JavaRanch Threads and Synchronization

Threads/Looping: How Can This Be?

Tue, 13 May 2008 09:07:11 MDT

I'm working my way through Head First Java (2nd ed.), and got a strange result when executing the following (Ch. 15 "Code Magnets" exercise):

code:

public class TestThreads {
 
 public static void main(String[] args) {
 
 ThreadOne t1 = new ThreadOne();
 Thread one = new Thread(t1);
 ThreadTwo t2 = new ThreadTwo();
 Thread two = new Thread(t2);
 
 one.start();
 
 two.start();
 
 } // close main
 
 
} // close class TestThreads
 
 class Accum {
 private int counter = 0; 
 
 private Accum() { }
 
 private static Accum a = new Accum();
...

'thread-safe' and 'synchronized'

Tue, 13 May 2008 08:59:11 MDT

what is threadSafe and what is synchonised ?

thread invoke later

Mon, 12 May 2008 23:43:10 MDT

I have create one new thread in invoke later mode but it is not calling

code:

 
 
public void showManageApp(final String path,final String temp){
 
 NativeInterfaceHandler.init();
 Toolkit.getDefaultToolkit().setDynamicLayout(true);
 Runnable xxx = new Runnable() {
 
 public void run() {
 ManageDocuments md = new ManageDocuments(path);
 md.webBrowser.setVisible(false);
 md.page.setVisible(true);
 md.sendPostRequest();
// cookie = UserStateManager.getInstance().getAuthKey();
 HTMLPath ...

Should a thread always sleep?

Mon, 12 May 2008 06:23:07 MDT

I'm just wondering, is it necessary for a thread to sleep, lets say after every iteration of an operation or calculation it is doing?

Thread concepts

Sat, 10 May 2008 04:43:01 MDT

hi all,
what is the diffenerce between Runnable interface and Thread class?
where do we use them?

Question about Design

Fri, 9 May 2008 16:02:59 MDT

Hello,

I am wondering which design would be better than the second for a business appliation managing user sessions (active/inactive etc):

1- Leave incoming threads to interact with sessions/database
2- Have internal threads managing incoming threads and serve their requests? How would I do that?
3- Any other idea is welcome.

Thanks!

Threads...

Fri, 9 May 2008 07:08:58 MDT

hi can any one answer to this...

How can two threads access a synchronized block simultaneuously i.e. one thread should give the chance for other thread to execute synchronized block even before this thread finishes the execution of the same synchronized block?

Doubt in synchronized....

Fri, 9 May 2008 03:12:58 MDT

Hi,
What is synchronized block? How it will work?Give me simple ex.
Why synchronized is not working in this below pgm.

public class test18 extends Thread
{

String setString="";
test18(String set)
{
setString=set;
start();

}
public void run()
{

try{
callLogic();

}catch(InterruptedException op)
{
System.out.println("Exception : "+op);
}
}
private synchronized void callLogic() throws InterruptedException
{
for(int i=0;i<100;i++)
{

System.out.print(setString);
Thread.sleep(100);

}

}

public static v...

query on sleep() method

Fri, 9 May 2008 02:48:58 MDT

When we call sleep(), it causes the running thread to transit to a ready to run state. Does the sleep() call will also relinquish the lock on the object if it is invoked from a synchronized method?

Wait , notify() ...explain this output

Wed, 7 May 2008 09:34:52 MDT

Hello everybody,
I understood the concept of notify() and wait(), but confused with the output ... why the statement "Entry of gettttttttttt:1" get printed...

I have bolded the line in the output statement where there is a confusion

code
class Q
{
int n;
boolean valueSet=false;

synchronized void put(int i)
{
if(valueSet)
{

try
{
System.out.println(" Entry of Put :"+i);
wait();
System.out.println("\n Exit of put ");
}
catch(InterruptedException ieo)
{
System.out.println("\n Interrupt Caught ");
}
}
<...

Objects and Threads

Mon, 5 May 2008 22:44:48 MDT

Acquiring the lock associated with an object does not of itself prevent other threads from accessing fields of the object or invoking unsynchronized methods on the object. Other threads can also use synchronized methods or the synchronized statement in a conventional manner to achieve mutual exclusion.
<a href=http:"//java.sun.com/docs/books/jls/second_edition/html/statements.doc.html#5984">
My views:
if ThreadA acquire a lock on Object A during invoking a synchronized method then another ThreadB can access or reference to Object A can invoke simultaneously Unsynchronized method on ObjectA.
Can any body elaborate more....

Class is Thread-Safe or not?

Mon, 5 May 2008 22:02:48 MDT

Hi All
Please let me know below my class code is thread-safe or not? if not,then let me know how to make it thread-safe?.

class Thread_safe {
private String name;
public synchronized void test(){
name="Test";
}
public void impl(){
name="Impl";
}
}
Thanks
Regards
Surender Singh

What is wrong with this code?

Sat, 3 May 2008 23:36:33 MDT

Hello,

I am thinking of writing a code which synchronize put and leaves get non synchronized except when put executes:

boolean put=false;
hashmap nonSynchronizedMap=new HashMap();

public void put(Ojbect Key,Object value)
{
synchronized(Lock)
{
put=true;
nonSynchronizedMap.put(Key,Value)
}
put=false;
}

public Object get(Object key)
{
if(put)
{
synchronized(Lock)
{
return nonSynchronizedMap.get(Lock);
}
}
else
return nonSynchronizedMap.get(Lock);
}

Thanks!

Marking a variable static final is thread safe

Sat, 3 May 2008 14:08:32 MDT

Folks,

I have a bit confusion that if i mark a variable "static final ", is it threadsafe?

like: public satic final MyObject myObj;

_____________________________
Mintoo
SCJP 1.4
_____________________________

stopping a runnable in ThreadPoolExecutor

Fri, 2 May 2008 06:32:28 MDT

hello,

let us say i have a runnable to execute and i want the ability to stop in the middle.

i execute the runnable using

code:


this.threadPool = new ThreadPoolExecutor(1, 1, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(1) );
this.threadPool.execute(searchRunnable);
[code]
 
inside the search runnable, there is code to wait() and look for any interrupts implementing like below..
private void doSearch() throws InterruptedException{
 ...
 ...
 
 // sleep 
 Thread.sleep(100);
 
}
 

I am not sure how to stop this Runnable. remove() on the thread pool using a Runnable only removes it from the q...

Shared cache

Thu, 1 May 2008 09:44:25 MDT

Hi Folks,

I am wondering what would be the best practice for below scenario in java:
we have a shared object for accessing cache but rarely updated based on users action.. I know that usually, we have to synchronize both, get and set, methods.. However, this way we oblige other threads to wait to get the cache at the same time .. In other words, I m looking for a way to share get method with all threads and block it when we modify it.. then, notify all threads to continue once set method is completed.

Thanks!

Confused in Thread join() code

Thu, 1 May 2008 08:32:25 MDT

Hi all,

I have copied a code from Khalid A. Mughal on join(), and a bit confused about how the output came. the code is as followed

code:

 
/**
 * 
 */
package threads;
 
/**
 * @author Sandeep Mukherji
 *
 */
public class Joining {
static Thread createThread(final int i, final Thread t1){ // function
Thread t2 = new Thread(){ // thread created in the function as a nested class
public void run(){
System.out.println(i+1);
try{
System.out.println(Thread.currentThread().getName()+" will join after the completion of other threads");
t1.join();
}catch(InterruptedException e){

}
System.out.println(i+2);...

Thread Pool in Java

Thu, 1 May 2008 04:52:24 MDT

Hello,

I want to write Thread Poolusing Java for some repititve task which is controlled through Job Scheduler.Job Scheduler will run after interval of 2 minutes.Can you please provide me few guidelines regarding how to write thread pool in Java.

My application will be deployed on Application Server and I am using JDK 1.4. I know there is a java.util.concurrent.Executors class which can be used for this. However, It's available in JDK 1.5 & not in 1.4

Thanks,
Shriniwas

Diffrence between calling the run() method.

Wed, 30 Apr 2008 09:00:22 MDT

Hi

If you extend any thread class , we should override the run method.

We can call that run method by using obj.start() or obj.run().

Just i want to know difference between them .

Is that start() method will create new Thread () ?

apache commons jci package

Tue, 29 Apr 2008 10:30:19 MDT

I am trying to test run an example given here:
on apache site (http://commons.apache.org/jci/usage.html)

In particular, I am trying to run the example link at the bottom on the above link..
test example (http://commons.apache.org/jci/usage.html)

I downloaded the commons package etc etc, compile goes fine, runs fine too, however, nothing happens when I change the test file (added new properties, deleted properties etc).. I don't even see the 'configuration changed' message. However the thread polling seems to work fine, but looks like control never goes to the hasChanged() conditional check.

Can someone please help me in understanding this behaviour? It is driving me nuts.

Threads' dependency on another thread

Tue, 29 Apr 2008 10:10:19 MDT

Hi There,

Can we make a thread dependent on other thread?
I mean to say, a thread can only perform a particular task, once another thread completes its task.
Is this kind of activity possible with thread management?

Thanks in advance,
Abra D.

Multi-Threading

Tue, 29 Apr 2008 06:48:18 MDT

Hi All,
I have change the single thread oriented code into Multi-threaded code, but i m stucked due to some issues.I m describing briefly the flow of the code.
Execution Start from - Main method i m reading msg and passed it to method ReceiveMsg() method and that method spawns 5 the threads of piece of code that process the msg.If the thread count is equal to 5 then the another
will not generated.
Problems:-
1)since i m generating five thread if any thread hang then how would i kill it.
2)My older code contain org.apache.common.collections.LRUMap that isnt synchronized.To get its synchronized version -the way is Collection.synchronizedMap(themaptosynchronize), but when use it i.e.(Collection.synchronizedMap(ne...

doubt on multithreading

Sat, 26 Apr 2008 07:50:08 MDT

what is the meaning of thread sharing the same address space?

How to Stop a thread

Fri, 25 Apr 2008 10:12:06 MDT

Hello,

I am using Executors.newFixedThreadPool(5) to create Thread pool. and then use ExecutorService.submit(Callable<> to start a thread out of that.
Now my question is - once one thread has completed its task, how can we kill that thread in this scenario.

Abra D.

Performance not improving after using Multithreading

Fri, 25 Apr 2008 01:00:05 MDT

Hi,
I have an application where i transfer documents from an ECM to another ECM(sort-of!).
For transferring 500 files, without multithreading the time taken is about 20 minutes.
After implementing multithreading (and limiting the max number of concurrent threads to 5 --a system requirement), theoretically the time taken should be around 4 minutes ( 20 divided by 5). But the time taken is still hanging at around 18 minutes.
I am not using any synchronised method or object.

Any suggestions?

Volatile Modifiers

Thu, 24 Apr 2008 04:42:03 MDT

I read a document which say Volatile Modifiers, are not cached by the JVM. When ever a volatile variable value is required by a thread it will access directly from the variable itself (without using the JVM cache)..

quote:
Volatile: Volatile modifier tells the compiler that the variable modified by
volatile can be changed unexpectedly by other parts of the program. For
example a Variable might be read from Cache and not update the content if it
has been changed by another thread. Specifying a variable as volatile tells
the JVM that any threads using that variable are not allowed to cache that
value at all. Making the Variable Volatile will ensure that the compiler
will get the cont...

ThreadCount for best Throughput??

Wed, 23 Apr 2008 08:31:59 MDT

Hi,

Suppose i have a Solaris box with 2 CPU's and i use Threads (Java threads or Websphere AsyncBeans) to do some work in parallel. Is there any means to arrive at the ThreadCount that would give me the best throughput.
Other than the Number of CPU's and the OS versions, is there any other variable which would contribute to deciding this. I know that there is no one single magic number to decide the ThreadCount to use, but if there is some guideline then it would be helpful

Thread Behaviour

Tue, 22 Apr 2008 04:36:23 MDT

Hi ,

I have checked the below code I got answer as "vandeleur".
but here in the mock test it is given as choice 4.
Can anyboby tell me which is correct answer..as i hav checked other papers they gav different answer..

What will happen when you attempt to compile and run the following code?

public class Tux extends Thread{
static String sName = "vandeleur";
public static void main(String argv[]){
Tux t = new Tux();
t.piggy(sName);
System.out.println(sName);
}
public void piggy(String sName){
sName = sName + " wiggy";
start();
}

publi...

Class.forName Thread Safe ?

Mon, 21 Apr 2008 22:52:22 MDT

I have the following code, it does a Class.forName based on the content-type of a particular attachment. So far so good.... I have few questions, Please answer at your convenience .

code:


 String nameOfClass = MIMETYPEMAP.get(mimetype);
 if (nameOfClass == null) {
 if(mimetype.equals("APPLICATION/OCTET-STREAM")
 && (fileName != null)) {
 String filextension = getFileExtension(fileName);
 if (filextension != null) {
 nameOfClass = FILEEXTENSIONMAP.get(filextension);
 } else {
 nameOfClass
 = MIME...

gremlin thread

Sat, 19 Apr 2008 19:10:25 MDT

basically i got a maze with a player and a gremlin. the gremlin moves very fast through the cells hence i put it to sleep for a second at each cell. but i get the problem that when my player walks past it when it is asleep, then the player does not die. but when he goes pat it when its awake it dies. i want it to go slowly through the maze hence stay asleep but yet still kill my player. i tried doing this by returning where the gremlin is and matchin it with the player but this also works only when the gremlin is not sleep. please help . its my first year in java. thanks

public void run()
{
while (true) {
getMaze().moveGremlins();
GremlinsEffect();
// game.v...

Explain me this output (notify)

Sat, 19 Apr 2008 14:42:25 MDT

Hello,
I didn't get the output ...Can anybody explain me ??
I have bolden the problem in output.

The code is from THE COMPLETE REFERENCE - HERBERT SCHILDT

My second question is what happens when a notify() is encountered before a wait() statement as in the program below ??

code:

class Q
{
 int n;
 boolean valueSet=false;
 
synchronized void put(int i)
{
 if(valueSet)
 {
 
 try
 {
 System.out.println(" Entry of Put :"+i); 
 
 wait();
 }
 catch(InterruptedException ieo)
 {
 System.out.println("\n Interrupt Caught ");
...

Doubt in synchronization of block

Thu, 17 Apr 2008 02:41:27 MDT

Hi
Lets take a code:

public abc()
{
List str=new List();
syncronized(str)
{
stmt 1..............
stmt 2..........
more code here....
}
}

My question is that..what is synchronized in above code
1)The List
2)Block of code in syncronized(str)()

Please give me more detail

Doubt in synchronization of block

Thu, 17 Apr 2008 02:39:30 MDT

Doubt in synchronization of block

Thu, 17 Apr 2008 01:07:03 MDT

Java threads

Thu, 17 Apr 2008 00:51:03 MDT

I have a main program which talks to a network device. I want to initiate a thread from the main program which checks if the main program has completed execution. If yes, i want to kill this thread else i want to throw an exception and stop termination of the main program. How can it be done?

Java threads

Wed, 16 Apr 2008 20:07:02 MDT

Live thread and GC root set

Wed, 16 Apr 2008 10:23:01 MDT

I came across a
thread asking:

quote:
Thread t = new Thread(); t.start(); t = null; now what will happen to the created thread?

One of the replies was that:

quote:
As long as it's not dead, it belongs to a ThreadGroup, which keeps it from being collected.

Is it true that a live thread doesn't get collected by GC just because
there is a reference from ThreadGroup to it?

I tried to prove it by using this program

code:

public static void main(String[] args) throws Exceptio...

wait() vs sleep()

Wed, 16 Apr 2008 02:25:00 MDT

Hi,
what is the difference between wait() and sleep() in threads.

wait() vs sleep()

Wed, 16 Apr 2008 02:15:00 MDT

Hi,
what is the difference between wait() and sleep() in threads.

wait() vs sleep()

Wed, 16 Apr 2008 01:49:00 MDT

Hi,
what is the difference between wait() and sleep() in threads.

Executing a Java Code by multiple threads

Wed, 16 Apr 2008 00:07:00 MDT

Hi ,

I have the following Java Program.

package com.citigroup.cai.core.util;

import java.io.IOException;
import java.net.InetAddress;
import java.util.Enumeration;

import netegrity.siteminder.javaagent.AgentAPI;
import netegrity.siteminder.javaagent.Attribute;
import netegrity.siteminder.javaagent.AttributeList;
import netegrity.siteminder.javaagent.InitDef;
import netegrity.siteminder.javaagent.RealmDef;
import netegrity.siteminder.javaagent.ResourceContextDef;
import netegrity.siteminder.javaagent.SessionDef;
import netegrity.siteminder.javaagent.UserCredentials;

import com.citigroup.cai.core.auth.ChangePasswordException;
import c...

wait() vs sleep()

Tue, 15 Apr 2008 22:59:00 MDT

Hi,
what is the difference between wait() and sleep() in threads.

Executing a Java Code by multiple threads

Tue, 15 Apr 2008 09:16:58 MDT

Question thread

Tue, 15 Apr 2008 06:23:20 MDT

How can i stop the Thread?
I have heard that Thread.stop() is unsafe.
What is the other way to stop the thread?
Which thread can we stop ?
If we write Thread.currentthread.stop(), which thread will stop?

Please expalin with example...

doubt in synchronization

Mon, 14 Apr 2008 23:47:19 MDT

what is the difference between syncronize a block and synchronize an entire method?

Newbie to ThreadLocal

Mon, 14 Apr 2008 11:43:17 MDT

Hi Gurus,
I start learning ThreadLocal. I have basic knowledge that is a separate thread to hold a separate value but I don't know if there its practical usage. Could you explain it? Is ThreadLocal useful?

I have the following code

code:

 
package general.thread;
 
public class ThreadLocalDemo extends Thread 
{
/** A serial number for clients */
private static int clientNum = 0;
 
 /** This ThreadLocal holds the Client reference for each Thread */
private ThreadLocal myClient = new ThreadLocal() 
{
 // The initialValue() method is called magically when you call get().
 protected synchronized Object initialValue()
 {
...

Live thread and GC root set

Mon, 14 Apr 2008 05:33:17 MDT

I came across a
thread asking:

quote:
Thread t = new Thread(); t.start(); t = null; now what will happen to the created thread?

One of the replies was that:

quote:
As long as it's not dead, it belongs to a ThreadGroup, which keeps it from being collected.

Is it true that a live thread doesn't get collected by GC just because
there is a reference from ThreadGroup to it?

I tried to prove it by using this program

code:

public static void main(String[] args) throws Exceptio...

Java threads

Mon, 14 Apr 2008 03:51:16 MDT

What is CAS not supported by Processor

Sun, 13 Apr 2008 22:45:16 MDT

Java 5 uses atomic object (Lock) to provide alternative to synchronization. Lock is nothing but the hardware level constructs of CAS. What is some processors do not support CAS and I have code with Lock object being used in it?

I think it will break. Any idea?

thread model in sun jvm for java6

Sun, 13 Apr 2008 12:29:15 MDT

what is the thread model used by sun jvm for java6?
does each user thread get mapped to a kernel thread?(1:1 model)
or are they multiplexed?(M:N model)

is there a documentation somewhere?

tia.