java.lang.RuntimeException：无法在 android 相机应用程序中实例化活动 ComponentInfo

Question

我正在尝试使用 android studio 从 android 开发者网站制作自定义 android 应用程序，但我的应用程序没有在模拟器中运行它显示错误

java.lang.RuntimeException: 无法实例化活动组件信息

这是我的主要活动

    package firstapp.boysjoys.com.waste;

import android.app.Activity;
import android.hardware.Camera;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.Button;
import android.widget.FrameLayout;



import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;


/**
 * Created by Varun on 12/24/14.
 */
public class MainActivity extends Activity {
    public static final int MEDIA_TYPE_IMAGE = 1;

    private Camera mcamera;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mcamera = getcam();
        cam mcam = new cam(this, mcamera);
        FrameLayout preview = (FrameLayout) findViewById(R.id.camPre);
        preview.addView(mcam);


    }

    public static Camera getcam() {
        Camera c = null;
        try {
            c = Camera.open();
        }
        catch (Exception e){
            e.getMessage();
        }
        return c;
    }

    private Camera.PictureCallback mpicturecallback = new Camera.PictureCallback() {
        @Override
        public void onPictureTaken(byte[] data, Camera camera) {

        File pictureFile=getOutPutMediaFile(MEDIA_TYPE_IMAGE);
            if (pictureFile==null){
                android.util.Log.d("Error","Problem");
                return;
            }

            try {
                FileOutputStream fos=new FileOutputStream(pictureFile);
                fos.write(data);
                fos.close();
            } catch (FileNotFoundException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    };



          //CREATE A FILE URI FOR SAVING AN IMAGE OR VIDEO
            private static Uri getOutPutMediaFileUri(int type){
            return Uri.fromFile(getOutPutMediaFile(type));

            }
    private static File getOutPutMediaFile(int type){

        File mediaStorage=new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),"MyCameraApp");

        if (!mediaStorage.exists()){
            if (!mediaStorage.mkdirs()) {
                android.util.Log.d("Mycamera", "Failed Error");
                return null;
            }
        }

        File mediaName;

        if (type==MEDIA_TYPE_IMAGE) {
            mediaName = new File(mediaStorage.getPath()+File.separator+"Rim_"+".jpg");
        }
        else {
            return null;
        }


        return mediaName;
    }

    //Button Listener to the capture button
    Button btn= (Button) findViewById(R.id.Capture);
    public void clickPic(View view){
        mcamera.takePicture(null,null,mpicturecallback);
    }
}

和相机预览活动

    package firstapp.boysjoys.com.waste;

import android.content.Context;
import android.hardware.Camera;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

import java.io.IOException;


public class cam extends SurfaceView implements SurfaceHolder.Callback{

    private SurfaceHolder mholder;
    private Camera mcamera;

    public cam(Context context,Camera camera){
        super(context);
        mholder=getHolder();
        mholder.addCallback(this);
        mholder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }


    /**
     * This is called immediately after the surface is first created.
     * Implementations of this should start up whatever rendering code
     * they desire.  Note that only one thread can ever draw into
     * a {@link android.view.Surface}, so you should not draw into the Surface here
     * if your normal rendering will be in another thread.
     *
     * @param holder The SurfaceHolder whose surface is being created.
     */
    @Override
    public void surfaceCreated(SurfaceHolder holder) {

        try {
            mcamera.setPreviewDisplay(holder);
            mcamera.startPreview();
        } catch (IOException e) {
            e.printStackTrace();
        }


    }

    /**
     * This is called immediately after any structural changes (format or
     * size) have been made to the surface.  You should at this point update
     * the imagery in the surface.  This method is always called at least
     * once, after {@link #surfaceCreated}.
     *
     * @param holder The SurfaceHolder whose surface has changed.
     * @param format The new PixelFormat of the surface.
     * @param width  The new width of the surface.
     * @param height The new height of the surface.
     */
    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
        if (mholder.getSurface()==null) {
            return;
        }
        mcamera.stopPreview();

        //Start Preview WIth New Setting
        try {
            mcamera.setPreviewDisplay(mholder);
            mcamera.startPreview();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    /**
     * This is called immediately before a surface is being destroyed. After
     * returning from this call, you should no longer try to access this
     * surface.  If you have a rendering thread that directly accesses
     * the surface, you must ensure that thread is no longer touching the
     * Surface before returning from this function.
     *
     * @param holder The SurfaceHolder whose surface is being destroyed.
     */
    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {

        if (mholder.getSurface()!=null){
            mholder.getSurface().release();
            mcamera=null;
        }
    }
}

这是清单文件

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

        <uses-permission android:name="android.permission.CAMERA" ></uses-permission>
        <uses-permission android:name="android.permission.write_external_storage"></uses-permission>
        <uses-feature android:name="android.hardware.camera2.full" android:required="false">
         </uses-feature>
        <uses-feature android:name="android.hardware.Camera" android:required="false"></uses-feature>

        <application
            android:allowBackup="true"
            android:icon="@drawable/ic_launcher"
            android:label="@string/app_name"
            android:theme="@style/AppTheme" >
            <activity android:name=".MainActivity"
                android:label="@string/app_name"
                android:screenOrientation="landscape"
                >


                <intent-filter>
                    <action android:name="android.intent.action.MAIN">
                    </action>
                    <category android:name="android.intent.category.LAUNCHER">

                    </category>

                </intent-filter>
                </activity>


        </application>

    </manifest>

这是来自 Android Studio 的完整 LogCat

  12-26 15:26:46.284      610-610/firstapp.boysjoys.com.waste D/dalvikvm﹕ Not late-enabling CheckJNI (already on)
    12-26 15:26:48.244      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ threadid=3: reacting to signal 3
    12-26 15:26:48.434      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ Wrote stack traces to '/data/anr/traces.txt'
    12-26 15:26:48.683      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ threadid=3: reacting to signal 3
    12-26 15:26:48.754      610-610/firstapp.boysjoys.com.waste D/AndroidRuntime﹕ Shutting down VM
    12-26 15:26:48.754      610-610/firstapp.boysjoys.com.waste W/dalvikvm﹕ threadid=1: thread exiting with uncaught exception (group=0x409c01f8)
    12-26 15:26:48.786      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ Wrote stack traces to '/data/anr/traces.txt'
    12-26 15:26:48.822      610-610/firstapp.boysjoys.com.waste E/AndroidRuntime﹕ FATAL EXCEPTION: main
        java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{firstapp.boysjoys.com.waste/firstapp.boysjoys.com.waste.MainActivity}: java.lang.NullPointerException
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1880)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
                at android.app.ActivityThread.access$600(ActivityThread.java:123)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
                at android.os.Handler.dispatchMessage(Handler.java:99)
                at android.os.Looper.loop(Looper.java:137)
                at android.app.ActivityThread.main(ActivityThread.java:4424)
                at java.lang.reflect.Method.invokeNative(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:511)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
                at dalvik.system.NativeStart.main(Native Method)
         Caused by: java.lang.NullPointerException
                at android.app.Activity.findViewById(Activity.java:1794)
                at firstapp.boysjoys.com.waste.MainActivity.<init>(MainActivity.java:102)
                at java.lang.Class.newInstanceImpl(Native Method)
                at java.lang.Class.newInstance(Class.java:1319)
                at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
                at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1871)
                at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981)
                at android.app.ActivityThread.access$600(ActivityThread.java:123)
                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147)
                at android.os.Handler.dispatchMessage(Handler.java:99)
                at android.os.Looper.loop(Looper.java:137)
                at android.app.ActivityThread.main(ActivityThread.java:4424)
                at java.lang.reflect.Method.invokeNative(Native Method)
                at java.lang.reflect.Method.invoke(Method.java:511)
                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
                at dalvik.system.NativeStart.main(Native Method)
    12-26 15:26:49.154      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ threadid=3: reacting to signal 3
    12-26 15:26:49.234      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ Wrote stack traces to '/data/anr/traces.txt'
    12-26 15:26:49.654      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ threadid=3: reacting to signal 3
    12-26 15:26:49.734      610-615/firstapp.boysjoys.com.waste I/dalvikvm﹕ Wrote stack traces to '/data/anr/traces.txt'
    12-26 15:27:18.314      610-610/firstapp.boysjoys.com.waste I/Process﹕ Sending signal. PID: 610 SIG: 9

我不明白它从哪里抛出 RuntimeException

在这一行，它在错误日志中显示 引起：android.app.Activity.findViewById(Activity.java:1794) 处的 java.lang.NullPointerException 说它在 xml 中返回 null 其按钮 id

    <?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <FrameLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/camPre"
        android:layout_weight="1"
        >
        <Button
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/click"
            android:text="Capture"

            android:layout_gravity="center"
            />


        </FrameLayout>

</LinearLayout>

我尝试调试应用程序，但仍然可以理解为什么它会抛出空指针异常 根据错误日志第 102 行，这可能是由于按钮 ID 或此行 在主要活动中

if (!mediaStorage.exists()){
        if (!mediaStorage.mkdirs()) {
            android.util.Log.d("Mycamera", "Failed Error");
            return null;
        }
    }

Answer 1

Caused by: java.lang.NullPointerException
  at android.app.Activity.findViewById(Activity.java:1794)
  at firstapp.boysjoys.com.waste.MainActivity.<init>(MainActivity.java:102)

您调用findViewById() 太早了，在活动<init> 阶段包括例如成员变量初始化。

您只能在onCreate() 或在活动生命周期的更高版本中调用findViewById() 等活动方法。

将 findViewById() 调用移至 onCreate() 以摆脱 NPE。把它放在setContentView() 之后，这样它就可以真正返回一个非空值了。