Question on synchronized method / block and Thread Cache
Hi all,
I came across a blog post at http://thejavacodemonkey.blogspot.com/2007/08/making-your-java-class-thread-safe.html which is talking about making Java classes thread safe. But, I was surprised to see that a thread can have my class' instance variable in its cache. Also, I have couple of questions on the things posted in the blog. I want to get the opinion from Java experts here.
1. Given the example class
class MyClass{
private int x;
public synchronized void setX(int X) { this.x = x; }
public synchronized int getX() { return this.x; }
Having the following instructions in two threads won't guarantee that it will print 1 and 2 as other thread can get the slot and call setX to modify the value - Am I right?
obj is an MyClass Instance available to both the threads
Thread 1:
obj.setX(1);
System.out.println(obj.getX());
Thread 2:
obj.setX(2);
System.out.println(obj.getX());
It will print 1 and 2 (in any order) only if I synchronize these calls on "obj" as follows - Is my understanding correct or ???
Thread 1:
synchronized(obj)
obj.setX(1);
System.out.println(obj.getX());
Thread 2:
synchronized(obj)
obj.setX(2);
System.out.println(obj.getX());
2. If my understanding on point 1 (given above) is right, What the blog-post says is wrong as I cannot even expect my thread 1 to print 1 and thread 2 to print 2. Then, again a question arises as why a thread cache has a object's instance variable value in its cache and need to make my instance variable volatile. Can anyone explain me in detail? Won't the thread always refer the heap for Object's instance variable value?
Thanks in advance,
With regards,
R Kaja Mohideen
your basic understanding (as far as i can understand what you've written) seems to be correct. if you run your first 2 threads, you can get "11", "12", "21", or "22" (ignoring newlines). if you run your second 2 threads, you can get "12" or "21".
i'm not sure i follow your second point about your thread's "cache". i think you are asking about the visibility of changes between threads, and, no, there is not concept of a shared "heap" in the memory model. basically, (conceptually) each thread has its own working memory, and it only shares updates to that memory when it has to (i.e. when a synchronization point is encountered, e.g. synchronized, volatile, etc). if every thread was forced to work out of a shared "heap", java on multi-core systems would be fairly useless.
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LOOP -- tant que l'item n'est pas le dernier du block on loop
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END;In my current project we work on quite huge forms which can have a consequent number of blocks and items.
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So my question :
This method seems to be quite efficient as it goes thrue blocks and items sequences as they are defined in the builder comparing to go_block -> go_item ->do_something which can easily turn into nightmare-programming.
But I don't really know about network roundtrips with this kind of method.
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Anyway; as said I just counted the roundtrips and looked where I can avoid them when I made our application ready for WAN.
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I would like to differences between Synchronized methods and blocks.
- MuniWell, you'll get so many of right answers in next ten to thirty minutes.
I like to yield :)
Oooo... Ten minutes has passed.
Synchronized block is a toilet room with a lock in a public lavatory.
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And, she/he have to receive the lock from a particular object obj like:
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Message was edited by:
hiwa
Message was edited by:
hiwa -
Question about synchronizing two methods...
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Any help is appreciated!799454 wrote:
So wait,
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synchronized method 1
synchronized method 2
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javanewbie80 wrote:
Great. Thanks. This whole explanation made a lot of sense to me.Cool, glad I was able to help!
Probably I was just trying to complicate things unnecessarily.It's a classic case of complexity inversion. It seems simpler to say something like "synchronization locks the class" or "...locks the method" than to give my explanation and then extrapolate the implications. Just like the seemingly simpler, but incorrect, "Java passes objects by reference," vs. the correct "Java passes references by value," or Java's seemingly complex I/O vs. other languages' int x = readInt(); or whatever.
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"Prashant" <[email protected]> wrote in message
news:[email protected]..
>
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I have seen a piece of code like this,
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If I am correct, I have a further question, a "synchronized" block means a set of operations that can not be interrupted, but when invoke "wait" method, the thread running in the "synchronized" block will be terminated (interrupted) by release the lock and other threads will have chance to enter the "synchronized" block. In short, the execution inside the "synchronized" block will be interrupted. So, I wonder whether the above piece of code is correct and conforms to the principle of "synchronized". And what is its real use?
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I just wondered whether "wait inside a synchronized
block" technology is thread safe. Please look at the
following example,wait and synchronized are thread safe.
public class Foo {
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may have tampered with T1's execution path because T2
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>
I think the most safest way is never wait inside a
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Synchronized method's block list implementation
In an object with synchronized methods, the OS maintains the list of blocked objects waiting to enter synchronzied methods. For this reason, I'm guesing, the Java spec seems to state the selection of these blocked objects as "arbitrary."
I am trying to determine that there is no risk of stavation in this blocked pool of threads. For example, a FIFO implementation of the blocked-object list would guarantee that all objects eventually entered their appropriate synchronized method.
Does anyone know for certain what blocked object selection algorithm is present on a JVM running on a Win32 platform? On SunOS 5.6? I believe the former uses a priority-based, preemptive thread scheduling with a round-robin-style time quantum to prevent starvation. Not sure what the latter uses.
Any ideas? Thanks!
-Jeff IshaqIn an object with synchronized methods, the OS
maintains the list of blocked objects waiting to enter
synchronzied methods. For this reason, I'm guesing,
the Java spec seems to state the selection of these
blocked objects as "arbitrary."
As a general rule, you can never depend on the underlying threading model. The trend is to use the native threading model but that is not guaranteed by the specification.
I am trying to determine that there is no risk of
stavation in this blocked pool of threads. For
example, a FIFO implementation of the blocked-object
list would guarantee that all objects eventually
entered their appropriate synchronized method.
When a thread attempts to acquire a lock, it only checks to see if another thread already holds the lock, and not if another thread is already waiting for the lock. Therefore you should always consider the possibility that starvation can occur.
Does anyone know for certain what blocked object
selection algorithm is present on a JVM running on a
Win32 platform? On SunOS 5.6? I believe the former
uses a priority-based, preemptive thread scheduling
with a round-robin-style time quantum to prevent
starvation. Not sure what the latter uses.
Again, you should avoid depending on the native threading model because the specification does not guarantee that the java threading model is going to behave the same way.
To guarantee that each thread can acquire the lock you will need to develop a lock that has a queue associated with it. When a thread attempts to acquire the lock it is placed in a queue waiting until it is the first element in the queue. When the thread releases the lock it removes itself from the queue and notifies the other threads that are waiting.
When using a queued lock you will not necessarily need to use the synchronized keyword on the method or block since the lock will take care of synchronization when a thread attempts to acquire the lock. -
Need blocking method without blocking main thread
I have this problem:
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frame.setVisible(true);
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return;
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I could solve my problem by using a modal Dialog and let the user enter some data there and then exit the Dialog. Dialog.show() would block until the user exits it again. But I would rather use my frame, so I took a look at the code for Dialog.show() method, and it does something like this:
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* pump events, filter out input events for
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dispatchThread.pumpEventsForHierarchy(new Conditional() {
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Any suggestions? I think there are circumstances where you would like a method block, but where it is the main thread that calls the method (out of my control), and you would not like the main thread block unless you could make sure the gui would still repaint and events would run.Ah, nm. The main thread is not the same as the event thread.
Whew! :)
The first who reply will get the dukes. -
I have a question regarding "static". for example, I have the following code
class Test{
public int i=0;
public static calculateInt(){
....some operations on i;
if two processes attempt to call Test.calculateInt() at the same time. What will the effects be? shall these two calls be implemented totally independent? each call holds it own "i" for calculation. OR there will be operation conflicit on i.
what if I change "public i" to :private i", the same effects?You can't operate on i in a static method, since i is an instance variable.
If you make i static, then you can operate on it in a static method. In that case, all threads will share a single i--there's only one i for the whole class, and threads don't change that. Making it private won't change that.
If you make the method into an instance (non-static) method, then any threads operating on the same instance will share the same i, regardless of whether it's public or private. Different threads operating on different instances will each have their own i.
Java's Thread Tutorial
JavaWorld: Introduction to Java threads
IBM: Introduction to Java threads
Google: java+threads+tutorial
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Thread caching generally and the print API specifically
hi - new slant on the old "printing" chestnut
code goes something like this.
a)
PrintService[] x = javax.print.PrintServiceLookup.lookupPrintServices(null, null);
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code end, then repeat
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PrintService[] z = javax.print.PrintServiceLookup.lookupPrintServices(null, null);
code end
example a)
if i run the same piece of code twice over, deleting a printer after the first time thru, i get the right answer on the second run.
example b)
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i think the JVM or thread is caching the answers to this static call "lookupPrintServices". so i get the same answer until the code ends and the JVM exits.
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thanksIn fact, these server process have multiple devices attached because there are different hardware requirements depending on metadata recorded in the XML file (paper size/type, duplex printing). When a new hardware requirement arises, we may need to install a different device. It may also be the case that an XML contains a new printer name but that an existing device matches the requirements. We then install a new (logical) printer and map it to the port of the existing matching device.
Any way, the device selection is data-driven and is somewhat dynamic (we will not add printer every day/hour but it will happen and the service should preferably not be interrupted)
The problem of a new report type with hardware requirements already supported by existing devices could be solved by a override method but the problem of new hardware specifications, requiring the installation of a new printer can currently be solved only by bouncing the VM, which must be done carefully (being sure that a report has been processed completely and that the next one has not yet been started).
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