Reinstate links to JUnit 5 Javadoc APIs

[sbrannen]

As a follow up to #27480 (see commit 030ba52), we need to determine why we cannot configure external Javadoc links to JUnit 5.

Note that code in the spring-test module (e.g., the SpringExtension for JUnit Jupiter) links to JUnit 5 APIs.

If we uncomment the following...

spring-framework/build.gradle

Lines 347 to 352 in 167e538

	// TODO Uncomment link to JUnit 5 docs once we have sorted out
	// the following warning in the build.
	//
	// warning: The code being documented uses packages in the unnamed module, but the packages defined in https://junit.org/junit5/docs/5.8.1/api/ are in named modules.
	//
	// "https://junit.org/junit5/docs/5.8.1/api/",

... we get the following Javadoc warning which fails the build.

warning: The code being documented uses packages in the unnamed module, but the packages defined in https://junit.org/junit5/docs/5.8.1/api/ are in named modules. 

Related JDK issues

• https://bugs.openjdk.java.net/browse/JDK-8240169
• https://bugs.openjdk.java.net/browse/JDK-8274639

[sbrannen]

/cc @sormuras

[sbrannen]

Update

Thanks to a tip from @sormuras, it turns out that the links are actually generated properly in the resulting HTML if we remove the use of -Werror for the javadoc task.

Thus, the warning is merely a "warning" that we are not interested in.

Unfortunately, (as far as I know) there is currently no way to instruct javadoc not to log that as a "warning".

[sormuras]

This https://bugs.openjdk.java.net/browse/JDK-8274639 describes the underlying cause and a potential fix.

[sbrannen]

Java 18 will introduce a --link-modularity-mismatch option that defaults to WARN (the status quo), and we will be able to set it to INFO to prevent such warnings from breaking our build.