我是Android和SQLite的新手。我试图在我的数据库中创建多个表，首先，我将电子邮件详细信息保存在数据库中，然后通过保存的电子邮件，我能够从服务器检索电子邮件。当我尝试将该电子邮件数据保存到第二个数据库表时，它会抛出空指针异常。这是因为Sqlitedatabase db = this.writeabledatabase;语句alwasy变为null，这意味着数据库无法获取路径。这是我的代码：
package com.elevenvalues.baig.db2listviewupdatedeleteinsert;
import android.content.ContentValues;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import java.sql.SQLException;
public class DbHelper extends SQLiteOpenHelper {
// table name
public static final String EMAIL_INSERT_TABLE = "EmailInsertDetail";
public static final String EMAIL_LOG = "EmailLog";
// Database Version
private static final int DATABASE_VERSION = 1;
// Database Name
public static String DATABASE_NAME = "My database";
public static final String KEY_USERNAME = "username";
public static final String KEY_EMAIL = "email";
public static final String KEY_PASSWORD = "password";
public static final String KEY_SERVER = "server";
public static final String KEY_PORT = "port";
public static final String KEY_SECURITY_TYPE = "securityType";
public static final String KEY_ACTIVE_TIME = "activeTime";
public static final String KEY_ID = "id";
public static final String KEY_ROWID = "id";
public static final String KEY_FROM = "froom";
public static final String KEY_TO = "too";
public static final String KEY_MESSAGE_BODY = "messagebody";
SQLiteDatabase db;
private boolean isUpdate;
String id;
public DbHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
String CREATE_TABLE = "CREATE TABLE " + EMAIL_INSERT_TABLE + " (" + KEY_ID + " INTEGER PRIMARY KEY, "
+ KEY_USERNAME + " TEXT, " + KEY_EMAIL + " TEXT," + KEY_PASSWORD + " TEXT," + KEY_SERVER + " TEXT," +
KEY_PORT + " TEXT," + KEY_SECURITY_TYPE + " TEXT," + KEY_ACTIVE_TIME + " TEXT);";
String CREATE_SERVER_DATABASE = "CREATE TABLE " + EMAIL_LOG + " (" + KEY_ROWID + " INTEGER PRIMARY KEY, "
+ KEY_TO + " TEXT, " + KEY_FROM + " TEXT," + KEY_MESSAGE_BODY + " TEXT);";
db.execSQL(CREATE_SERVER_DATABASE);
db.execSQL(CREATE_TABLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS " + EMAIL_INSERT_TABLE);
db.execSQL("DROP TABLE IF EXISTS " + EMAIL_LOG);
// create new tables
onCreate(db);
}
void EmailReadDataInsert(String from, String too, String messageBody) {
db=this.getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(DbHelper.KEY_FROM, from);
cv.put(DbHelper.KEY_TO, too);
cv.put(DbHelper.KEY_MESSAGE_BODY, messageBody);
// Inserting Row
db.insert(EMAIL_LOG, null, cv);
}
/**
* save data into SQLite
*/
public void AddEmailData(String username,String email,String password,String server,String port,String securityType,
String activeTime) {
db = this.getWritableDatabase();
ContentValues values = new ContentValues();
values.put(DbHelper.KEY_USERNAME, username);
values.put(DbHelper.KEY_EMAIL, email);
values.put(DbHelper.KEY_PASSWORD, password);
values.put(DbHelper.KEY_SERVER, server);
values.put(DbHelper.KEY_PORT, port);
values.put(DbHelper.KEY_SECURITY_TYPE, securityType);
values.put(DbHelper.KEY_ACTIVE_TIME, activeTime);
if (isUpdate) {
//update database with new data
db.update(DbHelper.EMAIL_INSERT_TABLE, values, DbHelper.KEY_ID + "=" + id, null);
} else {
//insert data into database
db.insert(DbHelper.EMAIL_INSERT_TABLE, null, values);
}
//close database
db.close();
}
//---opens the database---
public DbHelper open() throws SQLException
{
db = this.getWritableDatabase();
return this;
}
//---closes the database---
public void close()
{
this.close();
}
}
在db变为空的此方法中出现问题：
void EmailReadDataInsert(String from, String too, String messageBody) {
db=this.getWritableDatabase();
ContentValues cv = new ContentValues();
cv.put(DbHelper.KEY_FROM, from);
cv.put(DbHelper.KEY_TO, too);
cv.put(DbHelper.KEY_MESSAGE_BODY, messageBody);
// Inserting Row
db.insert(EMAIL_LOG, null, cv);
}
如何解决我的问题？有什么建议吗？
答案 0 :(得分：0)
如果您尝试在数据库中创建多个表，则不必始终创建新数据库。管理数据库中条目的类需要稍微调整一下，以便为两个表提供服务。尝试添加此代码：
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
public class Database {
static final String databaseName = "My database";
static final int databaseVersion = 0;
static final String rowId = “id”;
static final String rowUsername = “username”;
static final String rowEmail = “email”;
static final String rowPassword = "password";
static final String rowServer = “server”;
static final String rowPort = "port";
static final String rowSecurityType = "securityType";
static final String rowActiveTime = "activeTime";
static final String emailTable = "EmailInsertDetail";
static final String logTable = "EmailLog";
static final String rowFrom = "from";
static final String rowTo = "to";
static final String rowMessage = "message";
final Context context;
SqlLiteDatabase database;
DatabaseHelper helper;
public Database(Context context) {
this.context = context;
this.helper = new DatabaseHelper(context);
}
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context) {
super(context, databaseName, null, databaseVersion);
}
@Override
public void onCreate(SQLiteDatabase database) {
try {
database.execSQL("CREATE TABLE IF NOT EXISTS " + emailTable);
database.execSQL("CREATE TABLE IF NOT EXISTS " + logTable);
} catch (SQLException ex) {
ex.printStackTrace();
}
}
@Override
public void onUpgrade(SQLiteDatabase database, int oldVersion, int newVersion) {
Log.w("Database", “Upgrading database from version “ + oldVersion + “ to “ + newVersion + “, which will destroy all old data”);
db.execSQL(“DROP TABLE IF EXISTS " + emailTable + " (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT, email TEXT, password TEXT, server TEXT, port TEXT, securityType TEXT, activeTime TEXT)");
db.execSQL("DROP TABLE IF EXISTS " + logTable + " (id INTEGER PRIMARY KEY AUTOINCREMENT, from TEXT, to TEXT, message TEXT)");
onCreate(database);
}
}
public Database open() throws SQLException {
this.database = DatabaseHelper.getWritableDatabase();
return this;
}
public void close() {
DatabaseHelper.close();
}
public long insertEmail(String username, String email, String password, String server, String port, String securityType, String activeTime) {
ContentValues contentValues = new ContentValues();
contentValues.put(rowUsername, username);
contentValues.put(rowEmail, email);
contentValues.put(rowPassword, password);
contentValues.put(rowServer, server);
contentValues.put(rowPort, port);
contentValues.put(rowSecurityType, securityType);
contentValues.put(rowActiveTime, activeTime);
return database.insert(emailTable, null, contentValues);
}
public long insertData(String from, String to, String message) {
ContentValues contentValues = new ContentValues();
contentValues.put(rowFrom, from);
contentValues.put(rowTo, to);
contentValues.put(rowMessage, message);
return database.insert(logTable, null, contentValues);
}
public boolean updateEmail(long id, String username, String email, String password, String server, String port, String securityType, String activeTime) {
ContentValues contentValues = new ContentValues();
contentValues.put(rowUsername, username);
contentValues.put(rowEmail, email);
contentValues.put(rowPassword, password);
contentValues.put(rowServer, server);
contentValues.put(rowPort, port);
contentValues.put(rowSecurityType, securityType);
contentValues.put(rowActiveTime, activeTime);
return database.update(emailTable, contentValues, rowId + "=" + id, null) > 0;
}
public boolean updateEmail(long id, String from, String to, String message) {
ContentValues contentValues = new ContentValues();
contentValues.put(rowFrom, from);
contentValues.put(rowTo, to);
contentValues.put(rowMessage, message);
return database.update(logTable, contentValues, rowId + "=" + id, null) > 0;
}
public Cursor getAllEmails() {
return database.query(emailTable, new String[] {rowId, rowUsername, rowEmail, rowPassword, rowServer, rowPort, rowSecurityType, rowActiveTime}, null, null, null, null, null);
}
public Cursor getAllData() {
return database.query(logTable, new String[] {rowId, rowFrom, rowTo, rowMessage}, null, null, null, null, null);
}
public Cursor getEmail(long id) throws SQLException {
Cursor cursor = db.query(true, emailTable, new String[] {rowId, rowUsername, rowEmail, rowPassword, rowServer, rowPort, rowSecurityType, rowActiveTime}, rowId + “=” + id, null, null, null, null, null);
if (cursor != null) {
cursor.moveToFirst();
}
return cursor;
}
public Cursor getData(long id) throws SQLException {
Cursor cursor = db.query(true, logTable, new String[] {rowId, rowFrom, rowTo, rowMessage}, rowId + “=” + id, null, null, null, null, null);
if (cursor != null) {
cursor.moveToFirst();
}
return cursor;
}
}
在要插入或检索数据的活动或类中，键入：
Database database = new Database(context);
database.open();
// To insert whatever data you want
long insert = database.insertEmail(parametres go in here);
// To update whatever data you want
if (database.updateEmail(the parametres))
// more code to be executed
// To retrieve all items in a table
Cursor cursor = database.getAllEmails();
if (cursor.moveToFirst()) {
do {
Toast.makeText(this, "Id: " + cursor.getInt(0) + " Email: " + cursor.getString(2), Toast.LENGTH_SHORT).show();
} while (cursor.moveToNext();
}
// To retrieve a certain item
Cursor cursor = database.getEmail(the id);
Toast.makeText(this, "Id: " + cursor.getInt(0) + " From: " + cursor.getString(1), Toast.LENGTH_SHORT).show();
if (cursor.moveToFirst()) {
}
database.close();
请务必使用context，this或getContext()变量替换Context。请记住使用您自己的代码替换Toast语句。如果您仍然遇到问题，请务必参阅this电子书，它可以为您提供帮助或在下面留下一些评论。