Module java.base
Package java.util.concurrent

Class Executors


  • public class Executors
extends Object
    Factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes defined in this package. This class supports the following kinds of methods:
    • Methods that create and return an ExecutorService set up with commonly useful configuration settings.
    • Methods that create and return a ScheduledExecutorService set up with commonly useful configuration settings.
    • Methods that create and return a "wrapped" ExecutorService, that disables reconfiguration by making implementation-specific methods inaccessible.
    • Methods that create and return a ThreadFactory that sets newly created threads to a known state.
    • Methods that create and return a Callable out of other closure-like forms, so they can be used in execution methods requiring Callable.
    Since:
    1.5

    • Method Detail

      • newFixedThreadPool

        public static ExecutorService newFixedThreadPool​(int nThreads)
        Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue. At any point, at most nThreads threads will be active processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available. If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks. The threads in the pool will exist until it is explicitly shutdown.
        Parameters:
        nThreads - the number of threads in the pool
        Returns:
        the newly created thread pool
        Throws:
        IllegalArgumentException - if nThreads <= 0

      • newWorkStealingPool

        public static ExecutorService newWorkStealingPool​(int parallelism)
        Creates a thread pool that maintains enough threads to support the given parallelism level, and may use multiple queues to reduce contention. The parallelism level corresponds to the maximum number of threads actively engaged in, or available to engage in, task processing. The actual number of threads may grow and shrink dynamically. A work-stealing pool makes no guarantees about the order in which submitted tasks are executed.
        Parameters:
        parallelism - the targeted parallelism level
        Returns:
        the newly created thread pool
        Throws:
        IllegalArgumentException - if parallelism <= 0
        Since:
        1.8

      • newFixedThreadPool

        public static ExecutorService newFixedThreadPool​(int nThreads,
                                                 ThreadFactory threadFactory)
        Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue, using the provided ThreadFactory to create new threads when needed. At any point, at most nThreads threads will be active processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available. If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks. The threads in the pool will exist until it is explicitly shutdown.
        Parameters:
        nThreads - the number of threads in the pool
        threadFactory - the factory to use when creating new threads
        Returns:
        the newly created thread pool
        Throws:
        NullPointerException - if threadFactory is null
        IllegalArgumentException - if nThreads <= 0

      • newSingleThreadExecutor

        public static ExecutorService newSingleThreadExecutor​()
        Creates an Executor that uses a single worker thread operating off an unbounded queue. (Note however that if this single thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.) Tasks are guaranteed to execute sequentially, and no more than one task will be active at any given time. Unlike the otherwise equivalent newFixedThreadPool(1) the returned executor is guaranteed not to be reconfigurable to use additional threads.
        Returns:
        the newly created single-threaded Executor

      • newSingleThreadExecutor

        public static ExecutorService newSingleThreadExecutor​(ThreadFactory threadFactory)
        Creates an Executor that uses a single worker thread operating off an unbounded queue, and uses the provided ThreadFactory to create a new thread when needed. Unlike the otherwise equivalent newFixedThreadPool(1, threadFactory) the returned executor is guaranteed not to be reconfigurable to use additional threads.
        Parameters:
        threadFactory - the factory to use when creating new threads
        Returns:
        the newly created single-threaded Executor
        Throws:
        NullPointerException - if threadFactory is null

      • newCachedThreadPool

        public static ExecutorService 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. Calls to execute will reuse previously constructed threads if available. If no existing thread is available, a new thread will be created and added to the pool. Threads that have not been used for sixty seconds are terminated and removed from the cache. Thus, a pool that remains idle for long enough will not consume any resources. Note that pools with similar properties but different details (for example, timeout parameters) may be created using ThreadPoolExecutor constructors.
        Returns:
        the newly created thread pool

      • newCachedThreadPool

        public static ExecutorService newCachedThreadPool​(ThreadFactory threadFactory)
        Creates a thread pool that creates new threads as needed, but will reuse previously constructed threads when they are available, and uses the provided ThreadFactory to create new threads when needed.
        Parameters:
        threadFactory - the factory to use when creating new threads
        Returns:
        the newly created thread pool
        Throws:
        NullPointerException - if threadFactory is null

      • newSingleThreadScheduledExecutor

        public static ScheduledExecutorService newSingleThreadScheduledExecutor​()
        Creates a single-threaded executor that can schedule commands to run after a given delay, or to execute periodically. (Note however that if this single thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.) Tasks are guaranteed to execute sequentially, and no more than one task will be active at any given time. Unlike the otherwise equivalent newScheduledThreadPool(1) the returned executor is guaranteed not to be reconfigurable to use additional threads.
        Returns:
        the newly created scheduled executor

      • newSingleThreadScheduledExecutor

        public static ScheduledExecutorService newSingleThreadScheduledExecutor​(ThreadFactory threadFactory)
        Creates a single-threaded executor that can schedule commands to run after a given delay, or to execute periodically. (Note however that if this single thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks.) Tasks are guaranteed to execute sequentially, and no more than one task will be active at any given time. Unlike the otherwise equivalent newScheduledThreadPool(1, threadFactory) the returned executor is guaranteed not to be reconfigurable to use additional threads.
        Parameters:
        threadFactory - the factory to use when creating new threads
        Returns:
        a newly created scheduled executor
        Throws:
        NullPointerException - if threadFactory is null

      • newScheduledThreadPool

        public static ScheduledExecutorService newScheduledThreadPool​(int corePoolSize)
        Creates a thread pool that can schedule commands to run after a given delay, or to execute periodically.
        Parameters:
        corePoolSize - the number of threads to keep in the pool, even if they are idle
        Returns:
        a newly created scheduled thread pool
        Throws:
        IllegalArgumentException - if corePoolSize < 0

      • newScheduledThreadPool

        public static ScheduledExecutorService newScheduledThreadPool​(int corePoolSize,
                                                              ThreadFactory threadFactory)
        Creates a thread pool that can schedule commands to run after a given delay, or to execute periodically.
        Parameters:
        corePoolSize - the number of threads to keep in the pool, even if they are idle
        threadFactory - the factory to use when the executor creates a new thread
        Returns:
        a newly created scheduled thread pool
        Throws:
        IllegalArgumentException - if corePoolSize < 0
        NullPointerException - if threadFactory is null

      • unconfigurableExecutorService

        public static ExecutorService unconfigurableExecutorService​(ExecutorService executor)
        Returns an object that delegates all defined ExecutorService methods to the given executor, but not any other methods that might otherwise be accessible using casts. This provides a way to safely "freeze" configuration and disallow tuning of a given concrete implementation.
        Parameters:
        executor - the underlying implementation
        Returns:
        an ExecutorService instance
        Throws:
        NullPointerException - if executor null

      • unconfigurableScheduledExecutorService

        public static ScheduledExecutorService unconfigurableScheduledExecutorService​(ScheduledExecutorService executor)
        Returns an object that delegates all defined ScheduledExecutorService methods to the given executor, but not any other methods that might otherwise be accessible using casts. This provides a way to safely "freeze" configuration and disallow tuning of a given concrete implementation.
        Parameters:
        executor - the underlying implementation
        Returns:
        a ScheduledExecutorService instance
        Throws:
        NullPointerException - if executor null

      • defaultThreadFactory

        public static ThreadFactory defaultThreadFactory​()
        Returns a default thread factory used to create new threads. This factory creates all new threads used by an Executor in the same ThreadGroup. If there is a SecurityManager, it uses the group of System.getSecurityManager(), else the group of the thread invoking this defaultThreadFactory method. Each new thread is created as a non-daemon thread with priority set to the smaller of Thread.NORM_PRIORITY and the maximum priority permitted in the thread group. New threads have names accessible via Thread.getName() of pool-N-thread-M, where N is the sequence number of this factory, and M is the sequence number of the thread created by this factory.
        Returns:
        a thread factory

      • privilegedThreadFactory

        public static ThreadFactory privilegedThreadFactory​()
        Returns a thread factory used to create new threads that have the same permissions as the current thread. This factory creates threads with the same settings as defaultThreadFactory(), additionally setting the AccessControlContext and contextClassLoader of new threads to be the same as the thread invoking this privilegedThreadFactory method. A new privilegedThreadFactory can be created within an AccessController.doPrivileged action setting the current thread's access control context to create threads with the selected permission settings holding within that action.

        Note that while tasks running within such threads will have the same access control and class loader settings as the current thread, they need not have the same ThreadLocal or InheritableThreadLocal values. If necessary, particular values of thread locals can be set or reset before any task runs in ThreadPoolExecutor subclasses using ThreadPoolExecutor.beforeExecute(Thread, Runnable). Also, if it is necessary to initialize worker threads to have the same InheritableThreadLocal settings as some other designated thread, you can create a custom ThreadFactory in which that thread waits for and services requests to create others that will inherit its values.

        Returns:
        a thread factory
        Throws:
        AccessControlException - if the current access control context does not have permission to both get and set context class loader

      • callable

        public static <T> Callable<T> callable​(Runnable task,
                                       T result)
        Returns a Callable object that, when called, runs the given task and returns the given result. This can be useful when applying methods requiring a Callable to an otherwise resultless action.
        Type Parameters:
        T - the type of the result
        Parameters:
        task - the task to run
        result - the result to return
        Returns:
        a callable object
        Throws:
        NullPointerException - if task null

      • callable

        public static Callable<Object> callable​(Runnable task)
        Returns a Callable object that, when called, runs the given task and returns null.
        Parameters:
        task - the task to run
        Returns:
        a callable object
        Throws:
        NullPointerException - if task null

      • callable

        public static Callable<Object> callable​(PrivilegedAction<?> action)
        Returns a Callable object that, when called, runs the given privileged action and returns its result.
        Parameters:
        action - the privileged action to run
        Returns:
        a callable object
        Throws:
        NullPointerException - if action null

      • callable

        public static Callable<Object> callable​(PrivilegedExceptionAction<?> action)
        Returns a Callable object that, when called, runs the given privileged exception action and returns its result.
        Parameters:
        action - the privileged exception action to run
        Returns:
        a callable object
        Throws:
        NullPointerException - if action null

      • privilegedCallable

        public static <T> Callable<T> privilegedCallable​(Callable<T> callable)
        Returns a Callable object that will, when called, execute the given callable under the current access control context. This method should normally be invoked within an AccessController.doPrivileged action to create callables that will, if possible, execute under the selected permission settings holding within that action; or if not possible, throw an associated AccessControlException.
        Type Parameters:
        T - the type of the callable's result
        Parameters:
        callable - the underlying task
        Returns:
        a callable object
        Throws:
        NullPointerException - if callable null

      • privilegedCallableUsingCurrentClassLoader

        public static <T> Callable<T> privilegedCallableUsingCurrentClassLoader​(Callable<T> callable)
        Returns a Callable object that will, when called, execute the given callable under the current access control context, with the current context class loader as the context class loader. This method should normally be invoked within an AccessController.doPrivileged action to create callables that will, if possible, execute under the selected permission settings holding within that action; or if not possible, throw an associated AccessControlException.
        Type Parameters:
        T - the type of the callable's result
        Parameters:
        callable - the underlying task
        Returns:
        a callable object
        Throws:
        NullPointerException - if callable null
        AccessControlException - if the current access control context does not have permission to both set and get context class loader