Confused about static thread safe.help me

sample codes look like follows:
public static class test
public static method1(Object obj)
//some operations
}my question is that is method1 is thread safe?

Thread safety isn't about methods and it isn't about static. It's about being carefully about access to resources which may be accessed by more than one thread, and in particular may be changed by one thread and accessed by another.
Local variables are always thread safe, and there's no problem about serveral threads executing the same block of code concurently.
Thread safety is, in particular, about data items that are linked in some way so that there can be problems if one thread updates one piece of data on the basis of an out of date version of another.
Databases are, generally, designed to cope with that kind of thing.

Similar Messages

About the thread safe variable

Hello,
I have make 2 projects, the first is using to initialize the enviroument an loading some modules, and the 2nd using to execute a special work. In the first project, I use the DeclareThreadSafeScalarVar macro to define the handle of a file, I think the file handle can be shared between 2 processes, and each process can access data by the file handle.It is worth mentioning that the 2nd process loading is by the CreateProcess() API. My question is : if the safe variable is initialized in the first process, can I use the safe variable in the second process? Do I need to reinitialize the safe variable in the 2nd process? Thanks.
David

David,
Thread safe variables are only going to work for threads within the same process. Windows does not allow you to access memory outside of the virtual space of your application. Thus the processes act as if they are unaware of each other's data. Even if you send a pointer to the other application, it will not be able to access the data. There are a few other options for interapplication communication.
1. When you are calling CreateProcess() you can pass the new process some command line arguments. If the data you want to send can be serialized, then you can pass it to the other application at start using this command.
2. You can pass the data using WM_COPYDATA which allows you to set up a special structure and send it to the other application. This is similar to what you wanted to do with the thread safe variables, except that this data will be read only and must be sent to the other process using something like SendMessage(). This gets quite a bit more complicated.
3. You can set up a client server architecture and have all of the data management handled by the server process.
From what you have said about your application so far, it may be simpler to just create a single process with two threads. If the applications are dependent on each other, then they likely need to be part of the same process.
National Instruments
Product Support Engineer

Where to read about is-thread-safe tag?

Hello,
Does anybody know what tag <is-thread-safe> means and if it mandatory in ias-ejb-jar.xml runtime deployment descriptor. I've found it for helloworld sample for iAS 6.0 SP3 but it is not mentioned in AppServer Programmer's Guide for iAS 6.0 SP2 - the only last guide available on this subject for today.
Thank you,
Andrejus

The SQL plus reference guide immediately springs to mind, as I can see that you are using sqlplus for your output.
Actually, you don't need any sqlplus formatting. Gicven that you should know that an oracle table_name is never more than 30 characters, just rpad it to 30.
declare
cursor c1 is select owner, table_name
from dba_tables
where owner = 'SYSTEM'
group by owner, table_name;
sql_stmnt varchar2(4000);
cnt_val number;
begin
for c1_rec in c1 loop
sql_stmnt := 'select count(*) from '||c1_rec.owner||'.'||c1_rec.table_name;
execute immediate sql_stmnt into cnt_val;
dbms_output.put_line(rpad(C1_REC.TABLE_NAME,30)||' '||cnt_val||' rows');
end loop;
end;Message was edited by:
Keith Jamieson

Confusion About "Static"

If I have
public class abc
private static final int x=1;
public static def()
final int y=2;
}Since you can't make y static, and the method is static, does y become static automatically??

1. static methods are faster as there is no
polymorphismActually there is polymorphism from a Class P.O.V.
2. it's nice for helper classes like java.lang.Math
who have no instances.
3. They are needed for Objects without public
constructors (think Factory, Singleton, etc)The main reason for static methods (class methods) is to provide methods that are not instance specific. As phohmeyer mentioned classes like java.lang.Math have no instances because the needed funcitonality is provided by the class itself. There is nothing about the Math class that will change from instance to instance, so there is no need to ever instanciate one and use instance methods.
Think about the following:public class MyFirstClass extends Object{
static public void main(String[] args){
System.out.println("Hello World.");
}Normally, when you thing about using objects, you have to create one. However, how would you use MyFirstClass if it is the entry point of the system? In other words, where would you create an instance of MyFirstClass if your intent is to run MyFirstClass from the command prompt? Making main() a static (and public) method allows it to be called without creating an instance of MyFirstClass.
Likewise, the line that prints "Hello World." is making use of a static member. Note, an instance of System is never created. However, you are still allowed to access the output stream of System becasue out belongs to the class (that is, it exists as long as the class exists).

Confusion about make Thread running(waiting online)

the code like this
class a implements Runnable{
public void run(){
while(true){};
class App {
public static void main(String arg[]){
a mya=new a();
(new Thread(mya)).start();
(new Thread(mya)).start();
the two thread will run simultaneously .
could u tell me what's the order and step the two thread become runnable and then run.
why the two thread could run simultaneously?
They are the same priority and one is busy enough not to let others to run.
but actually they did run simultaneously.
why one thread could become runnable state and then running automaticatically!

I can't really understand your problem, but i think it this here:
A runnable class should look like this:
class myThread implements Runnable {
Thread thisThread
myThread () {
thisThread = new Thread(this);
thisThread.start();
public void run () {
while (Thread.currentThread() == this) {
//Do anything
class App {
public static void main (String a[]) {
(new myThread).start();
(new myThread).start();
This program will generate two threads, and as it's the sense of threads the will run simultaneously.
if you call start() of a thread, this will start the thread and goto next line, while the tread calls his run().
But where is the problem????

Confusion about statics

Hello,
Even though, we can use static variables in non-static methods , we are not allowed to use non-static variables in static methods. So Why??
Please expalin...

Even though, we can use static variables in
non-static methods , we are not allowed to use
e non-static variables in static methods. So Why??
Please expalin...A static variable "lives" in the static environment of the class. A non-static variable lives in the environment of an object. The latter could be a called a dynamic variable but it normally isn't.
You actually can access a non-static variable (the dynamic environment of an object) from a static method (the static environment of the class) if you have a reference to the object.

Confused about SOAP

Hi all,
As far as I know, SOAP is initially designed to be an XML based
RPC protocol.
But Java API for XML Messaging (JAXM), which is actually a
messaging API (and not RPC), is also based on SOAP.
How do they fit to each other?
In a few words, what does actually SOAP define?
I am confused about that, can you help me?
Any idea will be appreciated.
Thank you

In a few words -- no. Try here:
http://java.sun.com/webservices/docs/1.0/tutorial/index.html

Thread safe logging class, using final and static

Hi,
I'm looking through a logging class (which writes entries to a text file and so on) written by someone else, and I'm trying to determine whether it's thread-safe...
Essentially the class looks like this:
public final class Logger {
private static FileOutputStream fout;
private static PrinterWriter pout;
private static final long MaxLength = 100000;
static {
/* basically checks size of logfile, if bigger than 100k, then copy it as bak, and start again */
public void write(String msg) {
/* write entry into log file */
public void writeWithTimeStamp(string msg) {
/* write entry with time stamp into log file */
Now, I could be wrong, but I don't think that the class is thread-safe is it? in order to be thread-safe, I would have to use synchronized before write() and writeWithTimeStamp(), right?
My confusion arises from the use of the keyword "final" at the top of the class. This class isn't being inherited (or rather, there's no reason for the class not to be inheritable)... so what is it's purpose, if there is one?!
And my other question concerns the static block. But I need to describe the current setup first: The Logger class is being use by a file server. File server basically sits there, accepts socket connections from the outside, and takes text files and output them into a local directory. When a new socket connection is created, a new thread is created. As it stands right now, each thread instantiates its own Logger object which seems weird! Bringing me to my question which was, if each thread instantiates its own Logger object, the static block is only ran ONCE, regardless of how many threads (and therefore Logger objects) are created, right??
And wouldn't it be a better idea to simply create only ONE Logger object and pass it by reference to all newly created threads as they are needed so that all threads access the same and only Logger object? (and therefore utilize sychronization)
Thanks!

In JDK 1.4, there are already classes written that do all of that work. Check out the docs for the java.util.logging package. Specifically, check out FileHandler (which can rotate log files if they are greater than a specified size) and LogRecord (which is a standardized way to store logging entries). I believe the class is threadsafe, it doesn't say it isn't. Try it and see. If not, you can easily make a threadsafe wrapper for it (or any class for that matter).
The online documentation is at:
http://java.sun.com/j2se/1.4/docs/api/index.html
If you are curious, the simplest way to make any class threadsafe, if you don't want to or are not able to modify the class code itself is to define another class that calls methods of the original class, but that is synchronized.
Example:
    // NonThreadSafe.java
    public class NonThreadSafe {
        public NonThreadSafe (String s) {
            // Constructor
        public void SomeMethod (int param) {
            // Do some important stuff here...
        public int AnotherMethod (boolean param) {
            // Do some more important stuff here...
    // ThreadSafeWrapper.java
    public class ThreadSafeWrapper {
        protected NonThreadSafe nts;
        public ThreadSafeWrapper (String s) {
            nts = new NonThreadSafe(s);
        public synchronized void SomeMethod (int param) {
            nts.SomeMethod(param);
        public synchronized int AnotherMethod (boolean param) {
            return nts.AnotherMethod(param);
        public NonThreadSafe GetNonThreadSafe () {
            return nts;
    };Unfortunately, ThreadSafeWrapper isn't derived from NonThreadSafe, so you are somewhat limited in what you can do with it compared to what you could do with NonThreadSafe. AFAIK, you can't override unsynchronized methods with synchronized ones.
Another thing you could do is just write a method that writes to your logging class, and synchronize just that method. For example:
    // ThreadSafeLogger.java
    public class ThreadSafeLogger {
        public static synchronized void LogMessage (MyLogger log, String msg) {
            log.writeString(msg);
    // In another thread, far, far away:
        ThreadSafeLogger.LogMessage(theLog, "Blah");
        ...Hope that helps.
Jason Cipriani
[email protected]
[email protected]

Are static nested classes thread-safe?

There doesn't seem to be any definitive answer to this. Given the following code, is it thread-safe?
public class SomeMultiThreadedWebController {
public HttpServletResponse someMethodToExecuteViaWebRequest(HttpServletRequest request) {
    simpleQueryBuilder("SELECT...").addParameter("asdf","asdf").createQuery(EMF.getEntityManager()).executeUpdate();
protected static class SimpleQueryBuilder {
         private String queryString;
         private Map<String, Object> params = new HashMap<String, Object>();
         public SimpleQueryBuilder(String queryString) {
              this.queryString = queryString;
         public SimpleQueryBuilder addParameter(String name, Object value) {
              params.put(name, value);
              return this;
         public Query createQuery(EntityManager em) {
              Query query = em.createQuery(queryString);
              for (Entry<String, Object> entry : params.entrySet()) {
                   query.setParameter(entry.getKey(), entry.getValue());
              return query;
    public static SimpleQueryBuilder simpleQueryBuilder(String queryString) {
         return new SimpleQueryBuilder(queryString);
}Forget whether or not someone would do this, as this is just an example. I'm really trying to get at whether or not the instance variables inside the static nested class are thread-safe. Thanks for any responses.

Hello,
I believe you understand what you're talking about, but you state it in a way that is very confusing for others.
Let me correct this (essentially, incorrect uses of the terminology):
I agree that thread-safe or not is for an operation, for a member, it has some sort of contextual confusion.
Member has a much broader meaning in the [Java Language Specification|http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.4] . Even "class member" applies to both an attribute, a method, or an inner class or interface.
I think you mean "member variable" of a class (aka "attribute" or "field"). By the way, static or not is irrelevant to the rest of the discussion.
For an operation or a member, if there's only one thread could access it atomically in one moment, we could call it thread-safe.Mmm. I was tempted to say yes (I'm reluctant to commit myself). With an emphasis on "_The encapsulating class_ makes this member's usage thread-safe".
Still, just synchronizing each operation on a member is not enough to make all usages "thread-safe":
Consider a java.util.Vector: each add/get is synchronized, so it is atomic, fine.
However if one thread adds several values, let's say 3, one by one, to a vector that initially contains 0 values, and another thread reads the vector's size() (another properly synchronized method), the reader thread may witness a size anywhere among 0, 1, 2, 3, which, depending on the business logic, may be a severely inconsistent state.
The client code would have to make extra work (e.g. synchronizing on the vector's reference before the 3 adds) to guarantee that the usage is thread-safe.
Thus any synchronized method(With the limit stated above)
or immutable member (like primitive type) are thread-safe.
Additionally for a member, if it's immutable, then it's thread-safe. You mean, immutable primitive type, or immutable object. As stated previously, an immutable reference to a mutable object isn't thread-safe.
a static final HashMap still have thread-safe issue in practice because it's not a primitive.The underlined part is incorrect. A primitive may have thread-safety issues (unless it's immutable), and an object may not have such issues, depending on a number of factors.
The put, get methods, which will be invoked probably, are not thread-safe although the reference to map is.Yes. And even if the put/get methods were synchronized, the client code could see consistency issues in a concurrent scenario, as demonstrated above.
Additional considerations:
1) read/write of primitive types are not necessarily atomic: section [ §17.7 of the JLS|http://java.sun.com/docs/books/jls/third_edition/html/memory.html#17.7] explicitly states that writing a long or double value (2 32-bits words) may not be atomic, and may be subject to consistency issues in a concurrent scenario.
2) The Java Memory Model explicitly allows non-synchronized operations on non-volatile fields to be implemented in a "thread-unsafe" way by the JVM. Leading way to a lot of unintuitive problems such as the "Double-Checked Locking idiom is broken". Don't make clever guess on code execution path unless you properly synchronize access to variables across threads.
Edited by: jduprez on Mar 4, 2010 9:53 AM

Are Static methods Thread safe?

Hello All
I have a static method that takes a integer and returns a mathematically processed result.
Since this method is frequently required, I have placed it in a common class and declared it as static.
I want to know whether this method is thread safe?

There's nothing special about static methods, with regard to thread safety. If they access shared objects, then such access will usually need to be controlled by synchronisation; this might be on the object being accessed, some higher-level object or a special object allocated purely as a lock. The only way that you might think of them as special is that there's no instance of the Class on which you can synchronise.
You can declare a static method as synchronised. If you do, it will be synchronised on the single Class object of the class in which it is declared. This means that only one thread can be executing any part of the method at any one time. However, I understand that the Java Runtime itself sometimes synchronises on this Class object for its own reasons, therefore you might sometimes find a synchronised static method blocks when no other thread is executing it. Usually better, therefore, to explicitly synchronise on some object or other, than to use synchronised static methods.

Question about EJB and thread safe coding - asking again!

sorry everyone, no one seems to be interested in helping me on this, so i post it again.
i have a question about the EJB and thread safe coding. when we build EJBs, do we need to worry about thread safe issue? i know that EJBs are single threaded, and the container manages the thread for us. maybe i am wrong about this. if i wasnot, how can we program the EJB so that two or more instance of EJB are not going to have deadlock problem when accessing data resources. do we need to put syncronization in ours beans, do we even need to worry about creating a thread safe EJB? thanks in advance. the question really bothers me a lot lately.

sorry everyone, no one seems to be interested in
helping me on this, so i post it again.Excellent plan. Why not search a little bit on your own instead of waiting for your personal forum slaves to answer your call. See below.
i have a question about the EJB and thread safe
coding. when we build EJBs, do we need to worry about
thread safe issue? Read this: http://forum.java.sun.com/thread.jsp?forum=13&thread=562598&tstart=75&trange=15
i know that EJBs are single
threaded, and the container manages the thread for us.
maybe i am wrong about this. if i wasnot, how can we
program the EJB so that two or more instance of EJB
are not going to have deadlock problem when accessing
data resources. do we need to put syncronization in
ours beans, do we even need to worry about creating a
thread safe EJB? thanks in advance. the question
really bothers me a lot lately.
Java's Thread Tutorial
JavaWorld: Introduction to Java threads
IBM: Introduction to Java threads
Google: java+threads+tutorial

SingleThreadModel-static variables thread safe?

Hi..
I have one servlet which implements SingleThreadModel.My servlet contains two static variables .
public class MyServlet extends HttpServlet
implements SingleThreadModel {
private static String firstName="Umar hathab";
private static StringBuffer lastName= new StringBuffer("Abdullah");
I want to know whether this two variables will be thread safe? I know that static vars are shared in JVM..
Please help me..
Thanks in advance..
A.Umar

Hi heyad..
Static variables are shared among the instances.When we create two instances of an object which contains a static variable,both the instances share the same static variable.So there will be data corruption when two instances operate on the static variable at the same time.So I feel static variables are not thread safe.What I want to know is whether static variables are thread-safe when implemented by SingleThreadModel..
A.Umar

PSE 7 confusion about folder location, help needed

Here's some background: I'm using PhotoShop Elements 7 on Win XP. I have an external firewire hard drive I use for overflow/backups. Originally, this showed up as a K: drive on my system.
I added a RAID controller and two more drives to stripe as a single drive for temp space rendering movies. This ended up as a K: drive and the external moved to L: However, when I did this there were some pictures from my catalog that I had on the external drive. I have used File > reconnect missing files to handle the moves, but PSE is still confused (very).
PSE has started to mix up the RAID (K:) and external (L:) content. If I move a set of pictures (e.g. a folder of them) onto the external drive (L:) via drag and drop in the folder location view, the folder will show up empty on the L: heirarchy, but will show up as a gray folder icon in the K: RAID drive hierarchy. If I open up windows explorer, the RAID drive will appear empty. In other cases (I'm not sure when/how yet), pictures physically end up on the RAID drive itself instead of the external. I have uninstalled both the external drive and the RAID drive and reinstalled them both, but they are still getting confused in the folder view by PSE. I've also tried moving content outside of PSE and just reconnecting missing files, but it still wants to put things under the K: RAID drive instead of the external L:.
I've searched everywhere I can think of for a description of what gray folder icons in the folder location view means and can find none. I'm not sure how to convince PSE that there's nothing on the RAID K: drive and it shouldn't try to connect the two. Any suggestions?
Thanks,
Jim

Thank you! I ran psedbtool and the output shows the catalog has the wrong drive letter for the L: drive.
Is there a way to make PSE forget about the K: and L: drives that it is so confused about so that it can relearn of their existence? If I use the drive letter reassignment to fix PSE, it will break other apps that are now dependent on the current drive letter assignments. I've tried removing all files from the L: drive, but it's still remembered as part of the catalog.
Thanks,
Jim
psedbtool version 1.07
Opening catalog C:\Documents and Settings\All Users\Application Data\Adobe\Photoshop Elements\Catalogs\Moms pics\catalog.pse7db
Photoshop Elements version:      7
Total files               :   4833
Total files missing       :      0
Total files offline       :    278
Total files wrong volume :      0
****** Volumes with warnings: 1
PSE Volume Table
Path used by PSE        : F:
Type                    : builtin_drive
Status                  : online
Total files             : 0
Total files missing     : 0
Total files wrong volume: 0
Description             : MEDIA
Serial                  : 1C2F-9805
Drive_path_if_builtin   : F:
Id                      : 32520
Path used by PSE        : M:
Type                    : builtin_drive
Status                  : online
Total files             : 938
Total files missing     : 0
Total files wrong volume: 0
Description             : FreeAgent Drive
Serial                  : 7830-2970
Drive_path_if_builtin   : M:
Id                      : 93626
Path used by PSE        : E:
Type                    : builtin_drive
Status                  : online
Total files             : 0
Total files missing     : 0
Total files wrong volume: 0
Description             : GAMES
Serial                  : 025A-4B70
Drive_path_if_builtin   : E:
Id                      : 93625
Path used by PSE        : L:
Type                    : builtin_drive
Status                  : online
Total files             : 114
Total files missing     : 0
Total files wrong volume: 0
Description             : FreeAgent Drive
Serial                  : 60AE-A816
Drive_path_if_builtin   : K:
Id                      : 31354
***** WARNING *****     : Wrong drive letter in catalog
Path used by PSE        : \\dmzhost\media
Type                    : network_drive
Status                  : offline
Total files             : 278
Total files missing     : 0
Total files wrong volume: 0
Description             : //dmzhost/media
Serial                  : //dmzhost/media
Drive_path_if_builtin   :
Id                      : 26948
Path used by PSE        : J:
Type                    : builtin_drive
Status                  : online
Total files             : 3503
Total files missing     : 0
Total files wrong volume: 0
Description             : MEDIA2
Serial                  : BAC0-B5A3
Drive_path_if_builtin   : J:
Id                      : 108
Path used by PSE        : C:\Documents and Settings\All Users\Application Data\Adobe\Photoshop Elements\Catalogs\Moms pics
Type                    : database_relative_drive
Status                  : online
Total files             : 0
Total files missing     : 0
Total files wrong volume: 0
Description             : database relative
Serial                  : amoc:database_relative_volume
Drive_path_if_builtin   :
Id                      : 2
Windows Drives
Drive Type             Serial     Path
A:     removable_drive 0000-0000
C:     builtin_drive    9088-07F9
D:     builtin_drive    F408-1CB0
E:     builtin_drive    025A-4B70
F:     builtin_drive    1C2F-9805
G:     builtin_drive    242D-15ED
H:     readonly_drive   0000-0000
I:     builtin_drive    427D-3F6B
J:     builtin_drive    BAC0-B5A3
K:     builtin_drive    64E8-9846
L:     builtin_drive    60AE-A816
M:     builtin_drive    7830-2970
Thanks so much for your help!
Jim

I am confused about Icloud. I have a 5 year old Macbook, a 2 year old macbook and a 5 year old mac mini. What exactly do I need to do to get all this connected via Icloud. I also have an Iphone 3G. Help please!

I am confused about Icloud. I have a 5 year old Macbook, a 2 year old macbook and a 5 year old mac mini. What exactly do I need to do to get all this connected via Icloud. I also have an Iphone 3G. Help please! Also, I still don't understand exactly what the benefits are. At the moment it seems a lot of hassle to change a system that's been working perfectly well and that I'm very happy with. I'm not at all sure whether the mac mini (1.66 GHz INtel Core Duo) can be brought up to the relevant spec to run ICloud. Any thoughts?

There are two aspects to iCloud. One is simply getting an account, or migrating a mobileme account to iCloud. The second part is using the various features of iCloud to sync contacts, calendars, use music match or photo sharing, and store iWorks documents.
First to get an iCloud account:
If you have a mobileme account, you can migrate by simply logging onto www.icloud.com (ignore any references to Lion and just click through to complete the migration). If you have OS X Lion or an iOS 5 mobile device, you can create an iCloud account on that device using any valid AppleID.
Once you have an account, you can use JUST the mail account if you wishi - it is a standard IMAP account so you would set it up like any other IMAP email account you have, in whatever client program you use to read email.
To use the OTHER feautes of iCloud, your computers will have to be running OS X Lion (10.7). Your iPhone 3G will never be able to use iCloud as it cannot run iOS 5 (you need a min. of an iPhone 3GS to run iOS 5).
The requirements for OS X Lion are here - http://www.apple.com/macosx/specs.html You need a min. of a Core 2 duo processor so you mini is not capable of running Lion.
So, in your case, the single reasons for iCloud would be if you have a mobileme account, then use the online www.icloud.com to migrate your account so you do not lose your @me.com email address. You can use iCloud email (without using any other iCloud features) by setting it up as IMAP mail with these settings - http://support.apple.com/kb/HT4864
If you want an iCloud account and do NOT already have a mobileme account, you will need to upgrade one machine at least to Lion in order to be able to create an iCloud account.

Are static methods in Java thread safe?

Are static methods in Java thread safe?
thanks,
suresh

if static method use the instance variable
You mean member variable, where member variables are either class variables (static) or instance variables (non-static).
then you have to make it thread safe using
synchronization Not necessarily. Depends on requirements and usage context.
else in case of local var. it
is thread safe. Not necessarily. That local variable could refer to an object that's visible to multiple threads.
Statements like "Local variables are threadsafe but member variables aren't" are an oversimplification.
Something is threadsafe if it cannot be made to behave incorrectly simply by running it in a multithreaded context. Determining that can be very difficult.

Confused about static thread safe.help me

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