检查UTF-8数据类型3字节或4字节Unicode

Question

In my database I get the error

com.mysql.jdbc.MysqlDataTruncation: Data truncation: Data too long for column

I use Java and MySQL 5. As I know 4-byte Unicode is legal i Java, but illegal in MySQL 5, I think that it can cause my problem and I want to check type of my data, so here's my question: How can i check that my UTF-8 data is 3-byte or 4-byte Unicode?

Answer 1

UTF-8 encodes everything in the basic multilingual plane (ie U+0000 to U+FFFF inclusive) in 1-3 bytes. Therefore, you just need to check whether everything in your string is in the BMP.

In Java, that means checking whether any char (which is a UTF-16 code unit) is a high or low surrogate character, as Java will use surrogate pairs to encode non-BMP characters:

public static boolean isEntirelyInBasicMultilingualPlane(String text) {
    for (int i = 0; i < text.length(); i++) {
        if (Character.isSurrogate(text.charAt(i))) {
            return false;
        }
    }
    return true;
}

Answer 2

If you do not want to support beyond BMP, you can just strip those characters before handing it to MySQL:

public static String withNonBmpStripped( String input ) {
    if( input == null ) throw new IllegalArgumentException("input");
    return input.replaceAll("[^\\u0000-\\uFFFF]", "");
}

If you want to support beyond BMP, you need MySQL 5.5+ and you need to change everything that's utf8 to utf8mb4 (collations, charsets ...). But you also need the support for this in the driver that I am not familiar with. Handling these characters in Java is also a pain because they are spread over 2 chars and thus need special handling in many operations.

Answer 3

在我发现的 java中剥离非BMP charactres的最佳方法如下：

inputString.replaceAll("[^\\u0000-\\uFFFF]", "\uFFFD");