debug hanging multi-thread program with monitor
Hi all,
I am writting to seek help from other java developers.
I am currently using JDB to debug a hanging multi-thread program
running under jre 1.0.5_05.
I have created a class which acts as a lock for synchronization between two threads.
class myLockObject
{
public static Object aStaticVariable = new Object();
}
Two threads, namely Thread-1 and Thread-2, are basically doing similar things as followed:
while(true)
{
synchonized(myLockObject.aStaticVariable)
{
big chunk of code
};
Thread.sleep(10000);
}
But, the program sometimes hangs.
Using JDB "where" and "threadlocks" commands, I found that thread-1 is outside the
synchonized block and thread-2 is waiting to enter the synchronized block:
Thread-1[1] where
[1] java.lang.Thread.sleep (native method)
[2] myThread.run (myThread.java:845)
Thread-1[1] threadlocks
Monitor information for thread Thread-1:
No monitors owned
Not waiting for a monitor
Thread-2[1] threadlocks
Monitor information for thread Thread-2:
No monitors owned
Waiting for monitor: instance of java.lang.Object(id=2350)
However, when I use JDB "lock" command to show the lock info of myLockObject.aStaticVariable.
It shows that it is owned by Thread-1 (first confusing info).
However, the "entry count" is 0 (second confusing info):
Thread-1[1] lock myLockObject.aStaticVariable
com.sun.tools.example.debug.expr.ParseException: Unable to complete expression.
Thread not suspended for method invoke
Owned by: Thread-1, entry count: 0
Waiting thread: Thread-2
As Thread-1 is already outside the synchronized block,
it should have released the lock. Why does it show that the lock is still
being owned by Thread-1? It made me confused.
How to explain?
Could anyone help me? Thanks in advance.
MJ
Don't synchronize on an irrelevant object.Do synchronize on the critical data object itself.The data object might have to do wait() and notify() for the accessing threads for theirwell-ordered traffic control.
hiwaa at 2007-7-13 3:28:28 >
> class myLockObject
> {
> public static Object aStaticVariable = new
> w Object();
> }
>
> Two threads, namely Thread-1 and Thread-2, are
> basically doing similar things as followed:
>
> while(true)
> {
> synchonized(myLockObject.aStaticVariable)
> {
> big chunk of code
> };
> Thread.sleep(10000);
> }
>
>
Oh yeah. That's a helpful code snippet.
You are deadlocking. Why? Well because thread 1 is waiting for a lock on something thread 2 has but won't give up until thread 1 gives up it's lock that it has but it won't give that up until thread 2 releases the lock that it has but it won't give it up till thread 1 releases it's lock but it...
etc.
hiwaa at 2007-7-13 3:28:28 >
sorry I dont catch it.There is only one lock between two threads.IHow does it cause deadlocks ?
> Don't synchronize on an irrelevant object.
> Do synchronize on the critical data object itself.
> The data object might have to do wait() and notify()
> for the accessing threads for their
> well-ordered traffic control.
Thanks for your suggestion.
But, could you see what's wrong in my code?
I still don't know why it hangs. Thanks
> what's wrong in my code?
Read the scsi-boy's reply.
Because you didn't:
post a small demo code that is generally compilable, runnable and could reproduce your
problem. See: http://homepage1.nifty.com/algafield/sscce.html
Here is a stab in the dark, more or less:
public class BadKenny{
static Object aStaticVariable;
static Data data;
public static void main(String[] args){
aStaticVariable = new Object();
data = new Data("main");
ThreadX t1 = new ThreadX("T1");
ThreadX t2 = new ThreadX("T2");
t1.start();
t2.start();
}
static class ThreadX extends Thread{
String id;
public ThreadX(String s){
id = s;
}
public void run(){
while(true){
synchronized(aStaticVariable){
try{
data.setId(id);
data.printId();
Thread.sleep(10000); // a long task
}
catch (InterruptedException e){
e.printStackTrace();
}
}
}
}
}
static class Data{
String id;
public Data(String s){
id = s;
}
public void setId(String s){
id = s;
}
public void printId(){
System.out.println("I am a " + id);
}
}
}
public class GoodKenny{
static Data data;
public static void main(String[] args){
data = new Data("main");
ThreadX t1 = new ThreadX("T1");
ThreadX t2 = new ThreadX("T2");
t1.start();
t2.start();
}
static class ThreadX extends Thread{
String id;
public ThreadX(String s){
id = s;
}
public void run(){
while(true){
try{
data.setId(id);
data.printId();
Thread.sleep(10000); // long task
}
catch (InterruptedException e){
e.printStackTrace();
}
}
}
}
static class Data{
String id;
public Data(String s){
id = s;
}
public synchronized void setId(String s){
id = s;
}
public synchronized void printId(){
System.out.println("I am a " + id);
}
// more better
public synchronized setAndPrint(String s){
id = s;
System.out.println("I am a " + id);
}
}
}
hiwaa at 2007-7-13 3:28:28 >
> > what's wrong in my code?
> Read the scsi-boy's reply.
>
> Because you didn't:
> post a small demo code that is generally compilable,
> runnable and could reproduce your
> problem. See:
> http://homepage1.nifty.com/algafield/sscce.html
>
Thanks hiwa for your code suggestion. I will consider it seriously.
I know it is difficult for you to understand the problem fully.
But, it is difficult for me to isolate the relevant code.
So, lets ignore my deadlock problem.
If I just want to explain the following two conflicting JDB info,
any hints? Thanks in advance.
Thread-1[1] threadlocks
Monitor information for thread Thread-1:
No monitors owned
Not waiting for a monitor
Thread-1[1] lock myLockObject.aStaticVariable
com.sun.tools.example.debug.expr.ParseException: Unable to complete expression.
Thread not suspended for method invoke
Owned by: Thread-1, entry count: 0
Waiting thread: Thread-2
And, ,what does it meant by:
"Thread not suspended for method invoke"
in the following JDB message.
I am sure I have sent "suspend" command to suspend all threads already. Thanks.
Thread-1[1] lock myLockObject.aStaticVariable
com.sun.tools.example.debug.expr.ParseException: Unable to complete expression.
Thread not suspended for method invoke
Owned by: Thread-1, entry count: 0
Waiting thread: Thread-2
It's only based on my 'common sense':
> No monitors owned
Not executing synchronized block/method
> Not waiting for a monitor
Nor waiting for executing them
> Thread not suspended for method invoke
Method invoke() call is not suspended
> Owned by: Thread-1, entry count: 0
> Waiting thread: Thread-2
Thread-2 is waiting for acquiring monitor
hiwaa at 2007-7-13 3:28:28 >
