Synchronized method in Thread

Hi All,
Can u show how to use synchronized method in thread.So that only one thread can access the resource at a time so that other needs to wait.Can u give me a sample example regarding this.So that it will be much more useful.
Thanx,
m.ananthu

synchronized public void method1(){
// code for the method....
}Hope it helps.

Similar Messages

Synchronized methods and thread locking

Hi can someone please explain the difference between these two examples in the context of object locking
public void method1(){
synchronized(this){
}And
StringBuffer aStringBufferObject = new StringBuffer("A");
public void method2(){
synchronized(aStringBufferObject){
}I know the first example will obtain a lock on the this instance and the second will obtain a lock of the aStringBufferObject instance. But i dont really understand what the effect or the difference of teh two is.
For example, in the second example, will threads still be able to execute the code inside the synchronized block because the lock is not related to the 'this' instance?
I know that synchronizing a method or a block of code prevents multiple threads to access that block/method at the same time but what is the purpose of specifying the object to lock on and what is the difference in the way the object is specified as in teh above examples.
Thanks
Edited by: ziggy on Jul 24, 2011 3:23 PM

Shortly put, the synchronized(object) doesn't lock the object reference in any way. It locks the code inside the synchronized's brackets, so that code can run it only when they have locked object (or in truth, when they have acquired object's monitor).
Since only one thread at a time can lock an object (obtain an object's monitor), this means that 2 threads executing blocks that synchronize on the same object can't run at the same time, ensuring thread safety (among other things).

[svn:bz-trunk] 12951: Changed synchronized PropertyProxyRegistry#getRegistry method to non-synchronized to avoid threads blocking in message push .

Revision: 12951
Revision: 12951
Author:   [email protected]
Date:     2009-12-15 02:17:31 -0800 (Tue, 15 Dec 2009)
Log Message:
Changed synchronized PropertyProxyRegistry#getRegistry method to non-synchronized to avoid threads blocking in message push.
Checkintests: Pass with the usual 3-4 tests that time out with and without this change.
QA: Yes
Doc: No
Modified Paths:
    blazeds/trunk/modules/core/src/flex/messaging/io/PropertyProxyRegistry.java

Revision: 12951
Revision: 12951
Author:   [email protected]
Date:     2009-12-15 02:17:31 -0800 (Tue, 15 Dec 2009)
Log Message:
Changed synchronized PropertyProxyRegistry#getRegistry method to non-synchronized to avoid threads blocking in message push.
Checkintests: Pass with the usual 3-4 tests that time out with and without this change.
QA: Yes
Doc: No
Modified Paths:
    blazeds/trunk/modules/core/src/flex/messaging/io/PropertyProxyRegistry.java

Which object's monitor does a synchronized method acquire?

from the Java Tutorial for concurrency programming:
" When a thread invokes a synchronized method, it automatically acquires the intrinsic lock _for that method's object_ and releases it when the method returns. The lock release occurs even if the return was caused by an uncaught exception. "
what exactly does this mean?
do synchronized methods acquire the monitors for objects of type: java.lang.reflection.Method
please consider this code:
public class Foo {
private int counter = 0;
public synchronized void incriment() { counter++; }
public synchronized void decriment() { counter--; }
Foo f = new Foo();
Class[] sig = new Class[0];
Method m = f.getClass().getMethod("incriment", sig);
// ok. so "m" is the relevant method object.
f.incriment(); // <-- is the monitor for "m" ,
// or the monitor for "f", acquired?
.......my reading of the Concurrency Tutorial is that synchronized methods use the monitors of java.lang.reflection.Method objects?
and thus, Foo is not thread safe, right?
however, this simple change makes Foo thread-safe?
public class Foo {
private volatile int counter = 0; // "volatile"
public void incriment() { counter++; }
public void decriment() { counter--; }
}thanks.
Edited by: kogose on Feb 23, 2009 7:13 PM

tensorfield wrote:
jverd wrote:
tensorfield wrote:
kogose wrote:
what exactly does this mean?It means you're complicating things.
If a method is synchronized, it is. You don't need to go beyond that. The method is synchronized.Not true. You have to know what it means for a method to be synchronized. Often people come in with the erroneous impression that it somehow prevents you from using or accessing the object in any other thread.It's very simple. If a synchronized method is called at the same time from many threads only one call will be executed at a time. The calls will be lined up and performed one after the other in sequence.
AND because synchronization is on a per object basis, when one synchronized method is being called from one thread, all synchronized methods of that same object are blocked for calling from other threads.
Simple as that.No, it's not that simple, and as stated, that is not correct. In particular, you didn't mention that for an instance method, all the various threads have to be trying to call instance methods on the same object in order for execution to be sequential.
You really can't understand Java's syncing without understanding how it relates to locks, and what it means for a method to be synchronized in terms of which lock it acquires.
Edited by: jverd on Feb 25, 2009 2:47 PM

Synchronized method in a java class used by many interfaces

My interface (idoc to file) is using a java class, which has one method that reads a table from a central database and after doing some calculations updates it.
(The interface instantiate the class inside a user-defined function and calls the method there.)
The problem is that if somebody sends 100 idocs at the same time, there can be a dirty read, I mean, a read just before other interface updates the table.
We want the following:
Interface 1:
- Read counter from the table (counter = 5 )
- Increment counter (counter = 6)
- Update table with that counter (table with counter = 6)
Interface 2:
- Read counter from the table (counter = 6 )
- Increment counter (counter = 7)
- Update table with that counter (table with counter = 7)
RESULT: The table has the counter = 7
But what is happening is the following:
- Interface 1 reads (counter = 5)
- Interface 2 reads (counter = 5)
- Interface 1 increments counter (counter = 6)
- Interface 2 increments counter (counter = 6)
- Interface 1 updates table (table with counter = 6)
- Interface 2 updates table (table with counter = 6)
RESULT: The table has the counter = 6 (WRONG)
I made the method synchronized. What I was expecting was that only one interface (i1) could enter the method (read the table and update it) while other interfaces running at the same time would have to wait until i1 finished that method.
My first test indicates that's not happening. Can anybody help me to find a solution?

Hi Bhavesh,
If the QOS is EOIO this means that the integration engine manage the call to the mapping program (and all the other blocks) inside an "internal" synchronized method.
So this means that in this case you do not need to manage the queued access (synchronization) inside your custom java code because it is already enveloped in a queued block by XI.
The problem that Jorge had can be easily reproduced using the sample code that follows:
class Synch Object
import java.util.Date;
public class SynchObject {
 String strName;
 public SynchObject(String strName){
 this.strName = strName;
 public synchronized void syncWrite(String strCaller) throws InterruptedException{
 Date now;
 now = new Date();
 System.out.println("-- " + now.toLocaleString() + " " + strCaller + " entering syncWrite of " + strName);
 System.out.flush();
 Thread.sleep(1000);
 now = new Date();
 System.out.println("-- " + now.toLocaleString() + " syncWrite of " + strName + " called by " + strCaller );
 System.out.flush();
 Thread.sleep(1000);
 now = new Date();
 System.out.println("-- " + now.toLocaleString() + " " + strCaller + " leaving syncWrite of " + strName);
 System.out.println("");
 System.out.flush();
class Caller
public class Caller implements Runnable {
 String strName;
 SynchObject target;
 int intMax;
 public Caller(String strName, SynchObject target, int intMax) {
 this.strName = strName;
 this.target = target;
 this.intMax = intMax;
 public void run() {
 for(int i=0; i<intMax;i++)
 try {
 target.syncWrite(strName);
 } catch (InterruptedException e) {
 e.printStackTrace();
class Workbench
public class Workbench {
 public static void main(String[] args) {
 // TODO Auto-generated method stub
 SynchObject sObj1 = new SynchObject("syncObj1");
 SynchObject sObj2 = new SynchObject("syncObj2");
 Caller c1 = new Caller("caller1",sObj1,2);
 Caller c2 = new Caller("caller2",sObj1,2); '[*CHANGE*]
 Thread ct1 = new Thread(c1);
 Thread ct2 = new Thread(c2);
 ct1.start();
 ct2.start();
Run the workbench class to see what happen when setting QOS EOIO (the synch object is the same).
To see instead what happen now (missing synchronization) you have to change in Workbench class the statement
Caller c2 = new Caller("caller2",sObj1,2); '[*CHANGE*]
with
Caller c2 = new Caller("caller2",sObj2,2); '[*CHANGE*]
The reason is that every instance of the mapping program declare a new instance of the "Synchronized object" so the calls are synchronized inside the same mapping program but not between several mapping program.
Hope this give you a better idea on this problems with java synchronization, but if you have further doubts (I know it's a little bit tricky ) feel free to ask.
Kind Regards,
Sergio

How to know whether a method is thread-safe through the java-doc?

In some book, it says that SAXParserFactory.newSAXParser() is thread-safe,but in the java-doc,it doesn't say that.
newSAXParser
public abstract SAXParser newSAXParser()
throws ParserConfigurationException,
SAXExceptionCreates a new instance of a SAXParser using the currently configured factory parameters.
Returns:
A new instance of a SAXParser.
Throws:
ParserConfigurationException - if a parser cannot be created which satisfies the requested configuration.
SAXException - for SAX errors.
I want to know, how to know whether a method is thread-safe?

System.out is a PrintStream object. None of the methods there use the synchronized modifier, but if you look in the source code, you will find out it is thread-safe, because it will use synchronized blocks whenever it writes some text.
The source code is in the src.jar file which you can extract.
I didn't find any comments about that PrintStream is thread-safe in the API.

Use of 'static' keyword in synchronized methods. Does it ease concurrency?

Friends,
I have a query regarding the use of 'synchronized' keyword in a programme. This is mainly to check if there's any difference in the use of 'static' keyword for synchronized methods. By default we cannot call two synchronized methods from a programme at the same time. For example, in 'Program1', I am calling two methods, 'display()' and 'update()' both of them are synchronized and the flow is first, 'display()' is called and only when display method exits, it calls the 'update()' method.
But, things seem different, when I added 'static' keyword for 'update()' method as can be seen from 'Program2'. Here, instead of waiting for 'display()' method to finish, 'update()' method is called during the execution of 'display()' method. You can check the output to see the difference.
Does it mean, 'static' keyword has anything to do with synchronizaton?
Appreciate your valuable comments.
1. Program1
public class SynchTest {
     public synchronized void display() {
          try {
               System.out.println("start display:");
               Thread.sleep(7000);
               System.out.println("end display:");
          } catch (InterruptedException e) {
               e.printStackTrace();
     public synchronized void update() {
          try {
               System.out.println("start update:");
               Thread.sleep(2000);
               System.out.println("end update:");
          } catch (InterruptedException e) {
               e.printStackTrace();
     public static void main(String[] args) {
          System.out.println("Synchronized methods test:");
          final SynchTest synchtest = new SynchTest();
          new Thread(new Runnable() {
               public void run() {
                    synchtest.display();
          }).start();
          new Thread(new Runnable() {
               public void run() {
                    synchtest.update();
          }).start();
Output:
Synchronized methods test:
start display:
end display:
start update:
end update:
2. Program2
package camel.java.thread;
public class SynchTest {
     public synchronized void display() {
          try {
               System.out.println("start display:");
               Thread.sleep(7000);
               System.out.println("end display:");
          } catch (InterruptedException e) {
               e.printStackTrace();
     public static synchronized void update() {
          try {
               System.out.println("start update:");
               Thread.sleep(2000);
               System.out.println("end update:");
          } catch (InterruptedException e) {
               e.printStackTrace();
     public static void main(String[] args) {
          System.out.println("Synchronized methods test:");
          final SynchTest synchtest = new SynchTest();
          new Thread(new Runnable() {
               public void run() {
                    synchtest.display();
          }).start();
          new Thread(new Runnable() {
               public void run() {
                    synchtest.update();
          }).start();
Output:
Synchronized methods test:
start display:
start update:end update:
end display:

the synchronized method obtain the lock from the current instance while static synchronized method obtain the lock from the class
Below is some code for u to have better understanding
package facado.collab;
public class TestSync {
     public synchronized void add() {
          System.out.println("TestSync.add()");
          try {
               Thread.sleep(2000);
          } catch (InterruptedException e) {
               e.printStackTrace();
          System.out.println("TestSync.add() - end");
     public synchronized void update() {
          System.out.println("TestSync.update()");
          try {
               Thread.sleep(2000);
          } catch (InterruptedException e) {
               e.printStackTrace();
          System.out.println("TestSync.update() - end");
     public static synchronized void staticAdd() {
          System.out.println("TestSync.staticAdd()");
          try {
               Thread.sleep(2000);
          } catch (InterruptedException e) {
               e.printStackTrace();
          System.out.println("TestSync.staticAdd() - end");
     public static synchronized void staticUpdate() {
          System.out.println("TestSync.staticUpdate()");
          try {
               Thread.sleep(2000);
          } catch (InterruptedException e) {
               e.printStackTrace();
          System.out.println("TestSync.staticUpdate() - end");
     public static void main(String[] args) {
          final TestSync sync1 = new TestSync();
          final TestSync sync2 = new TestSync();
          new Thread(new Runnable(){
               public void run() {
                    sync1.add();
          }).start();
          new Thread(new Runnable(){
               public void run() {
                    sync2.update();
          }).start();
          try {
               Thread.sleep(3000);
          } catch (InterruptedException e) {
               e.printStackTrace();
          new Thread(new Runnable(){
               public void run() {
                    sync1.staticAdd();
          }).start();
          new Thread(new Runnable(){
               public void run() {
                    sync2.staticUpdate();
          }).start();
}

Need of more than one synchronized methods

Inside my run method I'm calling a synchronized method which in turn calls other synchronized methods of the same and another class.
Now if the top level method is synchronized, does the need to make other methods synchronized still remain ?
Won't the other threads wait till the thread using the top level synchronized method returns.
Vik

Well, if you can be sure that the second method is not going to be called some other way except within that first synchronized method, then no it's not needed.

How can I get to know if a method is threads-safe?

Hi, there.
How can I get to know if a method is threads-safe?
For example, in two different threads, is System.out.print() method safe or not?And where can I find the information with regard to this?
thanks very much.

System.out is a PrintStream object. None of the methods there use the synchronized modifier, but if you look in the source code, you will find out it is thread-safe, because it will use synchronized blocks whenever it writes some text.
The source code is in the src.jar file which you can extract.
I didn't find any comments about that PrintStream is thread-safe in the API.

Instance methods faster than sync. static methods in threaded env?

consider the following please:
(-) i have "lots" of instances of a single Runnable class running concurrently..
(-) each instance uses a common method: "exponential smoothing" and they do a lot of smoothing.
so:
(option #1): include a "smooth()" instance method in the Runnable class.
(option #2): include a "smooth()" synchronized static method in the Runnable class.
(option #3): create a MathUtility class, and have "smooth()" as an instance method in this class.
(option #4): make "smooth()" a synchronized static method in the MathUtility class.
from OOP point of view, i think i should externalize "smooth()" to a MathUtility class, and then make
is "synchronized static".
but then from a performance point of view....
would not it be optimal to make "smooth()" an instance method in MathUtility and then have each
instance of the Runnable create its own MathUtility instance so that each thread has its own copy
of the "smooth()" method??
well, i can't image there would be a measurable difference so maybe i should not post.
but, if there is a flaw in my thinking, please let me know.
thanks.

kogose wrote:
from OOP point of view, i think i should externalize "smooth()" to a MathUtility class, and then make
is "synchronized static".From an OOP point of view you should probably have a class that represents the data that provides a (non-static) smooth() method that either modifies the data or returns a new smoothed data object (depending on whether you want your data objects to be immutable or not).
but then from a performance point of view....
would not it be optimal to make "smooth()" an instance method in MathUtility and then have each
instance of the Runnable create its own MathUtility instance so that each thread has its own copy
of the "smooth()" method??No, methods are not "copied" for each instance. That just doesn't happen.
well, i can't image there would be a measurable difference so maybe i should not post.If you don't know, then you should probably try it.
but, if there is a flaw in my thinking, please let me know.The flaw in your thinking is that you can think that you can intuitively grasp the difference in performance of a change at that level.
I have yet to meet anyone who can reliably do that.
Performance optimization is not an intuitive task at that level and you should never do performance optimizations without proving that they are improvements for your particular use case.
First part: Is the smooth() method really thread-unsafe? Does it use some shared state? My guess would be that it uses only local state and therefore doesn't need any synchronization at all. That would also be the fastest alternative, most likely.

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. :-)

What is a lock in a synchronized method ??

Greetings,
I have a synchronized reset method whose job is to reset every variable i am using in that class.. its a synchronized method so when i am doing the reset stuff, no other method in that class can update that variable. But someone told me i need to also put lock inside the reset method and check for the lock everywhere where i update those variables ( So that variable doesn't update itself and lose its value as reset would be resetting that variable.) How do i lock all these variables in that reset method and how do i check for the lock where i would be updating that variable.
Example: Psedo code
class {
int a =0;
int b = 0;
method update {
a = 5;
method update1{
b = 6;
synchronized method reset{
a =0;
b = 0;

javanewbie83 wrote:
Greetings,
I have a synchronized reset method whose job is to reset every variable i am using in that class.. its a synchronized method so when i am doing the reset stuff, no other method in that class can update that variable. But someone told me i need to also put lock inside the reset method and check for the lock everywhere where i update those variables ( So that variable doesn't update itself and lose its value as reset would be resetting that variable.) How do i lock all these variables in that reset method and how do i check for the lock where i would be updating that variable.You don't need to "check" for the lock. When you have a synchronized block or method, the synhcronized keyword tells the VM to stop executing that thread until it can attain the object's lock, then give the object's lock to that thread, and then take it back when the thread leaves the synchronized block (or calls wait()). It's all automatic.
Note that you'll need to synchronize not only write access to the variable(s) in question, but read access also, in order to ensure that reader threads see the values written by writers.

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 Ishaq

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."
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.

Dont understand Synchronized method to complete program

I'm trying to finish theis program but i don't understand synchronized methods. I checked the java tutorial and i am still confused. Can someone help me approach finishing this program. Here is the spec and the first part of the program i have done.
===================================================================================
When the above is working, create a class called Food. The objective here is to demonstrate the behavior of threads that share data, and use synchronized methods.
Simulating an animal eating, simply means that the thread will sleep for some length of time.
There is one instance of the Food class that is shared by both of the animals. Pass it to the constructor of the Animal class.
There is a method in the Food class called eat(). This method is synchronized, i.e., only one Animal can be eating at a time.
The rabbit eats the food (the thread will sleep) for a longer time than the turtle, thus giving an advantage to the rabbit.
But, the turtle must wait until the rabbit is done eating until it can eat, so the advantage is reduced. Print out a message in the eat method when the animal begins to eat, and when it is done eating. Indicate which animal it is that starts to eat.
Try making the eat method not synchronized, and observe the different behavior if the eat method allows the rabbit to begin eating before the turtle is done eating
 import java.util.Random;
 public class Animal extends Thread implements Runnable{
 private String name;
 private int position;
 private int speed;
 private int restMax;
 public static boolean winner = false;
 Random random = new Random();
 public Animal (String name, int position, int speed,int restMax){
 this.name = name;
 this.position = position;
 this.speed = speed;
 this.restMax = restMax;
 public void run(){
 try{
 while( winner == false){
 if(position < 100){
 Thread.sleep(random.nextInt(restMax));
 position += speed ;
 System.out.println(name+" is in "+ position+" position ");
 if (position >= 100){
 winner = true;
 System.out.println(name+" is the winner");
 System.exit(0);
 catch(InterruptedException e){
 public static void main(String[] args){
 Animal rabbit = new Animal("trix", 0, 5, 150);
 Animal turtle = new Animal("maury",0, 3, 100);
 rabbit.start();
 turtle.start();
 }

Example:class Donkeyphile implements Runnable {
 private Donkey donkey;
 private long time;
 Donkeyphile(Donkey donkey, long time) {
 this.donkey = donkey;
 this.time = time;
 public void run() {
 for (int i = 0; i < 10; i++) {
 donkey.love(time);
 public static void main(String[] args) {
 Donkey donkey = new Donkey();
 Donkeyphile jverd = new Donkeyphile(donkey, 500);
 Donkeyphile yawmark = new Donkeyphile(donkey, 100);
 Thread j = new Thread(jverd, "Jverd");
 Thread y = new Thread(yawmark, "Yawmark");
 j.start();
 y.start();
class Donkey {
 synchronized void love(long time) {
 String name = Thread.currentThread().getName();
 System.out.println(name + " hugs the donkey.");
 try { Thread.sleep(time); } catch (InterruptedException e) { }
 System.out.println(name + " releases the donkey.");
}

How to use Synchronized method when creating our own controls

I don't know how to use the synchronized method when creating our own activex like controls to speed up the application.

[url http://java.sun.com/docs/books/tutorial/essential/threads/multithreaded.html] here you go

Synchronized method in Thread

Similar Messages

Maybe you are looking for