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Java: Threading in Shutdown Hook

I have a manager class that allows sub-modules to register a shutdown-hook using Runnable.
public class ApplicationManager() {
private final List<Runnable> shutdownHooks;
private ApplicationManager() {
// Other stuff
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
if (shutdownHooks != null && !shutdownHooks.isEmpty()) {
shutdownHooks.parallelStream()
.forEach(Runnable::run);
}
}));
}
// Other singleton stuff
public void registerShutdownHook(final Runnable hook) {
if (hook != null) {
this.shutdownHooks.add(hook);
}
}
public void resetApplication() {
// Reset stuff
shutdownHooks.parallelStream()
.forEach(Runnable::run);
shutdownHooks.clear();
}
}
The reason why this class does not accept Thread instances in registerShutdownHook() was mainly to reduce the complexity for the caller (so they do not need to wrap in an instance of Thread).
The application can be resetted, and I want to clean up the application by executing all the shutdown hooks. Although, I could wrap each Runnable with a Thread, register all of them with Runtime during registerShutdownHook(), and remove them from Runtime when resetApplication() is called, but I thought it may be neater that I control what needs to run.
In order to speed things up during cleanup, I used parallelStream(). Now I'm wondering if that is a bad idea to (during shutdown hook):
Use the thread pool;
Alternatively, create more threads.
Anyone experienced enough to give an advice?

java threading method within object with return value

I am pretty new to using multithreading, but I want to invoke a method asynchronously (in a separate Thread) rather than invoking it synchronously. The basic idea is that I'm creating a socket server with an object in memory, so for each client I will have to run something like object.getStuff() asynchronously.
The two constructs I found were:
having the class implement Runnable and threading this and
declaring a runnable class within a method.
Additionally this method needs a return value- will it be necessary to use Executor and Callable to achieve this? Could someone point me in the right direction for implementing this?
I have tried implement option 2, but this doesn't appear to be processing concurrently:
public class Test {
private ExecutorService exec = Executors.newFixedThreadPool(10);
public Thing getStuff(){
class Getter implements Callable<Thing>{
public Thing call(){
//do collection stuff
return Thing;
}
}
Callable<Thing> callable = new Getter();
Future<Thing> future = exec.submit(callable);
return future.get();
}
}
I am instantiating a single test object for the server and calling getStuff() for each client connection.

Threading Tutorial
The Java tutorial on concurrency has a good section on this. It's at https://docs.oracle.com/javase/tutorial/essential/concurrency/runthread.html. Essentially, you can either implement Runnable or Callable, or inherit from Thread.
Subclassing Thread
You can write a class, including an anonymous inner class, that extends Thread. Instantiate it, then invoke the start() method.
public class MyThread extends Thread {
public void run() {
System.out.println("This is a thread");
}
public static void main(String[] args) {
MyThread m = new MyThread();
m.start();
}
}
Implementing Runnable
You can write a class that implements Runnable, then wrap an instance in a Thread and invoke start(). Very much like the previous.
public class MyRunnable implements Runnable {
public void run() {
System.out.println("This is a thread");
}
public static void main(String[] args) {
MyRunnable r = new MyRunnable();
(new Thread(r)).start();
}
}
Return Value
Runnable doesn't allow for return values. If you need that, you need to implement Callable instead. Callable looks a lot like Runnable, except you override the call() method instead of the run() method, and you need to give it to an ExecutorService.
public class MyCallable implements Callable<Integer> {
public Integer call() {
System.out.println("A thread using Callable<Integer>");
return 42;
}
public static void main(String[] args) {
MyCallable c = new MyCallable();
Future<Integer> f = Executors.newSingleThreadExecutor().submit(c));
System.out.println("The thread returned: " +
f.get());
}
}

The two constructs I found were 1) having the class implement Runnable and threading 'this' and 2) declaring a runnable class within a method.
Option (2) probably is better. Most programs would be improved if they had more classes, not fewer. Each named entity in a program—each package, class, method, whatever—should have just one responsibility. In your option (1), you are asking the class to do two things.
For your option (2), you don't actually have to declare a whole class. You can either use an anonymous inner class, or if you can go with Java8 all the way, you can use a lambda expression. Google for either one to learn more.
Additionally this method needs a return value.
The classic way, is for the Runnable object to return the value through one of its own fields before the thread terminates. Then the parent thread, can examine the object and get the return value afterward.
Will it be necessary to use Executor and Callable to achieve this?
Necessary? A lot of people think that ExecutorService is a Good Thing.
Sounds like you are creating a server that serves multiple clients. Do these clients continually connect and disconnect? The advantage of using a thread pool (i.e., ThreadPoolExecutor) is that it saves your program from continually creating and destroying threads (e.g., every time a client connects/disconnects). Creating and destroying threads is expensive. If you have a lot of clients connecting and disconnecting, then using a thread pool could make a big difference in the performance of your server.

Creating and managing threads by yourself is generally bad approach.
As you already pointed - use Executors utility class to create executor and submit Callables to it.

public class RunWResult implements Runable{
private volatile ResultType var;
//the thread method
public void run(){
...
//generate a result and save it to var
var = someResult();
//notify waiting threads that a result has been generated
synchronized(this){
notify();
}
}
public ResultType runWithResult(){
//run the thread generating a result
Thread t = new Thread(this);
t.start();
//wait for t to create a result
try{
wait();
}catch(InterruptedException e){}
//return the result
return var;
}
}

Java new Thread using this - Any reason if it is good / bad?

Just a quick question look at the code below, is there any reason why wouldn't do this or is it fine?
public class MyClass implements Runnable, MyClassInterface {
Thread threader;
void start() {
threader = new Thread(this);
threader.start();
}
#Override
public void run() {
Thread current = Thread.getCurrentThread();
while (threader = current) {
..
}
}
}
The original logic was not to expose that fact it runs in a separate thread to the caller
who creates a "MyClass" but then there are doubts if that is a good thing or bad.
Can anyone see any good reason not to do it or is it acceptable. It can be expected that MyClass.start() maybe called a few times.
EDIT: Updated the code to show it is implementing Runnable and one other interface, the interface is used by client code, the actual implementation may run in a separate thread, same thread or any other way. The idea was to abstract that away from the client, as the client is simply an object that "MyClass" will notify and is not aware (currently) of the Runnable interface it implements.
Perhaps that abstraction is not needed and client should have more control?
EDIT: The start() was simply to tell the object it is ready to start receiving notifications rather than start a thread.
Have a look at this: http://docs.oracle.com/javase/8/docs/technotes/guides/concurrency/threadPrimitiveDeprecation.html

In my opinion, it is a bad design, because you are breaking encapsulation by implementing an interface (Runnable) and by providing a public method (run) that are of no use of the consumer of the class.
You can start a thread from the start method without inhering from Runnable:
public class MyClass {
private Thread thread;
public void start() {
thread = new Thread(this::doWork); // Java 8 method reference
thread.start();
}
private void doWork() {
// ...
}
}
If you can't use method references from Java 8, replace this::doWork with:
new Runnable() { public void run() { doWork(); } }

Using concurrent classes to process files in a directory in parallel

I am trying to figure out how to use the types from the java.util.concurrent package to parallelize processing of all the files in a directory.
I am familiar with the multiprocessing package in Python, which is very simple to use, so ideally I am looking for something similar:
public interface FictionalFunctor<T>{
void handle(T arg);
}
public class FictionalThreadPool {
public FictionalThreadPool(int threadCount){
...
}
public <T> FictionalThreadPoolMapResult<T> map(FictionalFunctor<T> functor, List<T> args){
// Executes the given functor on each and every arg from args in parallel. Returns, when
// all the parallel branches return.
// FictionalThreadPoolMapResult allows to abort the whole mapping process, at the least.
}
}
dir = getDirectoryToProcess();
pool = new FictionalThreadPool(10); // 10 threads in the pool
pool.map(new FictionalFunctor<File>(){
#Override
public void handle(File file){
// process the file
}
}, dir.listFiles());
I have a feeling that the types in java.util.concurrent allow me to do something similar, but I have absolutely no idea where to start.
Any ideas?
Thanks.
EDIT 1
Following the advices given in the answers, I have written something like this:
public void processAllFiles() throws IOException {
ExecutorService exec = Executors.newFixedThreadPool(6);
BlockingQueue<Runnable> tasks = new LinkedBlockingQueue<Runnable>(5); // Figured we can keep the contents of 6 files simultaneously.
exec.submit(new MyCoordinator(exec, tasks));
for (File file : dir.listFiles(getMyFilter()) {
try {
tasks.add(new MyTask(file));
} catch (IOException exc) {
System.err.println(String.format("Failed to read %s - %s", file.getName(), exc.getMessage()));
}
}
}
public class MyTask implements Runnable {
private final byte[] m_buffer;
private final String m_name;
public MyTask(File file) throws IOException {
m_name = file.getName();
m_buffer = Files.toByteArray(file);
}
#Override
public void run() {
// Process the file contents
}
}
private class MyCoordinator implements Runnable {
private final ExecutorService m_exec;
private final BlockingQueue<Runnable> m_tasks;
public MyCoordinator(ExecutorService exec, BlockingQueue<Runnable> tasks) {
m_exec = exec;
m_tasks = tasks;
}
#Override
public void run() {
while (true) {
Runnable task = m_tasks.remove();
m_exec.submit(task);
}
}
}
How I thought the code works is:
The files are read one after another.
A file contents are saved in a dedicated MyTask instance.
A blocking queue with the capacity of 5 to hold the tasks. I count on the ability of the server to keep the contents of at most 6 files at one time - 5 in the queue and another fully initialized task waiting to enter the queue.
A special MyCoordinator task fetches the file tasks from the queue and dispatches them to the same pool.
OK, so there is a bug - more than 6 tasks can be created. Some will be submitted, even though all the pool threads are busy. I've planned to solve it later.
The problem is that it does not work at all. The MyCoordinator thread blocks on the first remove - this is fine. But it never unblocks, even though new tasks were placed in the queue. Can anyone tell me what am I doing wrong?

The thread pool you are looking for is the ExecutorService class. You can create a fixed-size thread pool using newFixedThreadPool. This allows you to easily implement a producer-consumer pattern, with the pool encapsulating all the queue and worker functionality for you:
ExecutorService exec = Executors.newFixedThreadPool(10);
You can then submit tasks in the form of objects whose type implements Runnable (or Callable if you want to also get a result):
class ThreadTask implements Runnable {
public void run() {
// task code
}
}
...
exec.submit(new ThreadTask());
// alternatively, using an anonymous type
exec.submit(new Runnable() {
public void run() {
// task code
}
});
A big word of advice on processing multiple files in parallel: if you have a single mechanical disk holding the files it's wise to use a single thread to read them one-by-one and submit each file to a thread pool task as above, for processing. Do not do the actual reading in parallel as it will degrade performance.

A simpler solution than using ExecuterService is to implement your own producer-consumer scheme. Have a thread that create tasks and submits to a LinkedBlockingQueue or ArrayBlockingQueue and have worker threads that check this queue to retrieve the tasks and do them. You may need a special kind of tasks name ExitTask that forces the workers to exit. So at the end of the jobs if you have n workers you need to add n ExitTasks into the queue.

Basically, what #Tudor said, use an ExecutorService, but I wanted to expand on his code and I always feel strange editing other people's posts. Here's a sksleton of what you would submit to the ExecutorService:
public class MyFileTask implements Runnable {
final File fileToProcess;
public MyFileTask(File file) {
fileToProcess = file;
}
public void run() {
// your code goes here, e.g.
handle(fileToProcess);
// if you prefer, implement Callable instead
}
}
See also my blog post here for some more details if you get stuck
Since processing Files often leads to IOExceptions, I'd prefer a Callable (which can throw a checked Exception) to a Runnable, but YMMV.

Threads; Creating a separate thread to periodically do something

As an addition to my current application, I need to create a separate thread which will periodically do some processing
I've create a new class to do all this, and this class will be loaded on startup of my application.
This is what I have so far :
public class PeriodicChecker extends Thread
{
static
{
Thread t = new Thread(new PeriodicChecker());
while(true)
{
t.run();
try
{
Thread.sleep(5000l);
}
catch (InterruptedException e)
{
e.printStackTrace();
}
}
}
/**
* Private constructor to prevent instantiation
*/
private PeriodicChecker()
{
}
#Override
public void run()
{
System.out.println("Thread is doing something");
// Actual business logic here, that is repeated
}
}
I want to make constructor private to prevent other people from attempting to instantiate this class accidentally. How can I achieve this?
Also, is there anything bad about my implementation of such requirements? I'm only creating one thread which will run then sleep, have I missed anything obvious? I haven't worked with threads before

Java offers ScheduledExecutorService to schedule and run periodic tasks or tasks with delay. It should provide all the features you need. Timer is another class that offers similar functionalities, but I would recommend the ScheduledExecutorService over Timer for its flexibility of configuration and better error management.

You have some conceptual erros in your code... for example:
You should call start() and not run(), because you are running the method sequentially and not simultaneously.
You can call start() only once, not once in each loop iteration. After that, the thread is in state TERMINATED, you should create a new thread to run it again
You should not create the thread in the static block, it is a bad practice, and maybe the Thread is running before you want it to run.
You should read some examples about thread, it is a little difficult to unserstand at the beginning, and you can have undesired effects very easily.
Here is a little example, that may do something similar to that you want:
public class PeriodicChecker extends Thread
{
#Override
public void run()
{
while(true) {
System.out.println("Thread is doing something");
Thread.sleep(5000);
}
}
}
public OtherClass {
public static void main(String args[]) {
Thread t = new PeriodicChecker();
t.start();
}
}
If you want that none can create a new Thread, you could create a singleton, so you will be sure that none is creating more threads.

I'd recommend you to consider Timer class - it provides functionality for periodic tasks execution.
Also you may take a look at "Timer & TimerTask versus Thread + sleep in Java" question discussion - there you can find some arguments and examples.

First of all to answer your specific question, you have already achieved your objective. You have declared your constructor to be private meaning no external class can call it like new PeriodicChecker().
Looking at your code however, there are a number of other problems:
Firstly, you are creating an instance of your class within its own static constructor. The purpose of a static constructor is to initialise any static state that your class may have, which instances of your class may then depend on. By creating an instance of the class within the static constructor, all of these guarantees go out the window.
Secondly, I don't think your thread is going to behave in the way you expect it to behave, primarily because you don't actually start another thread :). If you intend to start a new thread, you need to call the start() method on that thread object. Calling run() as you do does not actually create a new thread, but simply runs the run() method in the current thread.
Nowadays when you want to create a new thread to do something, the reccomended way of achieving this is to not extend Thread, but instead implement the Runnable interface. This allows you to decouple the mechanism of the thread, from the behaviour you intend to run.
Based on your requirements, I would suggest doing away with a top-level class like this, and instead create either a private inner class within your application start-up code, or even go for an anonymous inner class:
public class Main {
public static void main(String[] args) {
new Thread(new Runnable() {
#Override
public void run() {
while(true) {
System.out.println("Thread is doing something");
Thread.sleep(5000);
}
}
}).start();
}
}

It is almost never right to extend Thread. If you ever find yourself doing this, step back, take a look and ask yourself if you really need to change the way the Thread class works.
Almost all occurances where I see extends Thread the job would be better done implementing the Runnable interface or using some form of Timer.