android - SQLite INNER JOIN 抛出 NullPointerException

Question

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 的包含单词；第二个名为 TABLE_TRANS 的包含翻译。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，并抛出一个 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;
}

Answer 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 在某处显示 rawQuery，否则 db 为空。您没有给出任何指示 db 应该在哪里分配非空值，但这是要检查的第一件事......

顺便说一句，请不要像那样在您的 SQL 中包含值 - 这是 SQL injection attacks 的秘诀.请改用参数化 SQL。