The system property `sun.misc.URLClassPath.disableJarChecking` controls whether the first 4 bytes of JAR files on the class path are checked to be the magic number for ZIP files ("PK"). The check is not performed by default.

If the system property is set on the command line to the value `false`, then JAR files on the class path are checked. JAR files used by `java.net.URLClassLoader` at run time are also checked. JAR files that fail the check are ignored.