The core class used to support loading nested jars is org.springframework.boot.loader.jar.JarFile.
It lets you load jar content from a standard jar file or from nested child jar data.
When first loaded, the location of each JarEntry is mapped to a physical file offset of the outer jar, as shown in the following example:
myapp.jar +-------------------+-------------------------+ | /BOOT-INF/classes | /BOOT-INF/lib/mylib.jar | |+-----------------+||+-----------+----------+| || A.class ||| B.class | C.class || |+-----------------+||+-----------+----------+| +-------------------+-------------------------+ ^ ^ ^ 0063 3452 3980
The preceding example shows how A.class can be found in /BOOT-INF/classes in myapp.jar at position 0063.
B.class from the nested jar can actually be found in myapp.jar at position 3452, and C.class is at position 3980.
Armed with this information, we can load specific nested entries by seeking to the appropriate part of the outer jar. We do not need to unpack the archive, and we do not need to read all entry data into memory.
Spring Boot Loader strives to remain compatible with existing code and libraries.
org.springframework.boot.loader.jar.JarFile extends from java.util.jar.JarFile and should work as a drop-in replacement.
The getURL() method returns a URL that opens a connection compatible with java.net.JarURLConnection and can be used with Java’s URLClassLoader.