SQLite INNER JOIN 抛出 NullPointerException答案

【问题标题】：SQLite INNER JOIN throws a NullPointerExceptionSQLite INNER JOIN 抛出 NullPointerException
【发布时间】：2012-03-05 17:20:37
【问题描述】：

我不明白为什么查询最终会抛出 NullPointerException。

我的数据库包含两个表。第一个名为TABLE_WORDS 包含单词；第二个名为TABLE_TRANS 包含翻译。一个词可以有多种翻译。

public static final String CREATE_TABLE_WORDS = "CREATE TABLE " + TABLE_WORDS + 
    " (" + WORDS_ID + " INTEGER PRIMARY KEY, " + WORDS_WORD + 
    " TEXT);";

public static final String CREATE_TABLE_TRANS = "CREATE TABLE " + TABLE_TRANS + 
    " (" + TRANS_ID + " INTEGER PRIMARY KEY, " + 
    TRANS_WORD + " TEXT, " + 
    TRANS_FOREIGN_ID_WORD + " INTEGER, " +
    "FOREIGN KEY (" + TRANS_FOREIGN_ID_WORD + ") REFERENCES " + TABLE_WORDS + "(" + WORDS_ID + "));";

我通过单词 ID 查询翻译，并抛出 NPE。

public Cursor queryTransesByWord(int idSend) {

    String query = "SELECT " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_WORD + ", " +
        DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_WORD + 
        " FROM " + DbHelper.TABLE_WORDS + 
        " INNER JOIN " + DbHelper.TABLE_TRANS + 
        " ON " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "=" + 
        DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_FOREIGN_ID_WORD + " WHERE " +
        DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "="+ idSend; 

    Cursor cur = db.rawQuery(query, null); // NullPointer always throws here

    if (cur != null)
        cur.moveToFirst();
    else
        Log.e(Constants.LOG_TAG, Constants.DB_ADAPTER_CLASS_NAME_SEP + "queryTransesByWord cursor is null");
    return cur;
}

【问题讨论】：

标签： android sqlite nullpointerexception

【解决方案1】：

好吧，给定你的代码：

Cursor cur = db.rawQuery(query, null); // NullPointer always throws here

这表明db 为空。该语句中没有其他取消引用操作 - 因此，除非堆栈跟踪 实际上 在其中某处显示 rawQuery，否则 db 为空。你没有给出任何迹象表明 db 应该被分配一个非空值，但这是首先要检查的事情......

顺便说一句，请不要在你的 SQL 中包含这样的值 - 这是SQL injection attacks 的秘诀。请改用参数化 SQL。

【讨论】：

db 不为空，我在查询之前打开它。公共类 DbAdapter { 私有 DbHelper 助手；私有 SQLiteDatabase 数据库；私有 int openStatus; public void openForReading() { if (openStatus == 0) { db = helper.getReadableDatabase();开放状态 = 1； } // if (openStatus == 0) else if (openStatus == 2) { helper.close(); db = helper.getReadableDatabase(); } // 嵌套 if (openStatus == 2) Log.i(Constants.LOG_TAG, Constants.DB_ADAPTER_CLASS_NAME_SEP + "openForReading succeed"); }
@RedPlanet：在这种情况下，您显示的行不会位于堆栈跟踪的顶部。（这就是为什么你应该在你的问题中包含堆栈跟踪。）但看起来如果你没有调用openForReading 那么db 将为空......

【解决方案2】：

在我的情况下，我应该使用

String query = "SELECT " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_WORD + ", " +
    DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_WORD + 
    " FROM " + DbHelper.TABLE_WORDS + 
    " INNER JOIN " + DbHelper.TABLE_TRANS + 
    " ON " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "=" + 
    DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_FOREIGN_ID_WORD + " WHERE " +
    DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "=?";    

Cursor cur = db.rawQuery(query, new String[] { Integer.toString(idSend) } );

我说的对吗？

【讨论】：

【解决方案3】：

String query = "SELECT " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_WORD + ", " +
DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_WORD + 
" FROM " + DbHelper.TABLE_WORDS + 
" INNER JOIN " + DbHelper.TABLE_TRANS + 
" ON " + DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "=" + 
DbHelper.TABLE_TRANS + "." + DbHelper.TRANS_FOREIGN_ID_WORD + " WHERE " +
DbHelper.TABLE_WORDS + "." + DbHelper.WORDS_ID + "=?";    

Cursor cur = db.rawQuery(query, new String[] { Integer.toString(idSend) } );

这对我有用:-)

【讨论】：