Deadlock describes a situation where two or more threads are blocked forever, waiting for each other. A Java multithreaded program may suffer from the deadlock condition because the synchronized keyword causes the executing thread to block while waiting for the lock, or monitor, associated with the specified object.
ExecutorService does not mandate or otherwise guarantee that all implementations are thread-safe, and it cannot as it is an interface. These types of contracts are outside of the scope of a Java interface. However, ThreadPoolExecutor both is and is clearly documented as being thread-safe.
Submit Method: This function executes the given command at some time in the future. The command may execute in a new thread, in a pooled thread, or in the calling thread, at the discretion of the Executor implementation. The future object is used to handle the task after the execution has started.
Example of assigning a task to ExecutorService using execute() method
- public class ExecutorServiceExample {
- public static void main(String[] args) {
- ExecutorService executorService = Executors.newSingleThreadExecutor();
- executorService.execute(new Runnable() {
- @Override.
- public void run() {
ForkJoinPoolIt is an implementation of the ExecutorService that manages worker threads and provides us with tools to get information about the thread pool state and performance. Worker threads can execute only one task at a time, but the ForkJoinPool doesn't create a separate thread for every single subtask.
Yes, documentation of Future. get() says: Waits if necessary for the computation to complete, and then retrieves its result. so, it will block until results of computation are available, or the computation was interrupted (cancelled or resulting in exception).
An executor is a person/institution who is the legal representative, named in a will or implied as such, to carry out the process of the distribution of the assets of the testator.
Executor Interfaces define the three executor object types. Thread Pools are the most common kind of executor implementation. Fork/Join is a framework (new in JDK 7) for taking advantage of multiple processors.
Types of Executors
- SingleThreadExecutor. This thread pool executor has only a single thread.
- FixedThreadPool(n) As the name indicates, it is a thread pool of a fixed number of threads.
- CachedThreadPool. This thread pool is mostly used where there are lots of short-lived parallel tasks to be executed.
- ScheduledExecutor.
To properly shut down an ExecutorService, we have the shutdown() and shutdownNow() APIs. The shutdown() method doesn't cause immediate destruction of the ExecutorService. It will make the ExecutorService stop accepting new tasks and shut down after all running threads finish their current work: executorService.
To use thread pools, we first create a object of ExecutorService and pass a set of tasks to it. ThreadPoolExecutor class allows to set the core and maximum pool size. The runnables that are run by a particular thread are executed sequentially. Method Description newFixedThreadPool(int) Creates a fixed size thread pool.
The ExecutorService class has 2 methods just for this: shutdown() and shutdownNow(). After using the shutdown() method, you can call awaitTermination() to block until all of the started tasks have completed. You can even provide a timeout to prevent waiting forever.
Executor Design pattern can be defined as a particular type of design pattern that serves the purpose of decoupling the execution of a task from the real task taken by the user with the help of executors. It decouples the submission of a command from the execution of that very command.
Future <V> is an interface that represents the result of an asynchronous computation. Once the computation is finished, you can obtain the result of it by using the get() method. Bear in mind that this is a blocking operation and waits until the outcome (V) is available.
How can we avoid a deadlock in Java?
- Avoid Nested Locks: A deadlock mainly happens when we give locks to multiple threads. Avoid giving a lock to multiple threads if we already have given to one.
- Avoid Unnecessary Locks: We can have a lock only those members which are required.
- Using Thread.
There is one more method to detect Deadlock in Java, it can be done by running the program in CMD. All we need to do is collect thread dumps and then we have to command to collect, depending upon the operating system. If we are running Java 8 on windows, a command would be jcmd $PID Thread. print.Apr 21, 2017
The difference is that execute simply starts the task without any further ado, whereas submit returns a Future object to manage the task. You can do the following things with the Future object: The Future interface is more useful if you submit a Callable to the pool.Sep 11, 2013
A thread goes through various stages in its lifecycle. For example, a thread is born, started, runs, and then dies. The following diagram shows the complete life cycle of a thread. New − A new thread begins its life cycle in the new state.
A Java Future, java. util. Future , represents the result of an asynchronous computation. When the asynchronous task is created, a Java Future object is returned. This Future object functions as a handle to the result of the asynchronous task.
How a Thread Pool Works. Instead of starting a new thread for every task to execute concurrently, the task can be passed to a thread pool. As soon as the pool has any idle threads the task is assigned to one of them and executed. The rest of the idle threads in the pool will be blocked waiting to dequeue tasks.
Executors provide a layer of indirection between a client and the execution of a task; instead of a client executing a task directly, an intermediate object executes the task. The program will exit as soon as all the tasks in the Executor complete.
Java Thread pool represents a group of worker threads that are waiting for the job and reuse many times. In case of thread pool, a group of fixed size threads are created. After completion of the job, thread is contained in the thread pool again.
We can create following 5 types of thread pool executors with pre-built methods in java. util. concurrent. Executors interface.Dec 26, 2020
Synchronization in java is the capability to control the access of multiple threads to any shared resource. In the Multithreading concept, multiple threads try to access the shared resources at a time to produce inconsistent results. The synchronization is necessary for reliable communication between threads.
Shutting down the ExecutorServiceshutdown() - when shutdown() method is called on an executor service, it stops accepting new tasks, waits for previously submitted tasks to execute, and then terminates the executor. shutdownNow() - this method interrupts the running task and shuts down the executor immediately.
The Java Callable interface, java. util. Callable , represents an asynchronous task which can be executed by a separate thread. For instance, it is possible to submit a Callable object to a Java ExecutorService which will then execute it asynchronously.
Difference between Callable and Runnable are following:Callable has call() method but Runnable has run() method. Callable has call method which returns value but Runnable has run method which doesn't return any value. call method can throw checked exception but run method can't throw checked exception.
A Semaphore is a thread synchronization construct that can be used either to send signals between threads to avoid missed signals, or to guard a critical section like you would with a lock. Java 5 comes with semaphore implementations in the java. util.
When finished using an ExecutorService , you need to shut it down explicitly. From its javadoc: "An unused ExecutorService should be shut down to allow reclamation of its resources." Calling shutdown initiates a gradual and orderly shutdown.
The Java volatile keyword is used to mark a Java variable as "being stored in main memory". Actually, since Java 5 the volatile keyword guarantees more than just that volatile variables are written to and read from main memory.
