Multiple static synchronized methods locking the same object ?

If I have multiple static synchronized methods in a
class will all the methods lock on the same (Class)
object ? I guess the answer to this would be yes. In
that case is it possible achieve synchronization
without an object ie code level synchronization ? If
yes, how ?There is nothing special about static synchronized methods vs. any other synchronization. It's just shorthand for syncing on the Class object for that class. The effects of synchronization are identical no matter what you sync on.
There's no way to achieve synchronization without an object. Synchronizing always obtains an object's lock. In the case of static synced methods, the object whose lock we obtain is the Class object for that class. As far as syncing goes, it's just another object.
There is no such thing as "code level synchronization" vs. any other kind. All syncing occurs in your Java code over blocks of code. For synced methods, those blocks are the entire respective method bodies.
Having said all that, I have no idea what you're really after or what you're trying to achieve.

Similar Messages

Calling static synchronized method in the constructor

Is it ok to do so ?
(calling a static synchronized method from the constructor of the same class)
Please advise vis-a-vis pros and cons.
regards,
s.giri

I would take a different take here. Sure you can do it but there are some ramifications from a best practices perspective. f you think of a class as a proper object, then making a method static or not is simple. the static variables and methods belong to the class while the non-static variables and methods belong to the instance.
a method should be bound to the class (made static) if the method operates on the class's static variables (modifies the class's state). an instance object should not directly modify the state of the class.
a method should be bound to the instance (made non-static) if it operates on the instance's (non-static) variables.
you should not modify the state of the class object (the static variables) within the instance (non-static) method - rather, relegate that to the class's methods.
although it is permitted, i do not access the static methods through an instance object. i only access static methods through the class object for clarity.
and since the instance variables are not part of the class object itself, the language cannot and does not allow access to non-static variables and methods through the class object's methods.

How do I use multiple classes to access the same object?

This should be staightforward. I have and object and I have multiple GUIs which operate on the same object. Do all the classes creating the GUIs need to be inner classes of the data class? I hope this question makes sense.
Thanks,
Mike

public final class SingletonClass {
private static SingletonClass instance;
private int lastIndex = 10;
public final static SingletonClass getInstance()
if (instance == null) instance = new SingletoClass();
return instance;
public int getLastIndex() {
return lastIndex;
}1) This won't work since one could create a new SingletonClass. You need to add a private constructor. Also, because the constructor is private the class doesn't have to be final.
2) Your design isn't thread-safe. You need to synchronize the access to the shared variable instance.
public class SingletonClass {
    private static SingletonClass instance;
    private static Object mutex = new Object( );
    private int lastIndex = 10;
    private SingletonClass() { }
    public static SingletonClass getInstance() {
        synchronized (mutex) {
            if (instance == null) {
                instance = new SingletonClass();
        return instance;
    public int getLastIndex() {
        return lastIndex;
}if you are going to create an instance of SingletonClass anyway then I suggest you do it when the class is loaded. This way you don't need synchronization.
public class SingletonClass {
    private static SingletonClass instance=new SingletonClass();
    private int lastIndex = 10;
    private SingletonClass() { }
    public static SingletonClass getInstance() {
        return instance;
    public int getLastIndex() {
        return lastIndex;
}If you don't really need an object, then you could just make getLastIndex() and lastIndex static and not use the singleton pattern at all.
public class SingletonClass {
    private static int lastIndex = 10;
    private SingletonClass() { }
    public static int getLastIndex() {
        return lastIndex;
}- Marcus

Intrinsic locks - static synchronized method

I am trying to understand the "static synchronized threads" - by theory when such a thread is invoked, it has to obtain a intrinsic lock on all the static variables. I wrote a sample program, but it is not giving me the desired results.
I have 3 threads, t1, t2, t3. t1 calls a static synchronized method crazy(), where i am using static int i. t2 and t3 calls a void function f2() and f3() which just prints i. Now i put a sleep in synchronized method crazy. I am expecting t1 to start and print i and go to sleep for 10 secs, release i and then t2 and t3 starts since crazy() holds an intrinsic lock on i. But the program calls t2 and t3 even if crazy puts the thread to sleep. What happend to the intrinsic lock on i ??
class RunnableThread implements Runnable{
static String i;
void f2() {
RunnableThread.i = "Two";
System.out.println(RunnableThread.i);
void f3() {
this.i = "three";
System.out.println(this.i);
static synchronized void crazy() {
try {
i = "One";
System.out.println(i);
Thread.sleep(10000);
System.out.println("Sleep done");
catch (Exception e ) {
e.printStackTrace();
public void run() {
System.out.println("Thread Name: " + Thread.currentThread().getName());
if (Thread.currentThread().getName().equals("two"))
f2();
} else if (Thread.currentThread().getName().equals("three"))
f3();
else if (Thread.currentThread().getName().equals("one"))
RunnableThread.crazy();
public static void main(String args[]) {
System.out.println("SOP from main");
RunnableThread rt1 = new RunnableThread();
RunnableThread rt2 = new RunnableThread();
RunnableThread rt3 = new RunnableThread();
Thread t1 = new Thread(rt1, "one");
t1.start();
Thread t2 = new Thread(rt2, "two");
t2.start();
Thread t3 = new Thread(rt3, "three");
t3.start();

lavanya.km wrote:
I am trying to understand the "static synchronized threads"Never heard of it. You might want to clarify your terminology.
- by theory when such a thread is invoked, it has to obtain a intrinsic lock on all the static variables. Nope. Doesn't happen.
I wrote a sample program,Ah, I see. You're creating synchronized static methods. Those do not even come close to "obtaining an intrinsic lock on all the static variables," even if there were such a thing as an "intrinsic lock," which there isn't. A synchronized method is just shorthand for enclosing the entire body in a sync block. In the case of a non-static method, it syncs on the "this" object. In the case of a static method, it syncs on the Class object for the class where the method is declared.
In no case does anything sync on "all the variables," static or not.

Multiple Transports for the Same Object.

I have ABAP program 123 and I make a change, create transport A and release the transport. I do not request transport A go to production.
I then modify ABAP program 123 again, create transport B and release the transport. I do not request transport B go to production.
Now I request transport A go to production.
Can someone explain to me why the changes in transport B are included when transport A is moved to production?
I thought each transport would be at its own change level.
Thank-you.

I can guarantee all of you that when Transport A was moved, it contained changes that were made in Transport B.
Surely you understand that Thomas is correct and that there's a logical reason, not related to transport layer configuration, for the issue? You CAN have multiple transports open for the same object without a lock by forcing it but check the dates as already suggested...

Can we have multiple transports for the same object.

Hi guys,
Can we have multiple transports for same object in dev system. Can anyone tell me how can this be done.
Thanks

Its not possible for the same development object. Only 1 person can access an object at a time and if mutiple users modify an object new TASKs are created under the same TRANSPORT.
Only after releasing the tr you can create a new tr on the same object.
Message was edited by:
Abhishek Jolly

If multiple users will be using the same iPad, how can I lock it so the settings cannot be changed, specifically the languages(students are changing the languages)?

If multiple users will be using the same iPad, how can I lock it so the settings cannot be changed, specifically the languages(students are changing the languages)?

Even with Apple Configurator most of the settings cannot be locked, and that includes the language settings. Most schools and organizations seem to just accept that they'll have to reconfigure the iPads on a regular basis.
Regards.

The same Object Version Number for the same person id multiple times

Hello all,
I am currently facing an issue with HRMS tables and the object version number for employees. I am trying to write a report but due to the same object version number for the same person appearing in several row i am getting duplicate information. For example,
person id 91 object version number 32 is on 4 rows and i have no idea why...help please guys, I'm at a lost so far 50 people are facing the same issue.

Hi Baal666bobo,
The person table is date-tracked, so the PK has effective start and end dates as well.
Get the correct record with sysdate(or any particular date) and then check the OVN.
Cheers,
Vignesh

Multiple inserts for the same object

We have observed that in certain cases, the same object is attempted to be inserted twice, resulting in Primary Key violation.
Here is one example -
A(1->1)B, unidirectional, A has the foreign key to B
new B
copyB = register(B) (also assign sequence number)
new A1, A2
A1.set(copyB), A2.set(copyB)
register(A1), register(A2)(also assign sequence number)
commit
it tries to insert the same B twice.
any clues whats wrong here?
thanks

Any chance your 1:1 from A to B is marked as privately-owned?
This would indicate to TopLink that the object in each relationship is unique and should be inserted.
Doug

Two Threads Sharing the Same Object

I am learning Java multithreading recently and I really hit the wall when I came to synchronizing data using the synchronized keyword. According to the explanation, synchronized will only work when 2 or more threads are accessing the same object. If there are accessing 2 different objects, they can run the synchronized method in parallel.
My question now is how to make sure for synchronized method to work, I am actually working with the same and only one object??
Imagine this:
Two person with the same account number are trying to access the very ONE account at the same time.
I suppose the logic will be:
Two different socket objects will be created. When it comes to the login or authentication class or method, how can I make sure in term of object that the login/authentication class or method will return them only ONE object (because they share the same account), so that they will be qualified for using the synchronized method further down the road?
Thanks in advance!

Actually your understanding is wrong. Consider:
public class MyClass {
private int someInt;
private float someFloat;
private synchronized void someMethod(final int value) {
    if (value > 2000) someInt = 2000;
private synchronized void someOtherMethod(final float value) {
    if (value > 2.0) someFloat = 1.999f;
}YOu might think that two different threads can enter this code, one can enter in someOtherMethod() while one is in someMethod(). That is wrong. The fact is that synchronization works by obtaining synchronization locks on a target object. In this case by putting it on the method declaration you are asking for the lock on the 'this' object. This means that only one of these methods may enter at a time. This code would be better written like so ...
public class MyClass {
private int someInt;
private float someFloat;
private void someMethod(final int value) {�
    synchronized(someInt) {
      if (value > 2000) someInt = 2000;
private void someOtherMethod(final float value) {
    synchronized(someFloat) {
      if (value > 2.0) someFloat = 1.999f;
}In this manner you are only locking on the pertinent objects to the method and not on 'this'. This means that both methods can be entered simultaneously by two different threads. However watch out for one little problem.
public class MyClass {
private int someInt;
private float someFloat;
private void someMethod(final int value) {�
    synchronized(someInt) {
      if (value > 2000) {
        someInt = 2000;
        synchronized (someFloat) {
          someFloat = 0.0f;
private void someOtherMethod(final float value) {
    synchronized(someFloat) {
      if (value > 2.0) {
        someFloat = 1.99999f;
        synchronized (someInt) {
          someInt = 0;
}In this case you can have a deadlock. If two threads enter one of these methods at the same time one would be waiting on the lock for someInt and the other on the lock for someFloat. Neither would proceed. The solution to the problem is simple. Acquire all locks in alphabetical order before you use the first.
public class MyClass {
private int someInt;
private float someFloat;
private void someMethod(final int value) {�
    synchronized (someFloat) {
      synchronized(someInt) {
        if (value > 2000) {
          someInt = 2000;
          someFloat = 0.0f;
private void someOtherMethod(final float value) {
    synchronized(someFloat) {
      if (value > 2.0) {
        someFloat = 1.99999f;
        synchronized (someInt) {
          someInt = 0;
}In this manner one thread will block waiting on someFloat and there can be no deadlock.

Using the same object in two actors at the same time

Hello,
I'm in doubt. In my application, for example i have three Hardware classes - Database, DAQ1, DAQ2.
Database class has one main method (except initialization methods) - Log Measure.
DAQ1 class also has one method - Get Voltage, and DAQ2 class has one method - Get Current.
Then i define two HAL (Hardware Abstraction Layer) classes - Measurement1 and Measurement2.
Each of those classes have two main methods, respectively - Measure Voltage and Measure Curent. They also have some initilziation methods.
In code, i create three objects of Hardware classes - Database, DAQ1 and DAQ2 and initiaze it. For example in Database private cluster i have DB RefNum, in DAQ1&2 i have device addresses etc.
Next I initialzie Measurement1 with two hardware objects - Database and DAQ1, and in the same way - Measurement2 with Database, and DAQ2 objects.
What Measure Voltage and Measure Curent (methods of Measurement1 and Measurement1 objects) do?
Measure Voltage method call DAQ1::Get Voltage method, then Database::Log Measure.
Similarly, Measure Current method call DAQ2::Get Voltage method, then Database::Log Measure.
Till now it's clear i think. It should work properly.
But now i incorporate Actor Framework, and define Measurement1 and Measurement2 as an actors.
Now i can run Measurement1 and Measurement2 at the same time, simultaneously, but this two actors share one object - Database. So my question - what is when actors want to use Database::Log measure method at the same time and this method is time cosuming (for example large pack of data).
Is one actor waits for second to stop executing this method of one shared object? I think yes because reentrancy setting (http://zone.ni.com/reference/en-XX/help/371361J-01/lvconcepts/reentrancy/)
But what, for example if i share one Hardware object of device (ADC converter) beetwen two HAL classes (Measurement1 and Measurement2). This Hardware object has two methods, for ex. Get Voltage At channel 1, and Get Voltage At Channel 2. Both of this methods cannot by call at the same time, because in physical device i can measure only at one channel in the same time.
Let the Get Voltage At channel X meausere take 10 seconds. Now i have huge chance, that Measurement 1 actor call Get Voltage At channel 1 method, and Measurement 2 wants to call Get Voltage At channel 2, when Get Voltage At channel 1 is in process. How to inform actor to wait till second actor release device resources?
Regards,
Kacper

Dear Kacper,
the way I understood, there are two separate questions in this post, namely:
I'm using a shared resource in multiple actors, and the code accessing the resource can take quite some time to execute. How can can I ensure this does not mess up the timing of one or all accessors?
I'm using a shared resource in multiple actors, and the code accessing the resource can generate errors if I try to access it from multiple places. How can I ensure synchronization between all accessors?
As for the first question, the optimal solution to separate the resource prone to timing issues in a different thread/actor. Put anything that is not strictly timed (file access, network comunication) in a separate loop. For each actor, you will get queue references to communicate with, so instead of logging the measurements in the actors where they are made, just queue them up for a different loop to process.
For the second part, the same logic still applies. Ideally, every singular resource should be handled by its own thread and nowhere else, so if anything else needs data from/to said resource, it can send a request to the dedicated thread or actor.
If, for some reason, this is not sufficient, you have to handle synchronization in some other way. There are a lot of techniques here, for example:
Create a named semaphore or lock as a part of the class. Have class functions use the semaphore before accessing the resource.
Use actor messages. Have a "resource in use" or "resource released" messages sent to all users whenever obtaining or releasing said resource.
Have a separate actor handle all resources, awarding them to threads needing them. This method also allows setting priorities between requests.
These are just a few examples, there are many other options.
Please let me know if this was helpful.
Kind regards:
Andrew Valko
National Instruments Hungary

Static synchronized methods VS non-static synchronized methods ??

what is the difference between static synchronized methods and non-static synchronized methods as far as the behavior of the threads is concerned? if a thread is in static synchronized method can another thread access simple (ie. non static) synchronized methods?

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.
In the seemingly complex case, the primitive construct is simpler, but the higher level construct requires more assembly or derivation of the primitive constructs, making that case seem more complicated.
Okay, I just re-read that, and it seems like I'm making no sense, but I'll leave it, just in case somebody can get some meaning out of it. :-)

Make sure 2 threads share the same object

I have been thinking about this for quite some time. However, I really couldn't figure it out due lack of experience and knowledge.
I just needed to know how can I make sure that 2 or more threads (after certain checkings that they should share a same common object) will really share the same object.
For example (to UlrikaJ if you are reading this):
How can I achieve the following?? Thanks!
"When James and Ada separately want to access their shared account,two transactions are started that accesses the object associated with that account number."

Thanks for the answer. You are welcome :-)
public class Account
   public Account getAccount(String userInput)
     // Some processes to verify the userInput go here.
     return account;
}//End of Account class
public UserThread implements Runnable
   private String name;
   private Semaphore s;
   private Account userAccount; //Reference for the Account class
   public UserThread(String name, Semaphore s)
      this.name = name;
      this.s = s;
   }//End of constructor
   public void run()
      for(;;)
          getUserInput(); //Ofcourse the implementation will depend on what you are doing
          s.lock(); //obtain a lock on Semaphore object
          //Entering critical section of the code
          this.userAccount = accountObject.getAccount(userInput);
          //End of critical section of the code.
          s.signal(); //Release the lock so that other thread
          //can access the getAccount() method...irrespective of the
          //variable or the object being used or referred to.
      }//End of for() loop
   }//End of run() method
} //End of UserThread class
public class AccountApplication
   public static void main(String args[])
      Semaphore sem = new Semaphore(0);
      //Always initialize the Semaphore with 0 when you create it.
      UserThread user1 = new UserThread("User Number 1",sem);
      UserThread user2 = new UserThread("User Number 2",sem);
      //All threads share the same Semaphore object
      Thread userThread1 = new Thread(user1);
      Thread userThread2 = new Thread(user2);
      userThread1.start();
      userThread2.start();
   }//End of main() method
}//End of classWell, I have given the outline of the code. I have not compiled or run the program so it might have errors in its syntax.
Obviously, you will definitely have to change it to suit your requirements. I have just shown you how to protect yur codes' critical section.
Hope this clears your doubts.
Vijay :-)

Static synchronized method

hi,
I am unable to understand the concept of static synchronized method.
A non-static synchronized method gets lock on this object before entering into the method. But how this is valid for static method.
thanks in advance

hi,
I am unable to understand the concept of static
atic synchronized method.
A non-static synchronized method gets lock on
this object before entering into the method.
But how this is valid for static method.
thanks in advanceIt locks the YourClassName.class object.

How can I use the same object in the different jsp files?

I am doing a project. I have finished my jave source files and compiled them successfully. And I also wrote a main method to test my classes, they also worked well. Now I am trying to use my jave code in the jsp files. But I meet a problem, in my method of java source file, I can generate a object of a class, and use it in the whole main method. But in the different jsp files, how can I do same thing?
For example, in the .java file,
Vector vl = new Vector();
While ...{
vl.add(...)
In each of my .jsp file I want to do one loop of the above, meanwhile I want to do that in the same object.
I hope you can understand what I mean. Really need your help!

put your object into a session and you can the use it in all the jsps as long as the session is valid. Or you could create a static variable in the class that only creates one instance off an object and then create a static method to return this object.

Multiple static synchronized methods locking the same object ?

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