Replacement for Finder
Does anyone know of a replacement for Finder? I've tried Forklift and like it. However, being the neat freak that I am, I don't like duplicates. I would like to replace Finder. Is that even possible?
you can quit finder completely. it will remain in the dock though. I don't use Pathfinder but I'm pretty sure its help pages explain how to do that. but in any case, to quit finder run the following terminal command
defaults write com.apple.finder QuitMenuItem -boolean True; killall Finder
this will add a Quit option to Finder's Finder menu and you can use this menu to quit Finder.
Similar Messages
-
Replacement for Finder's "Connect to Server" interface?
The Finder -> Go -> Connect to Server interface is a bit minimal, to put it mildly.
Ideally, I'd like to be able to:
• Rename the connections ("smb://192.168.1.32" isn't very helpful)
• Organize them in folders
• Sort by name, connection type, etc.
• Specify whether or not to save user/pwd in the Keychain
Anyone know of any third-party connection-manager utilities?That's what I was thinking. I have read numerous "how-to's" about this, and all of them say that this is possible, but I think what I am really reading is "How to copy files between computers" and not "How to use a PC as a server."
Thanks again! -
"Find and Replace" for field names in a fillable PDF
Is it possible to do a "Find and Replace" for the field names in a fillable PDF? For example, I have multiple fields that contain the word "Proposed Insured" as part of the field name and would like to find and replace all of them with "Owner". Is there an easy way to do this?
Not really. Even a script can't just rename a field. It needs to create a
new field on top of the old one, but then you lose all the associated
settings, like validation, calculation, format, keystroke, etc. -
Where can I find a replacement for this?
This adapter (I guess) broke/got bent so now I'm using the extension cord to charge my MacBook Pro.
Where can I buy a replacement for this?:
http://i41.tinypic.com/kced7k.jpg
http://i41.tinypic.com/2u90jsg.jpg
Also, what's it called?What about the world travel adapter
http://store.apple.com/us/product/MB974ZM/B?fnode=MTY1NDEwMQ
The Kit is designed to work with iPod, iPhone, iPad, Apple MagSafe Power Adapters (for MacBook and MacBook Pro), Portable Power Adapters (for iBook and PowerBook), and AirPort Express. -
Replacement for dvi to vga adapter for late 2008 17"Macbook pro
Hello,
I have a 17" inch late 2008 mack pro. The apple dvi to vga adapter cable is not working anymore. I can seem to find a replacement for it. I have tried the generic dvi to vga adapter (image attached). This does not work. Please adviseHello mshameel,
It may help to reset your system's SMC and NVRAM.
Intel-based Macs: Resetting the System Management Controller (SMC)
http://support.apple.com/kb/HT3964
About NVRAM and PRAM
http://support.apple.com/kb/HT1379
Cheers,
Allen -
Having spent a little time searching for a replacement download for iWeb (details of why below) it seems that I cannot find it anywhere in Apple Land. From my limited experience here (iphone, macbook pro, airport extreme and airport express) it looks like Apple gradually withdraws stuff prior to introducing something really exciting to replace it.
So
Question: Is there a new super replacement for iWeb about to be introduced?
I cannot believe that Apple would take away something as great as iWeb (having read reviews and used it myself as a complete web novice) without giving us something better.
I foolishly followed some genius advice and did a wipe/reload of Lion, prior to updating to Mountain Lion (my fault, I thought my lovely Macbook Pro was running a little slow - probably wasnt - just me being impatient.) Anyway, long story short, I lost iphoto (got it back by re-purchasing but would like a refund - another string running with this), iWeb - can't find it anywhere - asked questions here in Apple Land and am getting some conflicting answers.
I am sat in London without my iweb for a some days and would really like to get on with a bit of web stuff.
Question: is there anywhere in Apple Land where I can download iWeb
Question: can you confirm that the iweb programme will be on one of the disks that came with the machine? (they are 100 miles away so I will have to wait, unless there is an easier way)
Sorry, rambling a bit.
Hope you can help out there
Thanks
SteveThe iWeb application was specifically created to be used with .Mac and then MobileMe and Apple withdrew support for iWeb several months ago, when it was announced that MobileMe would be no longer and would be replaced by iCloud. MobileMe ceased at the end of June this year and has been replaced by iCloud, that does not offer web hosting, so no need for iWeb.
Apple stopped selling iWeb in its stores several months ago and has withdrawn support for iWeb, so no, you won't find either iWeb or iDVD for sale in the Mac App Store because Apple no longer sells it.
You cannot download iWeb, so if you want iWeb, you will have to purchase the iLife 11 boxed set from Amazon and this contains iWeb 09 which is the latest version of iWeb. Apple does not sell this either.
If your Mac was running Snow Leopard, then it would have come with installation CD's, so yes, you should be able to re-load the whole of the iLife series, which includes iWeb, iDVD and iPhoto back onto your Mac running Mountain Lion. You did not need to make an additional purcahse of iPhoto from the Mac App Store - you could have re-loaded from these CD's. The only place you will get a refund is by contacting the Mac App Store. Nobody here can help you with that.
So if you want iWeb back, check to see if it is on you installation CD's - it should be if you have a Mac running Snow Leopard and put it back on your Mac from there. If not, then your only option is to purchase the iLife 11 boxed set from Amazon and you can load iWeb from there.
As Apple no longer supports iWeb and there is no MobileMe any longer, then there are unikely to be any further updates to the programme and nothing else will come along. You can still use the app though if you have it and it works with Mountain Lion, however, there may come a time when it won't work with future iOS updates, so it would pay you to look at the alternatives out there which are many and varied. Take a look at RapidWeaver, Sandvox, Freeway Pro/Express, Flux 4 etc. You can download free trials of most and some are for sale in the Mac App Store.
Also, please be advised that this is a user to user forum and everyone here who replies to you is a user, so you are not talking directly to Apple here.
Give Apple feedback directly by going to http://www.apple.com/feedback. -
A replacement for the Quicksort function in the C++ library
Hi every one,
I'd like to introduce and share a new Triple State Quicksort algorithm which was the result of my research in sorting algorithms during the last few years. The new algorithm reduces the number of swaps to about two thirds (2/3) of classical Quicksort. A multitude
of other improvements are implemented. Test results against the std::sort() function shows an average of 43% improvement in speed throughout various input array types. It does this by trading space for performance at the price of n/2 temporary extra spaces.
The extra space is allocated automatically and efficiently in a way that reduces memory fragmentation and optimizes performance.
Triple State Algorithm
The classical way of doing Quicksort is as follows:
- Choose one element p. Called pivot. Try to make it close to the median.
- Divide the array into two parts. A lower (left) part that is all less than p. And a higher (right) part that is all greater than p.
- Recursively sort the left and right parts using the same method above.
- Stop recursion when a part reaches a size that can be trivially sorted.
The difference between the various implementations is in how they choose the pivot p, and where equal elements to the pivot are placed. There are several schemes as follows:
[ <=p | ? | >=p ]
[ <p | >=p | ? ]
[ <=p | =p | ? | >p ]
[ =p | <p | ? | >p ] Then swap = part to middle at the end
[ =p | <p | ? | >p | =p ] Then swap = parts to middle at the end
Where the goal (or the ideal goal) of the above schemes (at the end of a recursive stage) is to reach the following:
[ <p | =p | >p ]
The above would allow exclusion of the =p part from further recursive calls thus reducing the number of comparisons. However, there is a difficulty in reaching the above scheme with minimal swaps. All previous implementation of Quicksort could not immediately
put =p elements in the middle using minimal swaps, first because p might not be in the perfect middle (i.e. median), second because we don’t know how many elements are in the =p part until we finish the current recursive stage.
The new Triple State method first enters a monitoring state 1 while comparing and swapping. Elements equal to p are immediately copied to the middle if they are not already there, following this scheme:
[ <p | ? | =p | ? | >p ]
Then when either the left (<p) part or the right (>p) part meet the middle (=p) part, the algorithm will jump to one of two specialized states. One state handles the case for a relatively small =p part. And the other state handles the case for a relatively
large =p part. This method adapts to the nature of the input array better than the ordinary classical Quicksort.
Further reducing number of swaps
A typical quicksort loop scans from left, then scans from right. Then swaps. As follows:
while (l<=r)
while (ar[l]<p)
l++;
while (ar[r]>p)
r--;
if (l<r)
{ Swap(ar[l],ar[r]);
l++; r--;
else if (l==r)
{ l++; r--; break;
The Swap macro above does three copy operations:
Temp=ar[l]; ar[l]=ar[r]; ar[r]=temp;
There exists another method that will almost eliminate the need for that third temporary variable copy operation. By copying only the first ar[r] that is less than or equal to p, to the temp variable, we create an empty space in the array. Then we proceed scanning
from left to find the first ar[l] that is greater than or equal to p. Then copy ar[r]=ar[l]. Now the empty space is at ar[l]. We scan from right again then copy ar[l]=ar[r] and continue as such. As long as the temp variable hasn’t been copied back to the array,
the empty space will remain there juggling left and right. The following code snippet explains.
// Pre-scan from the right
while (ar[r]>p)
r--;
temp = ar[r];
// Main loop
while (l<r)
while (l<r && ar[l]<p)
l++;
if (l<r) ar[r--] = ar[l];
while (l<r && ar[r]>p)
r--;
if (l<r) ar[l++] = ar[r];
// After loop finishes, copy temp to left side
ar[r] = temp; l++;
if (temp==p) r--;
(For simplicity, the code above does not handle equal values efficiently. Refer to the complete code for the elaborate version).
This method is not new, a similar method has been used before (read: http://www.azillionmonkeys.com/qed/sort.html)
However it has a negative side effect on some common cases like nearly sorted or nearly reversed arrays causing undesirable shifting that renders it less efficient in those cases. However, when used with the Triple State algorithm combined with further common
cases handling, it eventually proves more efficient than the classical swapping approach.
Run time tests
Here are some test results, done on an i5 2.9Ghz with 6Gb of RAM. Sorting a random array of integers. Each test is repeated 5000 times. Times shown in milliseconds.
size std::sort() Triple State QuickSort
5000 2039 1609
6000 2412 1900
7000 2733 2220
8000 2993 2484
9000 3361 2778
10000 3591 3093
It gets even faster when used with other types of input or when the size of each element is large. The following test is done for random large arrays of up to 1000000 elements where each element size is 56 bytes. Test is repeated 25 times.
size std::sort() Triple State QuickSort
100000 1607 424
200000 3165 845
300000 4534 1287
400000 6461 1700
500000 7668 2123
600000 9794 2548
700000 10745 3001
800000 12343 3425
900000 13790 3865
1000000 15663 4348
Further extensive tests has been done following Jon Bentley’s framework of tests for the following input array types:
sawtooth: ar[i] = i % arange
random: ar[i] = GenRand() % arange + 1
stagger: ar[i] = (i* arange + i) % n
plateau: ar[i] = min(i, arange)
shuffle: ar[i] = rand()%arange? (j+=2): (k+=2)
I also add the following two input types, just to add a little torture:
Hill: ar[i] = min(i<(size>>1)? i:size-i,arange);
Organ Pipes: (see full code for details)
Where each case above is sorted then reordered in 6 deferent ways then sorted again after each reorder as follows:
Sorted, reversed, front half reversed, back half reversed, dithered, fort.
Note: GenRand() above is a certified random number generator based on Park-Miller method. This is to avoid any non-uniform behavior in C++ rand().
The complete test results can be found here:
http://solostuff.net/tsqsort/Tests_Percentage_Improvement_VC++.xls
or:
https://docs.google.com/spreadsheets/d/1wxNOAcuWT8CgFfaZzvjoX8x_WpusYQAlg0bXGWlLbzk/edit?usp=sharing
Theoretical Analysis
A Classical Quicksort algorithm performs less than 2n*ln(n) comparisons on the average (check JACEK CICHON’s paper) and less than 0.333n*ln(n) swaps on the average (check Wild and Nebel’s paper). Triple state will perform about the same number of comparisons
but with less swaps of about 0.222n*ln(n) in theory. In practice however, Triple State Quicksort will perform even less comparisons in large arrays because of a new 5 stage pivot selection algorithm that is used. Here is the detailed theoretical analysis:
http://solostuff.net/tsqsort/Asymptotic_analysis_of_Triple_State_Quicksort.pdf
Using SSE2 instruction set
SSE2 uses the 128bit sized XMM registers that can do memory copy operations in parallel since there are 8 registers of them. SSE2 is primarily used in speeding up copying large memory blocks in real-time graphics demanding applications.
In order to use SSE2, copied memory blocks have to be 16byte aligned. Triple State Quicksort will automatically detect if element size and the array starting address are 16byte aligned and if so, will switch to using SSE2 instructions for extra speedup. This
decision is made only once when the function is called so it has minor overhead.
Few other notes
- The standard C++ sorting function in almost all platforms religiously takes a “call back pointer” to a comparison function that the user/programmer provides. This is obviously for flexibility and to allow closed sourced libraries. Triple State
defaults to using a call back function. However, call back functions have bad overhead when called millions of times. Using inline/operator or macro based comparisons will greatly improve performance. An improvement of about 30% to 40% can be expected. Thus,
I seriously advise against using a call back function when ever possible. You can disable the call back function in my code by #undefining CALL_BACK precompiler directive.
- Like most other efficient implementations, Triple State switches to insertion sort for tiny arrays, whenever the size of a sub-part of the array is less than TINY_THRESH directive. This threshold is empirically chosen. I set it to 15. Increasing this
threshold will improve the speed when sorting nearly sorted and reversed arrays, or arrays that are concatenations of both cases (which are common). But will slow down sorting random or other types of arrays. To remedy this, I provide a dual threshold method
that can be enabled by #defining DUAL_THRESH directive. Once enabled, another threshold TINY_THRESH2 will be used which should be set lower than TINY_THRESH. I set it to 9. The algorithm is able to “guess” if the array or sub part of the array is already sorted
or reversed, and if so will use TINY_THRESH as it’s threshold, otherwise it will use the smaller threshold TINY_THRESH2. Notice that the “guessing” here is NOT fool proof, it can miss. So set both thresholds wisely.
- You can #define the RANDOM_SAMPLES precompiler directive to add randomness to the pivoting system to lower the chances of the worst case happening at a minor performance hit.
-When element size is very large (320 bytes or more). The function/algorithm uses a new “late swapping” method. This will auto create an internal array of pointers, sort the pointers array, then swap the original array elements to sorted order using minimal
swaps for a maximum of n/2 swaps. You can change the 320 bytes threshold with the LATE_SWAP_THRESH directive.
- The function provided here is optimized to the bone for performance. It is one monolithic piece of complex code that is ugly, and almost unreadable. Sorry about that, but inorder to achieve improved speed, I had to ignore common and good coding standards
a little. I don’t advise anyone to code like this, and I my self don’t. This is really a special case for sorting only. So please don’t trip if you see weird code, most of it have a good reason.
Finally, I would like to present the new function to Microsoft and the community for further investigation and possibly, inclusion in VC++ or any C++ library as a replacement for the sorting function.
You can find the complete VC++ project/code along with a minimal test program here:
http://solostuff.net/tsqsort/
Important: To fairly compare two sorting functions, both should either use or NOT use a call back function. If one uses and another doesn’t, then you will get unfair results, the one that doesn’t use a call back function will most likely win no matter how bad
it is!!
Ammar MuqaddasThanks for your interest.
Excuse my ignorance as I'm not sure what you meant by "1 of 5" optimization. Did you mean median of 5 ?
Regarding swapping pointers, yes it is common sense and rather common among programmers to swap pointers instead of swapping large data types, at the small price of indirect access to the actual data through the pointers.
However, there is a rather unobvious and quite terrible side effect of using this trick. After the pointer array is sorted, sequential (sorted) access to the actual data throughout the remaining of the program will suffer heavily because of cache misses.
Memory is being accessed randomly because the pointers still point to the unsorted data causing many many cache misses, which will render the program itself slow, although the sort was fast!!.
Multi-threaded qsort is a good idea in principle and easy to implement obviously because qsort itself is recursive. The thing is Multi-threaded qsort is actually just stealing CPU time from other cores that might be busy running other apps, this might slow
down other apps, which might not be ideal for servers. The thing researchers usually try to do is to do the improvement in the algorithm it self.
I Will try to look at your sorting code, lets see if I can compile it. -
Double Factory pattern purposal as replacement for Double Check #2
Hi All,
Here is the code for the pattern proposal, its intended as a replacement for double checked locking, which was proved to be broken in 2001. Here is the code...
public class DoubleFactory {
private static Object second_reference = null;
public static Object getInstance() {
Object toRet = second_reference;
if (toRet == null) {
second_reference = CreationFactory.createInstance();
toRet = second_reference;
return toRet;
private DoubleFactory() {}
public class CreationFactory {
private static Object instance = null;
public static synchronized Object createInstance() {
if (instance == null) {
instance = new Object();
return instance;
}Also I have spent several months discussing this with Peter Haggar, who believes that this code is not guaranteed to work. However I have been unable to discern from his message why he believes this will not be guaranteed to work, and I am posting this here to attempt to find a clearer explanation or confirmation that the pattern I am purposing (Double Factory) is guaranteed to work.
Thanks,
Scott
---------------------------- Original Message ----------------------------
Subject: Re: [Fwd: Double Factory replacement for Double Check #2] From:
"Scott Morgan" <[email protected]>
Date: Fri, January 25, 2008 10:36 pm
To: "Peter Haggar" <[email protected]>
Hi Peter,
I appologize if my last response came accross as rude or something. If
my code is not guaranteed to work ok, can you help me understand why. I
am after all looking for a solution for all of us.
If my solution is wrong as you say because the member variables of the
singleton are not up to date. I understand this to mean that the
second_reference pointer is assigned to the memory where the instance
object will get created before the instance object even starts the
creation process (when the jvm allocates memory and then enters the
constructor method of the Singleton). This doesn't seem possible to me.
Can you refrase your statments, to help me understand your points?
If not I am happy to turn to the original wiki for discussion.
Thanks for your effort,
Scott
Thanks for asking my opinion, many times, then telling me I'm
wrong...wonderful. You are a piece of work my friend. For what it'sworth, your email below shows you still don't understand these issues
or what I was saying in my emails. I've been more than patient.
>
All the best. And by the way, your code is not guaranteed to work. It's not just me that's "wrong", it's also the engineers at Sun who
designed Java, the JVM, and the memory model, and countless people who
have studied it. I'm glad you have it all figured out.
>
Peter
"Scott Morgan" <[email protected]>
01/18/2008 12:47 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
Thanks I understand your position now. However am I still believe that
it will work and be safe;
1) the Singleton you show would be fully constructed (having exited theSingleton() method) before the createInstance() method would have
returned.
2) The second_reference could not be assigned until the createInstance()
method returns.
3) So by the time second_reference points to Singleton all of the valueswill be set.
>
>
I do understand that if the createInstance method was not synchronized(at the CreationFactory class level) that my logic would be flawed, but
since there is synchronization on that method these points are true, and
your comments about up-to-date values are not accurate.
>
Cheers,
Scott
>In your listing from your latest email T2 does encounter a sync block
on createInstance.
>>>>
No. T2 will call getInstance and see second_reference as non-null.second_reference was made non-null by T1.
>>
>>>>
What are you exactly are you refering to with the phrase 'up-to-datevalues'?
>>>>
Assume my singleton ctor is thus:
public class Singleton
private int i;
private long l;
private String str;
public Singleton()
i = 5;
l = 10;
str = "Hello";
T2 will get a reference to the Singleton object. However, because youaccess second_reference without synchronization it may not see i as 5,
l as 10 and str as "Hello". It may see any of them as 0 or null. This
is not the out of order write problem, but is a general visibility
problem because you are accessing a variable without proper
synchronization.
>>
Peter
"Scott Morgan" <[email protected]>
01/16/2008 11:38 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
In your listing from your latest email T2 does encounter a sync blockon createInstance.
>>
What are you exactly are you refering to with the phrase 'up-to-datevalues'?
In this code the Singleton should also be
A) non mutable (as in the instance of class Object in the example).
If the singleton was more complex then the code to populate it'svalues
would go inside the sync of createInstance().
B) mutable with synchronization on it's mutator methods.
In your article you mention out of order writes, which doesn't occurin
this code.
Cheers,
Scott
You read it wrong.
- T1 calls getInstance which in turn calls createInstance.
- T1 constructs the singleton in createInstance and returns to
getInstance.
- T1 sets second_reference to the singleton returned in getInstance. -T1 goes about its business.
- T2 calls createInstance.
- T2 sees second_reference as non-null and returns it
- Since T2 accessed second_reference without sync, there is noguarantee
that T2 will see the up-to-date values for what this object refers to.
- Therefore the code is not guaranteed to work.
>>>
If this is not clear:
- Re-read my email below
- Re-read my article
- If still not clear, google on Double Checked Locking and readanything
from Brian Goetz or Bill Pugh.
Peter
"Scott Morgan" <[email protected]>
01/13/2008 05:26 AM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
Thanks for the reply, I don't see how T2 would see the a referenceto
a
partialy initialized object before the createInstance() method had
returned. If T1 was in createInstance() when T2 entered
getInstance(), T2 would wait on the CreationFactory's class monitor to
wait to enter createInstance().
Or in other words in the line of code ....
second_reference = CreationFactory.createInstance();
The pointer second_reference couldn't be assigned to the singleton
instance when the synchronized createInstance() had fully constructed,initialized and returned the singleton instance. Before that the the
second_reference pointer would always be assigned to null. So any
thread entering getInstance() before createInstance() had returned
(for the first time) would wait on the CreationFactory's class monitor
and enter the createInstance() method.
>>>
So T2 will wait for T1.
Cheers,
Scott
PS I think I am writing requirements for my next project :)
Sorry for the delay...been in all day meetings this week.
You are correct...I had been reading your code wrong, my apologies.
My explanations, although correct, did not exactly correspond to your
code.
However, the code is still not guaranteed to work. Here's why:
Assume T1 calls getInstance() which calls createInstance() and returnsthe
singelton. It then sets second_reference to refer to that singleton.
So
far, so good. Now, T2 executes and calls getInstance(). It can see
second_reference as non-null, so it simply returns it. But, there
was
no
synchronization in T2's code path. So there's no guarantee that even
if
T2 sees an up-to-date value for the reference, that it will seeup-to-date
values for anything else, ie what the object refers to...it's
instance data. If T2 used synchronization, it would ensure that it
read
up-to-date
values when it obtained the lock. Because it didn't, it could see
stale
values for the object's fields, which means it could see a partially
constructed object.
>>>>
In the typical double-checked locking, the mistake is to assume theworst
case is that two threads could race to initialize the object. But
the worst case is actually far worse -- that a thread uses an object
which
it
believes to be "fully baked" but which is in fact not.
Peter
"Scott Morgan" <[email protected]>
01/03/2008 06:33 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
Thanks for responding, I am still thinking that your mis
interpreting
the code so I have rewritten it here (Replacing
DoubleFactory.instance with DoubleFactory.second_reference for
clarity). If the T1 burps (gets interrupted) in the createInstance
method it wouldn't have returned so the second_reference pointer
would have never been
assigned
so T2 would just try again upon entering the getInstance method. Orif
it had already entered getInstance it would be waiting to enter
(until T1 releases the lock on CreationFactory.class ) on the
createInstance method.
>>>>
public class DoubleFactory {
private static Object second_reference = null;
public static Object getInstance() {
Object toRet = second_reference;
if (toRet == null) {
second_reference =
CreationFactory.createInstance();
toRet = second_reference;
return toRet;
private DoubleFactory() {}
public class CreationFactory {
private static Object instance = null;
public static synchronized Object createInstance() {
if (instance == null) {
instance = new Object();
return instance;
Does this clear up my idea at all?
second_reference should be always pointing to
null
or
a fully initialized Object
(also referenced by the pointer named 'instance' ), I don't see howit would end up partially initialized.
>>>>
Thanks Again,
Scott
"It" refers to T2.
Your createInstance method is identical to my Listing 2 and is fine
and
will work.
Yes, the problem with your code is in getInstance.
>I don't see how the DoubleFactory getInstance method could bereturning
a partially initialized object at this point. If CreationFactoryalways
returns a fully initialized object and DoubleFactory only assigns a
new
reference/pointer to it how could DoubleFactory getInstance return a
reference/pointer to partially initialized object?
>>>>>>>
>>>>>
The reason it is not guaranteed to work is explained in my previousemails
and in detail in the article. However, I'll try again. Anytime you
access shared variables from multiple threads without proper
synchronization, your code is not guaranteed to work. Threads are
allowed
to keep private working memory separate from main memory. There are
2
distinct points where private working memory is reconciled with main
memory:
- When using a synchronized method or block - on acquisition of thelock
and when it is released.
- If the variable is declared volatile - on each read or write of
that
volatile variable. (Note, this was broken in pre 1.5 JVMs which isthe
reason for the caveat I previously mentioned)
Your createInstance method uses synchronization, therefore, the
reconciliation happens on lock acquisition and lock release. T1 can
acquire the lock in createInstance, make some updates (ie create an
object, run it's ctor etc), but then get interrupted before exiting
createInstance and therefore before releasing the lock. Therefore,
T1
has
not released the lock and reconciled its private working memory withmain
memory. Therefore, you have ZERO guarantee about the state of mainmemory
from another threads perspective. Now, T2 comes along and accesses
"instance" from main memory in your getInstance method. What will
T2
see?
Since it is not properly synchronized, you cannot guarantee that T2sees
the values that T1 is working with since T1 may not have completely
flushed its private working memory back to main memory. Maybe it
did completely flush it, maybe it didn't. Since T1 still hold the
lock,
you
cannot guarantee what has transpired. Maybe your JVM is not usingprivate
working memory. However, maybe the JVM your code runs on does or
will
some day.
Bottom line: Your code is not properly synchronized and is notguaranteed
to work. I hope this helps.
Peter
"Scott Morgan" <[email protected]>
01/03/2008 12:49 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
Thanks for your response, I don't follow what 'it' refers to in
the
phrase 'It can see'. So for the same reason that you state that
example 2 from your article I believe this class CreationFactory to
work flawlessly when a client object calls the createInstance
method.
>>>>>
I see this CreationFactory code as identical to your example 2 doyou agree with this?
>>>>>
public class CreationFactory {
private static Object instance = null;
public static synchronized Object createInstance() {
if (instance == null) {
instance = new Object();
return instance;
}Then my rational in the DoubleFactory class is that it can obtain a
reference/pointer to the fully initialized object returned bycalling the above code. I believe you think that the problem with
my code is
in
the DoubleFactorys getInstance method, is this correct?
I don't see how the DoubleFactory getInstance method could bereturning
a partially initialized object at this point. If CreationFactory
always
returns a fully initialized object and DoubleFactory only assigns a
new
reference/pointer to it how could DoubleFactory getInstance return a
reference/pointer to partially initialized object?
>>>>>
Thanks again,
Scott
public static synchronized Singleton getInstance() //0
if (instance == null) //1
instance = new Singleton(); //2
return instance; //3
This above code is fine and will work flawlessly.
Annotating my paragraph:
T1 calls getInstance() and obtains the class lock at //0. T1 "sees"
instance as null at //1 and therefore executes: instance = new
Singleton() at //2. Now, instance = new Singleton() is made up of
several lines of non-atomic code. Therefore, T1 could be
interrupted
after Singleton is created but before Singleton's ctor isrun...somewhere
before all of //2 completes. T1 could also be interrupted after
//2 completes, but before exiting the method at //3. Since T1 has
not
exited
its synchronized block it has not flushed its cache. Now assume T2
then
calls getInstance().
All still true to this point. However, with your code the nextparagraph
is possible, with the code above, it's not. The reason is that T2
would
never enter getInstance() above at //0 because T1 holds the lock. T2will
block until T1 exits and flushes it's cache. Therefore, the code
above
is
properly thread safe.
It can "see" instance to be non-null and thus
return it. It will return a valid object, but one in which its ctor
has
not yet run or an object whose
values have not all been fully flushed since T1 has not exited itssync
block.
"Scott Morgan" <[email protected]>
01/02/2008 06:10 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
Thanks for the response I understand the rational for inventing
the
double check anti pattern, I am sorry I still don't understand the
difference between your solution #2 and my CreationFactory class.
>>>>>>
From your article figure 2.public static synchronized Singleton getInstance() //0
if (instance == null) //1
instance = new Singleton(); //2
return instance; //3
If I understand your email correctly this figure 2 is also flawed,since...
>>>>>>
T1 calls getInstance() and obtains the class lock at //0. T1 "sees"
instance as null at //1 and therefore executes: instance = new
Singleton() at //2. Now, instance = new Singleton() is made up ofseveral lines of non-atomic code. Therefore, T1 could be
interrupted
after Singleton is created but before Singleton's ctor isrun...somewhere
before all of //2 completes. T1 could also be interrupted after
//2 completes, but before exiting the method at //3. Since T1 has
not
exited
its synchronized block it has not flushed its cache. Now assume T2
then
calls getInstance(). It can "see" instance to be non-null and thus
return it. It will return a valid object, but one in which its
ctor
has
not yet run or an object whose
values have not all been fully flushed since T1 has not exited itssync
block.
So is #2 is also flawed for this reason?
If so please revise your article, since I interpreted #2 as a
plausible
solution recommended by you (which lead me to the DoubleFactory
idea).
If not please help me understand the difference between #2 and my
CreationFactory class.
>>>>>>
Thanks,
Scott
#2 is in Listing 2 in the article. What I meant was to forget the
DCL
idiom, and just synchronize the method...that's what listing 2
shows.
DCL
was invented to attempt to get rid of the synchronization for 99.9%
of
the
accesses.
The solution I outlined in my email is using the DCL idiom, but on
a
1.5
or later JVM and using volatile.
You solution is not guaranteed to work. Here's why:
public class DoubleFactory {
private static Object instance = null;
public static Object getInstance() {
Object toRet = instance;
if (toRet == null) {
instance =
CreationFactory.createInstance();
toRet = instance;
return toRet;
private DoubleFactory() {}
public class CreationFactory {
private static Object instance = null;
public static synchronized ObjectcreateInstance()
//1
if (instance == null) {
instance = new Object(); //2
return instance;
} //3
}T1 calls createInstance() and obtains the class lock at //1. T1"sees"
instance as null and therefore executes: instance = new Object() at//2.
Now, instance = new Object() is made up of several lines of
non-atomic
code. Therefore, T1 could be interrupted after Object is created
but
before Object's ctor is run...somewhere before all of //2
completes.
T1
could also be interrupted after //2 completes, but before exiting
the
method at //3. Since T1 has not exited its synchronized block ithas
not
flushed its cache. Now assume T2 then calls getInstance(). It can"see"
instance to be non-null and thus return it. It will return a
valid object, but one in which its ctor has not yet run or an
object
whose
values have not all been fully flushed since T1 has not exited itssync
block.
The bottom line is that if you are accessing shared variables
between
multiple threads without proper protection, you are open for aproblem.
Proper protection is defined as: proper synchronization pre 1.5,
and
proper synchronization or proper use of volatile 1.5 or after.
Therefore, if you must use the DCL idiom you have one option: -
Use DCL with volatile on a 1.5 or later JVM.
>>>>>>>
You can also forget about DCL and just use synchronization (listing2
in
my article) or use a static field (listing 10 in my article).
I hope this clears it up.
Peter
"Scott Morgan" <[email protected]>
01/02/2008 04:00 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
Re: [Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
I apologies for not understanding but I don't see what is
different
between the solution you purposed...
2) Don't use DCL but use synchronization
and the code that I am putting forward. Perhaps I do just notunderstand
but you seem to be contradicting yourself in this email?
I understand that you are saying in #2 that this will always 'work'
with
out any issues...
public static Object instance = null;
public static synchronized Object getInstance() {
if (instance == null) {
instance = new Object();
return instance;
But first you seem to say in the email that if T1 gets
interrupted
it
may leave the instance pointing to a partially initialized object?
So as far as I understand it the createInstance method in my
CreationFactory class should be successful (always retuning a
fully initialized object) for the same reason #2 is successful.
Please keep in mind that there are two different instancepointers
in
the code I sent you, one is part of the DoubleFactory class and
the other is part of the CreationFactory class.
>>>>>>>
Thanks for your time, just looking for better solutions!
Scott
Scott,
Your solution is not guaranteed to work for various reasons
outlined
in
the article. For example, you can still return from your code apartially
initialized object. This can occur if T1 gets interrupted beforeleaving
the synchronized method createInstance() and T2 calls
getInstance().
T2
can "see" toRet/instance as non-null but partially initialized
since
T1
has not fully flushed its values.
As of 1.5, Sun fixed various issues with the memory model that
were
broken. Double Checked Locking will still break unless you usevolatile
(which was fixed in 1.5). Therefore, the following code works:
volatile Helper helper;
Helper getHelper() {
if (helper == null)
synchronized(this) {
if (helper == null)
helper = new Helper();
return helper;
but the original DCL idiom will not work. So, your options are:
1) Use DCL with volatile (above)
2) Don't use DCL but use synchronization
3) Don't use DCL, but use a static field.
#2 and #3 are outlined in my article from 2002.
Hope this helps,
Peter
"Scott Morgan" <[email protected]>
12/26/2007 04:12 PM
Please respond to
[email protected]
To
Peter Haggar/Raleigh/IBM@IBMUS
cc
Subject
[Fwd: Double Factory replacement for Double Check #2]
Hi Peter,
Thanks for the article on the out of order write problem. Whatdo
you
think of this as a solution?
TIA,
Scott
---------------------------- Original Message----------------------------
Subject: Double Factory replacement for Double Check #2
From: "Scott Morgan" <[email protected]>
Date: Wed, December 26, 2007 2:55 pm
To: [email protected]
Hi Ward,
Here is a pattern submission
Double Factory
Lazy initialization of singletons in accepted for a while usingthe
double check pattern. However it has been discovered that the
double
check pattern isn't thread safe because of the out of order write
problem. This problem occurs when Threads entering the Singleton
Factory method return with a fully constructed, but partially
initialized, Singleton object.
>>>>>>>>
Therefore: It makes sense to look for a way to initializeSingletons
in
a Lazy and Thread Safe manor. The following illustrates a fairly
simple
solution...
package foo;
public class DoubleFactory {
private static Object instance = null;
public static Object getInstance() {
Object toRet = instance;
if (toRet == null) {
instance =
CreationFactory.createInstance();
toRet = instance;
return toRet;
private DoubleFactory() {}
public class CreationFactory {
private static Object instance = null;
public static synchronized ObjectcreateInstance()
if (instance == null) {
instance = new Object();
return instance;
This gets around the out of order write problem because all
Threads
waiting on the CreationFactory's Class monitor will have a fully
constructed and initialized instance when they actually exit the
createInstance method.
>>>>>>>>
>>>>>>>>
During runtime while the Singleton instance is getting created(constructed and initialized) there may be a few Threads waiting
on
the
CreationFactory Class's objects monitor. After that period all
the
Treads
accessing
the Singleton will have unsynchronized reads to the instance,
which
will
optimize execution.
References:
http://www.ibm.com/developerworks/java/library/j-dcl.html
Copyright 2007 Adligo Inc.Scott-Morgan wrote:
Hi All,
Thanks for your comments, here are some more....
jtahlborn you state that
the only way to guarantee that a (non-final) reference assignment is visible across threads is through the use of volatile and synchronized,
From the jvm spec
http://java.sun.com/docs/books/jls/third_edition/html/memory.html
17.4.1 Shared Variables
Memory that can be shared between threads is called shared memory or heap memory.
All instance fields, static fields and array elements are stored in heap memory.
Since both the second_reference and instance fields are both static, they are shared and visible across all threads.Yes, all these things are shared across threads, however, if you keep reading, there is a notion of "correct" sharing. obviously these values may be visible, that's why double-checked locking was used for so long before people realized it was broken. it worked most of the time, except when it didn't, and that's what i'm trying to show. that the only way to correctly share state between threads is via synchronization points, the most common being volatile and synchronized (there are a couple of other less used ones which don't apply here). The articles you linked to below from ibm cover the "visibility" in great depth, this is exactly what i am referring to.
You also state that volatile is a solution, but you seem to rebut your self in stating that the overhead for volatile is almost as great as synchronization.
This article illustrates the solution, and also comments on the overhead of volatile.
http://www.ibm.com/developerworks/library/j-jtp03304/
linked from
http://www.ibm.com/developerworks/java/library/j-dcl.html
volatile is a solution, in that it is correct, and you avoid the appearance of synchronization each time. however, since the semantics of volatile were strengthened in the new memory model, using volatile will perform practically (if not exactly) the same as simply synchronizing each time. the article you link to says exactly this under the heading "Does this fix the double-checked locking problem?".
Also could you be more specific about the example at the end of the jvm memory spec page, like a section number?It's the very last thing on the page, the "discussion" under 17.9, where it mentions that changes to "this.done" made by other threads may never be visible to the current thread. -
Dv6 hard drive failing how do i transferrer os 7 and hp info ??? i have a 500g wd replacement for it
dv6 hard drive failing how do i transferrer os 7 and hp info ??? I have a 500G wd replacement for it.
I have transferrer my self using WD Smart ware and a separate win 7 backed up to a WD 2T external ( win 7 backup is now struggling to back up but I do have one copy)
when new HD was put in, and repair disk put in it wouldn't pick up the back up off the WD 2 T. WIN 7 or my manual backup.
So I loaded up a win 7 32bit (off an older computer which is now parts) but it wouldn't reconis Win 7 back up on the WD 2 T.
Using WD smart ware. I did transferrer all the files over to the new HD but could only make an put it in a new folder. Not reload back to the same as it was before I had to transferrer every thing.
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Here is a topic which helps you to clone the old HDD:
http://www.makeuseof.com/tag/5-ways-to-clone-and-copy-your-hard-drive/
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**Click the KUDOS thumb up on the left to say 'Thanks'**
Make it easier for other people to find solutions by marking a Reply 'Accept as Solution' if it solves your problem. -
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Need replacement for function module "WWW_GET_SCRIPT_AND_HTML"
Hi,
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EXPORTING
obj_name = 'SDCAS_XML_TEMPLATE'
TABLES
html = xml_output.
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Hello,
Likely I should be using a search button but too much go over and well I am lazy doing so, so I'm apologies for this.
So I have Directv and I'm watching most are NHL (it's essy to get on Apple TV), Dicovery, History, National Geographic, Travel Channel, TLC, NET Geo Wild, SCI, VEL, NBC Sports, some local like NBC, FOX, ABC, BBCAmerica and some other channels cant remember now, but it's all in HD. Also my kids are watching all kids channels and kids shows.
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--TimFWIW, if you've found a bug, please contact Apple Support folks or (if you have access) use the BugReport tool.
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One convenient feature of it is that it can move/copy photos from your existing MMe galleries as well as your iPhoto library and other sources:
As for for iWeb can do after MMe is gone read the following:
On June 30, 2012 MobileMe will be shutdown. HOWEVER, iWeb will still continue to work but without the following:
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◼ Password protection
◼ Blog and photo comments
◼ Blog search
◼ Hit counter
◼ MobileMe Gallery
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Click to view full size
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OT -
Photo sharing replacement for mobileme galleries
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What resolution are My Photo Stream photos?
On your Mac or PC, your photos are downloaded and stored in full resolution. On your iPhone, iPad, iPod touch, and Apple TV, your Photo Stream photos are delivered in a device-optimized resolution that speeds downloads and saves storage space. While actual dimensions will vary, an optimized version of a photo taken by a standard point-and-shoot camera will have a 2048 x 1536 pixel resolution when pushed to your devices. Panoramic photos can be up to 5400 pixels wide.
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