question: public boolean removeShutdownHook ( Thread hook ) { SecurityManager sm = System . getSecurityManager ( ) ; if ( sm != null ) { sm . checkPermission ( RuntimePermissionCollection . SHUTDOWN_HOOKS_PERMISSION ) ; } if ( hook == null ) throw new NullPointerException ( "null is not allowed here" ) ; if ( VMState > 0 ) { throw new IllegalStateException ( ) ; } synchronized ( hooksList ) { return hooksList . remove ( hook ) ; } }
target:  De-registers a previously-registered virtual-machine shutdown hook.