#{} 和 ${} 的区别
#{}
匹配的是一个占位符,相当于 JDBC 中的一个?,会对一些敏感字符进行过滤,编译过后会对传递的值加上双引号,因此可以防止 SQL 注入问题。
${}
匹配的是真实传递的值,传递过后,会与 SQL 语句进行字符串拼接。${} 会与其他 SQL 进行字符串拼接,无法防止 SQL 注入问题。
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< mapper namespace = "com.gitee.shiayanga.mybatis.wildcard.dao.UserDao" > < select id = "findByUsername" resultType = "com.gitee.shiayanga.mybatis.wildcard.entity.User" parameterType = "string" > select * from user where username like #{userName} </ select > < select id = "findByUsername2" resultType = "com.gitee.shiayanga.mybatis.wildcard.entity.User" parameterType = "string" > select * from user where username like '%${userName}%' </ select > </ mapper > |
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==> Preparing: select * from user where username like ? ==> Parameters: '%小%' or 1=1 --(String) <== Total: 0 ==> Preparing: select * from user where username like '%aaa' or 1=1 -- %' ==> Parameters: <== Total: 4 |
#{} 底层是如何防止 SQL 注入的?
- #{} 底层采用的是 PreparedStatement,因此不会产生 SQL 注入问题
- #{} 不会产生字符串拼接,而 ${} 会产生字符串拼接
为什么能防止SQL注入?
以MySQL为例,#{} 使用的是 com.mysql.cj.ClientPreparedQueryBindings#setString 方法,在这里会对一些特殊字符进行处理:
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public void setString( int parameterIndex, String x) { if (x == null ) { setNull(parameterIndex); } else { int stringLength = x.length(); if ( this .session.getServerSession().isNoBackslashEscapesSet()) { // Scan for any nasty chars boolean needsHexEscape = isEscapeNeededForString(x, stringLength); if (!needsHexEscape) { StringBuilder quotedString = new StringBuilder(x.length() + 2 ); quotedString.append( '\'' ); quotedString.append(x); quotedString.append( '\'' ); byte [] parameterAsBytes = this .isLoadDataQuery ? StringUtils.getBytes(quotedString.toString()) : StringUtils.getBytes(quotedString.toString(), this .charEncoding); setValue(parameterIndex, parameterAsBytes, MysqlType.VARCHAR); } else { byte [] parameterAsBytes = this .isLoadDataQuery ? StringUtils.getBytes(x) : StringUtils.getBytes(x, this .charEncoding); setBytes(parameterIndex, parameterAsBytes); } return ; } String parameterAsString = x; boolean needsQuoted = true ; if ( this .isLoadDataQuery || isEscapeNeededForString(x, stringLength)) { needsQuoted = false ; // saves an allocation later StringBuilder buf = new StringBuilder(( int ) (x.length() * 1.1 )); buf.append( '\'' ); // // Note: buf.append(char) is _faster_ than appending in blocks, because the block append requires a System.arraycopy().... go figure... // for ( int i = 0 ; i < stringLength; ++i) { char c = x.charAt(i); switch (c) { case 0 : /* Must be escaped for 'mysql' */ buf.append( '\\' ); buf.append( '0' ); break ; case '\n' : /* Must be escaped for logs */ buf.append( '\\' ); buf.append( 'n' ); break ; case '\r' : buf.append( '\\' ); buf.append( 'r' ); break ; case '\\' : buf.append( '\\' ); buf.append( '\\' ); break ; case '\'' : buf.append( '\'' ); buf.append( '\'' ); break ; case '"' : /* Better safe than sorry */ if ( this .session.getServerSession().useAnsiQuotedIdentifiers()) { buf.append( '\\' ); } buf.append( '"' ); break ; case '\032' : /* This gives problems on Win32 */ buf.append( '\\' ); buf.append( 'Z' ); break ; case '\u00a5' : case '\u20a9' : // escape characters interpreted as backslash by mysql if ( this .charsetEncoder != null ) { CharBuffer cbuf = CharBuffer.allocate( 1 ); ByteBuffer bbuf = ByteBuffer.allocate( 1 ); cbuf.put(c); cbuf.position( 0 ); this .charsetEncoder.encode(cbuf, bbuf, true ); if (bbuf.get( 0 ) == '\\' ) { buf.append( '\\' ); } } buf.append(c); break ; default : buf.append(c); } } buf.append( '\'' ); parameterAsString = buf.toString(); } byte [] parameterAsBytes = this .isLoadDataQuery ? StringUtils.getBytes(parameterAsString) : (needsQuoted ? StringUtils.getBytesWrapped(parameterAsString, '\'' , '\'' , this .charEncoding) : StringUtils.getBytes(parameterAsString, this .charEncoding)); setValue(parameterIndex, parameterAsBytes, MysqlType.VARCHAR); } } |
所以 '%小%' or 1=1 --
经过处理之后就变成了 '''%小%'' or 1=1 --'
而 ${} 只是简单的拼接字符串,不做其他处理。
这样,它们就变成了:
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-- %aaa' or 1=1 -- select * from user where username like '%aaa ' or 1=1 -- %' -- '%小%' or 1=1 -- select * from user where username like '' '%小%' ' or 1=1 --' |
所以就避免了 SQL 注入的风险。
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原文链接:https://juejin.cn/post/7095247748126998535