如何使用Java中的编码将Clob转换为字符串

Question

We are doing massive batch of xml processing and the logic to convert clob to string is shown below.

import java.sql.Clob
import org.apache.commons.io.IOUtils

String extractXml(Clob xmlClob) {

    log.info "DefaultCharset: " + groovy.util.CharsetToolkit.getDefaultSystemCharset()

    String sourceXml
    try {
        sourceXml = new String(IOUtils.toByteArray(xmlClob?.getCharacterStream()), encoding)            // 1. Encoding not working
        sourceXml = new String(IOUtils.toByteArray(xmlClob?.getCharacterStream(), encoding), encoding)  // 2. Encoding working
    } catch (Exception e) {
        ...
    }

    return sourceXml
}

My queries:

a. I am not sure why (1) doesn't work even though I am using getCharacterStream() instead of getAsciiStream(). but (2) seems to work fine may be I am using explicit overriding of system encoding ?

b. The solution (2) looks bit odd as you are specifing 2 times the encoding format (one for bytes array and one for string creation). I am not sure if there are any performance issues or wondered if there are better ways to write them?

c. I thought of not using the Apache-commons libraries and use a simple java package solution. But the suprising thing is, I did not give any explicit encoding but it seems to work perfectly. Is it because It does "streams character -> straight to string buffering" ?

/*
 * working perfectly and retuns encoding correctly
 */
String extractXmlWithoutApacheCommons(Clob xmlClob) {

    log.info "DefaultCharset: " + groovy.util.CharsetToolkit.getDefaultSystemCharset()

    StringBuffer sb = new StringBuffer((int) xmlClob.length())
    try {
        Reader r = xmlClob.getCharacterStream()
        char[] cbuf = new char[2048]
        int n = 0

        while ((n = r.read(cbuf, 0, cbuf.length)) != -1) {
            if (n > 0) {
                sb.append(cbuf, 0, n)
            }
        }

    } catch (Exception e) {
        ...
    }

    return sb.toString()
}    

Can you guys please shed some light to understand them.

Answer 1

The Clob already has an encoding. It's whatever you've specified in the database, and once you read it on Java side it'll be a String (with the implicit UTF-16 encoding, not that it matters at all).

Whatever you think you're doing with all those encoding tricks is wrong and useless. You only need to specify an encoding when turning bytes to chars or the other way around. You're dealing with chars only (except in your first example where you for some unknown reason want to turn them to bytes).

If you want to use IOUtils , then readFully(Reader input, char[] buffer) would be the method to use.

The platform default encoding has no effect in this whole question, since you shouldn't be working with bytes at all.

Edit: A slightly more modern way with the standard JDK classes would be to use Reader.read(CharBuffer target) like

CharBuffer cb = CharBuffer.allocate((int) xmlClob.length());
while(r.read(cb) != -1)
    ;
return cb.toString();

but it doesn't really make a huge difference (it's a bit nicer looking).