ADDITIONAL SYSTEM INFORMATION :
OS: Generic
java -version:
  openjdk version "15-ea" 2020-09-15
  OpenJDK Runtime Environment (build 15-ea+27-1372)
  OpenJDK 64-Bit Server VM (build 15-ea+27-1372, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
javadoc generates invalid HTML pages, if java source files have ftp:// links.
The link is translated into a relative path as follows:
 java source file:
    <a href="ftp://www.domain.com/">FTP Site</a>
 generated page:
    <a href="../a/ftp://www.domain.com">FTP Site</a>

The issue has reported in JDK-5044953, but yet has to be fixed.
These errors may be included in the Java API documents in the future.
It's a waste of time to modify the source every time a problem is found,
that I would like to suggest the fix.
The problem is that HtmlDocletWriter only checks certain tagas, mailto, http, https, and file.
Regular expressions should be used to support all protocols.

src/jdk.javadoc/share/classes/jdk/javadoc/internal/doclets/formats/html/HtmlDocletWriter.java
 
@@ -1623,11 +1623,7 @@ public class HtmlDocletWriter {
        if (redirectPathFromRoot == null) {
            return text;
        }
- String lower = Utils.toLowerCase(text);
- if (!(lower.startsWith("mailto:")
- || lower.startsWith("http:")
- || lower.startsWith("https:")
- || lower.startsWith("file:"))) {
+ if (!text.matches("^[^:/?#]+:.+$")) {
            text = "{@" + (new DocRootTaglet()).getName() + "}/"
                    + redirectPathFromRoot.resolve(text).getPath();
            text = replaceDocRootDir(text);

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. unzip attached JavadocTest.zip
   I will send it later.
2. execute javadoc
   cd JavadocTest
   javadoc -J-Duser.language=us -d doc -notimestamp -use a
3. check generated doc/a/package-use.html, doc/a/class-use/B1.StaticInnerB1.html
   and doc/index-all.html.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The link should be translated into an absolute path:
  <a href="ftp://www.domain.com">FTP Site</a>
ACTUAL -
The link becomes a relative path:
  <a href="../a/ftp://www.domain.com">FTP Site</a>

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I can send JavadocTest.zip as a test case.
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FREQUENCY : occasionally