Question

我熟悉Android和ContectProviders的编程。我已经创建了代码（用于测试目的）来检查数据库的读/写

这是代码 TestProvider.java

package com.example.testapp;

import android.content.ContentProvider;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.net.Uri;

public class TestProvider extends ContentProvider
{
    private static final String DBNAME = "testdb";
    private static final String SQL_CREATE_MAIN = "CREATE TABLE if not exists test(id INTEGER PRIMARY KEY, word TEXT)";
    private MainDatabaseHelper helper;

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs)
    {
        return 0;
    }

    @Override
    public String getType(Uri uri)
    {
        return null;
    }

    @Override
    public Uri insert(Uri uri, ContentValues values)
    {
        return null;
    }

    @Override
    public boolean onCreate()
    {
        helper = new MainDatabaseHelper(getContext());
        return false;
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder)
    {
        return helper.getReadableDatabase().rawQuery("SELECT * FROM test", null);
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs)
    {
        return 0;
    }

    protected static final class MainDatabaseHelper extends SQLiteOpenHelper
    {
        MainDatabaseHelper(Context context)
        {
            super(context, DBNAME, null, 1);
        }

        public void onCreate(SQLiteDatabase db)
        {
            db.execSQL(SQL_CREATE_MAIN);
            db.execSQL("INSERT INTO test (word) VALUES ('AAA')");
            db.execSQL("INSERT INTO test (word) VALUES ('BBB')");
        }

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

        }
    }
}

这是MainActivity.java

package com.example.testapp;

import android.app.Activity;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;

public class MainActivity extends Activity {

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

        TestProvider provider = new TestProvider();
        Cursor c = provider.query(Uri.parse("content://com.example.testapp.provider/test"), null, null, null, null);
        do
        {
            Log.d("MainActivity", String.format("ID:%s / Word:%s", c.getInt(c.getColumnIndex("id")), c.getString(c.getColumnIndex("word"))));
        }
        while(c.moveToNext());
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

来自logcat的Stacktrace：

07-30 15:38:25.695: E/Trace(3533): error opening trace file: No such file or directory (2)
07-30 15:38:26.255: D/AndroidRuntime(3533): Shutting down VM
07-30 15:38:26.305: W/dalvikvm(3533): threadid=1: thread exiting with uncaught exception (group=0x40a71930)
07-30 15:38:26.345: E/AndroidRuntime(3533): FATAL EXCEPTION: main
07-30 15:38:26.345: E/AndroidRuntime(3533): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.testapp/com.example.testapp.MainActivity}: java.lang.NullPointerException
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.os.Looper.loop(Looper.java:137)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.ActivityThread.main(ActivityThread.java:5041)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at java.lang.reflect.Method.invokeNative(Native Method)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at java.lang.reflect.Method.invoke(Method.java:511)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at dalvik.system.NativeStart.main(Native Method)
07-30 15:38:26.345: E/AndroidRuntime(3533): Caused by: java.lang.NullPointerException
07-30 15:38:26.345: E/AndroidRuntime(3533):     at com.example.testapp.TestProvider.query(TestProvider.java:45)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at com.example.testapp.MainActivity.onCreate(MainActivity.java:19)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.Activity.performCreate(Activity.java:5104)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
07-30 15:38:26.345: E/AndroidRuntime(3533):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
07-30 15:38:26.345: E/AndroidRuntime(3533):     ... 11 more

问题出在哪里？感谢

Android - rawQuery到ContentProvider会抛出NullPointerException

0 个答案: