一个Activity被启动时的堆栈信息。这有利于理解Activity的生命周期。

这不就是很多面试官最喜欢问的问题么？ 建议读者一定要深刻理解这些错误信息。

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注：重视错误信息，重视警告信息，重视提示信息。