为什么在应用程序运行时不显示activity_main.xml

Question

我正在开发这个简单的应用程序。在这个项目中，android-23的所有隐藏API都是打开的。当我简单地在布局上显示按钮时，它只显示白屏。

activity_main.xml中

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/activity_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.himanshusrivastava.ethernetlollipop.MainActivity"
android:background="@color/colorPrimaryDark">


<Button
    android:text="check ethernet"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:layout_marginLeft="100dp"
    android:layout_marginStart="100dp"
    android:layout_marginTop="35dp"
    android:id="@+id/button" />

</RelativeLayout>

MainActivity.java

package com.example.himanshusrivastava.ethernetlollipop;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;


public class MainActivity extends Activity{
Button btn;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    btn = (Button) findViewById(R.id.button);
    btn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Toast.makeText(MainActivity.this, "Ethernet is off", Toast.LENGTH_SHORT).show();
        }
    });
  }
}

build.gradle（Ethernetlollipop）

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
repositories {
    maven {
        url 'https://dl.bintray.com/anggrayudi/maven/'
    }
    jcenter()
}
dependencies {
    classpath 'com.android.tools.build:gradle:2.2.3'

    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}
}

allprojects {
repositories {
    jcenter()
}
}

task clean(type: Delete) {
delete rootProject.buildDir
}

build.gradle（Module：app）

apply plugin: 'com.android.application'

android {
compileSdkVersion 23
buildToolsVersion "23.0.2"
defaultConfig {
    applicationId "com.example.himanshusrivastava.ethernetlollipop"
    minSdkVersion 17
    targetSdkVersion 23
    versionCode 1
    versionName "1.0"
   testInstrumentationRunner"android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
       proguardFiles getDefaultProguardFile('proguardandroid.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
    exclude group: 'com.android.support', module: 'support-annotations'
})
compile 'com.android.support:appcompat-v7:23.0.2'
testCompile 'junit:junit:4.12'
compile 'com.anggrayudi:android-hidden-api:0.0.5'
}

AndroidMainfest.xml

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

<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.WRITE_SETTINGS" />
<uses-permission android:name="android.permission.WRITE_SECURE_SETTINGS" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    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>