java thread reusage via executor

java thread reusage via executor - java

I am confused on the following:
To use threads in a Java program, the simplest way is to extend Thread class and implement the runnable interface (or simply implement runnable).
To start the thread's execution. we must call the Thread's method start(), which in turn calls method run() of the thread. And so the thread starts.
The method start() (unless I am wrong) must be called exactly and only once for each thread. As a result, thread instances can not be reused unless somehow the run method itself runs in some-short of infinite loop that facilitates a custom implementation of the thread's reusage.
Now the javadoc
link text
says
Calls to execute will reuse previously constructed threads if available
I do not understand how this is implemented.
I provide in the execute method of the executor method my custom thread e.g.
ExecutorService myCachedPool = Executors.newCachedThreadPool();
myCachedPool.execute(new Runnable(){public void run(){
//do something time consuming
}});
How can this custom thread I delegeate to the executor framework be reused?
Is Executor is allowed to call method start() more than 1 time, while we can not in our programs?
Am I misunderstanding something?
Thank you.

Note that it's not Executor that calls start() - it's ExecutorService. And no, it's not calling start() twice. It doesn't start the task that you give it directly using Thread.start()... instead, it starts a thread which knows about that thread pool's queue of work. The thread will basically wait until there's some work to do, then pick it up and execute it, before going back to waiting. So although the thread performs several tasks, Thread.start() is only called once.
EDIT: Judging by the comments, you're a bit confused about the difference between a Runnable (which is a task to be executed) and a Thread (which is what executes tasks).
The same thread can execute multiple tasks. For a very simple example not using a thread pool, consider this:
public class MultiRunnable implements Runnable
{
private final List<Runnable> runnables;
public MultiRunnable(List<Runnable> runnables)
{
this.runnables = runnables;
}
public void run()
{
for (Runnable runnable : runnables)
{
runnable.run();
}
}
}
(Ignore the potential thread safety issues of using a List<T> from multiple threads.)
You could create a whole bunch of Runnable tasks capable of doing different things, then create a single MultiRunnable to run them in turn. Pass that instance of MultiRunnable into the Thread constructor, and then when you start the thread, it will execute each of the original runnable tasks. Does that help?

It is not calling start() more than once; instead the Thread in the pool never completes, but just stays alive---waiting. The source code is available for download if you want to look at it.
Each Thread in the thread pool can simply wait() for the Executor to hand it a new Runnable, but the Thread's own run() method has not completed. It simply waits for a new Runnable to be given to the Executor.

To "start" a thread more than once, create a runnable. For example:
//NO
private class T extends Thread { //not necessary to implement runnable
public void run(){
//...
}
}
void someMethod(){
T a = new T();
a.start();
a.start(); //NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO NO
}
Instead,
//Yes
private class T implements Runnable {
public void run(){
//...
}
}
void someMethod(){
T a = new T();
new Thread(a).start();
new Thread(a).start(); //YES YES YES
}
It is also possible to do this:
void someMethod(){
final Runnable r = new Runnable(){
public void run(){
//...
}
};
new Thread(r).start();
new Thread(r).start();
}
// r could also be a field of you class.

Related

Java - are there different ways of starting threads?

Ok, guys so my teacher uses this code to start a thread if a thread is not already active. But i have been taught that to run threads no matter if its runnable or extending thread, you start it by the start method and not run. But in this case he starts it with run, why is that?
public void start2(Runnable r) {
if (thread == null) {
thread = new Thread(new Runnable() {
public void run() {
r.run();
thread = null;
}
});
thread.start();
}
}

Your teacher starts thread with thread.start() . He just implemented the runnable interface inside the Thread object initialization which is the absolutely correct approach.

A more modern approach would be to use an Executor to run the thread:
ExecutorService executor = Executors.newSingleThreadExecutor();
executor.submit(() -> {
String threadName = Thread.currentThread().getName();
System.out.println("Hello " + threadName);
});
You have a better control of the thread:
Can retrieve some result (with futures)
Know if your thread is over (executor.isTerminated())
Request/force a shutdown (executor.awaitTermination()/executor.shutdownNow()).
These functionalities are not natively supported by the thread.start() that your teacher shows you (which is, by the way, a good way to launch a thread).
For more about Executors, I advice this excellent tutorial.

The r.run(); part in your code is just a method call to your Runnable r input parameter, which will be an implementation of the Runnable interface.
This does not start a thread
It's just a method call that is executes the input's implementation of Run method.
It will execute just like any other method.
Then, the actual thread will start at thread.start();
Long answer:
What is happening here is, first checking the thread variable.
If its null then initialize a new Thread with an anonymus class thread = new Thread(/*here --> */new Runnable() {.... and implementing the run() method.
Inside the run() there is a call, made to the outer method's input param, called Runnable r with r.run(); then set the thread variable to null.
Just outside of the if statement, the teacher starts the thread with thread.start();.

What is happening in below code of threading?

Code:
public class ThreadTest {
public static void main(String[] args) {
MyImlementThread mit = new MyImlementThread();
Thread t = new Thread(mit);
t.start();
t = new Thread(mit);
t.start();
}
}
// MyImlementThread
class MyImlementThread implements Runnable {
public void run() {
System.out.println("This is implemented run() method");
}
}
/*
Output
This is implemented run() method
This is implemented run() method
*/

What happens here is the main thread starts two threads and exits. Each of the new threads writes a message to stdout, then ends. At that point since all the non-daemon threads have finished the JVM exits.
The posted code is confusing on account of it defining a Runnable but giving it a name ending in Thread.
A Thread object relates to an os-level thread, calling start on a Thread makes the code in the run method of the passed-in Runnable execute run on a separate thread from the one that called start.
The Runnable defines a task but doesn't specify how it runs. It could be passed into a specific Thread's constructor or submitted to an Executor or run by the current thread.
In this case the Runnable declared has no state, no instance variables are declared. Here two threads can execute the same Runnable without a conflict because there is no shared state. The printstream that writes to the console is synchronized, so the lines written by the threads each get written one at a time and don't get jumbled together.

How do I run a thread in java that lets me know when another thread has died?

Assume that I have a thread A in java. This thread continues to perform some task A.
I have another thread B that must perform a task B only after Task A is finished. How do I implement this in Java?

You can use Thread.join() to basically block one thread until another thread terminates:
// In thread B
threadA.join();
doStuff();
Note that this won't work properly if you use a thread pool e.g. via an executor service. If you need to do that (and I'd generally recommend using executors instead of "raw" threads) you'd need to make the executor task notify any listeners that it has completed (e.g. via a CountDownLatch).
If you use Guava, you should look at ListenableFuture, too, which simplifies things.

You can use Thread Exceutor to acieve this. Executor keep the value in thread pool. Refer this link, It may help you
http://www.journaldev.com/1069/java-thread-pool-example-using-executors-and-threadpoolexecutor
see also
How to run thread after completing some specific worker thread

In Java SE 7 you could use CountDownLatch. Here is an example. Good thing that comes with using of CountDownLatch is that you can initialize it with certain number of required countdowns, so you can wait for a set of threads. Also it doesn't require thread to be completed (like in join()), thread can call countDown() in any place you want and continue execution.
Also, another approach is CyclicBarrier.
class Starter {
public static void main(String[] args) {
CountDownLatch signal = new CountDownLatch();
Thread a = new Worker(signal);
Thread b = new AnotherWorker(signal);
a.start();
b.start();
//doSomethingElse
}
}
class Worker extends Thread {
CountDownLatch signal;
Worker(CountDownLatch signal) {
this.signal = signal;
}
public void run(){
//doSomething
signal.await(); //wait until thread b dies
//doSomethingElse
}
}
class AnotherWorker extends Thread {
CountDownLatch signal;
AnotherWorker(CountDownLatch signal) {
this.signal = signal;
}
public void run(){
//doSomething
signal.countDown(); //notify a about finish
}
}

How do I get the java.concurrency.CyclicBarrier to work as expected

I am writing code that will spawn two thread and then wait for them to sync up using the CyclicBarrier class. Problem is that the cyclic barrier isn't working as expected and the main thread doesnt wait for the individual threads to finish. Here's how my code looks:
class mythread extends Thread{
CyclicBarrier barrier;
public mythread(CyclicBarrier barrier) {
this.barrier = barrier;
}
public void run(){
barrier.await();
}
}
class MainClass{
public void spawnAndWait(){
CyclicBarrier barrier = new CyclicBarrier(2);
mythread thread1 = new mythread(barrier).start();
mythread thread2 = new mythread(barrier).start();
System.out.println("Should wait till both threads finish executing before printing this");
}
}
Any idea what I am doing wrong? Or is there a better way to write these barrier synchronization methods? Please help.

During execution of your main thread you create two other threads and tell them to wait for each other. But you wrote nothing to make your main thread to wait for them and complain it doesn't wait. Try
CyclicBarrier barrier = new CyclicBarrier(3);
mythread thread1 = new mythread(barrier).start();
mythread thread2 = new mythread(barrier).start();
barrier.await(); // now you wait for two new threads to reach the barrier.
System.out.println("Should wait till both threads finish executing before printing this");
BTW. Don't extend Thread class unless you have to. Implement Runnable and pass implementations to Thread objects. Like this:
class MyRunnable implements Runnable {
public void run(){
// code to be done in thread
}
}
Thread thread1 = new Thread(MyRunnable);
thread1.start();
EDIT
Justification for avoiding extending Thread.
The rule of thumb is as little coupling as possible. Inheritance is a very strong connection between classes. You have to inherit from Thread if you want to change some of its default behaviour (i.e. override some methods) or want to access some protected fields of class Thread. If you don't want it, you choose looser coupling - implementing Runnable and passing it as a constructor parameter to Thread instance.

Pass a Runnable instance to the constructor of your CyclicBarrier like this.
CyclicBarrier barrier = new CyclicBarrier(2, new Runnable() {
#Override
public void run() {
System.out.println("Should wait till both threads finish executing before printing this");
}
});
new mythread(barrier).start();
new mythread(barrier).start();

You are looking for Thread.join() method...
thread1.join();
thread2.join();
System.out.println("Finished");
EDIT: because of the comments...
And if you don't want to wait forever you can also specify the maximum number of milliseconds plus nanoseconds to wait for the thread to die

Cyclic barrier is not the correct choice in this case. You must use CountDownLatch here.
I assume you have are invoking spawnAndWait method from the main method.
The reason this wont work is that the CyclicBarrier has 2 constructors. To perform post-operations you must use a 2 parameter constructor. The most important thing to remember is that the main thread will not wait by the await method; but will continue to execute. However the Thread specified in the CyclicBarrier constructor will only run when all spawned threads stop at the barrier(by the await method)

How to know if other threads have finished?

I have an object with a method named StartDownload(), that starts three threads.
How do I get a notification when each thread has finished executing?
Is there a way to know if one (or all) of the thread is finished or is still executing?

There are a number of ways you can do this:
Use Thread.join() in your main thread to wait in a blocking fashion for each Thread to complete, or
Check Thread.isAlive() in a polling fashion -- generally discouraged -- to wait until each Thread has completed, or
Unorthodox, for each Thread in question, call setUncaughtExceptionHandler to call a method in your object, and program each Thread to throw an uncaught Exception when it completes, or
Use locks or synchronizers or mechanisms from java.util.concurrent, or
More orthodox, create a listener in your main Thread, and then program each of your Threads to tell the listener that they have completed.
How to implement Idea #5? Well, one way is to first create an interface:
public interface ThreadCompleteListener {
void notifyOfThreadComplete(final Thread thread);
}
then create the following class:
public abstract class NotifyingThread extends Thread {
private final Set<ThreadCompleteListener> listeners
= new CopyOnWriteArraySet<ThreadCompleteListener>();
public final void addListener(final ThreadCompleteListener listener) {
listeners.add(listener);
}
public final void removeListener(final ThreadCompleteListener listener) {
listeners.remove(listener);
}
private final void notifyListeners() {
for (ThreadCompleteListener listener : listeners) {
listener.notifyOfThreadComplete(this);
}
}
#Override
public final void run() {
try {
doRun();
} finally {
notifyListeners();
}
}
public abstract void doRun();
}
and then each of your Threads will extend NotifyingThread and instead of implementing run() it will implement doRun(). Thus when they complete, they will automatically notify anyone waiting for notification.
Finally, in your main class -- the one that starts all the Threads (or at least the object waiting for notification) -- modify that class to implement ThreadCompleteListener and immediately after creating each Thread add itself to the list of listeners:
NotifyingThread thread1 = new OneOfYourThreads();
thread1.addListener(this); // add ourselves as a listener
thread1.start(); // Start the Thread
then, as each Thread exits, your notifyOfThreadComplete method will be invoked with the Thread instance that just completed (or crashed).
Note that better would be to implements Runnable rather than extends Thread for NotifyingThread as extending Thread is usually discouraged in new code. But I'm coding to your question. If you change the NotifyingThread class to implement Runnable then you have to change some of your code that manages Threads, which is pretty straightforward to do.

Solution using CyclicBarrier
public class Downloader {
private CyclicBarrier barrier;
private final static int NUMBER_OF_DOWNLOADING_THREADS;
private DownloadingThread extends Thread {
private final String url;
public DownloadingThread(String url) {
super();
this.url = url;
}
#Override
public void run() {
barrier.await(); // label1
download(url);
barrier.await(); // label2
}
}
public void startDownload() {
// plus one for the main thread of execution
barrier = new CyclicBarrier(NUMBER_OF_DOWNLOADING_THREADS + 1); // label0
for (int i = 0; i < NUMBER_OF_DOWNLOADING_THREADS; i++) {
new DownloadingThread("http://www.flickr.com/someUser/pic" + i + ".jpg").start();
}
barrier.await(); // label3
displayMessage("Please wait...");
barrier.await(); // label4
displayMessage("Finished");
}
}
label0 - cyclic barrier is created with number of parties equal to the number of executing threads plus one for the main thread of execution (in which startDownload() is being executed)
label 1 - n-th DownloadingThread enters the waiting room
label 3 - NUMBER_OF_DOWNLOADING_THREADS have entered the waiting room. Main thread of execution releases them to start doing their downloading jobs in more or less the same time
label 4 - main thread of execution enters the waiting room. This is the 'trickiest' part of the code to understand. It doesn't matter which thread will enter the waiting room for the second time. It is important that whatever thread enters the room last ensures that all the other downloading threads have finished their downloading jobs.
label 2 - n-th DownloadingThread has finished its downloading job and enters the waiting room. If it is the last one i.e. already NUMBER_OF_DOWNLOADING_THREADS have entered it, including the main thread of execution, main thread will continue its execution only when all the other threads have finished downloading.

You should really prefer a solution that uses java.util.concurrent. Find and read Josh Bloch and/or Brian Goetz on the topic.
If you are not using java.util.concurrent.* and are taking responsibility for using Threads directly, then you should probably use join() to know when a thread is done. Here is a super simple Callback mechanism. First extend the Runnable interface to have a callback:
public interface CallbackRunnable extends Runnable {
public void callback();
}
Then make an Executor that will execute your runnable and call you back when it is done.
public class CallbackExecutor implements Executor {
#Override
public void execute(final Runnable r) {
final Thread runner = new Thread(r);
runner.start();
if ( r instanceof CallbackRunnable ) {
// create a thread to perform the callback
Thread callerbacker = new Thread(new Runnable() {
#Override
public void run() {
try {
// block until the running thread is done
runner.join();
((CallbackRunnable)r).callback();
}
catch ( InterruptedException e ) {
// someone doesn't want us running. ok, maybe we give up.
}
}
});
callerbacker.start();
}
}
}
The other sort-of obvious thing to add to your CallbackRunnable interface is a means to handle any exceptions, so maybe put a public void uncaughtException(Throwable e); line in there and in your executor, install a Thread.UncaughtExceptionHandler to send you to that interface method.
But doing all that really starts to smell like java.util.concurrent.Callable. You should really look at using java.util.concurrent if your project permits it.

Many things have been changed in last 6 years on multi-threading front.
Instead of using join() and lock API, you can use
1.ExecutorService invokeAll() API
Executes the given tasks, returning a list of Futures holding their status and results when all complete.
2.CountDownLatch
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
A CountDownLatch is initialized with a given count. The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a CyclicBarrier.
3.ForkJoinPool or newWorkStealingPool() in Executors is other way
4.Iterate through all Future tasks from submit on ExecutorService and check the status with blocking call get() on Future object
Have a look at related SE questions:
How to wait for a thread that spawns it's own thread?
Executors: How to synchronously wait until all tasks have finished if tasks are created recursively?

Do you want to wait for them to finish? If so, use the Join method.
There is also the isAlive property if you just want to check it.

You can interrogate the thread instance with getState() which returns an instance of Thread.State enumeration with one of the following values:
* NEW
A thread that has not yet started is in this state.
* RUNNABLE
A thread executing in the Java virtual machine is in this state.
* BLOCKED
A thread that is blocked waiting for a monitor lock is in this state.
* WAITING
A thread that is waiting indefinitely for another thread to perform a particular action is in this state.
* TIMED_WAITING
A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.
* TERMINATED
A thread that has exited is in this state.
However I think it would be a better design to have a master thread which waits for the 3 children to finish, the master would then continue execution when the other 3 have finished.

You could also use the Executors object to create an ExecutorService thread pool. Then use the invokeAll method to run each of your threads and retrieve Futures. This will block until all have finished execution. Your other option would be to execute each one using the pool and then call awaitTermination to block until the pool is finished executing. Just be sure to call shutdown() when you're done adding tasks.

I would suggest looking at the javadoc for Thread class.
You have multiple mechanisms for thread manipulation.
Your main thread could join() the three threads serially, and would then not proceed until all three are done.
Poll the thread state of the spawned threads at intervals.
Put all of the spawned threads into a separate ThreadGroup and poll the activeCount() on the ThreadGroup and wait for it to get to 0.
Setup a custom callback or listener type of interface for inter-thread communication.
I'm sure there are plenty of other ways I'm still missing.

I guess the easiest way is to use ThreadPoolExecutor class.
It has a queue and you can set how many threads should be working in parallel.
It has nice callback methods:
Hook methods
This class provides protected overridable beforeExecute(java.lang.Thread, java.lang.Runnable) and afterExecute(java.lang.Runnable, java.lang.Throwable) methods that are called before and after execution of each task. These can be used to manipulate the execution environment; for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method terminated() can be overridden to perform any special processing that needs to be done once the Executor has fully terminated.
which is exactly what we need. We will override afterExecute() to get callbacks after each thread is done and will override terminated() to know when all threads are done.
So here is what you should do
Create an executor:
private ThreadPoolExecutor executor;
private int NUMBER_OF_CORES = Runtime.getRuntime().availableProcessors();
private void initExecutor() {
executor = new ThreadPoolExecutor(
NUMBER_OF_CORES * 2, //core pool size
NUMBER_OF_CORES * 2, //max pool size
60L, //keep aive time
TimeUnit.SECONDS,
new LinkedBlockingQueue<Runnable>()
) {
#Override
protected void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
//Yet another thread is finished:
informUiAboutProgress(executor.getCompletedTaskCount(), listOfUrisToProcess.size());
}
}
};
#Override
protected void terminated() {
super.terminated();
informUiThatWeAreDone();
}
}
And start your threads:
private void startTheWork(){
for (Uri uri : listOfUrisToProcess) {
executor.execute(new Runnable() {
#Override
public void run() {
doSomeHeavyWork(uri);
}
});
}
executor.shutdown(); //call it when you won't add jobs anymore
}
Inside method informUiThatWeAreDone(); do whatever you need to do when all threads are done, for example, update UI.
NOTE: Don't forget about using synchronized methods since you do your work in parallel and BE VERY CAUTIOUS if you decide to call synchronized method from another synchronized method! This often leads to deadlocks
Hope this helps!

Here's a solution that is simple, short, easy to understand, and works perfectly for me. I needed to draw to the screen when another thread ends; but couldn't because the main thread has control of the screen. So:
(1) I created the global variable: boolean end1 = false; The thread sets it to true when ending. That is picked up in the mainthread by "postDelayed" loop, where it is responded to.
(2) My thread contains:
void myThread() {
end1 = false;
new CountDownTimer(((60000, 1000) { // milliseconds for onFinish, onTick
public void onFinish()
{
// do stuff here once at end of time.
end1 = true; // signal that the thread has ended.
}
public void onTick(long millisUntilFinished)
{
// do stuff here repeatedly.
}
}.start();
}
(3) Fortunately, "postDelayed" runs in the main thread, so that's where in check the other thread once each second. When the other thread ends, this can begin whatever we want to do next.
Handler h1 = new Handler();
private void checkThread() {
h1.postDelayed(new Runnable() {
public void run() {
if (end1)
// resond to the second thread ending here.
else
h1.postDelayed(this, 1000);
}
}, 1000);
}
(4) Finally, start the whole thing running somewhere in your code by calling:
void startThread()
{
myThread();
checkThread();
}

You could also use SwingWorker, which has built-in property change support. See addPropertyChangeListener() or the get() method for a state change listener example.

Look at the Java documentation for the Thread class. You can check the thread's state. If you put the three threads in member variables, then all three threads can read each other's states.
You have to be a bit careful, though, because you can cause race conditions between the threads. Just try to avoid complicated logic based on the state of the other threads. Definitely avoid multiple threads writing to the same variables.

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java thread reusage via executor - java

Related

Java - are there different ways of starting threads?

What is happening in below code of threading?

How do I run a thread in java that lets me know when another thread has died?

How do I get the java.concurrency.CyclicBarrier to work as expected

How to know if other threads have finished?

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