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HealthChecker for Java Process

I want to create a health checker, which will check the health of a java process. My process does a lot of things and is multi threaded. Various exceptions could be thrown, like Service / SQL / IO, etc. My plan is to call the HealthChecker to check for the process, from the catch block, in the individual threads. This will check for all the different healths, and in the case where there is any issue it will pause the threads, and log appropriately. There will be other processes which will read the logs by the process, and alert support to take appropriate actions.
Below is the general structure of the java process.
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Schedular {
private static int numOfTasks = 10 ;
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(5);
while(true){
for(int i=0;i<numOfTasks;i++){
service.execute(new Workers());
}
}
}
}
class Workers implements Runnable{
#Override
public void run() {
/*
* This can throw different exceptions , eg:
*/
try{
}catch(Exception e){
e.printStackTrace();
HealthChecker.checkHealth();
}
}
}
class HealthChecker{
public static void checkHealth() {
//Check health and then , log and pause all the threads
}
}
I am not able to figure out a way to pause all the threads. If there is a db exception I want all the threads to pause. I am requesting some suggestions.

You need a way to block the threads until some event occurs that allows the threads to continue. I see some major issues with the code:
1) The while(true) in your main thread might lead to a StackOverflowError. With each iteration of the while loop, you will add 10 more threads to the executor, and this will just continue unbounded.
2) There is no loop in your run() so that even if an exception is caught and we wait for the HealthCheck, the run() method would still exit. While a loop is not needed in your run() if you can constantly execute new Threads from your main thread to take the place of the terminated one, but that logic is not presently there in the main loop.
But setting those concerns aside here is one way to block worker threads until some event (presumably a HealthCheck all clear) occurs.
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Schedular {
private static int numOfTasks = 10 ;
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(5);
HealtchChecker hChecker = new HealthChecker();
for(int i=0;i<numOfTasks;i++){
service.execute(new Workers(hChecker));
}
}
}
class Workers implements Runnable{
private HealtchChecker hChecker;
public Workers(HealtchChecker hChecker){
this.hChecker = hChecker;
}
#Override
public void run() {
/*
* This can throw different exceptions , eg:
*/
while(true) {
try{
}catch (InterruptedException ie) {
throw ie;
}catch(Exception e){
e.printStackTrace();
HealthChecker.checkHealth();
}
}
}
}
class HealthChecker implements Runnable {
private final Semaphore semaphore = new Semaphore(1, true);
public void checkHealth() {
try {
semaphore.acquire();
} finally {
semaphore.release();
}
}
#Override
public void run(){
//code to check for errors that cause threads to pause.
if (inErrorState) {
semaphore.acquire();
} else {
semaphore.release();
}
}
}
A few things worth mentioning.
1) The main thread only creates 10 threads, versus an unbounded amount. You can adjust this as needed.
2) The Worker thread is long lived, meaning it will continue running even if it encounters Exceptions, except for an InterruptException.
3) HealthCheck is no longer a static object. it is instead a shared object.
4) HealthCheck is a runnable that can be executed in its own thread for monitoring for errors. I did not add the code to execute this thread.
5) HealCheck uses a Semaphore to cause the threads to block until the error state is cleared. I looked for other objects that can do this, like CountDownLatch or CyclicBarrier or Phaser, but this one came closest to giving us what we need to block all the threads from one point (the run() method).
Its not perfect but I think it gets you a little bit closer to what you want.

You're venturing pretty far afield from best practices, but you didn't ask about best practices for monitoring the health of threads - so I won't answer that question. Instead, I'll just answer the question you asked: how can I pause a set of threads managed by an ExecutorService?
Assuming that your Workers.run() will eventually end without intervention (in other words, it's not in an infinite loop - intentional or otherwise), the right thing to do is to call service.shutdown() (where service is your instance of ExecutorService). To do this, you can pass service in to HealthCheck.healthCheck() as a new parameter. Calling shutdown() will allow the currently-running threads to complete, then stop the executor.
If Workers.run() will not naturally complete, best practice says that you need to change your code such that it will. There is a Thread.stop() method you can call to halt the thread and a Thread.suspend() method you can call to suspend the thread. Both of these are double-bad ideas for you to use for two reasons:
They are Deprecated and will leave the Threads in a super-unhealthy state. You will have very difficult problems in the future if you use them.
You are using ExecutorService. That means you are delegating thread management to that class. If you go messing with the state of the Threads underneath ExecutorService, it can't manage the thread pool for you and, again, you will have very difficult problems in the future.

How to change my Thread implementation to use ExecutorService

I had been making a game, and was using Threads in my program to carry out tasks. So let me explain the scenario a bit. I have a BattleManager class which implements Runnable and keep looping in the battle queue for battles, if there are any.
#Override
public void run() {
while(serverRunning){
synchronized (battleQueue) {
for(Battle battle : battleQueue){
if(battle != null){
if (battle instanceof WildBattle) {
if(!((WildBattle) battle).isBattleOver()){
((WildBattle) battle).tryExecuteBattleTurn();
}else{
battleQueue.remove(battle);
battle = null;
}
}
}
}
}
try {
Thread.sleep(3);
} catch (InterruptedException e)
e.printStackTrace();
}
}
currentThread = null;
}
Then I check if battle is not over, and if not I try to execute the battle turn. Since there can be more than 100 battles running at the same time and there are complex calculations inside every battle, I inside WildBattle class spawn a child thread to execute the task, so that the battles run in parallel.
Here is the method which is invoked inside wild battle class, which spawns a new thread.
public void tryExecuteBattleTurn() {
if (!isBattleTurnRunning && battleThread == null) {
battleThread = new Thread(new Runnable() {
#Override
public void run() {
//long startTime = System.currentTimeMillis();
executeBattle();
battleLog.setBattleLog("");
battleThread = null;
//System.err.println("Total execution time : " +(System.currentTimeMillis() - startTime));
}
}, "Battle thread");
battleThread.start();
}
}
Now the main question is, I want to learn about executor service and I read at few places that it is always better to use executor service rather than spawning new child threads. How can I change this to use executor service.
I am not sure though. I am not a java expert and still learning the language so spare me if you see something is wrong, and please let me know if I can change anything to make it more efficient.
Let me know if you are not clear about anything.

I'll show you a basic example and you'll manage how to integrate it with your code
First you create ExecutorService somewhere in your application.
ExecutorService executorService = Executors.newFixedThreadPool(NUMBER_OF_THREADS);
You should choose NUMBER_OF_THREADS based on your application needs. Threads are not created immediately - only when you submit a task to service and there are no available threads for it. If all NUMBER_OF_THREADS are busy, task will wait in queue until one of the threads will be able to handle it. ExecutorService will reuse threads, this will save time on thread instantiation and is a generally good concept to work with threads.
Then you manage how to access executor service from your battles. Then, when you need to perform an asynchronous work you submit task to service:
executorService.submit(new Runnable() {
#Override public void run() {
// your code here
}
}
If your application has a lifecycle and can be somehow shutdown, you'd like to shutdown ExecutorService as well. There are two options - shutdown() and shutdownNow(), first one waits for all current tasks to be executed, second one performs shutdown immediately and returns list of tasks that were not completed.
As was mentioned in comments, you should figure out how to preserve model state and organize thread synchronization based on your real situation.

Need to execute a piece of code repeatedly for fixed duration in java

I have written a piece of code . How can I get that code to run for certain duration repeatedly, say for 10 second?

The ExecutorService seems to provide methods which execute tasks until they are either completed or else a timeout occurs (such as the invokeAll).

You can give a try to Quartz Job Scheduler
Quartz is a richly featured, open source job scheduling library that
can be integrated within virtually any Java application - from the
smallest stand-alone application to the largest e-commerce system.
Quartz can be used to create simple or complex schedules for executing
tens, hundreds, or even tens-of-thousands of jobs; jobs whose tasks
are defined as standard Java components that may execute virtually
anything you may program them to do. The Quartz Scheduler includes
many enterprise-class features, such as support for JTA transactions
and clustering.
If you are familiar with Cron in Linux , this will be a cakewalk for you .

Use a worker and start it in a thread, wait in the main thread for the specific time and stop the worker after this.
MyRunnable task = new MyRunnable();
Thread worker = new Thread(task);
// Start the thread, never call method run() direct
worker.start();
Thread.sleep(10*1000); //sleep 10s
if (worker.isAlive()) {
task.stopPlease(); //this method you have to implement
}

Not too sure why people downvoted the question. Be sure to in the future provide some sample code. Your answer however is simple here. Create a new thread to watch the wait. In simple code:
public class RunningClass {
public static void runThis(){
TimerThread tt = new TimerThread();
tt.timeToWait = 10000;
new Thread(tt).start();
while (!TimerThread.isTimeOver){
\\Code to execute for time period
}
}
class TimerThread implements Runnable {
int timeToWait = 0;
boolean isTimeOver = false;
#override
public void run(){
Thread.sleep(timeToWait);
}
}
The code above can be put in the same class file. Change the 10000 to whatever time you require it to run for.
You could use other options, but it would require you to have knowledge on workers and tasks.

not sure what was the exact requirement, but
if your req was to cancel only a long running task
you could use ExecutorService & Future (in jdk 5) as follows.
ExecutorService fxdThrdPl = Executors.newFixedThreadPool(2);
// actual task .. which just prints hi but after 100 mins
Callable<String> longRunningTask = new Callable<String>() {
#Override
public String call() throws Exception {
try{
TimeUnit.MINUTES.sleep(100); // long running task .......
}catch(InterruptedException ie){
System.out.println("Thread interrupted");
return "";
}
return "hii"; // result after the long running task
}
};
Future<String> taskResult = fxdThrdPl.submit(longRunningTask); // submitting the task
try {
String output = taskResult.get(***10**strong text**, TimeUnit.SECONDS***);
System.out.println(output);
} catch (InterruptedException e) {
} catch (ExecutionException e) {
} catch (TimeoutException e) {
***taskResult.cancel(true);***
}

How to run concurrent job with dependent tasks?

I have a situation that I need to work on
I have a class which has send method, example
#Singleton
class SendReport {
public void send() {}
}
The send method is called from a user click on web page, and must return immediately, but must start a sequence of tasks that will take time
send
->|
| |-> Task1
<-| |
<-|
|
|-> Task2 (can only start when Task1 completes/throws exception)
<-|
|
|-> Task3 (can only start when Task2 completes/throws exception)
<-|
I am new to Java concurrent world and was reading about it. As per my understanding, I need a Executor Service and submit() a job(Task1) to process and get the Future back to continue.
Am I correct?
The difficult part for me to understand and design is
- How and where to handle exceptions by any such task?
- As far as I see, do I have to do something like?
ExecutorService executorService = Executors.newFixedThreadPool(1);
Future futureTask1 = executorService.submit(new Callable(){
public Object call() throws Exception {
System.out.println("doing Task1");
return "Task1 Result";
}
});
if (futureTask1.get() != null) {
Future futureTask2 = executorService.submit(new Callable(){
public Object call() throws Exception {
System.out.println("doing Task2");
return "Task2 Result";
}
}
... and so on for Task 3
Is it correct?
if yes, is there a better recommended way?
Thanks

Dependent task execution is made easy with Dexecutor
Disclaimer : I am the owner
Here is an example, it can run the following complex graph very easily, you can refer this for more details
Here is an example

If you just have a line of tasks that need to be called on completion of the previous one than as stated and discussed in the previous answers I don't think you need multiple threads at all.
If you have a pool of tasks and some of them needs to know the outcome of another task while others don't care you can then come up with a dependent callable implementation.
public class DependentCallable implements Callable {
private final String name;
private final Future pre;
public DependentCallable(String name, Future pre) {
this.name = name;
this.pre = pre;
}
#Override
public Object call() throws Exception {
if (pre != null) {
pre.get();
//pre.get(10, TimeUnit.SECONDS);
}
System.out.println(name);
return name;
}
A few other things you need to take care of based on the code in your question, get rid of future.gets in between submits as stated in previous replies. Use a thread pool size of which is at least greater than the depth of dependencies between callables.

Your current approach will not work as it will block till the total completion which you wanted to avoid.
future.get() is blocking();
so after submitting first Task, your code will wait till its finished and then next task will be submitted, again wait, so there is no advantage over single thread executing the tasks one by one.
so if anything the code would need to be:
Future futureTask2 = executorService.submit(new Callable(){
public Object call() throws Exception {
futureTask1.get()
System.out.println("doing Task2");
return "Task2 Result";
}
}
your graph suggests that the subsequent task should execute despite exceptions. The ExecutionException will be thrown from get if there was problem with computation so you need to guard the get() with appropriate try.
Since Task1, Task2 have to completed one after another, why you do you want them exececuted in different threads. Why not have one thread with run method that deals with Task1,Task2.. one by one. As you said not your "main" thread, it can be in the executor job but one that handles all the tasks.
I personally don't like anonymous inner classes and callback (that is what you kind of mimic with chain of futures). If I would have to implement sequence of tasks I would actually implement queue of tasks and processors that executes them.
Mainly cause it is "more manageable", as I could monitor the content of the queue or even remove not necessary tasks.
So I would have a BlockingQueue<JobDescription> into which I would submit the JobDescription containing all the data necessary for the Task execution.
I would implement threads (Processors) that in their run() will have infinitive loop in which they take the job from the queue, do the task, and put back into the queue the following task. Something in those lines.
But if the Tasks are predefined at the send method, I would simply have them submitted as one job and then execute in one thread. If they are always sequential then there is no point in splitting them between different threads.

You need to add one more task if you want to return send request immediately. Please check the following example. It submits the request to the background thread which will execute the tasks sequentially and then returns.
Callable Objects for 3 long running tasks.
public class Task1 implements Callable<String> {
public String call() throws Exception {
Thread.sleep(5000);
System.out.println("Executing Task1...");
return Thread.currentThread().getName();
}
}
public class Task2 implements Callable<String> {
public String call() throws Exception {
Thread.sleep(5000);
System.out.println("Executing Task2...");
return Thread.currentThread().getName();
}
}
public class Task3 implements Callable<String> {
public String call() throws Exception {
Thread.sleep(5000);
System.out.println("Executing Task3...");
return Thread.currentThread().getName();
}
}
Main method that gets request from the client and returns immediately, and then starts executing tasks sequentially.
public class ThreadTest {
public static void main(String[] args) {
final ExecutorService executorService = Executors.newFixedThreadPool(5);
executorService.submit(new Runnable() {
public void run() {
try {
Future<String> result1 = executorService.submit(new Task1());
if (result1.get() != null) {
Future<String> result2 = executorService.submit(new Task2());
if (result2.get() != null) {
executorService.submit(new Task3());
}
}
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
});
System.out.println("Submitted request...");
}
}

Future.get() does not return

I have the following piece of code:
public class Test {
List<Future> future = new ArrayList<Future>();
public static void main(String args[]) throws Exception {
Adapter b1 = new Adapter();
final ExecutorService threadPool = Executors.newCachedThreadPool();
for(//iterate for number of files) {
while(data exists in file) {
//Call a function to process and update values in db
future.add(threadPool.submit(new Xyz(b1)));
//read next set of data in file;
}
}
try {
for(Future f: future) {
f.get();
}
}
catch(Exception e) {
throw e;
}
}
}
class Xyz implements Runnable {
private Adapter a1;
public Xyz(Adapter al) {
this.a1=a1;
}
#Override
public void run() {
try {
a1.abc();
} catch (Exception e) {
throw new RuntimeException(e);
}
}
}
When the number of files is 1 (for loop runs for 1 time), the code runs fine.
But, when the number of files increases, the code never returns back from future.get() method.

just out of curiosity.. do i need to shutdown the executor somewhere ??
Yes, and this is likely the problem. Each Future.get() will block until the corresponding task is complete, then once all the tasks are complete your main thread will exit. But your java process will not exit because the thread pool threads are still active in the background. You should shut down the executor once you have finished with it, most likely as the last thing in your main method.
I also note that you're submitting many tasks that wrap the same Adapter instance and all call its abc() method - check that there's nothing in there that will deadlock when called simultaneously in more than one thread.

Your Callable::call / Runable::run does not return. Otherwise the corresponding future would not block.
Additional executor.shutdown or future.cancel will thow an InterruptedException to stop the thread processing the object you submitted but it is up to you if to catch it or not. Your are responsible for making the jobs you submitted stop.
When you submit thousands Callables/Runnables to a CachedExecutor that it might spawn so many threads that your machine gets so slow that you think it takes forever. But you would have noticed that.
When dealing with an undefined number of parallelizable tasks i suggest to use a FixedThreadPool with not much more threads that there are cpu cores.
Edit: Therefore when you set a breakpoints at a1.abc(); and step forward you will probably find out that it never returns.