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?
最佳答案
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。
关于java - Android 另一个 Activity 调用主 Activity Databasehelper 函数失败,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/21048988/