Class Cleaner
Cleaner
manages a set of object references and corresponding cleaning actions.
Cleaning actions are registered
to run after the cleaner is notified that the object has become
phantom reachable.
The cleaner uses PhantomReference
and ReferenceQueue
to be
notified when the reachability
changes.
Each cleaner operates independently, managing the pending cleaning actions
and handling threading and termination when the cleaner is no longer in use.
Registering an object reference and corresponding cleaning action returns
a Cleanable
. The most efficient use is to explicitly invoke
the clean
method when the object is closed or
no longer needed.
The cleaning action is a Runnable
to be invoked at most once when
the object has become phantom reachable unless it has already been explicitly cleaned.
Note that the cleaning action must not refer to the object being registered.
If so, the object will not become phantom reachable and the cleaning action
will not be invoked automatically.
The execution of the cleaning action is performed by a thread associated with the cleaner. All exceptions thrown by the cleaning action are ignored. The cleaner and other cleaning actions are not affected by exceptions in a cleaning action. The thread runs until all registered cleaning actions have completed and the cleaner itself is reclaimed by the garbage collector.
The behavior of cleaners during System.exit
is implementation specific. No guarantees are made relating
to whether cleaning actions are invoked or not.
Unless otherwise noted, passing a null
argument to a constructor or
method in this class will cause a
NullPointerException
to be thrown.
- API Note:
- The cleaning action is invoked only after the associated object becomes
phantom reachable, so it is important that the object implementing the
cleaning action does not hold references to the object.
In this example, a static class encapsulates the cleaning state and action.
An "inner" class, anonymous or not, must not be used because it implicitly
contains a reference to the outer instance, preventing it from becoming
phantom reachable.
The choice of a new cleaner or sharing an existing cleaner is determined
by the use case.
If the CleaningExample is used in a try-finally block then the
close
method calls the cleaning action. If theclose
method is not called, the cleaning action is called by the Cleaner when the CleaningExample instance has become phantom reachable.
The cleaning action could be a lambda but all too easily will capture the object reference, by referring to fields of the object being cleaned, preventing the object from becoming phantom reachable. Using a static nested class, as above, will avoid accidentally retaining the object reference.public class CleaningExample implements AutoCloseable { // A cleaner (preferably one shared within a library, // but for the sake of example, a new one is created here) private static final Cleaner cleaner = Cleaner.create(); // State class captures information necessary for cleanup. // It must hold no reference to the instance being cleaned // and therefore it is a static inner class in this example. static class State implements Runnable { State(...) { // initialize State needed for cleaning action } public void run() { // cleanup action accessing State, executed at most once } } private final State state; private final Cleaner.Cleanable cleanable; public CleaningExample() { this.state = new State(...); this.cleanable = cleaner.register(this, state); } public void close() { cleanable.clean(); } }
Cleaning actions should be prepared to be invoked concurrently with other cleaning actions. Typically the cleaning actions should be very quick to execute and not block. If the cleaning action blocks, it may delay processing other cleaning actions registered to the same cleaner. All cleaning actions registered to a cleaner should be mutually compatible.
- Since:
- 9
-
Nested Class Summary
Modifier and TypeClassDescriptionstatic interface
Cleanable
represents an object and a cleaning action registered in aCleaner
. -
Method Summary
Modifier and TypeMethodDescriptionstatic Cleaner
create()
Returns a newCleaner
.static Cleaner
create
(ThreadFactory threadFactory) Returns a newCleaner
using aThread
from theThreadFactory
.Registers an object and a cleaning action to run when the object becomes phantom reachable.
-
Method Details
-
create
Returns a newCleaner
.The cleaner creates a
daemon thread
to process the phantom reachable objects and to invoke cleaning actions. The context class loader of the thread is set to the system class loader. The thread has no permissions, enforced only if aSecurityManager
is set.The cleaner terminates when it is phantom reachable and all of the registered cleaning actions are complete.
- Returns:
- a new
Cleaner
- Throws:
SecurityException
- if the current thread is not allowed to create or start the thread.
-
create
Returns a newCleaner
using aThread
from theThreadFactory
.A thread from the thread factory's
newThread
method is set to be a daemon thread and started to process phantom reachable objects and invoke cleaning actions. On each call the thread factory must provide a Thread that is suitable for performing the cleaning actions.The cleaner terminates when it is phantom reachable and all of the registered cleaning actions are complete.
- Parameters:
threadFactory
- aThreadFactory
to return a newThread
to process cleaning actions- Returns:
- a new
Cleaner
- Throws:
IllegalThreadStateException
- if the thread from the thread factory was not a new thread.SecurityException
- if the current thread is not allowed to create or start the thread.
-
register
Registers an object and a cleaning action to run when the object becomes phantom reachable. Refer to the API Note above for cautions about the behavior of cleaning actions.- Parameters:
obj
- the object to monitoraction
- aRunnable
to invoke when the object becomes phantom reachable- Returns:
- a
Cleanable
instance
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