Hi Oliver, thanks for the explanation. The application itself is not single-threaded.
It is a servlet engine with potentially multiple simultaneous threads. What is intended
to be "single-threaded" is usage of a Node, e.g.:
Node n = store.getPath(path);
List l = n.children();
Node child = (Node) l.get(0);
child.write(...);
(I believe this is similar to JNDI...imagine another request thread doing the
same - with different Node instances which refer to the same resource) I am
not intending, for example, the child object to be handed to another thread.
Each Node implementation has its own lock member field, so with respect to
parent/child relationships, usage is single-threaded (although clearly
re-entrant as children must obtain parent locks).
I will use Thread.currentThread() for the "owner". I'm not clear as to the
utility of non-thread owners...what is the semantics of synchronization if the owners are
not threads? Or is the intention that owners ultimately must be associated with unique
threads?
E.g.
ReadWriteLock lock = ...;
String ownerA = "a";
String ownerB = "b";
lock.acquireWrite(ownerA, Long.MAX_VALUE);
lock.acquireWrite(ownerB, Long.MAX_VALUE);
Is the intention that the above statement deadlock? Whereas if the owner was
Thread.currentThread (or any 2 identical objects for that matter), I would
assume the call we be considered a re-entrant acquisition and not deadlock (?).
I guess I have always thought of the Thread (or some other concurrency
construct, such as Process) as the implicit owner in any synchronization scheme.
Aaron
Oliver Zeigermann wrote:
Hi Aaron!
On 6/24/05, Aaron Hamid <[EMAIL PROTECTED]> wrote:
Hi all,
I suspect I just reimplemented FileResourceManager, because I was not aware of
it until recently (despite using the org.apache.commons.transaction.locking
package). Basically I created a hierarchical Node interface, with operations
like: children() and read(), which implicitly obtain a readlock; write(), which
upgrades to a write lock; and close() which releases the locks.
First, I'm wondering if there is a way to list a directory through the API,
which itself requires a read lock, or whether I must explicitly obtain the
lock, and then perform the listing externally (which is cumbersome).
The FileResourceManager is more low level than your approach. It does
not have the notion of hierarchichal data and thus no idea of
children.
Also, I'm curious as to the significance of the "owner" parameter to the lock
API. For instance, I keep lock objects as a member field of Node implementations, using
the unique path as a resource id:
The "owner" is whatever object owns, i.e. holds, the lock. This can
e.g. be a thread, a process, a transaction, an application or
whatever. In "normal" Java locking this would be the thread.
public class NodeImpl {
private ReadWriteLock lock;
private Node parent;
public NodeImpl(Node parent, Path path, Store store) {
lock = new ReadWriteLock(new StorePath(store, path), null);
}
public void open() {
// should I send 'this' as owner?
lock.acquireRead(this, Long.MAX_VALUE);
}
public void openForWriting() {
// should I send 'this' as owner?
lock.acquireWrite(this, Long.MAX_VALUE);
}
public void write() {
parent.openForWriting();
this.openForWriting();
// .. do some writing ..
}
public void close() {
lock.release();
}
}
Now...what should the value of the "owner" object be? Should it naively be the 'this'
object? Or should it be the Thread.currentThread(), or is it completely irrelevant and only for
informational purposes? As you can see, to perform any writing, I must also lock the parent node
(for instance, if the write creates a new file...we can't allow reads/listings to be happening).
However, the parent node, in this implementation, will obtain the lock with an owner value of
itself. Now, I intend Node usage to only be single-threaded anyway (the strategy being other
threads would obtain a distinct node object, and not share node objects accross threads)...but in
any case, is this still legitimate, or should I be obtaining the parent lock with some /other/
value of owner? If I used distinct owners with that prevent the child not from acquiring the lock,
even though it is in its own thread? The semantics of "owner" don't seem to be very well
documented.
I thought the notion of an "owner" was rahter obvious. Anyway, if you
have a single threaded application, why use locking?
Oliver
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