Sun J2SE(1.4)的实现对于字符集转换非常缓慢，我用Jfluid做Profile发现一个简单的数据库程序花费在字符集转换上的时间竟然占用了60%以上，因此有必要对转换结果缓存：

package com.sybase.jdbc2.jdbc;

import java.io.UnsupportedEncodingException;

import java.util.*;

public class CachedCharset {

    public String convertTo(String src) {

        try {

            String dest = (String)cacheConvertTo.get(src);

            if (dest == null) {

                byte[] bytes = src.getBytes(destCharset);

                dest = new String(bytes, RAW);

                if (cacheConvertTo.size() > EXPIRE_EVERY)

                    cacheConvertTo.clear();

                cacheConvertTo.put(src, dest);

            }

            return dest;

        } catch (UnsupportedEncodingException e) {

            throw new RuntimeException(e.getMessage(), e);

        }

    }

    public String castFrom(String dest) {

        try {

            String src = (String)cacheCastFrom.get(dest);

            if (src == null) {

                byte[] bytes = dest.getBytes(RAW);

                src = new String(bytes, destCharset);

                if (cacheCastFrom.size() > EXPIRE_EVERY)

                    cacheCastFrom.clear();

                cacheCastFrom.put(dest, src);

            }

            return src;

        } catch (UnsupportedEncodingException e) {

            throw new RuntimeException(e.getMessage(), e);

        }

    }

    public static String convertTo(String src, String charset) {

        if (charset == null) return src;

        if (src == null) return null;

        CachedCharset cc = (CachedCharset)poolCC.get(charset);

        if (cc == null) {

            cc = new CachedCharset(charset);

            poolCC.put(charset, cc); 

        }

        return cc.convertTo(src);

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