Hey I am trying to make 10 threads in a synchronized manner and I came up with the code below however I am not able to understand a part of it as mentioned below. I am still really new to java, I tried looking up synchronized threading from
Here but still I am clueless.
class question3 {
public static void main(String arg[]) throws Exception {
for (int i = 0; i < 11; i++) {
data di = new data();
System.out.println(di.count);
}
}
}
class item {
static int count = 0;
}
class data extends item implements Runnable {
item d = this;
Thread t;
data() {
t = new Thread(this);
t.start();
}
public void run() {
d = syn.increment(d);
}
}
class syn {
synchronized static item increment(item i) {
i.count++;
return (i);
}
}
I am not sure what this part of code does?
public void run() {
d = syn.increment(d);
}
}
class syn {
synchronized static item increment(item i) {
i.count++;
return (i);
}
}
the run function is used when starting the thread, this is a must function you need to override when implement Runnable. When calling Thread.start(), the run function will be called.
The class syn contains a synchronized method, it is simply mean that only one thread can access it each time, thus make the incerment function thread safe.
object d has a static variable count meaning all instances of item class (and data ) share the same count, so all threads increment the same variable
the line d = syn.increment(d); is basically count++ but in a thread safe way
My code basically goes like this:
//in Main Thread: (myList is a volatile field)
myList = new ArrayList<myClass>();
Thread myThread = new Thread(new MyCustomRunnable(myList));
myThread.start();
//some lines of code NOT involving myList
myThread.join();
//myList appears empty here even though I can see that the list has been populated
//in the other thread
Is there a reason for this behavior? Like I said, i can see in the debugger that the list has been populated in the called thread, but these changes don't appear in the caller thread after the join() method. MyCustomRunnable also declares the ArrayList as a volatile field, assigned by the constructor.
UPDATE:
Ok, I made a simpler program, replacing the ArrayList with a simple Integer and the results are the same...
public class Threading {
private volatile static Integer i = 0;
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(new MyCustomRunnable(i));
t.start();
t.join();
System.out.println(i); //prints '0', I expected '1'
}
}
public class MyCustomRunnable implements Runnable {
private volatile Integer i;
public MyCustomRunnable(Integer i) {
this.i = i;
}
public void run() {
this.i = 1;
}
}
Why isn't the Integer's value updated in the main Thread?
Add
public static void setI(int i) {
Threading.i = i;
}
to your Threading class and in your runnable add
public void run() {
this.i = 1;
Threading.setI(1);
}
This has nothing to do with multithreading, just variable scoping. i in the Threading class is not updated in MyCustomRunnable's run method.
Is there a way to enforce only a single execution of a thread object?
Something like a thread singleton?
To illustrate, consider below example:
I have a runnable implemented class.
I would like that I will be able to call start() method only one time of the object.
You can put a boolean as attribute to check if the thread has already been launch
Add a static boolean field in your Runnable and check it at the start of the run method like this:
synchronized(MyRunnable.class) {
if(alreadyRan) {
return;
}
alreadyRan = true;
}
Well, with the tips of my friends here in this thread, I reached the following:
public class TestThread extends Thread {
static private TestThread _instance = null;
private TestThread() {}
public static TestThread getThread(){
if(_instance == null)
_instance = new TestThread();
return _instance;
}
#Override
public void run()
{
System.out.println("Hello");
}
}
And this is an example of using it, when calling start for the second time throws an exception:
public class Main {
public static void main(String[] args) {
try {
TestThread.getThread().start();
TestThread.getThread().start();
} catch (IllegalThreadStateException e) {
System.out.println("Error: Tried to start more than one instance of this thread!");
e.printStackTrace();
}
}
}
Your comments are welcomed.
Why does a call to removeListener() in the following code throw a ConcurrentModificationException when another thread is using the iterator in fireEvent()?
public class MyClass {
private Set<Object> synchronizedListeners;
public MyClass() {
synchronizedListeners = Collections.synchronizedSet(
new LinkedHashSet<Object>());
}
public void addListener(Object listener) {
synchronizedListeners.add(listener);
}
public synchronized void removeListener(Object listener) {
synchronizedListeners.remove(listener);
}
public void fireEvent() {
synchronized (synchronizedListeners) {
for (Object listener : synchronizedListeners) {
// do something with listener
}
}
}
}
From my understanding, since I'm using synchronized (synchronizedListeners) in fireEvent(), this should block any other thread that calls removeListener(), till the iteration in fireEvent() is complete at which point it should be safe to remove an element from this Set. But this doesn't seem to be the case. What am I doing wrong?
Possibly related: Java synchronized block vs. Collections.synchronizedMap
Edit: It was pointed out that I was unnecessarily synchronizing the removeListener() method. So I tried this version:
public void removeListener(Object listener) {
synchronizedListeners.remove(listener);
}
But still got the same error.
Edit 2: As assylias pointed out, the problem isn't visible in the above code. I was calling removeListener() from inside the for loop in the synchronized (synchronizedListeners) block which was causing the error. The fix I ended up using in this case is to remove the listener from another thread:
public void removeListener(final Object listener) {
new Thread() {
#Override
public void run() {
synchronizedListeners.remove(listener);
}
}.start();
}
You are synchronizing on two different objects.
The removeListener method is synchronized on the MyClass instance, whereas the loop inside the fireEvent is synchronized on synchronizedListeners set.
What you need to do is to synchronize every method that uses the synchronizedListeners on the set itself.
I can't reproduce what you describe - the code at the bottom gives the output shown below - which shows that remove is called in the middle of the iteration, but does not complete until after the iteration because you use a synchronized collection. That's the behaviour one would expect and no ConcurrentModificationException is thrown. Note that I have removed the synchronized keyword from the removeListener method as it is useless here.
fire 100000
remove 100000
done fire 100000
done remove 99999
Conclusion: the problem is somewhere else. For example, if you have a subclass that overrides the fireEvent method, or if you construct the synchronized set not exactly as in the code you posted.
public static void main(String[] args) {
final MyClass mc = new MyClass();
final Object o = new Object();
mc.addListener(o);
for (int i = 0; i < 99999; i++) {
Object o1 = new Object();
mc.addListener(o1);
}
Runnable remove = new Runnable() {
#Override
public void run() {
mc.removeListener(o);
}
};
new Thread(remove).start();
mc.fireEvent();
}
public static class MyClass {
protected Set<Object> synchronizedListeners = Collections.synchronizedSet(new LinkedHashSet<Object>());
public void addListener(Object listener) {
synchronizedListeners.add(listener);
}
public void removeListener(Object listener) {
System.out.println("remove " + synchronizedListeners.size());
synchronizedListeners.remove(listener);
System.out.println("done remove " + synchronizedListeners.size());
}
public void fireEvent() {
System.out.println("fire " + synchronizedListeners.size());
synchronized (synchronizedListeners) {
for (Object listener : synchronizedListeners) {
// do something with listener
}
}
System.out.println("done fire " + synchronizedListeners.size());
}
}
Can anyone suggest to me how I can pass a parameter to a thread?
Also, how does it work for anonymous classes?
You need to pass the parameter in the constructor to the Runnable object:
public class MyRunnable implements Runnable {
public MyRunnable(Object parameter) {
// store parameter for later user
}
public void run() {
}
}
and invoke it thus:
Runnable r = new MyRunnable(param_value);
new Thread(r).start();
For Anonymous classes:
In response to question edits here is how it works for Anonymous classes
final X parameter = ...; // the final is important
Thread t = new Thread(new Runnable() {
p = parameter;
public void run() {
...
};
t.start();
Named classes:
You have a class that extends Thread (or implements Runnable) and a constructor with the parameters you'd like to pass. Then, when you create the new thread, you have to pass in the arguments, and then start the thread, something like this:
Thread t = new MyThread(args...);
t.start();
Runnable is a much better solution than Thread BTW. So I'd prefer:
public class MyRunnable implements Runnable {
private X parameter;
public MyRunnable(X parameter) {
this.parameter = parameter;
}
public void run() {
}
}
Thread t = new Thread(new MyRunnable(parameter));
t.start();
This answer is basically the same as this similar question: How to pass parameters to a Thread object
via constructor of a Runnable or Thread class
class MyThread extends Thread {
private String to;
public MyThread(String to) {
this.to = to;
}
#Override
public void run() {
System.out.println("hello " + to);
}
}
public static void main(String[] args) {
new MyThread("world!").start();
}
This answer comes very late, but maybe someone will find it useful. It is about how to pass a parameter(s) to a Runnable without even declaring named class (handy for inliners):
String someValue = "Just a demo, really...";
new Thread(new Runnable() {
private String myParam;
public Runnable init(String myParam) {
this.myParam = myParam;
return this;
}
#Override
public void run() {
System.out.println("This is called from another thread.");
System.out.println(this.myParam);
}
}.init(someValue)).start();
Of course you can postpone execution of start to some more convenient or appropriate moment. And it is up to you what will be the signature of init method (so it may take more and/or different arguments) and of course even its name, but basically you get an idea.
In fact there is also another way of passing a parameter to an anonymous class, with the use of the initializer blocks. Consider this:
String someValue = "Another demo, no serious thing...";
int anotherValue = 42;
new Thread(new Runnable() {
private String myParam;
private int myOtherParam;
// instance initializer
{
this.myParam = someValue;
this.myOtherParam = anotherValue;
}
#Override
public void run() {
System.out.println("This comes from another thread.");
System.out.println(this.myParam + ", " + this.myOtherParam);
}
}).start();
So all happens inside of the initializer block.
When you create a thread, you need an instance of Runnable. The easiest way to pass in a parameter would be to pass it in as an argument to the constructor:
public class MyRunnable implements Runnable {
private volatile String myParam;
public MyRunnable(String myParam){
this.myParam = myParam;
...
}
public void run(){
// do something with myParam here
...
}
}
MyRunnable myRunnable = new myRunnable("Hello World");
new Thread(myRunnable).start();
If you then want to change the parameter while the thread is running, you can simply add a setter method to your runnable class:
public void setMyParam(String value){
this.myParam = value;
}
Once you have this, you can change the value of the parameter by calling like this:
myRunnable.setMyParam("Goodbye World");
Of course, if you want to trigger an action when the parameter is changed, you will have to use locks, which makes things considerably more complex.
I know that I'm a few years late, but I came across this issue and took an unorthodox approach. I wanted to do it without making a new class, so this is what I came up with:
int x = 0;
new Thread((new Runnable() {
int x;
public void run() {
// stuff with x and whatever else you want
}
public Runnable pass(int x) {
this.x = x;
return this;
}
}).pass(x)).start();
You can either extend the Thread class or the Runnable class and provide parameters as you want. There are simple examples in the docs. I'll port them here:
class PrimeThread extends Thread {
long minPrime;
PrimeThread(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
. . .
}
}
PrimeThread p = new PrimeThread(143);
p.start();
class PrimeRun implements Runnable {
long minPrime;
PrimeRun(long minPrime) {
this.minPrime = minPrime;
}
public void run() {
// compute primes larger than minPrime
. . .
}
}
PrimeRun p = new PrimeRun(143);
new Thread(p).start();
To create a thread you normally create your own implementation of Runnable. Pass the parameters to the thread in the constructor of this class.
class MyThread implements Runnable{
private int a;
private String b;
private double c;
public MyThread(int a, String b, double c){
this.a = a;
this.b = b;
this.c = c;
}
public void run(){
doSomething(a, b, c);
}
}
Either write a class that implements Runnable, and pass whatever you need in a suitably defined constructor, or write a class that extends Thread with a suitably defined constructor that calls super() with appropriate parameters.
In Java 8 you can use lambda expressions with the Concurrency API & the ExecutorService as a higher level replacement for working with threads directly:
newCachedThreadPool() Creates a thread pool that creates new threads
as needed, but will reuse previously constructed threads when they are
available. These pools will typically improve the performance of programs that execute many short-lived asynchronous tasks.
private static final ExecutorService executor = Executors.newCachedThreadPool();
executor.submit(() -> {
myFunction(myParam1, myParam2);
});
See also executors javadocs.
As of Java 8, you can use a lambda to capture parameters that are effectively final. For example:
final String param1 = "First param";
final int param2 = 2;
new Thread(() -> {
// Do whatever you want here: param1 and param2 are in-scope!
System.out.println(param1);
System.out.println(param2);
}).start();
Parameter passing via the start() and run() methods:
// Tester
public static void main(String... args) throws Exception {
ThreadType2 t = new ThreadType2(new RunnableType2(){
public void run(Object object) {
System.out.println("Parameter="+object);
}});
t.start("the parameter");
}
// New class 1 of 2
public class ThreadType2 {
final private Thread thread;
private Object objectIn = null;
ThreadType2(final RunnableType2 runnableType2) {
thread = new Thread(new Runnable() {
public void run() {
runnableType2.run(objectIn);
}});
}
public void start(final Object object) {
this.objectIn = object;
thread.start();
}
// If you want to do things like setDaemon(true);
public Thread getThread() {
return thread;
}
}
// New class 2 of 2
public interface RunnableType2 {
public void run(Object object);
}
You can derive a class from Runnable, and during the construction (say) pass the parameter in.
Then launch it using Thread.start(Runnable r);
If you mean whilst the thread is running, then simply hold a reference to your derived object in the calling thread, and call the appropriate setter methods (synchronising where appropriate)
There is a simple way of passing parameters into runnables.
Code:
public void Function(final type variable) {
Runnable runnable = new Runnable() {
public void run() {
//Code adding here...
}
};
new Thread(runnable).start();
}
No you can't pass parameters to the run() method. The signature tells you that (it has no parameters). Probably the easiest way to do this would be to use a purpose-built object that takes a parameter in the constructor and stores it in a final variable:
public class WorkingTask implements Runnable
{
private final Object toWorkWith;
public WorkingTask(Object workOnMe)
{
toWorkWith = workOnMe;
}
public void run()
{
//do work
}
}
//...
Thread t = new Thread(new WorkingTask(theData));
t.start();
Once you do that - you have to be careful of the data integrity of the object you pass into the 'WorkingTask'. The data will now exist in two different threads so you have to make sure it is Thread Safe.
One further option; this approach lets you use the Runnable item like an asynchronous function call. If your task does not need to return a result, e.g. it just performs some action you don't need to worry about how you pass back an "outcome".
This pattern lets you reuse an item, where you need some kind of internal state. When not passing parameter(s) in the constructor care is needed to mediate the programs access to parameters. You may need more checks if your use-case involves different callers, etc.
public class MyRunnable implements Runnable
{
private final Boolean PARAMETER_LOCK = false;
private X parameter;
public MyRunnable(X parameter) {
this.parameter = parameter;
}
public void setParameter( final X newParameter ){
boolean done = false;
synchronize( PARAMETER_LOCK )
{
if( null == parameter )
{
parameter = newParameter;
done = true;
}
}
if( ! done )
{
throw new RuntimeException("MyRunnable - Parameter not cleared." );
}
}
public void clearParameter(){
synchronize( PARAMETER_LOCK )
{
parameter = null;
}
}
public void run() {
X localParameter;
synchronize( PARAMETER_LOCK )
{
localParameter = parameter;
}
if( null != localParameter )
{
clearParameter(); //-- could clear now, or later, or not at all ...
doSomeStuff( localParameter );
}
}
}
Thread t = new Thread(new MyRunnable(parameter));
t.start();
If you need a result of processing, you will also need to coordinate completion of MyRunnable when the sub-task finishes. You could pass a call back or just wait on the Thread 't', etc.
Specially for Android
For callback purposes I usually implement my own generic Runnable with input parameter(s):
public interface Runnable<TResult> {
void run(TResult result);
}
Usage is simple:
myManager.doCallbackOperation(new Runnable<MyResult>() {
#Override
public void run(MyResult result) {
// do something with the result
}
});
In manager:
public void doCallbackOperation(Runnable<MyResult> runnable) {
new AsyncTask<Void, Void, MyResult>() {
#Override
protected MyResult doInBackground(Void... params) {
// do background operation
return new MyResult(); // return resulting object
}
#Override
protected void onPostExecute(MyResult result) {
// execute runnable passing the result when operation has finished
runnable.run(result);
}
}.execute();
}
Create a local variable in your class that extends Thread or implements Runnable.
public class Extractor extends Thread {
public String webpage = "";
public Extractor(String w){
webpage = w;
}
public void setWebpage(String l){
webpage = l;
}
#Override
public void run() {// l is link
System.out.println(webpage);
}
public String toString(){
return "Page: "+webpage;
}}
This way, you can pass a variable when you run it.
Extractor e = new Extractor("www.google.com");
e.start();
The output:
"www.google.com"
First I want to point out that other answers are true.
However, using the parameter in the constructor may not be the best idea for all of you.
In many scenarios you will want to use "Anonymous Inner Class", and override the run() method, because defining specific class for every use is painful.
(new MyRunnable(){...})
And at the time you create that Runnable, the parameter may not be available to you to pass it in the constructor. If for example, you pass this object to a method, that will perform some work in separate thread and then call your runnable, applying the result from that work to it.
In that case, using a method like this one:
public MyRunnable withParameter(Object parameter), may turn out to be far more useful choice.
I do not claim that this is the best solution to the problem, but it will get the job done.