访问外部已创建的数据库。但代码不起作用

Question

这是我编写的主要活动，但它不起作用！请帮忙

import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends Activity {
Button button1;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    button1.findViewById(R.id.button1);
    button1.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            // TODO Auto-generated method stub 
            Toast.makeText(getApplicationContext(),"onClick aaaayaaaa", Toast.LENGTH_LONG).show();

            try {
                MyDatabase db=new MyDatabase(MainActivity.this);
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }
    });



    }



}

这是我为访问使用不同应用程序预先填充的外部数据库而编写的 MyDatabase java 文件

import android.database.sqlite.SQLiteException;
import android.widget.Toast;

import com.readystatesoftware.sqliteasset.SQLiteAssetHelper;

public class MyDatabase extends SQLiteAssetHelper {
 private Context mycontext;
    private String DB_PATH="/mnt/sdcard/Kitchen_Data.db"; 

    private static String DB_NAME = "Kitchen_Data.db";
    public SQLiteDatabase myDataBase;


    public MyDatabase(Context context) throws IOException  {

        super(context,DB_NAME,null,1);
        this.mycontext=context;
        Toast.makeText(mycontext, "Constructor called", Toast.LENGTH_LONG).show();

        boolean dbexist = checkdatabase();
        if (dbexist) {
              opendatabase(); 
        } else {
            System.out.println("Database doesn't exist");
            createdatabase();
        }
    }

    public void createdatabase() throws IOException {
        boolean dbexist = checkdatabase();
        if(!dbexist) {
            this.getReadableDatabase();
            try {
                copydatabase();
            } catch(IOException e) {
                throw new Error("Error copying database");
            }
        }
    }   

    private boolean checkdatabase() {

        boolean checkdb = false;
        try {
            String myPath = DB_PATH;
            File dbfile = new File(myPath);
            Toast.makeText(mycontext, "Database exists", Toast.LENGTH_LONG).show();
            checkdb = dbfile.exists();
        } catch(SQLiteException e) {Toast.makeText(mycontext, "Error in database", Toast.LENGTH_LONG).show();
        }
        return checkdb;
    }

    private void copydatabase() throws IOException {
        //Open your local db as the input stream
        InputStream myinput = mycontext.getAssets().open(DB_NAME);

        // Path to the just created empty db
        String outfilename = DB_PATH;

        //Open the empty db as the output stream
        OutputStream myoutput = new FileOutputStream(outfilename);

        // transfer byte to inputfile to outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myinput.read(buffer))>0) {
            myoutput.write(buffer,0,length);
        }

        //Close the streams
        myoutput.flush();
        myoutput.close();
        myinput.close();
    }

    public void opendatabase() throws SQLException {
        //Open the database
        String mypath = DB_PATH;
        myDataBase = SQLiteDatabase.openDatabase(mypath, null, SQLiteDatabase.OPEN_READWRITE);
        Toast.makeText(mycontext, "database opeened", Toast.LENGTH_LONG).show();
    }

    public synchronized void close() {
        if(myDataBase != null) {
            myDataBase.close();
        }
        super.close();
    }

}

这是日志：

03-31 12:34:37.045: D/AndroidRuntime(15922): Shutting down VM
03-31 12:34:37.045: W/dalvikvm(15922): threadid=1: thread exiting with uncaught exception (group=0x40d79ae0)
03-31 12:34:37.055: E/AndroidRuntime(15922): FATAL EXCEPTION: main
03-31 12:34:37.055: E/AndroidRuntime(15922): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitchen_application/com.example.kitchen_application.MainActivity}: java.lang.NullPointerException
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2186)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.app.ActivityThread.access$600(ActivityThread.java:145)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1238)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.os.Handler.dispatchMessage(Handler.java:99)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.os.Looper.loop(Looper.java:137)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at android.app.ActivityThread.main(ActivityThread.java:5099)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at java.lang.reflect.Method.invokeNative(Native Method)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at java.lang.reflect.Method.invoke(Method.java:511)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
 03-31 12:34:37.055: E/AndroidRuntime(15922):   at dalvik.system.NativeStart.main(Native Method)
03-31 12:34:37.055: E/AndroidRuntime(15922): Caused by: java.lang.NullPointerException
03-31 12:34:37.055: E/AndroidRuntime(15922):    at com.example.kitchen_application.MainActivity.onCreate(MainActivity.java:30)
03-31 12:34:37.055: E/AndroidRuntime(15922):    at android.app.Activity.performCreate(Activity.java:5117)
03-31 12:34:37.055: E/AndroidRuntime(15922):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-31 12:34:37.055: E/AndroidRuntime(15922):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
03-31 12:34:37.055: E/AndroidRuntime(15922):    ... 11 more
03-31 12:35:38.650: D/AndroidRuntime(16560): Shutting down VM
03-31 12:35:38.650: W/dalvikvm(16560): threadid=1: thread exiting with uncaught exception (group=0x40d79ae0)
03-31 12:35:38.660: E/AndroidRuntime(16560): FATAL EXCEPTION: main
03-31 12:35:38.660: E/AndroidRuntime(16560): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitchen_application/com.example.kitchen_application.MainActivity}: java.lang.NullPointerException
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2186)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.ActivityThread.access$600(ActivityThread.java:145)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1238)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.os.Looper.loop(Looper.java:137)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.ActivityThread.main(ActivityThread.java:5099)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at java.lang.reflect.Method.invokeNative(Native Method)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at java.lang.reflect.Method.invoke(Method.java:511)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at dalvik.system.NativeStart.main(Native Method)
03-31 12:35:38.660: E/AndroidRuntime(16560): Caused by: java.lang.NullPointerException
03-31 12:35:38.660: E/AndroidRuntime(16560):    at com.example.kitchen_application.MainActivity.onCreate(MainActivity.java:30)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.Activity.performCreate(Activity.java:5117)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-31 12:35:38.660: E/AndroidRuntime(16560):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
03-31 12:35:38.660: E/AndroidRuntime(16560):    ... 11 more
03-31 12:35:39.982: I/Process(16560): Sending signal. PID: 16560 SIG: 9
03-31 12:36:19.714: D/AndroidRuntime(17185): Shutting down VM
03-31 12:36:19.714: W/dalvikvm(17185): threadid=1: thread exiting with uncaught exception (group=0x40d79ae0)
03-31 12:36:19.794: E/AndroidRuntime(17185): FATAL EXCEPTION: main
03-31 12:36:19.794: E/AndroidRuntime(17185): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.kitchen_application/com.example.kitchen_application.MainActivity}: java.lang.NullPointerException
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2186)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2236)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.ActivityThread.access$600(ActivityThread.java:145)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1238)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.os.Handler.dispatchMessage(Handler.java:99)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.os.Looper.loop(Looper.java:137)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.ActivityThread.main(ActivityThread.java:5099)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at java.lang.reflect.Method.invokeNative(Native Method)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at java.lang.reflect.Method.invoke(Method.java:511)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:803)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:570)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at dalvik.system.NativeStart.main(Native Method)
03-31 12:36:19.794: E/AndroidRuntime(17185): Caused by: java.lang.NullPointerException
03-31 12:36:19.794: E/AndroidRuntime(17185):    at com.example.kitchen_application.MainActivity.onCreate(MainActivity.java:28)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.Activity.performCreate(Activity.java:5117)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1081)
03-31 12:36:19.794: E/AndroidRuntime(17185):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)
03-31 12:36:19.794: E/AndroidRuntime(17185):    ... 11 more
03-31 12:36:21.236: I/Process(17185): Sending signal. PID: 17185 SIG: 9

Answer 1

在logcat 中没有任何说明错误是因为您的External database。简单的错误在于onCreate() 方法。在您刚刚声明button1 的那个方法上，您需要初始化按钮。所以你需要从

button1.findViewById(R.id.button1);

到

button1 = (Button)findViewById(R.id.button1);

Answer 2

试试这样：

 private static final int DATABASE_VERSION=1;
    private SQLiteDatabase OurDb;
    private final Context myContext;
    private static String DB_PATH = "/data/data/com.example.classroom/databases/";



            /**
             * Constructor
             * Takes and keeps a reference of the passed context in order to access to the application assets and resources.
             * @param context
             */
            public DbHelper(Context context) {

                super(context, DATABASE_NAME, null, 1);
                this.myContext = context;
            }   

          /**
             * Creates a empty database on the system and rewrites it with your own database.
             * */

            /**
             * Check if the database already exist to avoid re-copying the file each time you open the application.
             * @return true if it exists, false if it doesn't
             */
            private boolean checkDataBase(){

                SQLiteDatabase checkDB = null;

                try{
                    String myPath = DB_PATH + DATABASE_NAME;
                    checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);

                }catch(SQLiteException e){

                    //database does't exist yet.

                }

                if(checkDB != null){

                    checkDB.close();

                }

                return checkDB != null ? true : false;
            }

            /**
             * Copies your database from your local assets-folder to the just created empty database in the
             * system folder, from where it can be accessed and handled.
             * This is done by transfering bytestream.
             * */
            private void copyDataBase() throws IOException{

                //Open your local db as the input stream
                InputStream myInput = myContext.getAssets().open(DATABASE_NAME);

                // Path to the just created empty db
                String outFileName = DB_PATH + DATABASE_NAME;

                //Open the empty db as the output stream
                OutputStream myOutput = new FileOutputStream(outFileName);

                //transfer bytes from the inputfile to the outputfile
                byte[] buffer = new byte[1024];
                int length;
                while ((length = myInput.read(buffer))>0){
                    myOutput.write(buffer, 0, length);
                }
                 Log.v("log", "copy finish");
                //Close the streams
                myOutput.flush();
                myOutput.close();
                myInput.close();

            }



            @Override
            public synchronized void close() {

                    if(OurDb != null)
                        OurDb.close();

                    super.close();

            }

            @Override
            public void onCreate(SQLiteDatabase db) {

            }

            @Override
            public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

            }

            public void openDataBase() throws SQLException{
                // TODO Auto-generated method stub
                //Open the database
                String myPath = DB_PATH + DATABASE_NAME;
                OurDb = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);

            }

            public void createDataBase()  throws IOException{
                // TODO Auto-generated method stub

                boolean dbExist = checkDataBase();

                if(dbExist){
                    //do nothing - database already exist
                    Toast.makeText(myContext, "exist", Toast.LENGTH_SHORT).show();
                }else{

                    //By calling this method and empty database will be created into the default system path
                       //of your application so we are gonna be able to overwrite that database with our database.
                    Toast.makeText(myContext, "creating", Toast.LENGTH_SHORT).show();
                    this.getReadableDatabase();
                    try {

                        copyDataBase();

                    } catch (IOException e) {

                        Log.v("log",e.toString());
                        throw new Error("Error copying database");


                    }
                }
            }