/*
* Copyright 1999-2004 The Apache Software Foundation.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/*
* $Id: CoroutineManager.java,v 1.10 2004/02/16 23:06:11 minchau Exp $
*/
package com.sun.org.apache.xml.internal.dtm.ref;
import java.util.BitSet;
import com.sun.org.apache.xml.internal.res.XMLErrorResources;
import com.sun.org.apache.xml.internal.res.XMLMessages;
/**
* <p>Support the coroutine design pattern.</p>
*
* <p>A coroutine set is a very simple cooperative non-preemptive
* multitasking model, where the switch from one task to another is
* performed via an explicit request. Coroutines interact according to
* the following rules:</p>
*
* <ul>
* <li>One coroutine in the set has control, which it retains until it
* either exits or resumes another coroutine.</li>
* <li>A coroutine is activated when it is resumed by some other coroutine
* for the first time.</li>
* <li>An active coroutine that gives up control by resuming another in
* the set retains its context -- including call stack and local variables
* -- so that if/when it is resumed, it will proceed from the point at which
* it last gave up control.</li>
* </ul>
*
* <p>Coroutines can be thought of as falling somewhere between pipes and
* subroutines. Like call/return, there is an explicit flow of control
* from one coroutine to another. Like pipes, neither coroutine is
* actually "in charge", and neither must exit in order to transfer
* control to the other. </p>
*
* <p>One classic application of coroutines is in compilers, where both
* the parser and the lexer are maintaining complex state
* information. The parser resumes the lexer to process incoming
* characters into lexical tokens, and the lexer resumes the parser
* when it has reached a point at which it has a reliably interpreted
* set of tokens available for semantic processing. Structuring this
* as call-and-return would require saving and restoring a
* considerable amount of state each time. Structuring it as two tasks
* connected by a queue may involve higher overhead (in systems which
* can optimize the coroutine metaphor), isn't necessarily as clear in
* intent, may have trouble handling cases where data flows in both
* directions, and may not handle some of the more complex cases where
* more than two coroutines are involved.</p>
*
* <p>Most coroutine systems also provide a way to pass data between the
* source and target of a resume operation; this is sometimes referred
* to as "yielding" a value. Others rely on the fact that, since only
* one member of a coroutine set is running at a time and does not
* lose control until it chooses to do so, data structures may be
* directly shared between them with only minimal precautions.</p>
*
* <p>"Note: This should not be taken to mean that producer/consumer
* problems should be always be done with coroutines." Queueing is
* often a better solution when only two threads of execution are
* involved and full two-way handshaking is not required. It's a bit
* difficult to find short pedagogical examples that require
* coroutines for a clear solution.</p>
*
* <p>The fact that only one of a group of coroutines is running at a
* time, and the control transfer between them is explicit, simplifies
* their possible interactions, and in some implementations permits
* them to be implemented more efficiently than general multitasking.
* In some situations, coroutines can be compiled out entirely;
* in others, they may only require a few instructions more than a
* simple function call.</p>
*
* <p>This version is built on top of standard Java threading, since
* that's all we have available right now. It's been encapsulated for
* code clarity and possible future optimization.</p>
*
* <p>(Two possible approaches: wait-notify based and queue-based. Some
* folks think that a one-item queue is a cleaner solution because it's
* more abstract -- but since coroutine _is_ an abstraction I'm not really
* worried about that; folks should be able to switch this code without
* concern.)</p>
*
* <p>%TBD% THIS SHOULD BE AN INTERFACE, to facilitate building other
* implementations... perhaps including a true coroutine system
* someday, rather than controlled threading. Arguably Coroutine
* itself should be an interface much like Runnable, but I think that
* can be built on top of this.</p>
* */
public class CoroutineManager
{
/** "Is this coroutine ID number already in use" lookup table.
* Currently implemented as a bitset as a compromise between
* compactness and speed of access, but obviously other solutions
* could be applied.
* */
BitSet m_activeIDs=new BitSet();
/** Limit on the coroutine ID numbers accepted. I didn't want the
* in-use table to grow without bound. If we switch to a more efficient
* sparse-array mechanism, it may be possible to raise or eliminate
* this boundary.
* @xsl.usage internal
*/
static final int m_unreasonableId=1024;
/** Internal field used to hold the data being explicitly passed
* from one coroutine to another during a co_resume() operation.
* (Of course implicit data sharing may also occur; one of the reasons
* for using coroutines is that you're guaranteed that none of the
* other coroutines in your set are using shared structures at the time
* you access them.)
*
* %REVIEW% It's been proposed that we be able to pass types of data
* other than Object -- more specific object types, or
* lighter-weight primitives. This would seem to create a potential
* explosion of "pass x recieve y back" methods (or require
* fracturing resume into two calls, resume-other and
* wait-to-be-resumed), and the weight issue could be managed by
* reusing a mutable buffer object to contain the primitive
* (remember that only one coroutine runs at a time, so once the
* buffer's set it won't be walked on). Typechecking objects is
* interesting from a code-robustness point of view, but it's
* unclear whether it makes sense to encapsulate that in the
* coroutine code or let the callers do it, since it depends on RTTI
* either way. Restricting the parameters to objects implementing a
* specific CoroutineParameter interface does _not_ seem to be a net
* win; applications can do so if they want via front-end code, but
* there seem to be too many use cases involving passing an existing
* object type that you may not have the freedom to alter and may
* not want to spend time wrapping another object around.
* */
Object m_yield=null;
// Expose???
final static int NOBODY=-1;
final static int ANYBODY=-1;
/** Internal field used to confirm that the coroutine now waking up is
* in fact the one we intended to resume. Some such selection mechanism
* is needed when more that two coroutines are operating within the same
* group.
*/
int m_nextCoroutine=NOBODY;
/** <p>Each coroutine in the set managed by a single
* CoroutineManager is identified by a small positive integer. This
* brings up the question of how to manage those integers to avoid
* reuse... since if two coroutines use the same ID number, resuming
* that ID could resume either. I can see arguments for either
* allowing applications to select their own numbers (they may want
* to declare mnemonics via manefest constants) or generating
* numbers on demand. This routine's intended to support both
* approaches.</p>
*
* <p>%REVIEW% We could use an object as the identifier. Not sure
* it's a net gain, though it would allow the thread to be its own
* ID. Ponder.</p>
*
* @param coroutineID: If >=0, requests that we reserve this number.
* If <0, requests that we find, reserve, and return an available ID
* number.
*
* @return If >=0, the ID number to be used by this coroutine. If <0,
* an error occurred -- the ID requested was already in use, or we
* couldn't assign one without going over the "unreasonable value" mark
* */
public synchronized int co_joinCoroutineSet(int coroutineID)
{
if(coroutineID>=0)
{
if(coroutineID>=m_unreasonableId || m_activeIDs.get(coroutineID))
return -1;
}
else
{
// What I want is "Find first clear bit". That doesn't exist.
// JDK1.2 added "find last set bit", but that doesn't help now.
coroutineID=0;
while(coroutineID<m_unreasonableId)
{
if(m_activeIDs.get(coroutineID))
++coroutineID;
else
break;
}
if(coroutineID>=m_unreasonableId)
return -1;
}
m_activeIDs.set(coroutineID);
return coroutineID;
}
/** In the standard coroutine architecture, coroutines are
* identified by their method names and are launched and run up to
* their first yield by simply resuming them; its's presumed that
* this recognizes the not-already-running case and does the right
* thing. We seem to need a way to achieve that same threadsafe
* run-up... eg, start the coroutine with a wait.
*
* %TBD% whether this makes any sense...
*
* @param thisCoroutine the identifier of this coroutine, so we can
* recognize when we are being resumed.
* @exception java.lang.NoSuchMethodException if thisCoroutine isn't
* a registered member of this group. %REVIEW% whether this is the
* best choice.
* */
public synchronized Object co_entry_pause(int thisCoroutine) throws java.lang.NoSuchMethodException
{
if(!m_activeIDs.get(thisCoroutine))
throw new java.lang.NoSuchMethodException();
while(m_nextCoroutine != thisCoroutine)
{
try
{
wait();
}
catch(java.lang.InterruptedException e)
{
// %TBD% -- Declare? Encapsulate? Ignore? Or
// dance widdershins about the instruction cache?
}
}
return m_yield;
}
/** Transfer control to another coroutine which has already been started and
* is waiting on this CoroutineManager. We won't return from this call
* until that routine has relinquished control.
*
* %TBD% What should we do if toCoroutine isn't registered? Exception?
*
* @param arg_object A value to be passed to the other coroutine.
* @param thisCoroutine Integer identifier for this coroutine. This is the
* ID we watch for to see if we're the ones being resumed.
* @param toCoroutine. Integer identifier for the coroutine we wish to
* invoke.
* @exception java.lang.NoSuchMethodException if toCoroutine isn't a
* registered member of this group. %REVIEW% whether this is the best choice.
* */
public synchronized Object co_resume(Object arg_object,int thisCoroutine,int toCoroutine) throws java.lang.NoSuchMethodException
{
if(!m_activeIDs.get(toCoroutine))
throw new java.lang.NoSuchMethodException(XMLMessages.createXMLMessage(XMLErrorResources.ER_COROUTINE_NOT_AVAIL, new Object[]{Integer.toString(toCoroutine)})); //"Coroutine not available, id="+toCoroutine);
// We expect these values to be overwritten during the notify()/wait()
// periods, as other coroutines in this set get their opportunity to run.
m_yield=arg_object;
m_nextCoroutine=toCoroutine;
notify();
while(m_nextCoroutine != thisCoroutine || m_nextCoroutine==ANYBODY || m_nextCoroutine==NOBODY)
{
try
{
// System.out.println("waiting...");
wait();
}
catch(java.lang.InterruptedException e)
{
// %TBD% -- Declare? Encapsulate? Ignore? Or
// dance deasil about the program counter?
}
}
if(m_nextCoroutine==NOBODY)
{
// Pass it along
co_exit(thisCoroutine);
// And inform this coroutine that its partners are Going Away
// %REVIEW% Should this throw/return something more useful?
throw new java.lang.NoSuchMethodException(XMLMessages.createXMLMessage(XMLErrorResources.ER_COROUTINE_CO_EXIT, null)); //"CoroutineManager recieved co_exit() request");
}
return m_yield;
}
/** Terminate this entire set of coroutines. The others will be
* deregistered and have exceptions thrown at them. Note that this
* is intended as a panic-shutdown operation; under normal
* circumstances a coroutine should always end with co_exit_to() in
* order to politely inform at least one of its partners that it is
* going away.
*
* %TBD% This may need significantly more work.
*
* %TBD% Should this just be co_exit_to(,,CoroutineManager.PANIC)?
*
* @param thisCoroutine Integer identifier for the coroutine requesting exit.
* */
public synchronized void co_exit(int thisCoroutine)
{
m_activeIDs.clear(thisCoroutine);
m_nextCoroutine=NOBODY; // %REVIEW%
notify();
}
/** Make the ID available for reuse and terminate this coroutine,
* transferring control to the specified coroutine. Note that this
* returns immediately rather than waiting for any further coroutine
* traffic, so the thread can proceed with other shutdown activities.
*
* @param arg_object A value to be passed to the other coroutine.
* @param thisCoroutine Integer identifier for the coroutine leaving the set.
* @param toCoroutine. Integer identifier for the coroutine we wish to
* invoke.
* @exception java.lang.NoSuchMethodException if toCoroutine isn't a
* registered member of this group. %REVIEW% whether this is the best choice.
* */
public synchronized void co_exit_to(Object arg_object,int thisCoroutine,int toCoroutine) throws java.lang.NoSuchMethodException
{
if(!m_activeIDs.get(toCoroutine))
throw new java.lang.NoSuchMethodException(XMLMessages.createXMLMessage(XMLErrorResources.ER_COROUTINE_NOT_AVAIL, new Object[]{Integer.toString(toCoroutine)})); //"Coroutine not available, id="+toCoroutine);
// We expect these values to be overwritten during the notify()/wait()
// periods, as other coroutines in this set get their opportunity to run.
m_yield=arg_object;
m_nextCoroutine=toCoroutine;
m_activeIDs.clear(thisCoroutine);
notify();
}
}
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