Retrieving static resource via ServletContext root creates (many) jar_cache tmp files

[BastiaanProot]

Since version 1.4.5 we notice our application produces jar_cache*.tmp files. In previous versions it did not. In our case the jar_cache*.tmp files risk filling the disk after a while.

We found the cause:

1. Locations for static content are auto configured by Spring Boot.
   Default locations for static resources in the resolveChain (in order) are:
   • (ServletContext) /
   • classpath:/META-INF/resources/
   • classpath:/resources/
   • classpath:/static/
   • classpath:/public/
2. In version 1.4.4 getting a resource from ServletContext root returned instances of EmptyResource and getUrl() returned null. So the ServletContext location got skipped and then resources were retrieved from classpath:/META-INF/resources/. This changed in version 1.4.5: Resource handling is inconsistent across different embedded containers #8299. ServletContext now returns a valid URL to the resource.
3. Our static resources are contained in a jar in our application jar. So jar-in-jar. (URL to the resource looks like this: jar:war:file:application.jar*/BOOT_INF/lib/resources.jar!/META-INF/resources/favicon.ico) Retrieving the static resource requires JVM to use jar_cache*.tmp files. (using File.deleteOnExit). This seems to occur when you do resource.contentLength().
   On Debian/OpenJDK we can only see these jar_cache*.tmp files using lsof; they are open but deleted files. These do not get released until the application exits. So they still take up space and fill the disk. Similarly on Windows/SunJDK we can see the jar_cache*.tmp files accumulate and they are only deleted when the application exits. This seems to be a known/old phenomenon with the JVM.

In short:

Due to changes in the behaviour of ServletContext static resources our application now performs jar-in-jar lookups and accumulates tmp files.

Our solution:

We used the spring.resources.static-locations application property to customize the order of the locations and put classpath:/META-INF/resources/ first.

[wilkinsona]

@BastiaanProot Thank you very much for the detailed analysis. We'd already seen the symptom you've described in #9751 but were some distance away from identifying the cause.

I see that you've mentioned EmptyResource so I assume that you're using Tomcat. Can you please confirm that's the case?

[BastiaanProot]

Indeed, we're using embedded Tomcat.

[wilkinsona]

The default ordering has been changed in 9dd3fb7 that closed #9240.