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Thread.sleep is blocking other thread also, working on other method, along with itself callled inside synchronized method

class Common
{
public synchronized void synchronizedMethod1()
{
System.out.println("synchronized Method1 called");
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("synchronized Method1 done");
}
public synchronized void synchronizedMethod2()
{
System.out.println("synchronized Method2 called");
try
{
Thread.sleep(1000);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
System.out.println("synchronized Method2 done");
}
}
In the above class I have two synchronized methods which I am calling from run method of another class. Other class code is given below:
public class ThreadClass implements Runnable
{
private int id = 0;
private Common common;
public ThreadClass(int no, Common object)
{
common = object;
id = no;
}
public void run()
{
System.out.println("Running Thread " + Thread.currentThread().getName());
try
{
if (id == 11)
{
common.synchronizedMethod1();
}
else
{
common.synchronizedMethod2();
}
}
catch (Exception e)
{
e.printStackTrace();
}
}
public static void main(String[] args)
{
Common c = new Common();
ThreadClass tc = new ThreadClass(11, c);
ThreadClass tc1 = new ThreadClass(20, c);
Thread t1 = new Thread(tc, "Thread 1");
Thread t2 = new Thread(tc1, "Thread 2");
t1.start();
t2.start();
}
}
From main method I am starting two different threads. In run method I have given a condition to send both different threads to different synchronized methods. Output produced by the code is:
Running Thread Thread 2
Running Thread Thread 1
synchronized Method2 called
synchronized Method2 done
synchronized Method1 called
synchronized Method1 done
MY QUESTION FOR THE OUTPUT IS:
When thread 2 goes to synchronized Method2 it prints 3rd line of output and goes to sleep for 1 second. Now since thread 1 is not blocked by anything so it should execute and print 5th line of the output just after 3rd line of output and should go to sleep then but this is not happening instead when thread 2 goes to sleep it make's thread 1 also sleep then first thread 2 complete's its execution after which thread 1 completes its execution.
Such a behavior is not happening if I remove synchronized keyword from methods.
Can you please explain me the reason behind different way of processing the code with and without synchronized keywords.
Thanks in advance.

Such a behavior is not happening if I remove synchronized keyword from methods. Can you please explain me the reason behind different way of processing the code with and without synchronized keywords.
This is actually the entire purpose of the synchronized keyword. When you have several synchronized instance methods of the same class, only one may be executing at a time. You have written this:
class Common {
public synchronized void synchronizedMethod1(){}
public synchronized void synchronizedMethod2(){}
}
Because both methods are synchronized, only one may be executed at once. One of them can't start the other one is done.
How does this work? In short, you have a Common object and call a synchronized instance method of it. When you call synchronzedMethod1, that method will "lock" the Common object (called "acquiring the lock"). While that method has that lock on that Common object, if you try to call any other synchronized method on that same object, it will try to lock it and it will find that it's already locked. So any other attempt to lock the object will hang until they can do so. Once synchronizedMethod1 finishes, it will unlock the Common object (called "releasing the lock") and anybody can then try to lock it, such as synchronzedMethod2.
So in short, synchronized specifically makes it so you can't have two synchronized methods of the same class happening at once. This is useful because some problematic behavior can come from not doing this. As an example, ArrayList does not do this, so if one thread tries to add an object to an ArrayList while another tries to iterate over it, it might throw a ConcurrentModificationException and make everyone sad.

A sleeping thread does not release its locks, but you can replace your sleep(...) calls with wait(...). Keep in mind, though, that only the lock of the object having wait(...) called on it will be released, so you'd have to devise a different solution if you expected multiple locks to be released while waiting.

synchronising a method doesnt mean just the method itself synchronised
synchronized void x(){}
equals to:
void x(){
synchronised(this){}
}
Since both thread access same Common instance first thread will get the ownership of the Common object lock doesnt matter which synchronised method called and it will just release this lock after this method body completed its job.
If you would send two Common instance there would not be a problem since they are not static. Also you might be interested in ReentrantLock

First of all synchronized keyword is used to define mutual exclusion. Here mutual exclusion achieved by Monitor concept. One more thing is sleep does not release monitor. It just pause the execution of current thread for some time. Other threads which requires the monitor have to wait until the thread which acquired monitor release it.
There is two ways to use synchronized...
First one is using synchronized blocks.
synchronized(obj){...}
Here if any thread want to enter into synchronized block it have to get monitor of obj.
Second one is to using synchronized method.
synchronized void meth(){...}
Main difference between synchronised method & block is synchronised method use monitor of object it self & synchronised block can have monitor of any object.
Synchronized method can be defined using synchronized block as follows...
void meth(){
synchronized (this){
//method body
}
}
Now you can use the synchronised block to prevent the problem of blocking another method. Here you have to define synchronised block on different objects so both methods can execute concurrently but multiple threads can not execute same method concurrently.

One thread updates variable and another read it, do I need something special

I have a class that has the object "Card". This class keeps checking to see if the object is not null anymore. Only one other thread can update this object. Should I just do it like the code below? Use volatile?Syncronized? lock (which I dont know how to use really)? What do you recommend as easiest solution?
Class A{
public Card myCard = null;
public void keepCheck(){
while(myCard == null){
Thread.sleep(100)
}
//value updated
callAnotherMethod();
}
Another thread has following:
public void run(){
a.myCard = new Card(5);
}
What do you suggest?

You should use a proper wait event (see the Guarded Block tutorial), otherwise you run the risk of the "watching" thread seeing the reference before it sees completely initialized member fields of the Card. Also wait() will allow the thread to sleep instead of sucking up CPU in a tight while loop.
For example:
Class A {
private final Object cardMonitor = new Object();
private volatile Card myCard;
public void keepCheck () {
synchronized (cardMonitor) {
while (myCard == null) {
try {
cardMonitor.wait();
} catch (InterruptedException x) {
// either abort or ignore, your choice
}
}
}
callAnotherMethod();
}
public void run () {
synchronized (cardMonitor) {
myCard = new Card(5);
cardMonitor.notifyAll();
}
}
}
I made myCard private in the above example. I do recommend avoiding lots of public fields in a case like this, as the code could end up getting messy fast.
Also note that you do not need cardMonitor -- you could use the A itself, but having a separate monitor object lets you have finer control over synchronization.
Beware, with the above implementation, if run() is called while callAnotherMethod() is executing, it will change myCard which may break callAnotherMethod() (which you do not show). Moving callAnotherMethod() inside the synchronized block is one possible solution, but you have to decide what the appropriate strategy is there given your requirements.

The variable needs to be volatile when modifying from a different thread if you intend to poll for it, but a better solution is to use wait()/notify() or even a Semaphore to keep your other thread sleeping until myCard variable is initialized.

Looks like you have a classic producer/consumer case.
You can handle this case using wait()/notify() methods. See here for an example: How to use wait and notify in Java?
Or here, for more examples: http://www.programcreek.com/2009/02/notify-and-wait-example/

If I synchronized two methods on the same class, can they run simultaneously?

If I synchronized two methods on the same class, can they run simultaneously on the same object? For example:
class A {
public synchronized void methodA() {
//method A
}
public synchronized void methodB() {
// method B
}
}
I know that I can't run methodA() twice on same object in two different threads. same thing in methodB().
But can I run methodB() on different thread while methodA() is still running? (same object)

Both methods lock the same monitor. Therefore, you can't simultaneously execute them on the same object from different threads (one of the two methods will block until the other is finished).

In the example methodA and methodB are instance methods (as opposed to static methods). Putting synchronized on an instance method means that the thread has to acquire the lock (the "intrinsic lock") on the object instance that the method is called on before the thread can start executing any code in that method.
If you have two different instance methods marked synchronized and different threads are calling those methods concurrently on the same object, those threads will be contending for the same lock. Once one thread gets the lock all other threads are shut out of all synchronized instance methods on that object.
In order for the two methods to run concurrently they would have to use different locks, like this:
class A {
private final Object lockA = new Object();
private final Object lockB = new Object();
public void methodA() {
synchronized(lockA) {
//method A
}
}
public void methodB() {
synchronized(lockB) {
//method B
}
}
}
where the synchronized block syntax allows specifying a specific object that the executing thread needs to acquire the intrinsic lock on in order to enter the block.
The important thing to understand is that even though we are putting a "synchronized" keyword on individual methods, the core concept is the intrinsic lock behind the scenes.
Here is how the Java tutorial describes the relationship:
Synchronization is built around an internal entity known as the intrinsic lock or monitor lock. (The API specification often refers to this entity simply as a "monitor.") Intrinsic locks play a role in both aspects of synchronization: enforcing exclusive access to an object's state and establishing happens-before relationships that are essential to visibility.
Every object has an intrinsic lock associated with it. By convention, a thread that needs exclusive and consistent access to an object's fields has to acquire the object's intrinsic lock before accessing them, and then release the intrinsic lock when it's done with them. A thread is said to own the intrinsic lock between the time it has acquired the lock and released the lock. As long as a thread owns an intrinsic lock, no other thread can acquire the same lock. The other thread will block when it attempts to acquire the lock.
The purpose of locking is to protect shared data. You would use separate locks as shown in the example code above only if each lock protected different data members.

Java Thread acquires an object level lock when it enters into an instance synchronized java method and acquires a class level lock when it enters into static synchronized java method.
In your case, the methods(instance) are of same class. So when ever a thread enters into java synchronized method or block it acquires a lock(the object on which the method is called). So other method cannot be called at the same time on the same object until the first method is completed and lock(on object) is released.

In your case you synchronized two method on the same instance of class. So, these two methods can't run simultaneously on different thread of the same instance of class A. But they can on different class A instances.
class A {
public synchronized void methodA() {
//method A
}
}
is the same as:
class A {
public void methodA() {
synchronized(this){
// code of method A
}
}
}

Think of your code as the below one:
class A {
public void methodA() {
synchronized(this){
//method A body
}
}
public void methodB() {
synchronized(this){
// method B body
}
}
So, synchronized on method level simply means synchronized(this).
if any thread runs a method of this class, it would obtain the lock before starting the execution and hold it until the execution of the method is finished.
But can I run methodB() on different thread while methodA() is still
running? (same object)
Indeed, it is not possible!
Hence, multiple threads will not able to run any number of synchronized methods on the same object simultaneously.

From oracle documentation link
Making methods synchronized has two effects:
First, it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.
Second, when a synchronized method exits, it automatically establishes a happens-before relationship with any subsequent invocation of a synchronized method for the same object. This guarantees that changes to the state of the object are visible to all threads
This will answer your question: On same object, You can't call second synchronized method when first synchronized method execution is in progress.
Have a look at this documentation page to understand intrinsic locks and lock behavior.

Just to all clarity, It’s possible that both static synchronized and non static synchronized method can run simultaneously or concurrently because one is having object level lock and other class level lock.

The key idea with synchronizing which does not sink in easily is that it will have effect only if methods are called on the same object instance - it has already been highlighted in the answers and comments -
Below sample program is to clearly pinpoint the same -
public class Test {
public synchronized void methodA(String currentObjectName) throws InterruptedException {
System.out.println(Thread.currentThread().getName() + "->" +currentObjectName + "->methodA in");
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + "->" +currentObjectName + "->methodA out");
}
public synchronized void methodB(String currentObjectName) throws InterruptedException {
System.out.println(Thread.currentThread().getName() + "->" +currentObjectName + "->methodB in");
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + "->" +currentObjectName + "->methodB out");
}
public static void main(String[] args){
Test object1 = new Test();
Test object2 = new Test();
//passing object instances to the runnable to make calls later
TestRunner runner = new TestRunner(object1,object2);
// you need to start atleast two threads to properly see the behaviour
Thread thread1 = new Thread(runner);
thread1.start();
Thread thread2 = new Thread(runner);
thread2.start();
}
}
class TestRunner implements Runnable {
Test object1;
Test object2;
public TestRunner(Test h1,Test h2) {
this.object1 = h1;
this.object2 = h2;
}
#Override
public void run() {
synchronizedEffectiveAsMethodsCalledOnSameObject(object1);
//noEffectOfSynchronizedAsMethodsCalledOnDifferentObjects(object1,object2);
}
// this method calls the method A and B with same object instance object1 hence simultaneous NOT possible
private void synchronizedEffectiveAsMethodsCalledOnSameObject(Test object1) {
try {
object1.methodA("object1");
object1.methodB("object1");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// this method calls the method A and B with different object instances object1 and object2 hence simultaneous IS possible
private void noEffectOfSynchronizedAsMethodsCalledOnDifferentObjects(Test object1,Test object2) {
try {
object1.methodA("object1");
object2.methodB("object2");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
Notice the difference in output of how simultaneous access is allowed as expected if methods are called on different object instances.
Ouput with noEffectOfSynchronizedAsMethodsCalledOnDifferentObjects() commented -the output is in order methodA in > methodA Out .. methodB in > methodB Out
and Ouput with synchronizedEffectiveAsMethodsCalledOnSameObject() commented -
the output shows simultaneous access of methodA by Thread1 and Thread0 in highlighted section -
Increasing the number of threads will make it even more noticeable.

You are synchronizing it on object not on class. So they cant run simultaneously on the same object

No it is not possible, if it were possible then both method could be updating same variable simultaneously which could easily corrupt the data.

Yes, they can run simultaneously both threads. If you create 2 objects of the class as each object contains only one lock and every synchronized method requires lock.
So if you want to run simultaneously, create two objects and then try to run by using of those object reference.

Two different Threads executing a common synchronized method on the single object, since the object is same, when one thread uses it with synchronized method, it will have to verify the lock, if the lock is enabled, this thread will go to wait state, if lock is disabled then it can access the object, while it will access it will enable the lock and will release the lock
only when it's execution is complete.
when the another threads arrives, it will verify the lock, since it is enabled it will wait till the first thread completes his execution and releases the lock put on the object, once the lock is released the second thread will gain access to the object and it will enable the lock until it's execution.
so the execution will not be not concurrent, both threads will execute one by one, when both the threads use the synchronized method on different objects, they will run concurrently.

Concurrent access to static methods

I have a static method with the following signature:
public static List<ResultObjects> processRequest(RequestObject req){
// process the request object and return the results.
}
What happens when there are multiple calls made to the above method concurrently? Will the requests be handled concurrently or one after the other?

Answering exactly your question:
Method will be executed concurrently (multiple times in the same time if you have several threads).
Requests will be handled concurrently.
You need to add the synchronized modifier if you are working with objects that require concurrent access.

All your calls to the method will be executed concurrently... but:
You may have concurrency issue (and being in non thread-safe situation) as soon as the code of your static method modify static variables. And in this case, you can declare your method as synchronized
If your method only use local variables you won't have concurrency issues.

If you need to avoid concurrent execution, you need to explicitly synchronize. The fact that the method is static has nothing to do with it. If you declare the method itself to be synchronized, then the synchronization will be on the class object. Otherwise you will need to synchronize on some static object (since this doesn't exist for static methods).

I see a lot of answers but none really pointing out the reason.
So this can be thought like this,
Whenever a thread is created, it is created with its own stack (I guess the size of the stack at the time of creation is ~2MB). So any execution that happens actually happens within the context of this thread stack.
Any variable that is created lives in the heap but it's reference lives in the stack with the exceptions being static variables which do not live in the thread stack.
Any function call you make is actually pushed onto the thread stack, be it static or non-static. Since the complete method was pushed onto the stack, any variable creation that takes place lives within the stack (again exceptions being static variables) and only accessible to one thread.
So all the methods are thread safe until they change the state of some static variable.

You can check it yourself:
public class ConcurrentStatic {
public static void main(String[] args) {
for (String name: new String[] {"Foo", "Bar", "Baz"}) {
new Thread(getRunnable(name)).start();
}
}
public static Runnable getRunnable(final String name) {
return new Runnable() {
public void run() {
longTask(name);
}
};
}
public static void longTask(String label) {
System.out.println(label + ": start");
try {
Thread.sleep(10000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(label + ": end");
}
}

all method invocations from separate threads in java are concurrent by default.

Why is this thread allowing another one to access its synchronized method?

I have the following codes. I expected one thread to execute its synchronized method completely and then allow another one to access the same method. However, this is not the case.
public class Threads {
/**
* #param args
*/
public static void main(String[] args) {
//Thread Th = new Threads();
Thread th = new Thread (new thread1 ());
th.start();
Thread th1 = new Thread (new thread1 ());
th1.start();
}
}
class thread1 implements Runnable{
String name = "vimal";
public void run() {
System.out.println("Runnable "+this.name);
setNAme("Manish");
}
public synchronized void setNAme(String name){
try {
System.out.println("Thread "+Thread.currentThread().getName());
wait(1000);
this.name = name;
System.out.println("Name "+this.name);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
I have one output as
Runnable vimal
Thread Thread-0
Runnable vimal
Thread Thread-1
Name Manish
Name Manish
What is the use of synchronized here and how do I make my method to run completely before another accesses it?

synchronized has no effect here because you are not synchronizing on the same object in both cases. When applied to an instance method, the synchronized keyword causes the method to be synchronized on this. So in each case you are synchronizing on the instance of thread1, and there are two of those.
The more interesting test would be when you run the same instance of thread1 in two threads simultaneously. In that case, calling wait(1000) is a very bad thing to do because (as documented) it releases the lock on this. You want to use Thread.sleep(1000) instead in your code.
If you need to have two instances of thread1, you need to synchronize on some shared object, possibly like this:
private static final Object lockObject = new Object();
public void setName(String newName) {
synchronized(lockObject) {
doSetName(newName);
}
}

You will have to remove the call to wait(1000). It looks like what you actually want is a call to Thread.sleep(1000), if you simply want to pause the current thread, this does not release ownership of any monitors.
From the javadoc for Object.wait().
This method causes the current thread (call it T) to place itself in
the wait set for this object and then to relinquish any and all
synchronization claims on this object. Thread T becomes disabled for
thread scheduling purposes and lies dormant until one of four things
happens:
Some other thread invokes the notify method for this object and thread T happens to be arbitrarily chosen as the thread to be
awakened.
Some other thread invokes the notifyAll method for this object.
Some other thread interrupts thread T.
The specified amount of real time has elapsed, more or less. If timeout is zero, however, then real time is not taken into
consideration and the thread simply waits until notified.
The thread T is then removed from the wait set for this object and
re-enabled for thread scheduling. It then competes in the usual manner
with other threads for the right to synchronize on the object; once it
has gained control of the object, all its synchronization claims on
the object are restored to the status quo ante - that is, to the
situation as of the time that the wait method was invoked. Thread T
then returns from the invocation of the wait method. Thus, on return
from the wait method, the synchronization state of the object and of
thread T is exactly as it was when the wait method was invoked.
UPDATE: As has been mentioned in other answers, you are not synchronizing on the same object. Once you do, you will still suffer the same output, due to the issue I have mentioned. You will need to fix both for your desired results.

The output is correct, you are creating to independent threads that do not share any data. Thus both threads start with first string, and after some time, the string is changed and printed.

You're creating 2 thread1 objects. They each have their own setNAme method. Synchronized methods only synchronize on the object, not the class. Unless the method is static.

You have two Threads here with independent name variables and independent monitors, so each Thread is only accessing its own members. If you want to have the threads interact with each other you'll have to implement such an interaction.

you are creating two separate thread1 objects and running them. Each thread has it's own copy of the name variable as well as the setName function. Make them both static and you will see the effects of synchronization.

You are locking on two different instance of the objects where you dont need any synchronization at all. You need to synchronize only if you are working on a shared data. I think you meant to write a test like the below.
If you test this, you will realize that the second thread will wait until the first thread is completed with the synchronized method. Then take out the synchronized word and you will see both threads are executing at the same time.
public class SynchronizeTest {
public static void main(String[] args) {
Data data = new Data();
Thread task1 = new Thread(new UpdateTask(data));
task1.start();
Thread task2 = new Thread(new UpdateTask(data));
task2.start();
}
}
class UpdateTask implements Runnable {
private Data data;
public UpdateTask(Data data) {
this.data = data;
}
public void run() {
try {
data.updateData();
} catch (Exception e) {
e.printStackTrace();
}
}
}
class Data {
public synchronized void updateData() throws InterruptedException {
for (int i = 0; i < 5; i++) {
Thread.sleep(5000);
System.out.println(i);
}
}
}