org.python.modules

Class gc

java.lang.Object

org.python.modules.gc

public class gc
extends java.lang.Object

In Jython, the gc module notably differs from that in CPython. This comes from the different ways Jython and CPython perform garbage collection. While CPython's garbage collection is based on reference counting, Jython is backed by Java's gc, which is based on a mark-and-sweep approach.

This difference becomes most notable if finalizers are involved that perform resurrection. While the resurrected object itself behaves rather similar between Jython and CPython, things are more delicate with objects that are reachable (i.e. strongly referenced) via the resurrected object exclusively. While in CPython such objects do not get their finalizers called, Jython/Java would call all their finalizers. That is because Java detects the whole unreachable subgraph as garbage and thus calls all their finalizers without any chance of direct intervention. CPython instead detects the unreachable object and calls its finalizer, which makes the object reachable again. Then all other objects are reachable from it and CPython does not treat them as garbage and does not call their finalizers at all. This further means that in Jython weak references to such indirectly resurrected objects break, while these persist in CPython.

As of Jython 2.7, the gc module offers some options to emulate CPython behavior. Especially see the flags PRESERVE_WEAKREFS_ON_RESURRECTION, DONT_FINALIZE_RESURRECTED_OBJECTS and DONT_FINALIZE_CYCLIC_GARBAGE for this.

Another difference is that CPython's gc module offers some debug features like counting of collected cyclic trash, which are hard to support by Jython. As of Jython 2.7 the introduction of a traverseproc mechanism (c.f. Traverseproc) made support of these features feasible. As support of these features comes with a significant emulation cost, one must explicitly tell gc to perform this. To make objects subject to cyclic trash counting, these objects must be gc-monitored in Jython. See monitorObject(PyObject), unmonitorObject(PyObject), MONITOR_GLOBAL and stopMonitoring() for this.

If at least one object is gc-monitored, collect() works synchronously in the sense that it blocks until all gc-monitored objects that are garbage actually have been collected and had their finalizers called and completed. collect() will report the number of collected objects in the same manner as in CPython, i.e. counts only those that participate in reference cycles. This allows a unified test implementation across Jython and CPython (which applies to most tests in test_gc.py). If not any object is gc-monitored, collect() just delegates to System.gc(), runs asynchronously (i.e. non-blocking) and returns UNKNOWN_COUNT. See also DEBUG_SAVEALL for a useful gc debugging feature that is supported by Jython from version 2.7 onwards.

Implementing all these features in Jython involved a lot of synchronization logic. While care was taken to implement this without using timeouts as far as possible and rely on locks, states and system/hardware independent synchronization techniques, this was not entirely feasible.
The aspects that were only feasible using a timeout are waiting for gc to enqueue all collected objects (i.e. weak references to monitored objects that were gc'ed) to the reference queue and waiting for gc to run all PyObject finalizers.

Waiting for trash could in theory be strictly synchronized by using MXBeans, i.e. GarbageCollectionNotificationInfo and related API. However, experiments showed that the arising gc notifications do not reliably indicate when enqueuing was done for a specific gc run. We kept the experimental implementation in source code comments to allow easy reproducibility of this issue. (Note that out commented code contradicts Jython styleguide, but this one - however - is needed to document this infeasible approach and is explicitly declared accordingly).

But how is sync done now? We insert a sentinel before running gc and wait until this sentinel was collected. Timestamps are taken to give us an idea at which time scales the gc of the current JVM performs. We then wait until twice the measured time (i.e. duration from call to System.gc() until the sentinel reference was enqueued) has passed after the last reference was enqueued by gc. While this approach is not entirely safe in theory, it passes all tests on various systems and machines we had available for testing so far. We consider it more robust than a fixed-length timeout and regard it the best known feasible compromise to emulate synchronous gc runs in Java.

The other timing-based synchronization issue - waiting for finalizers to run - is solved as follows. Since PyObject finalizers are based on FinalizeTriggers, Jython has full control about these finalization process from a central point. Before such a finalizer runs, it calls notifyPreFinalization() and when it is done, it calls notifyPostFinalization(). While processing of a finalizer can be of arbitrary duration, it widely holds that Java's gc thread calls the next finalizer almost instantaneously after the former. That means that a timestamp taken in notifyPreFinalization() is usually delayed only few milliseconds - often even reported as 0 milliseconds - after the last taken timestamp in notifyPostFinalization() (i.e. that was called by the previous finalizer). Jython's gc module assumes the end of Java's finalization process if postFinalizationTimeOut milliseconds passed after a call of notifyPostFinalization() without another call to notifyPreFinalization() in that time. The default value of postFinalizationTimeOut is 100, which is far larger than the usual almost-zero duration between finalizer calls.
This process can be disturbed by third-party finalizers of non-PyObjects brought into the process by external libraries. If these finalizers are of short duration (which applies to typical finalizers), one can deal with this by adjusting postFinalizationTimeOut, which was declared public for exactly this purpose. However if the external framework causing the issue is Jython aware, a cleaner solution would be to let its finalizers call notifyPreFinalization() and notifyPostFinalization() appropriately. In that case these finalizers must not terminate by throwing an exception before notifyPostFinalization() was called. This is a strict requirement, since a deadlock can be caused otherwise.

Note that the management API (c.f. com.sun.management.GarbageCollectionNotificationInfo) does not emit any notifications that allow to detect the end of the finalization phase. So this API provides no alternative to the described technique.

Usually Java's gc provides hardly any guarantee about its collection and finalization process. It not even guarantees that finalizers are called at all (c.f. http://howtodoinjava.com/2012/10/31/why-not-to-use-finalize-method-in-java). While at least the most common JVM implementations usually do call finalizers reliably under normal conditions, there still is no specific finalization order guaranteed (one might reasonably expect that this would be related to reference connection graph topology, but this appears not to be the case). However Jython now offers some functionality to compensate this situation. Via registerPreFinalizationProcess(Runnable) and registerPostFinalizationProcess(Runnable) and related methods one can now listen to beginning and end of the finalization process. Note that this functionality relies on the technique described in the former paragraph (i.e. based on calls to notifyPreFinalization() and notifyPostFinalization()) and thus underlies its unsafety, if third-party finalizers are involved. Such finalizers can cause false-positive runs of registered (pre/post) finalization processes, so this feature should be used with some care. It is recommended to use it only in such a way that false-positive runs would not cause serious harm, but only some loss in performance or so.

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	gc.CycleMarkAttr

Field Summary

Fields

Modifier and Type	Field and Description
static java.lang.String	__doc__
static PyString	__doc__collect
static PyString	__doc__disable
static PyString	__doc__enable
static PyString	__doc__get_count
static PyString	__doc__get_debug
static PyString	__doc__get_objects
static PyString	__doc__get_referents
static PyString	__doc__get_referrers
static PyString	__doc__get_thresh
static PyString	__doc__is_tracked
static PyString	__doc__isenabled
static PyString	__doc__set_debug
static PyString	__doc__set_thresh
static java.lang.String	__name__
static int	DEBUG_COLLECTABLE print collectable objects (in Jython scoped on monitored objects)
static int	DEBUG_INSTANCES print instances (in Jython scoped on monitored objects)
static int	DEBUG_LEAK Bit combination of the flags DEBUG_COLLECTABLE, DEBUG_UNCOLLECTABLE, DEBUG_INSTANCES, DEBUG_OBJECTS, DEBUG_SAVEALL.
static int	DEBUG_OBJECTS print other objects (in Jython scoped on monitored objects)
static int	DEBUG_SAVEALL save all garbage in gc.garbage (in Jython scoped on monitored objects)
static int	DEBUG_STATS print collection statistics (in Jython scoped on monitored objects)
static int	DEBUG_UNCOLLECTABLE print uncollectable objects (in Jython scoped on monitored objects)
static short	DONT_FINALIZE_CYCLIC_GARBAGE CPython prior to 3.4 does not finalize cyclic garbage PyObjects, while Jython does this by default.
static short	DONT_FINALIZE_RESURRECTED_OBJECTS If in CPython an object is resurrected via its finalizer and contained strong references to other objects, these are also resurrected and not finalized in CPython (as their reference count never drops to zero).
static short	DONT_TRAVERSE_BY_REFLECTION Reflection-based traversal is an inefficient fallback method to traverse PyObject subtypes that don't implement Traverseproc and are not marked as Untraversable.
static short	FORCE_DELAYED_FINALIZATION
static short	FORCE_DELAYED_WEAKREF_CALLBACKS
static PyList	garbage list of uncollectable objects
static long	gcRecallTime
static short	INSTANCE_TRAVERSE_BY_REFLECTION_WARNING Makes gc emit reflection-based traversal warning for every traversed object instead of only once per class.
static short	MONITOR_GLOBAL This flag tells every newly created PyObject to register for gc monitoring.
static long	postFinalizationTimeOut
static short	PRESERVE_WEAKREFS_ON_RESURRECTION If a PyObject is resurrected during its finalization process and was weakly referenced, Jython breaks the weak references to the resurrected PyObject by default.
static short	SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING If this flag is not set, gc warns whenever an object would be subject to reflection-based traversal.
static int	UNKNOWN_COUNT A constant that can occur as result of collect() and indicates an unknown number of collected cyclic trash.
static short	USE_PY_WRITE_DEBUG In Jython one usually uses Py.writeDebug for debugging output.
static short	VERBOSE Bit combination of the flags VERBOSE_COLLECT, VERBOSE_WEAKREF, VERBOSE_DELAYED, VERBOSE_FINALIZE.
static short	VERBOSE_COLLECT Enables collection-related verbose output.
static short	VERBOSE_DELAYED Enables delayed finalization related verbose output.
static short	VERBOSE_FINALIZE Enables finalization-related verbose output.
static short	VERBOSE_WEAKREF Enables weakref-related verbose output.

Constructor Summary

Constructors

Constructor and Description
gc()

Method Summary

Modifier and Type	Method and Description
static void	abortDelayedFinalization(PyObject ob)
static void	addJythonGCFlags(short flags) This is a convenience method to add flags via bitwise or.
static boolean	canLinkToPyObject(java.lang.Class<?> cls, boolean actual) This method checks via type-checking-only, whether an object of the given class can in principle hold a ref to a PyObject.
static int	collect() If no objects are monitored, this just delegates to System.gc() and returns UNKNOWN_COUNT as a non-erroneous default value.
static int	collect(int generation) The generation parameter is only for compatibility with CPython collect() and is ignored.
static boolean	delayedFinalizationEnabled()
static boolean	delayedWeakrefCallbacksEnabled()
static void	disable() Not supported by Jython.
static void	enable() Does nothing in Jython as Java gc is always enabled.
static java.util.Set<PyObject>	findCyclicObjects(PyObject start) Return objects that are reachable from start AND can reach start, thus participate in a cycle with start.
static PyObject	get_count() Not supported by Jython.
static int	get_debug() Copied from CPython doc: Get the garbage collection debugging flags.
static PyObject	get_objects() Only works reliably if monitorGlobal is active, as it depends on monitored objects to search for referrers.
static PyObject	get_referents(PyObject[] args, java.lang.String[] kwargs) Only works reliably if all objects in args properly implement the Traverseproc mechanism (unless reflection-based traversal is activated and works stable).
static PyObject	get_referrers(PyObject[] args, java.lang.String[] kwargs) Only works reliably if monitorGlobal is active, as it depends on monitored objects to search for referrers.
static PyObject	get_threshold() Not supported by Jython.
static short	getJythonGCFlags() Gets the current Jython specific gc flags.
static boolean	getMonitorGlobal()
static org.python.modules.gc.WeakReferenceGC	getMonitorReference(PyObject ob) Avoid to use this method.
static int	indexOfPostFinalizationProcess(java.lang.Runnable process)
static int	indexOfPreFinalizationProcess(java.lang.Runnable process)
static PyObject	is_tracked(PyObject[] args, java.lang.String[] kwargs) is_tracked is - in Jython case - interpreted in the sense that gc.collect will be able to count the object as collected if it participates in a cycle.
static boolean	isenabled() Always returns true in Jython.
static boolean	isMonitored(PyObject ob)
static boolean	isMonitoring()
static boolean	isTraversable(PyObject ob)
static void	markCyclicObjects(PyObject start, boolean uncollectable) Mark all objects that are reachable from start AND can reach start, thus participate in a cycle with start.
static void	monitorObject(PyObject ob)
static void	monitorObject(PyObject ob, boolean initString)
static void	notifyFinalize(PyObject finalized) Do not call this method manually.
static void	notifyPostFinalization()
static void	notifyPreFinalization()
static void	registerForDelayedFinalization(PyObject ob)
static void	registerPostFinalizationProcess(java.lang.Runnable process) Registers a process that will be called after all finalization during gc run is done ("finalization" refers to Jython style finalizers ran by FinalizeTriggers; to care for other finalizers these must call gc.notifyPreFinalization() before anything else is done and gc.notifyPostFinalization() afterwards; between these calls the finalizer must not terminate by throwing an exception).
static void	registerPostFinalizationProcess(java.lang.Runnable process, int index) See doc of registerPostFinalizationProcess(Runnable).
static void	registerPreFinalizationProcess(java.lang.Runnable process) Registers a process that will be called before any finalization during gc run takes place ("finalization" refers to Jython style finalizers ran by FinalizeTriggers; to care for other finalizers these must call gc.notifyPreFinalization() before anything else is done and gc.notifyPostFinalization() afterwards; between these calls the finalizer must not terminate by throwing an exception).
static void	registerPreFinalizationProcess(java.lang.Runnable process, int index) See doc of registerPreFinalizationProcess(Runnable).
static void	removeJythonGCFlags(short flags) This is a convenience method to remove flags via bitwise and-not.
static void	restoreFinalizer(PyObject obj) In addition to what FinalizeTrigger.ensureFinalizer(PyObject) does, this method also restores the finalizer's FinalizeTrigger's flags by taking the values from the former finalizer.
static void	restoreWeakReferences(PyObject rst) Restores weak references pointing to rst.
static void	set_debug(int flags) Copied from CPython doc: Set the garbage collection debugging flags.
static void	set_threshold(PyObject[] args, java.lang.String[] kwargs) Not supported by Jython.
static void	setJythonGCFlags(short flags) Sets the current Jython specific gc flags.
static void	setMonitorGlobal(boolean flag)
static void	stopMonitoring()
static int	traverse(PyObject ob, Visitproc visit, java.lang.Object arg) Does its best to traverse the given PyObject ob.
static int	traverseByReflection(java.lang.Object ob, Visitproc visit, java.lang.Object arg) This method recursively traverses fields of ob.
static void	unmonitorAll()
static boolean	unmonitorObject(PyObject ob)
static boolean	unregisterPostFinalizationProcess(java.lang.Runnable process)
static void	unregisterPostFinalizationProcessAfterNextRun(java.lang.Runnable process) Useful if a process wants to remove another one or itself during its execution.
static boolean	unregisterPreFinalizationProcess(java.lang.Runnable process)
static void	unregisterPreFinalizationProcessAfterNextRun(java.lang.Runnable process) Useful if a process wants to remove another one or itself during its execution.
static void	writeDebug(java.lang.String type, java.lang.String msg) Works like PrePy.writeDebug(String, String), but prints to PrePy.writeDebug(String, String) (i.e. subject to Jython's verbose level) or directly to System.err, according to USE_PY_WRITE_DEBUG.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

UNKNOWN_COUNT

public static final int UNKNOWN_COUNT

A constant that can occur as result of collect() and indicates an unknown number of collected cyclic trash. It is intentionally not valued -1 as that value is reserved to indicate an error.

See Also:

Constant Field Values

MONITOR_GLOBAL

public static final short MONITOR_GLOBAL

This flag tells every newly created PyObject to register for gc monitoring. This allows collect() to report the number of collected objects.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

DONT_FINALIZE_CYCLIC_GARBAGE

public static final short DONT_FINALIZE_CYCLIC_GARBAGE

CPython prior to 3.4 does not finalize cyclic garbage PyObjects, while Jython does this by default. This flag tells Jython's gc to mimic CPython <3.4 behavior (i.e. add such objects to gc.garbage list instead).

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

PRESERVE_WEAKREFS_ON_RESURRECTION

public static final short PRESERVE_WEAKREFS_ON_RESURRECTION

If a PyObject is resurrected during its finalization process and was weakly referenced, Jython breaks the weak references to the resurrected PyObject by default. In CPython these persist, if the object was indirectly resurrected due to resurrection of its owner. This flag tells Jython's gc to preserve weak references to such resurrected PyObjects. It only works if all involved objects implement the traverseproc mechanism properly (see Traverseproc). Note that this feature comes with some cost as it can delay garbage collection of some weak referenced objects for several gc cycles if activated. So we recommend to use it only for debugging.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

DONT_FINALIZE_RESURRECTED_OBJECTS

public static final short DONT_FINALIZE_RESURRECTED_OBJECTS

If in CPython an object is resurrected via its finalizer and contained strong references to other objects, these are also resurrected and not finalized in CPython (as their reference count never drops to zero). In contrast to that, Jython calls finalizers for all objects that were unreachable when gc started (regardless of resurrections and in unpredictable order). This flag emulates CPython behavior in Jython. Note that this emulation comes with a significant cost as it can delay collection of many objects for several gc cycles. Its main intention is for debugging resurrection-sensitive code.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

FORCE_DELAYED_FINALIZATION

public static final short FORCE_DELAYED_FINALIZATION

See Also:

Constant Field Values

FORCE_DELAYED_WEAKREF_CALLBACKS

public static final short FORCE_DELAYED_WEAKREF_CALLBACKS

See Also:

Constant Field Values

DONT_TRAVERSE_BY_REFLECTION

public static final short DONT_TRAVERSE_BY_REFLECTION

Reflection-based traversal is an inefficient fallback method to traverse PyObject subtypes that don't implement Traverseproc and are not marked as Untraversable. Such a situation indicates that the programmer was not aware of Jython's traverseproc mechanism and reflection is used to compensate this.

This flag allows to inhibit reflection-based traversal. If it is activated, objects that don't implement Traverseproc are always treated as if they were marked as Untraversable.

Note that reflection-based traversal fallback is performed by default. Further note that Jython emits warning messages if reflection-based traversal occurs or if an object is encountered that neither implements Traverseproc nor is marked as Untraversable (even if reflection-based traversal is inhibited). See SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING and INSTANCE_TRAVERSE_BY_REFLECTION_WARNING to control these warning messages.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING, INSTANCE_TRAVERSE_BY_REFLECTION_WARNING, Constant Field Values

SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING

public static final short SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING

If this flag is not set, gc warns whenever an object would be subject to reflection-based traversal. Note that if this flag is not set, the warning will occur even if reflection-based traversal is not active. The purpose of this behavior is to identify objects that don't properly support the traverseproc mechanism, i.e. instances of PyObject subclasses that neither implement Traverseproc, nor are annotated with the Untraversable annotation.

A SUPPRESS flag was chosen rather than a WARN flag, so that warning is the default behavior - the user must actively set this flag in order to not to be warned. This is because in an ideal implementation reflection-based traversal never occurs; it is only an inefficient fallback.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), INSTANCE_TRAVERSE_BY_REFLECTION_WARNING, Constant Field Values

INSTANCE_TRAVERSE_BY_REFLECTION_WARNING

public static final short INSTANCE_TRAVERSE_BY_REFLECTION_WARNING

Makes gc emit reflection-based traversal warning for every traversed object instead of only once per class. A potential reflection-based traversal occurs whenever an object is traversed that neither implements Traverseproc, nor is annotated with the Untraversable annotation.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING, Constant Field Values

USE_PY_WRITE_DEBUG

public static final short USE_PY_WRITE_DEBUG

In Jython one usually uses Py.writeDebug for debugging output. However that method is only verbose if an appropriate verbose level was set. In CPython it is enough to set gc DEBUG flags to get gc messages, no matter what overall verbose level is selected. This flag tells Jython to use Py.writeDebug for debugging output. If it is not set (default case), gc debugging output (if gc VERBOSE or DEBUG flags are set) is directly written to System.err.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

VERBOSE_COLLECT

public static final short VERBOSE_COLLECT

Enables collection-related verbose output.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

VERBOSE_WEAKREF

public static final short VERBOSE_WEAKREF

Enables weakref-related verbose output.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

VERBOSE_DELAYED

public static final short VERBOSE_DELAYED

Enables delayed finalization related verbose output.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

VERBOSE_FINALIZE

public static final short VERBOSE_FINALIZE

Enables finalization-related verbose output.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

VERBOSE

public static final short VERBOSE

Bit combination of the flags VERBOSE_COLLECT, VERBOSE_WEAKREF, VERBOSE_DELAYED, VERBOSE_FINALIZE.

See Also:

setJythonGCFlags(short), getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short), Constant Field Values

DEBUG_STATS

public static final int DEBUG_STATS

print collection statistics (in Jython scoped on monitored objects)

See Also:

set_debug(int), get_debug(), Constant Field Values

DEBUG_COLLECTABLE

public static final int DEBUG_COLLECTABLE

print collectable objects (in Jython scoped on monitored objects)

See Also:

set_debug(int), get_debug(), Constant Field Values

DEBUG_UNCOLLECTABLE

public static final int DEBUG_UNCOLLECTABLE

print uncollectable objects (in Jython scoped on monitored objects)

See Also:

set_debug(int), get_debug(), Constant Field Values

DEBUG_INSTANCES

public static final int DEBUG_INSTANCES

print instances (in Jython scoped on monitored objects)

See Also:

set_debug(int), get_debug(), Constant Field Values

DEBUG_OBJECTS

public static final int DEBUG_OBJECTS

print other objects (in Jython scoped on monitored objects)

See Also:

set_debug(int), get_debug(), Constant Field Values

DEBUG_SAVEALL

public static final int DEBUG_SAVEALL

save all garbage in gc.garbage (in Jython scoped on monitored objects)

See Also:

set_debug(int), get_debug(), Constant Field Values

DEBUG_LEAK

public static final int DEBUG_LEAK

Bit combination of the flags DEBUG_COLLECTABLE, DEBUG_UNCOLLECTABLE, DEBUG_INSTANCES, DEBUG_OBJECTS, DEBUG_SAVEALL.

See Also:

set_debug(int), get_debug(), Constant Field Values

gcRecallTime

public static long gcRecallTime

garbage

public static PyList garbage

list of uncollectable objects

postFinalizationTimeOut

public static long postFinalizationTimeOut

__doc__

public static final java.lang.String __doc__

See Also:

Constant Field Values

__name__

public static final java.lang.String __name__

See Also:

Constant Field Values

__doc__enable

public static final PyString __doc__enable

__doc__disable

public static final PyString __doc__disable

__doc__isenabled

public static final PyString __doc__isenabled

__doc__collect

public static final PyString __doc__collect

__doc__get_count

public static final PyString __doc__get_count

__doc__set_debug

public static final PyString __doc__set_debug

__doc__get_debug

public static final PyString __doc__get_debug

__doc__set_thresh

public static final PyString __doc__set_thresh

__doc__get_thresh

public static final PyString __doc__get_thresh

__doc__get_objects

public static final PyString __doc__get_objects

__doc__is_tracked

public static final PyString __doc__is_tracked

__doc__get_referrers

public static final PyString __doc__get_referrers

__doc__get_referents

public static final PyString __doc__get_referents

Constructor Detail

gc

public gc()

Method Detail

writeDebug

public static void writeDebug(java.lang.String type,
                              java.lang.String msg)

Works like PrePy.writeDebug(String, String), but prints to PrePy.writeDebug(String, String) (i.e. subject to Jython's verbose level) or directly to System.err, according to USE_PY_WRITE_DEBUG.

See Also:

USE_PY_WRITE_DEBUG, PrePy.writeDebug(String, String)

restoreFinalizer

public static void restoreFinalizer(PyObject obj)

In addition to what FinalizeTrigger.ensureFinalizer(PyObject) does, this method also restores the finalizer's FinalizeTrigger's flags by taking the values from the former finalizer. On the other hand - in contrast to FinalizeTrigger.ensureFinalizer(PyObject) - this method would not create a FinalizeTrigger for an object that did not have one before (i.e. the method checks for an old (dead) trigger before it creates a new one.

If a new finalizer is needed due to an ordinary resurrection (i.e. the object's finalizer was called), FinalizeTrigger.ensureFinalizer(PyObject) is the right choice. If a finalization was vetoed in context of delayed finalization (i.e. a resurrection that pretends not to be one and didn't run the finalizer), this method is the better choice as it helps to make the new FinalizeTrigger look exactly like the old one regarding flags etc. E.g. this method is called by abortDelayedFinalization(PyObject).

See Also:

abortDelayedFinalization(PyObject)

restoreWeakReferences

public static void restoreWeakReferences(PyObject rst)

Restores weak references pointing to rst. Note that this does not prevent callbacks, unless it is called during finalization phase (e.g. by a finalizer) and delayedWeakrefCallbacksEnabled() returns true. In a manual fashion, one can enforce this by using the gc flag FORCE_DELAYED_WEAKREF_CALLBACKS. Alternatively, one can use the automatic way via the gc flag PRESERVE_WEAKREFS_ON_RESURRECTION, but then one would not need to call this method anyway. The manual way has better performance, but also brings more responsibilies.

See Also:

delayedWeakrefCallbacksEnabled(), FORCE_DELAYED_WEAKREF_CALLBACKS, PRESERVE_WEAKREFS_ON_RESURRECTION

delayedWeakrefCallbacksEnabled

public static boolean delayedWeakrefCallbacksEnabled()

delayedFinalizationEnabled

public static boolean delayedFinalizationEnabled()

registerForDelayedFinalization

public static void registerForDelayedFinalization(PyObject ob)

abortDelayedFinalization

public static void abortDelayedFinalization(PyObject ob)

registerPreFinalizationProcess

public static void registerPreFinalizationProcess(java.lang.Runnable process)

Registers a process that will be called before any finalization during gc run takes place ("finalization" refers to Jython style finalizers ran by FinalizeTriggers; to care for other finalizers these must call gc.notifyPreFinalization() before anything else is done and gc.notifyPostFinalization() afterwards; between these calls the finalizer must not terminate by throwing an exception). This works independently from monitoring, which is mainly needed to allow counting of cyclic garbage in collect().

This feature compensates that Java's gc does not provide any guarantees about finalization order. Java not even guarantees that when a weak reference is added to a reference queue, its finalizer already ran or not yet ran, if any.

The only guarantee is that PhantomReferences are enqueued after finalization of their referents, but this happens in another gc cycle then.

Actually there are still situations that can cause pre finalization process to run again during finalization phase. This can happen if external frameworks use their own finalizers. This can be cured by letting these finalizers call gc.notifyPreFinalization() before anything else is done and gc.notifyPostFinalization() right before the finalization method returns. Between these calls the finalizer must not terminate by throwing an exception.

We recommend to use this feature in a way such that false-positive runs are not critically harmful, e.g. use it to enhance performance, but don't let it cause a crash if preprocess is rerun unexpectedly.

registerPreFinalizationProcess

public static void registerPreFinalizationProcess(java.lang.Runnable process,
                                                  int index)

See doc of registerPreFinalizationProcess(Runnable).

indexOfPreFinalizationProcess

public static int indexOfPreFinalizationProcess(java.lang.Runnable process)

unregisterPreFinalizationProcess

public static boolean unregisterPreFinalizationProcess(java.lang.Runnable process)

unregisterPreFinalizationProcessAfterNextRun

public static void unregisterPreFinalizationProcessAfterNextRun(java.lang.Runnable process)

Useful if a process wants to remove another one or itself during its execution. This asynchronous unregister method circumvents the synchronized state on pre finalization process list.

registerPostFinalizationProcess

public static void registerPostFinalizationProcess(java.lang.Runnable process)

Registers a process that will be called after all finalization during gc run is done ("finalization" refers to Jython style finalizers ran by FinalizeTriggers; to care for other finalizers these must call gc.notifyPreFinalization() before anything else is done and gc.notifyPostFinalization() afterwards; between these calls the finalizer must not terminate by throwing an exception). This works independently from monitoring (which is mainly needed to allow garbage counting in collect()).

This feature compensates that Java's gc does not provide any guarantees about finalization order. Java not even guarantees that when a weak reference is added to a reference queue, its finalizer already ran or not yet ran, if any.

The only guarantee is that PhantomReferences are enqueued after finalization of the referents, but this happens - however - in another gc cycle then.

There are situations that can cause post finalization process to run already during finalization phase. This can happen if external frameworks use their own finalizers. This can be cured by letting these finalizers call gc.notifyPreFinalization() before anything else is done and gc.notifyPostFinalization() right before the finalization method returns. Between these calls the finalizer must not terminate by throwing an exception.

If it runs too early, we can at least guarantee that it will run again after finalization was really done. So we recommend to use this feature in a way such that false-positive runs are not critically harmful.

registerPostFinalizationProcess

public static void registerPostFinalizationProcess(java.lang.Runnable process,
                                                   int index)

See doc of registerPostFinalizationProcess(Runnable).

indexOfPostFinalizationProcess

public static int indexOfPostFinalizationProcess(java.lang.Runnable process)

unregisterPostFinalizationProcess

public static boolean unregisterPostFinalizationProcess(java.lang.Runnable process)

unregisterPostFinalizationProcessAfterNextRun

public static void unregisterPostFinalizationProcessAfterNextRun(java.lang.Runnable process)

Useful if a process wants to remove another one or itself during its execution. This asynchronous unregister method circumvents the synchronized state on post finalization process list.

notifyPreFinalization

public static void notifyPreFinalization()

notifyPostFinalization

public static void notifyPostFinalization()

monitorObject

public static void monitorObject(PyObject ob)

monitorObject

public static void monitorObject(PyObject ob,
                                 boolean initString)

getMonitorReference

public static org.python.modules.gc.WeakReferenceGC getMonitorReference(PyObject ob)

Avoid to use this method. It is inefficient and no intended purpose of the backing Set of objects. In normal business it should not be needed and only exists for bare debugging purposes.

isMonitoring

public static boolean isMonitoring()

isMonitored

public static boolean isMonitored(PyObject ob)

unmonitorObject

public static boolean unmonitorObject(PyObject ob)

unmonitorAll

public static void unmonitorAll()

stopMonitoring

public static void stopMonitoring()

getMonitorGlobal

public static boolean getMonitorGlobal()

setMonitorGlobal

public static void setMonitorGlobal(boolean flag)

getJythonGCFlags

public static short getJythonGCFlags()

Gets the current Jython specific gc flags.

See Also:

MONITOR_GLOBAL, DONT_FINALIZE_CYCLIC_GARBAGE, PRESERVE_WEAKREFS_ON_RESURRECTION, DONT_FINALIZE_RESURRECTED_OBJECTS, DONT_TRAVERSE_BY_REFLECTION, SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING, INSTANCE_TRAVERSE_BY_REFLECTION_WARNING, USE_PY_WRITE_DEBUG, VERBOSE_COLLECT, VERBOSE_WEAKREF, VERBOSE_DELAYED, VERBOSE_FINALIZE, VERBOSE, setJythonGCFlags(short), addJythonGCFlags(short), removeJythonGCFlags(short)

setJythonGCFlags

public static void setJythonGCFlags(short flags)

Sets the current Jython specific gc flags.
flags is a short and can have the following bits turned on:

MONITOR_GLOBAL - Automatically monitors all PyObjects created from now on.
DONT_FINALIZE_CYCLIC_GARBAGE - Adds cyclic finalizable PyObjects to garbage.
PRESERVE_WEAKREFS_ON_RESURRECTION - Keeps weak references alive if the referent is resurrected.
DONT_FINALIZE_RESURRECTED_OBJECTS - Emulates CPython behavior regarding resurrected objects and finalization.
DONT_TRAVERSE_BY_REFLECTION - Inhibits reflection-based traversal.
SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING - Suppress warnings for PyObjects that neither implement Traverseproc nor are marked as Untraversable.
USE_PY_WRITE_DEBUG - uses PrePy.writeDebug(String, String) for debugging output instead of directly writing to System.err.
VERBOSE_COLLECT - Enable collection-related verbose output.
VERBOSE_WEAKREF - Enable weakref-related verbose output.
VERBOSE_DELAYED - Enable delayed finalization-related verbose output.
VERBOSE_FINALIZE - Enable finalization-related verbose output.
VERBOSE - All previous verbose-flags combined.

See Also:

MONITOR_GLOBAL, DONT_FINALIZE_CYCLIC_GARBAGE, PRESERVE_WEAKREFS_ON_RESURRECTION, DONT_FINALIZE_RESURRECTED_OBJECTS, DONT_TRAVERSE_BY_REFLECTION, SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING, INSTANCE_TRAVERSE_BY_REFLECTION_WARNING, USE_PY_WRITE_DEBUG, VERBOSE_COLLECT, VERBOSE_WEAKREF, VERBOSE_DELAYED, VERBOSE_FINALIZE, VERBOSE, getJythonGCFlags(), addJythonGCFlags(short), removeJythonGCFlags(short)

addJythonGCFlags

public static void addJythonGCFlags(short flags)

This is a convenience method to add flags via bitwise or.

See Also:

MONITOR_GLOBAL, DONT_FINALIZE_CYCLIC_GARBAGE, PRESERVE_WEAKREFS_ON_RESURRECTION, DONT_FINALIZE_RESURRECTED_OBJECTS, DONT_TRAVERSE_BY_REFLECTION, SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING, INSTANCE_TRAVERSE_BY_REFLECTION_WARNING, USE_PY_WRITE_DEBUG, VERBOSE_COLLECT, VERBOSE_WEAKREF, VERBOSE_DELAYED, VERBOSE_FINALIZE, VERBOSE, getJythonGCFlags(), setJythonGCFlags(short), removeJythonGCFlags(short)

removeJythonGCFlags

public static void removeJythonGCFlags(short flags)

This is a convenience method to remove flags via bitwise and-not.

See Also:

MONITOR_GLOBAL, DONT_FINALIZE_CYCLIC_GARBAGE, PRESERVE_WEAKREFS_ON_RESURRECTION, DONT_FINALIZE_RESURRECTED_OBJECTS, DONT_TRAVERSE_BY_REFLECTION, SUPPRESS_TRAVERSE_BY_REFLECTION_WARNING, INSTANCE_TRAVERSE_BY_REFLECTION_WARNING, USE_PY_WRITE_DEBUG, VERBOSE_COLLECT, VERBOSE_WEAKREF, VERBOSE_DELAYED, VERBOSE_FINALIZE, VERBOSE, getJythonGCFlags(), setJythonGCFlags(short), addJythonGCFlags(short)

notifyFinalize

public static void notifyFinalize(PyObject finalized)

Do not call this method manually. It should only be called by FinalizeTrigger.

enable

public static void enable()

Does nothing in Jython as Java gc is always enabled.

disable

public static void disable()

Not supported by Jython.

Throws:

PyException - NotImplementedError

isenabled

public static boolean isenabled()

Always returns true in Jython.

collect

public static int collect(int generation)

The generation parameter is only for compatibility with CPython collect() and is ignored.

Parameters:

generation - (ignored)

Returns:

Collected monitored cyclic trash objects or gc.UNKNOWN_COUNT if nothing is monitored or -1 if an error occurred and collection did not complete.

See Also:

collect()

collect

public static int collect()

If no objects are monitored, this just delegates to System.gc() and returns UNKNOWN_COUNT as a non-erroneous default value. If objects are monitored, it emulates a synchronous gc run in the sense that it waits until all collected monitored objects were finalized.

Returns:

Number of collected monitored cyclic trash objects or UNKNOWN_COUNT if nothing is monitored or -1 if an error occurred and collection did not complete.

See Also:

UNKNOWN_COUNT, collect(int)

get_count

public static PyObject get_count()

Not supported by Jython.

Throws:

PyException - NotImplementedError

set_debug

public static void set_debug(int flags)

Copied from CPython doc:

Set the garbage collection debugging flags. Debugging information is written to System.err.

flags flags is an integer and can have the following bits turned on:

DEBUG_STATS - Print statistics during collection.
DEBUG_COLLECTABLE - Print collectable objects found.
DEBUG_UNCOLLECTABLE - Print unreachable but uncollectable objects found.
DEBUG_INSTANCES - Print instance objects.
DEBUG_OBJECTS - Print objects other than instances.
DEBUG_SAVEALL - Save objects to gc.garbage rather than freeing them.
DEBUG_LEAK - Debug leaking programs (everything but STATS).

See Also:

DEBUG_STATS, DEBUG_COLLECTABLE, DEBUG_UNCOLLECTABLE, DEBUG_INSTANCES, DEBUG_OBJECTS, DEBUG_SAVEALL, DEBUG_LEAK

get_debug

public static int get_debug()

Copied from CPython doc:

Get the garbage collection debugging flags.

set_threshold

public static void set_threshold(PyObject[] args,
                                 java.lang.String[] kwargs)

Not supported by Jython.

Throws:

PyException - NotImplementedError

get_threshold

public static PyObject get_threshold()

Not supported by Jython.

Throws:

PyException - NotImplementedError

get_objects

public static PyObject get_objects()

Only works reliably if monitorGlobal is active, as it depends on monitored objects to search for referrers. It only finds referrers that properly implement the traverseproc mechanism (unless reflection-based traversal is activated and works stable).

Throws:

PyException - NotImplementedError

get_referrers

public static PyObject get_referrers(PyObject[] args,
                                     java.lang.String[] kwargs)

Only works reliably if monitorGlobal is active, as it depends on monitored objects to search for referrers. It only finds referrers that properly implement the traverseproc mechanism (unless reflection-based traversal is activated and works stable). Further note that the resulting list will contain referrers in no specific order and may even include duplicates.

get_referents

public static PyObject get_referents(PyObject[] args,
                                     java.lang.String[] kwargs)

Only works reliably if all objects in args properly implement the Traverseproc mechanism (unless reflection-based traversal is activated and works stable). Further note that the resulting list will contain referents in no specific order and may even include duplicates.

is_tracked

public static PyObject is_tracked(PyObject[] args,
                                  java.lang.String[] kwargs)

is_tracked is - in Jython case - interpreted in the sense that gc.collect will be able to count the object as collected if it participates in a cycle. This mimics CPython behavior and passes the corresponding unit test in test_gc.py.

markCyclicObjects

public static void markCyclicObjects(PyObject start,
                                     boolean uncollectable)

Mark all objects that are reachable from start AND can reach start, thus participate in a cycle with start.

findCyclicObjects

public static java.util.Set<PyObject> findCyclicObjects(PyObject start)

Return objects that are reachable from start AND can reach start, thus participate in a cycle with start. Returns null if start does not participate in any cycle.

traverse

public static int traverse(PyObject ob,
                           Visitproc visit,
                           java.lang.Object arg)

Does its best to traverse the given PyObject ob. It exploits both Traverseproc.traverse(Visitproc, Object) and TraverseprocDerived.traverseDerived(Visitproc, Object). If ob neither implements Traverseproc nor Traverseproc and is not annotated with Untraversable, reflection-based traversal via traverseByReflection(Object, Visitproc, Object) may be attempted according to DONT_TRAVERSE_BY_REFLECTION.

See Also:

Traverseproc.traverse(Visitproc, Object), TraverseprocDerived.traverseDerived(Visitproc, Object), DONT_TRAVERSE_BY_REFLECTION, Untraversable, traverseByReflection(Object, Visitproc, Object)

traverseByReflection

public static int traverseByReflection(java.lang.Object ob,
                                       Visitproc visit,
                                       java.lang.Object arg)

This method recursively traverses fields of ob. If a field is a PyObject, it is passed to visit. and recursion ends in that branch. If a field is an array, the elements are checked whether they are PyObjects. PyObject elements are passed to visit. Elements that are arrays themselves are again processed elementwise and so on.

Through the whole search this method fails fast if visit returns non-zero.

Note that we intentionally don't traverse iterables by iterating them. Since we perform recursion, this should reach all contained references anyway - in Java every object can only contain references as fields or arrays. On the one hand, exploiting iterables would ease the access to private fields, but on the other hand during iteration they might change inner state, create new (Py)Objects or obtain objects from native methods. Additionally we might run into concurrent modification issues. So all in all the traversal is cleaner and safer if just fields and arrays are traversed.

canLinkToPyObject

public static boolean canLinkToPyObject(java.lang.Class<?> cls,
                                        boolean actual)

This method checks via type-checking-only, whether an object of the given class can in principle hold a ref to a PyObject. Especially if arrays are involved, this can safe a lot performance. For now, no generic type info is exploited.

If actual is true, the answer will hold for an object that is an instance of the given class. Otherwise it is assumed that cls is the type of a field holding an object, so cls is considered as upper bound for an objects actual type.

One should call with actual == true, if cls was obtained by ob.getClass() and with actual == false, if cls was obtained as a field type or component type of an array.

isTraversable

public static boolean isTraversable(PyObject ob)