Nested synchronized method class

Hi. I have a situation where there are two synchronized methods: Method A and Method B. In the course of execution, Method A calls Method B. Method B calls wait(). This causes a classical deadlock as wait() releases the monitor for Method B, but no for Method A. What are my options to work around this problem?
Thanks in Advance,
Paul0al

A and B cannot synchronize on the same object because if A acquires the monitor, then B cannot obtain the same monitor without A releasing the monitor. My problem is that A and B are synchronized on different objects. A calls B and B calls wait(). A deadlock occurs because A's monitor is not released.
Thanks,
Paul0al

Similar Messages

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

If there are two synchronized methods in one class.

If there are two synchronized methods in one class then what will be the beheviour of the threads accessing the methods.

Synchronization is on objects, not methods or classes. A thread, entering a synchronized method, synchronizes on the object on which that method is called. Another thread, attempting to synchronize on that object, will be made to wait until the first thread releases it.

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

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.

Release memory for nested top-level classes

Hello experts.
I have a top-level class which consists of several static methods. Inside such a method I create several new instances of a nested top-level class inside my original class. Later I want to release one of these instances i.e. destroy instance and free memory. How can I do that?
Thanks.

sure, but I do not see any references to this nested top-level class. With "Releasing" I mean something like directly assigning a NULL value to the nested top-level clas in order to enable GC.
But as I said there are no references and while monitoring the memory I see that the GC does not collect some memory. (I know that GC is not reliable, but after ten attempts of this scenario the memory allocation is equal the whole time).
I guess the GC does not work with classes, but the classloader. But how can I control releasing memory in this scenario?
Any further hints?

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.

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.

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.");
}

Doubt: About synchronized methods?

Hi,
Can any one let me know, Is there any difference between static and non-static synchronized methods? If yes, what is the difference?

One is static and one isn't. What did you think?
As I recall, non-static methods lock on the object, whereas static methods lock on the class object, if that's what you're wondering about.

Synchronized methods trouble

hey guys,
I was under the impression that when u use synchronized methods, when a thread calls it any other threads calling it have to wait. If thats true then can someone help me and point out why my threads aren't waiting :(.
import java.util.*;
public class cannibal implements Runnable {
 private static int nonCannibal = 0;
 cannibal(){}
 public synchronized void eat(){
 boolean flag = false;
 try{
 while(nonCannibal == 0){
 if(flag == false){
 System.out.println("A cannibal waits in the darkness...");
 flag = true;
 wait();
 }catch(InterruptedException e){
 System.out.println("Some Exception!");
 nonCannibal--;
 System.out.println("Cannibal just ate someone in the woods!");
 System.out.println(nonCannibal +" survivors left!");
 public synchronized void dontEatMe(){
 nonCannibal++;
 notify();
 System.out.println("Unsuspecting person ventures to far out in the woods!");
 public synchronized void run(){
 Random generator = new Random();
 int x = generator.nextInt(2);
 if(x == 0){
 //System.out.println("Cannibal was just summoned!");
 eat();
 }else if(x == 1){
 // System.out.println("Unsuspecting Person just summoned!");
 dontEatMe();
 public static void main(String[] args) {
 Thread []x = new Thread[10];
 for(int i = 0; i < 10; i++){
 x[i] = new Thread(new cannibal());
 x.start();
try{
Thread.currentThread().sleep(3000);
}catch(InterruptedException e){

Demo:
import java.util.Random;
class Account {
 private int balance;
 public synchronized void deposit(int amount) {
 if (amount < 0) throw new IllegalArgumentException();
 System.out.println("increasing balance " + balance + " by " + amount);
 balance += amount;
 notifyAll();
 public synchronized void withdraw(int amount) throws InterruptedException {
 if (amount < 0) throw new IllegalArgumentException();
 while (balance < amount) {
 wait();
 System.out.println("decreasing balance " + balance + " by " + amount);
 balance -= amount;
class Depositer implements Runnable {
 private Account account;
 public Depositer(Account account) {
 this.account = account;
 public void run() {
 try {
 Random rnd = new Random();
 for(int i=0; i<10; ++i) {
 account.deposit(rnd.nextInt(100));
 Thread.sleep(1000);
 } catch(InterruptedException e) {
 e.printStackTrace();
class Withdrawer implements Runnable {
 private Account account;
 public Withdrawer(Account account) {
 this.account = account;
 public void run() {
 try {
 Random rnd = new Random();
 for(int i=0; i<10; ++i) {
 account.withdraw(rnd.nextInt(200));
 } catch(InterruptedException e) {
 e.printStackTrace();
class MainClass {
 public static void main(String[] args) {
 Account acct = new Account();
 Withdrawer out = new Withdrawer(acct);
 Depositer in = new Depositer(acct);
 new Thread(out).start();
 new Thread(in).start();
}

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.

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 ?

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.

Nested synchronized method class

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