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.

Similar Messages

How are static methods handled in a multithreaded application?

hi
i have a class Drawer with a static method public static draw(Graphics g). now assume there are more thrads callin at the same time Drawer.draw(g). What happens? have some threads to wait until the others have finished calling the method, or can static methods be multithreaded, (can they perform simultaniously)?
thanks, jo

ups, i am not drawing on the screen, but in every thread i am drawing a Image. this means before i call the static method in every thread there will be created a BufferedImage and then i pass the Graphics from this images to the static method. also in this case the method performs simultaniously? every thread has its own image and graphics, so the static method should be perform at the same time, isn't it?

Is getString() method in ResourceBundle thread safe?

Is getString() method in ResourceBundle thread safe?

i was just thinking about the following scenario.
Lets assume that there are 2 threads accessing the same ResourceBundle object.
and are calling getString(key) method. In this case if thread1 passes key1 and thread2 passes key2, there might be a chance that both the threads get the same value as the counter is shared across these threads. I believe if ResourceBundle uses some synchronized objects like HashTable, then we are sure that it is thread safe.
Let me know your comments on this pls.

Accesing database from static method on java collaboration

Hi *,
I want cache data from a oracle database in a java collaboratioon when java collaboration is enabled by emanager.
My java collaboration has a oracleOTD
I did an static method on java collaboration, but I can't access database from static method.
Thanks
Hector

hi hector,
this wont work!
regards chris

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.

Are DLL methods called as instance or static methods from Java ?

Hi,
I have only a small question.
When I call a C++ DLL method from Java using JNI, do I instantiate it like an object method or is it called as a static method ?
I am asking you this because I will use servlets (Java 1.2.2) to call a DLL. These servlets may run concurrently in any moment, and I pass some data to this DLL. I want to know if there is any possibility of this data becoming corrupt, because of one concurrent servlet rewriting the data another servlet has passed.
Thanks and regards.

The question is not specific enough.
Methods in C/C++ can be qualified with the following forms:
-global (C/C++)
-static member(C++ class method)
-instance member(C++ class method)
Since you didn't provide your code I couldn't possible determine which of the above you are using, but presumably you know.
On the other hand a java native method is either static or non-static depending on how you defined it in the class. Once again without the code there is no way to say and presumably you already know this.
The context of the methods is the same context that a similar servlet method has. But naturally in C/C++ is is easy to circumvent the context - for example by using a global variable.
Data in C/C++ can always become corrupt and in the vast (all?) cases it is because something is wrong in the C/C++ code. Like a pointer that is not initialized, or a pointer used after it is free'd. Or writing past the end of a buffer, or writing before the beginning of a buffer.

Static methods in multi-threaded environment

Hi,
I am wondering what happens when several threads try to access the same static method ?
The static method is not synchronized.
Will this create any performance issues ?
The actual scenario is of a J2EE server, where several Session Bean instances try to access a static method in a utility class. Will some session beans have to wait till others are being serviced ?
thnx n regards
s giri

thanx for replying.
yes. the operations are thread-safe. we do not change the state of any object.
but we make an rmi call to another application thru this static method.
is it ok?
Currently, my session bean has too many private methods - each calling the other application. Due to some quality metrics (a class must be restricted to 700 lines) we have to consider moving these private methods into some Helper class in the form of "public static methods". We have made many utility methods as static, but have some reservations abt doing a similar thing for these methods that call other application.
regards
Shivraman

Static methods in multi-thread environment.

Sorry for this silly question, but I have no clue...
If you're using classes with static methods in a application server environment, would that imply all threads use that same 'one in the jvm' method and would there be a possibility for performance loss?
Is calling another static method from a static method a good idea in a multi-thread environment, with respect to performance? Somehow I seem to get this java.lang.NoClassDefFoundError. Any ideas? The classes are in the same package. I only get these kind of problems calling static methods from static methods in a multi-thread environmnent. The static methods are pure utility functions and of course do not use class state.
Thanks for your help.

Sorry for this question, wasn't thinking straight. I have the answer, won't post question on multiple forums.

Are i++, i-- atomic and thread safe for int i

I want to minimize the sync time multiple threads take to contend for a shared pool, which is implemented as a FIFO. My plan is to:
1. Use a int variable "count" to trace the free obj count. Hence eliminates the need of synchonized methods size() and isEmpty()
2. Increment & decrement count when implemeting push() and pop()
3. Minimize the number of statements in synchonized block as possible within push() and pop(), which in this case, excludes count++ and count-- provided these operations are atomic and thread safe
Have I lost something ?

So, may I make a conclusion.
To minimize contention, I use FIFO instead LIFO or stack and build it from scratch without extending from existing Collection-related class
1 public class FIFO {
2 . . private volatile int count;
3 . . private Object head, tail;
4 . . public Object pop() {
5 . . . .synchonized (tail) {
6 . . . . .if (count >= 3) {
7 . . . . . .Object tail_origin = tail;
8 . . . . . .// Re-organizing the linked list skipped
9 . . . . . .synchonized (this) {
10 . . . . . . count--;
11 . . . . . }
12 . . . . . return tail_origin;
13 . . . . }
14 . . . . else {
15 . . . . . throw new PoolEmptyException();
16 . . . . }
17 . . . }
18 . . }
19 . . public void push(Object o) {
20 . . . synchronized (head) {
21 . . . . . // Re-organizing the linked list skipped
22 . . . . . head = o;
23 . . . . . synchonized (this) {
24 . . . . . . count++;
25 . . . . . }
26 . . . }
27 . . }
28 . . public int sizeOf() {
29 . . . return count;
30 . . }
31 }
1. The reason I choose FIFO is to avoid the contention between push() and pop() -- they are NOT declared as synchonized, instead they only lock head and tail respectively. (Sorry I borrow the name push and pop from stack)
2. pop() only works when count >= 3 to avoid head == tail after pop() and hence a further call to push() block concurrent call to pop() and vice versa (see line 5 and 20)
3. The goal is to minimize the time and scope of contention. In line 9 and 23, I lock the whole pool object only when modify count, which is not atomic.
4. This pool is used for a thread pool supporting many concurrent short-lived HTTP request
Have I missed something ?
Thanks a lot!

Why are static methods called with null references,valid ?

This is my code :
package inheritance;
public class inh6{
     public static void method(){
     System.out.println("Called");
     public static void main(String[] args){
          inh6 t4 = null;
     t4.method();
O/P :
CalledHere t4 is a null reference and yet we are able to call a method on it,why is null pointerexception not thrown.Why are we able to call static methods using null references ?
t4 is null means it doesnot refer to any memeory address,hence how is method() called correctly.
I hope i am clear. :)
Thank you for your consideration.

punter wrote:
jverd wrote:
Memory addresses have nothing to do with it. I doubt memory addresses are even mentioned once in the JLS.
By memory address i mean the memory location the reference is pointing to.I know what you mean. But if you think it's relevant, can you show me where in the JLS it says anything about memory locations?
>
You can do that because a) t4's type is "reference to inh6" and b) method() is declared static, which means that you don't need an object to call it, just the class. That class comes from the compile time type of t4. The fact that t4 is null at runtime is irrelevant.
So at compile time the type of t4 is inh6 and hence the method is called.Is it ? Had method() not been static a NullPointerException would have been thrown.Correct.
With non-static, non-private, non-final methods, which implementation of the method gets called is determined at runtime, buy the class of the object on which it's being called. If any one of those "non"s goes away, then the method is entirely determined at compile time, and in the case of static methods, there's no instance necessary to call the method in the first place.

Static method and threads?

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
Also check out http://java.sun.com/j2se/1.4.2/docs/api/java/lang/ThreadLocal.html

Please confirm that stubs are thread-safe

Can someone please confirm that if one is to invoke the same remote method from more than one thread in a client application that one does not need to get multiple remote stubs for the same RMI server object. As long as the implementation of the remote method is thread safe, everything okay, correct?
In otherwords, are remote stubs them selves thread safe? I always assumed they were.
Thanks in advance

Stubs are threadsafe. The reason is that a second concurrent invocation via the same stub will use a new TCP connection. I took this up with Sun once and the response was along the lines 'if it doesn't say it isn't thread-safe, it's thread-safe'.
It's the method implementation at the server that isn't thread-safe, unless you make it so.

Thread Safe Testing

I havn't written much mult-threaded code but I realise it's important when many web round-trips are required.
How do I design, test and verify that I have a thread-safe application? I don't like relying on the testing statistics of chance unless the odds of failure are 0.0001%!
Is there a correct way to verify a thread-safe application?

I agree it is a lofty, though sadly in all probability too time consuming, goal to achieve. However, there are a few pitfalls to watch out for that should get most of the common, pernicious thread safety issues:
Synchronize data that must be shared. This will involved performance bottle-necks. If the data is read-mostly, consider a Singleton cache. Though pitfalls will always remain here. Data sharing among threads, IMO, is the single greatest issue.
Use static methods and instances wisely. If an object has instance variables that are used from method to method call (either within the class or from the caller), then state exists for that object. It is inherently a candidate to look at in terms of thead-safety. This class should be created for each logical unit of work that depends on its state. So, do not reuse these classes among threads (or at least provide an init() or reset() method to clear the state). The same applies to static methods. You can safely use a static method from multiple threads if it does not modify other static variables (e.g., changing the state of a static variable).
Within the Collections API, pay attention to which types are synchronized and which are not. The Javadocs will specify.
Try to write methods thread-safe. Have them only use the arguments provided in the method's signature.
Watch for relatively simple race conditions that can easily be spotted.
static private Singleton INSTANCE;
private Singleton() {
    super();
    // Now I will fetch values from a database, perhaps taking seconds
static final public Singleton getInstance() {
      if (INSTANCE == null) {
         INSTANCE = new Singleton(); // what if this takes some time, e.g. database query to get cache values?
      return INSTANCE;
}Not good. Here's a possible refactoring:
private static final Singleton INSTANCE; // bonus, we can now make it final
static {
   INSTANCE = new SIngleton(); // Occurs only when class is loaded, atomic
public static final Singleton getInstance() {
return INSTANCE; // no race condition
}The above would be even more applicable and pronounced if you offered a static refresh() method on the instance, say to refresh its cache of data from a database.
Seems easy, right? The rub is that you need to verify that a given method does not call other methods that are not thread-safe. If every class you design follows the above precepts, then you will probably only be chasing down rarer thread-safety issues.
- Saish

Is KeyStore thread safe?

Hi all,
My class has KeyStore instance as static field.
When KeyStore#getKey(String alias, char[] password) mthod is called with right alias and password on multi-thread environment, normaly I can get Key object successfully.
But rarely following UnrecoverableKeyException is thrown from that method.
Caused by: java.security.UnrecoverableKeyException: Given final block not properly padded
     at com.sun.crypto.provider.SunJCE_z.a(DashoA13*..)
     at com.sun.crypto.provider.JceKeyStore.engineGetKey(DashoA13*..)
     at java.security.KeyStore.getKey(KeyStore.java:763)Is KeyStore (specifically JceKeyStore) thread safe?
Thanks in advance.

In general a Java API class or method isn't thread safe unless it is specifically so documented.
You could always synchronize it and see if it helps ...

Thread safe servlets

I have a question regarding the semantice of servlets. Consider you are calling a method
public StringmodifyAddress(){......}in the service method of the servlet. Now considering that a servlet could be sericing multiple requests would this method be technically thread safe? If not what could we do to make it safe.
cheers

Ok let me rephrase and provide a little more details. Sorry I messed up the question the first time
private String email;
public void doPost(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
   User user = new User;
   String address = user.modifyAddress(email);
}Now in the class User
public String modifyAddress(String email)
//writes the email address to a database
}Now I feel since user is a variable local to the service method it is thread safe, but a coworker feels that since email is a member variable it would not be thread safe. Please advise it seems that he might be correct.

Are static methods in Java thread safe?

Similar Messages

Maybe you are looking for