votorola.g.hold

Interface Spool

All Known Implementing Classes:

Spool0, Spool1, SpoolT

public interface Spool

An unwindable spool of holds. It contols and implements the closure of an object - the "unwinding" of its state. As an object takes hold of resources that later need to be released, it constructs a Hold for each, and winds it onto a spool. At closure the object, the spool is unwound, and each hold is released.

An example is listener registration. This is state that often needs to be unwound, in order to prevent memory leaks. Using a spool, both registration and unregistration may be coded together in line. For example:

     registry.addListener( C.this );
     spool.add( new Hold()
     {
         public void release() { registry.removeListener( C.this ); }
     });
     

The life cycle of spools follows two common patterns. In one, a spool is created internally by the object, and later unwound by it (in a public close() method, for example) In the other pattern, a spool is passed to the object by a construction parameter, and later unwound by its creator (effectively destroying the object, and any others that had wound themselves onto it). The latter is the more common pattern; but the codebase [of textbender] has examples of both. In any case, once unwound, a spool is never resused.

Field Summary

Fields

Modifier and Type	Field and Description
static Catcher0R<Hold>	CATCHER_0 A common instance of a null catcher.

Method Summary

Methods

Modifier and Type	Method and Description
boolean	add(Hold hold) Adds the hold to the spool or releases it immediately.
boolean	isUnwinding() Returns true if the spool is unwinding or already unwound.
boolean	unwind() Commences to unwind this spool.
boolean	unwind(Catcher<Hold> catcher) Commences to unwind this spool, removing and releasing each of its holds.

Field Detail

CATCHER_0

static final Catcher0R<Hold> CATCHER_0

A common instance of a null catcher.

Method Detail

add

boolean add(Hold hold)

Adds the hold to the spool or releases it immediately. If the spool is unwinding then the hold is released immediately, otherwise it is added.

Returns:

true if the hold is added; false if it is released instead.

Throws:

IllegalStateException - if this contract cannot be met (e.g. if the spool runs out of space, and no other unchecked exception applies).

isUnwinding

boolean isUnwinding()

Returns true if the spool is unwinding or already unwound. Once true, it never reverts to false.

unwind

boolean unwind()

Commences to unwind this spool. Equivalent to unwind(CATCHER_0).

Returns:

true if unwinding commences with this call; false if it had already commenced.

unwind

boolean unwind(Catcher<Hold> catcher)

Commences to unwind this spool, removing and releasing each of its holds. Works in LIFO order. Once unwinding is commenced, subsequent calls to this method have no effect.

Parameters:

catcher - the catcher for any errors or exceptions that occur during unwinding.

Returns:

true if unwinding commences with this call; false if it had already commenced.