E.2 Spring Boot’s “JarFile” class

The core class used to support loading nested jars is org.springframework.boot.loader.jar.JarFile. It allows you to 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:

myapp.jar
+-------------------+-------------------------+
| /BOOT-INF/classes | /BOOT-INF/lib/mylib.jar |
|+-----------------+||+-----------+----------+|
||     A.class      |||  B.class  |  C.class ||
|+-----------------+||+-----------+----------+|
+-------------------+-------------------------+
 ^                    ^           ^
 0063                 3452        3980

The example above shows how A.class can be found in /BOOT-INF/classes in myapp.jar position 0063. B.class from the nested jar can actually be found in myapp.jar position 3452 and C.class is at position 3980.

Armed with this information, we can load specific nested entries by simply seeking to the appropriate part of the outer jar. We don’t need to unpack the archive and we don’t need to read all entry data into memory.

E.2.1 Compatibility with the standard Java “JarFile”

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 will return a URL that opens a java.net.JarURLConnection compatible connection and can be used with Java’s URLClassLoader.