URL 转码遇到的坑

1. 背景

做图片中心时, 需要根据图片URL去下载图片, 发现部分URL浏览器里面能访问,但是代码无法下载

原因: 需要对URL进行转码

2. 使用UrlEncode

一开始直接使用UrlEncode对url转码, 如下

• 原始链接

https://s3.amazonaws.com/fromfactory.club.image/9f/0f/9fbd598c4681773f6371c27dfb64180f.webp

• UrlEncode后

https%3a%2f%2fs3.amazonaws.com%2ffromfactory.club.image%2f9f%2f0f%2f9fbd598c4681773f6371c27dfb64180f.webp

2.1 问题

斜杠,冒号等字符也被UrlEncode进行了转码, 显然这样的转码后是不能下载的

3. 原因分析

编码的意义在于，假如URL的参数中的中文或特殊字符在发送到服务端时，服务端无法解析它的真正意义，会导致服务端不能理解客户端的请求, 此时, 需要对它进行编码

3.1 哪些字符需要编码

RFC3986文档规定，URL中只允许包含英文字母（a-zA-Z）、数字（0-9）、-_.~4个特殊字符以及所有保留字符。

• 保留字符：Url可以划分成若干个组件，协议、主机、路径等。有一些字符（:/?#[]@）是用作分隔不同组件的。
  当组件中的普通数据包含这些特殊字符时，需要对其进行编码。

• RFC3986中指定了以下字符为==保留字符：! * ’ ( ) ; : @ & = + $ , / ? # [ ]==

对于图片下载中, 如下都不转码

• 英文字母 a-zA-Z
• 数字 0-9
• 4个特殊字符 _.~、
• 保留字符 ! * ’ ( ) ; : @ & = + $ , / ? # [ ]==

3.2 UrlEncode不转码字符

UrlEncode中维护了dontNeedEncoding, 但是dontNeedEncoding只包含了

• 英文字母（a-zA-Z）
• 数字（0-9）
• 空格 - _ . *
  ab8f20ac3c5a4b0f9beb02bd9be627fb_image.png

如下字符也需要不转码: ! ~ ’ ( ) ; : @ & = + $ , / ? # [ ]==

4. 解决方案

重写UrlEncode添加不转码字符

7be93685d8544476aa88d21c35ec3dd1_image.png

4.1 代码

package com.clubfactory.center.product.util;


import sun.security.action.GetPropertyAction;

import java.io.CharArrayWriter;
import java.io.UnsupportedEncodingException;
import java.nio.charset.Charset;
import java.nio.charset.IllegalCharsetNameException;
import java.nio.charset.UnsupportedCharsetException;
import java.security.AccessController;
import java.util.BitSet;

/**
 * Utility class for HTML form encoding. This class contains static methods
 * for converting a String to the <CODE>application/x-www-form-urlencoded</CODE> MIME
 * format. For more information about HTML form encoding, consult the HTML
 * <A HREF="http://www.w3.org/TR/html4/">specification</A>.
 *
 * <p>
 * When encoding a String, the following rules apply:
 *
 * <ul>
 * <li>The alphanumeric characters &quot;{@code a}&quot; through
 *     &quot;{@code z}&quot;, &quot;{@code A}&quot; through
 *     &quot;{@code Z}&quot; and &quot;{@code 0}&quot;
 *     through &quot;{@code 9}&quot; remain the same.
 * <li>The special characters &quot;{@code .}&quot;,
 *     &quot;{@code -}&quot;, &quot;{@code *}&quot;, and
 *     &quot;{@code _}&quot; remain the same.
 * <li>The space character &quot; &nbsp; &quot; is
 *     converted into a plus sign &quot;{@code +}&quot;.
 * <li>All other characters are unsafe and are first converted into
 *     one or more bytes using some encoding scheme. Then each byte is
 *     represented by the 3-character string
 *     &quot;<i>{@code %xy}</i>&quot;, where <i>xy</i> is the
 *     two-digit hexadecimal representation of the byte.
 *     The recommended encoding scheme to use is UTF-8. However,
 *     for compatibility reasons, if an encoding is not specified,
 *     then the default encoding of the platform is used.
 * </ul>
 *
 * <p>
 * For example using UTF-8 as the encoding scheme the string &quot;The
 * string &#252;@foo-bar&quot; would get converted to
 * &quot;The+string+%C3%BC%40foo-bar&quot; because in UTF-8 the character
 * &#252; is encoded as two bytes C3 (hex) and BC (hex), and the
 * character @ is encoded as one byte 40 (hex).
 *
 * @author  Herb Jellinek
 * @since   JDK1.0
 */
public class MyURIEncoder {
    static BitSet dontNeedEncoding;
    static final int caseDiff = ('a' - 'A');
    static String dfltEncName = null;

    static {

        /* The list of characters that are not encoded has been
         * determined as follows:
         *
         * RFC 2396 states:
         * -----
         * Data characters that are allowed in a URI but do not have a
         * reserved purpose are called unreserved.  These include upper
         * and lower case letters, decimal digits, and a limited set of
         * punctuation marks and symbols.
         *
         * unreserved  = alphanum | mark
         *
         * mark        = "-" | "_" | "." | "!" | "~" | "*" | "'" | "(" | ")"
         *
         * Unreserved characters can be escaped without changing the
         * semantics of the URI, but this should not be done unless the
         * URI is being used in a context that does not allow the
         * unescaped character to appear.
         * -----
         *
         * It appears that both Netscape and Internet Explorer escape
         * all special characters from this list with the exception
         * of "-", "_", ".", "*". While it is not clear why they are
         * escaping the other characters, perhaps it is safest to
         * assume that there might be contexts in which the others
         * are unsafe if not escaped. Therefore, we will use the same
         * list. It is also noteworthy that this is consistent with
         * O'Reilly's "HTML: The Definitive Guide" (page 164).
         *
         * As a last note, Intenet Explorer does not encode the "@"
         * character which is clearly not unreserved according to the
         * RFC. We are being consistent with the RFC in this matter,
         * as is Netscape.
         *
         */

        dontNeedEncoding = new BitSet(256);
        int i;
        for (i = 'a'; i <= 'z'; i++) {
            dontNeedEncoding.set(i);
        }
        for (i = 'A'; i <= 'Z'; i++) {
            dontNeedEncoding.set(i);
        }
        for (i = '0'; i <= '9'; i++) {
            dontNeedEncoding.set(i);
        }
        //dontNeedEncoding.set(' '); /* encoding a space to a + is done * in the encode() method */
        dontNeedEncoding.set('-');
        dontNeedEncoding.set('_');
        dontNeedEncoding.set('.');
        dontNeedEncoding.set('*');

        //对以下在 URI 中具有特殊含义的 ASCII 标点符号    ;/?:@&=+$,#  不需要转义
        dontNeedEncoding.set(';');
        dontNeedEncoding.set('/');
        dontNeedEncoding.set('?');
        dontNeedEncoding.set(':');
        dontNeedEncoding.set('@');
        dontNeedEncoding.set('&');
        dontNeedEncoding.set('=');
        dontNeedEncoding.set('+');
        dontNeedEncoding.set('$');
        dontNeedEncoding.set(',');
        dontNeedEncoding.set('#');
        dontNeedEncoding.set('!');
        dontNeedEncoding.set('\'');
        dontNeedEncoding.set('(');
        dontNeedEncoding.set(')');
        dontNeedEncoding.set('%');
        dontNeedEncoding.set('[');
        dontNeedEncoding.set(']');

        dfltEncName = AccessController.doPrivileged(
                new GetPropertyAction("file.encoding")
        );
    }

    /**
     * You can't call the constructor.
     */
    private MyURIEncoder() { }

    /**
     * Translates a string into {@code x-www-form-urlencoded}
     * format. This method uses the platform's default encoding
     * as the encoding scheme to obtain the bytes for unsafe characters.
     *
     * @param   s   {@code String} to be translated.
     * @deprecated The resulting string may vary depending on the platform's
     *             default encoding. Instead, use the encode(String,String)
     *             method to specify the encoding.
     * @return  the translated {@code String}.
     */
    @Deprecated
    public static String encode(String s) {

        String str = null;

        try {
            str = encode(s, dfltEncName);
        } catch (UnsupportedEncodingException e) {
            // The system should always have the platform default
        }

        return str;
    }

    /**
     * Translates a string into {@code application/x-www-form-urlencoded}
     * format using a specific encoding scheme. This method uses the
     * supplied encoding scheme to obtain the bytes for unsafe
     * characters.
     * <p>
     * <em><strong>Note:</strong> The <a href=
     * "http://www.w3.org/TR/html40/appendix/notes.html#non-ascii-chars">
     * World Wide Web Consortium Recommendation</a> states that
     * UTF-8 should be used. Not doing so may introduce
     * incompatibilities.</em>
     *
     * @param   s   {@code String} to be translated.
     * @param   enc   The name of a supported
     *    <a href="../lang/package-summary.html#charenc">character
     *    encoding</a>.
     * @return  the translated {@code String}.
     * @exception  UnsupportedEncodingException
     *             If the named encoding is not supported
     * @since 1.4
     */
    public static String encode(String s, String enc)
            throws UnsupportedEncodingException {

        boolean needToChange = false;
        StringBuffer out = new StringBuffer(s.length());
        Charset charset;
        CharArrayWriter charArrayWriter = new CharArrayWriter();

        if (enc == null)
            throw new NullPointerException("charsetName");

        try {
            charset = Charset.forName(enc);
        } catch (IllegalCharsetNameException e) {
            throw new UnsupportedEncodingException(enc);
        } catch (UnsupportedCharsetException e) {
            throw new UnsupportedEncodingException(enc);
        }

        for (int i = 0; i < s.length();) {
            int c = (int) s.charAt(i);
            if (dontNeedEncoding.get(c)) {
                out.append((char)c);
                i++;
            } else {
                // convert to external encoding before hex conversion
                do {
                    charArrayWriter.write(c);
                    /*
                     * If this character represents the start of a Unicode
                     * surrogate pair, then pass in two characters. It's not
                     * clear what should be done if a bytes reserved in the
                     * surrogate pairs range occurs outside of a legal
                     * surrogate pair. For now, just treat it as if it were
                     * any other character.
                     */
                    if (c >= 0xD800 && c <= 0xDBFF) {
                        /*
                          System.out.println(Integer.toHexString(c)
                          + " is high surrogate");
                        */
                        if ( (i+1) < s.length()) {
                            int d = (int) s.charAt(i+1);
                            /*
                              System.out.println("\tExamining "
                              + Integer.toHexString(d));
                            */
                            if (d >= 0xDC00 && d <= 0xDFFF) {
                                /*
                                  System.out.println("\t"
                                  + Integer.toHexString(d)
                                  + " is low surrogate");
                                */
                                charArrayWriter.write(d);
                                i++;
                            }
                        }
                    }
                    i++;
                } while (i < s.length() && !dontNeedEncoding.get((c = (int) s.charAt(i))));

                charArrayWriter.flush();
                String str = new String(charArrayWriter.toCharArray());
                byte[] ba = str.getBytes(charset);
                for (int j = 0; j < ba.length; j++) {
                    out.append('%');
                    char ch = Character.forDigit((ba[j] >> 4) & 0xF, 16);
                    // converting to use uppercase letter as part of
                    // the hex value if ch is a letter.
                    if (Character.isLetter(ch)) {
                        ch -= caseDiff;
                    }
                    out.append(ch);
                    ch = Character.forDigit(ba[j] & 0xF, 16);
                    if (Character.isLetter(ch)) {
                        ch -= caseDiff;
                    }
                    out.append(ch);
                }
                charArrayWriter.reset();
                needToChange = true;
            }
        }

        return (needToChange? out.toString() : s);
    }
}