Object-Oriented Techniques - Java - page 300

Discussion

One way of performing application-level cleanup is the runtime method addShut-
downHook() , to which you pass a nonstarted Thread subclass object. If the virtual machine
has a chance , it runs your shutdown hook code as part of JVM termination. This normally
works, but won't happen if the VM was terminated abruptly as by a kill signal on Unix or a
KillProcess on Win32, or the VM aborts due to detecting internal corruption of its data struc-
tures.

Program ShutdownDemo shown in Example 8-1 contains both a finalize() method (I told
you not to use those earlier in this chapter) and a shutdown hook. The program normally
exits while holding a reference to the object with the finalize() method. If run with -f as
an argument, it “frees” the object and “forces” a GC run by calling System.gc() ; only in
this case does the finalize() method run. The shutdown hook is run in every case.

Example 8-1. ShutdownDemo
public public class class ShutdownDemo ShutdownDemo {

public public static static void void main ( String [] args ) throws throws Exception {

// Create an Object with a finalize() method - Bad idea!
Object f = new new Object () {

public public void throws Throwable {

System . out . println ( "Running finalize()" );
super

void finalize () throws

super . finalize ();

}

};

// Add a shutdownHook to the JVM
Runtime . getRuntime (). addShutdownHook ( new new Thread () {

public public void void run () {

System . out . println ( "Running Shutdown Hook" );

}

});

// Unless the user puts -f (this-program-specific argument for "free") on
// the command line, call System.exit while holding a reference to
// Object f, which can therefore not be finalized().

iif ( args . length == 1 && args [ 0 ]. equals ( "-f" )) {
f = null

null ;
System . gc ();

}