java - Android 另一个 Activity 调用主 Activity Databasehelper 函数失败

Question

数据库帮助.java

package com.example.abc2;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;

public class DataBaseHelper extends SQLiteOpenHelper{

//The Android's default system path of your application database.
private static String DB_PATH = "/data/data/com.example.abc2/databases/";

private static String DB_NAME = "DB_BusData";

private SQLiteDatabase myDataBase;

private final Context myContext;

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

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

/**
  * Creates a empty database on the system and rewrites it with your own database.
  * */
public void createDataBase() throws IOException{

boolean dbExist = checkDataBase();

if(dbExist){
//do nothing - database already exist
}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.
this.getReadableDatabase();

try {

copyDataBase();

} catch (IOException e) {

throw new Error("Error copying 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 + DB_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(DB_NAME);

// Path to the just created empty db
String outFileName = DB_PATH + DB_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);
}

//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();

}

public void openDataBase() throws SQLException{

//Open the database
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);

}

@Override
public synchronized void close() {

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

super.close();

}

@Override
public void onCreate(SQLiteDatabase db) {

}

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

}

// Add your public helper methods to access and get content from the database.
// You could return cursors by doing "return myDataBase.query(....)" so it'd be easy
// to you to create adapters for your views.

}

MainActivity.java

public void printInspectorFormat(){
    try {
        Log.d(null,"1234545");
        DataBaseHelper myDbHelper = new DataBaseHelper(this.getApplicationContext());
        myDbHelper = new DataBaseHelper(this);
        try { 
        myDbHelper.createDataBase();  
        } catch (IOException ioe) { 
        throw new Error("Unable to create database"); 
        }  
        try {  
        myDbHelper.openDataBase();         
        }catch(SQLException sqle){  
        throw sqle;  
        }
}

UtililyActivity.java

MainActivity cls2= new MainActivity();
cls2.printInspectorFormat();

当在实用程序 Activity 中时，我尝试调用主 Activity 函数，但返回错误

java.lang.NullPointerException

也许databasehelper不共享？或者是什么？怎么解决这个问题？

Answer 1

您无法自己创建 Activity 实例。它们缺少系统在创建它们时添加的Context。如果您通过 new Activity() 自己创建了 Activity，那么您几乎可以对它执行的任何操作都会失败。

但是您可以将 printInspectorFormat 移动到另一个负责数据库访问的类中，并添加 Context 参数。现在，每个 Activity 都可以通过将自身作为上下文传递来使用该方法。

class DbBackend {
    public static void printInspectorFormat(Context context) {
        try {
            Log.d(null,"1234545");
            DataBaseHelper myDbHelper = new DataBaseHelper(context.getApplicationContext());
            try { 
            myDbHelper.createDataBase();  
            } catch (IOException ioe) { 
            throw new Error("Unable to create database"); 
            }  
            try {  
            myDbHelper.openDataBase();         
            }catch(SQLException sqle){  
            throw sqle;  
            }
    }
}

来自任何 Activity

DbBackend.printInspectorFormat(this);

您还应该考虑对基于 Assets 的数据库使用可靠的实现，例如 https://github.com/jgilfelt/android-sqlite-asset-helper因为你的看起来不太安全。也许可以阅读类似 http://www.vogella.de/articles/AndroidSQLite/article.html 的教程它还向您展示了如何使用 SQLiteOpenHelper 的一些模式。