浅谈为什么#{}可以防止SQL注入

<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>

==>  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

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);
        }
    }

-- %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 --'