在清单文件中找不到MainActivity，但是我声明了它

Question

我正在使用Android Studio 3.2。我无法通过显示错误来运行android项目：

MainActivity not found in the Manifest file

但是我也声明，然后也不能创建新项目。 因此，我将卸载并重新安装android studio并清理android studio设置。但这是行不通的。

当我还创建一个新项目时，显示以下错误：

> MyApplication\app\src\main\res\values\colors.xml:1:1: Error: Content is not allowed in prolog.

当我运行现有项目时，此消息显示：

MainActivity not found in Manifest file

清单：

<?xml version="1.0" encoding="utf-8"?>
<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/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

Answer 1

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

<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/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

Answer 2

  <?xml version="1.0" encoding="utf-8"?>

<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/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

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

Answer 3

请确认您colours.xml中的第一行是2018-10-11 13:07:56.834 WARN 16756 --- [nio-8081-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[1]->com.activeboss.model.pm.ProjectTasks["projecttaskcommentsCollection"]->org.hibernate.collection.internal.PersistentBag[0]->com.activeboss.model.pm.ProjectTaskComments["commentedEmployee"]) 2018-10-11 13:07:56.853 WARN 16756 --- [nio-8081-exec-2] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved exception caused by Handler execution: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException); nested exception is com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[1]->com.activeboss.model.pm.ProjectTasks["projecttaskcommentsCollection"]->org.hibernate.collection.internal.PersistentBag[0]->com.activeboss.model.pm.ProjectTaskComments["commentedEmployee"]) ？

错误提示<?xml version="1.0" encoding="utf-8"?>显示了您无法运行此APP的原因。