java resultset.getstring（“col_name”）查询 [英] java resultset.getstring("col_name") query

问题描述

我有一个关于Java中的 ResultSet.getString（）方法的简单查询。

假设数据库列中的值具有 \ 这是javas转义字符例如 \\\
或 \t 等

当我将值检索为 getString（）我看到一个更多的转义字符被添加，这个 \\\
的实际含义现在是字符串文字只有。
所以我不得不解压缩java，然后正确使用它。

I have a simple query regarding ResultSet.getString() method in java for JDBC.
Suppose the value in the Database column is having a \ which is javas escape character e.g. \n or \t etc.
When i retrieve the value as getString() i see one more escape character is getting added and the actual meaning of this \n is now a string literal only.
So i had to unescape java and then use it properly.

String s= rs.getString("col_name");

当 s 包含`\\\
' ：

When s contains `\n':

System.out.println(s)

输出：

\n

使用apache进行解压缩后，常用 StringEscapeUtils

输出： / p>

After unescaping java using apache common StringEscapeUtils
output:

System.out.println("hi"+s+"hello");
hi
hello

我的问题是谁添加了这个额外的$ $ c> \ before unescaping？
对不起，如果这是一个愚蠢的问题。

My question is who is adding this extra \ before unescaping?? Sorry if it is a dumb question.

推荐答案

JDBC驱动程序不会在结果集。如果这样做会是非常糟糕的。看看这个简单的例子：

The JDBC driver does no escaping in the ResultSet. It would be very bad if it does. Look at this simple example:

数据库表 x 确实包含一列 value 。有两行：一个有两个字符的字符串（'\'和'n'）和一个女巫一个字符串（换行符）。我已经向输出添加了一个字符串长度来进行说明。

The database table x does contain one column value. There are two rows: One with a two character string ('\' and 'n') and one witch a one character string (the newline character). I've added a string-length to the output for clarification.

select *, length(value) from x;
 value | length 
-------+--------
 \n    |      2
      +|      1
       | 

此表使用JDBC进行读取：

This table is read with JDBC:

    Connection db = DriverManager.getConnection( /* these parameters are up to you */ );

    Statement st = db.createStatement();
    ResultSet rs = st.executeQuery("select value, length(value) from x");
    while(rs.next()){
        String value = rs.getString(1);
        int length = rs.getInt(2);

        System.out.println("length="+length+", value='"+value+"'");
    }

你看到：到目前为止，没有显式转义到代码的任何地方。输出是：

You see: no explicit escaping anywhere in the code so far. And the output is:

length=2, value='\n'
length=1, value='
'

你看：还没有任何东西被转义 - JDBC。

You see: There is still nothing escaped - neither by the code nor by JDBC.

但是

如果您使用Java文字：如果你这样做：

Things get a bit murky if you mix in Java literals: If you do something like this:

st.execute("insert into x values ('\n')");

然后猜测发生了什么？您还有另一行一个字符！

then guess what happened? You have another row with one character!

length=2, value='\n'
length=1, value='
'
length=1, value='
'

这是因为Java 编译器将两个字符\转换为一个换行符。因此，JDBC驱动程序和数据库只能看到一个字符。

This is because the Java compiler translated the two characters '\n' into one newline character. Hence the JDBC driver and the database only see one character.

但是，如果您已经阅读了一些用户输入，用户将输入 \ 和 n 然后没有人会撤销这个，行将包含两个字符。

But if you would have read some userinput and the user would have typed \ and n then nobody would de-escape this and the row would have contained two characters.

下一步

你说，你使用 StringEscapeUtils 进行明确的转义。那么这将发生：

You say, you do explicit de-escaping using StringEscapeUtils. Then this would happen:

• 具有一个字符的行将从DB传递到屏幕。


• 具有两个字符的行将由 StringEscapeUtils 更改为包含一个字符的字符串。之后两行看起来和你一样。

• The row with one character will pass unaltered from the DB up to the screen.
• The row with two characters will be changed by StringEscapeUtils into a string containing one characters. After that both row look like the same to you.

摘要

不要将编译器的String-Literal转义与JDBC进行转义（不会发生）。

Do not confuse String-Literal escaping by the compiler with escaping by JDBC (which won't happen).

不要添加不必要的转义图层。

Do not add unnecessary escaping layers.

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