src/share/classes/com/sun/tools/jdi/VMState.java

/*
 * Copyright (c) 1999, 2011, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package com.sun.tools.jdi;

import com.sun.jdi.*;

import java.lang.ref.WeakReference;
import java.util.*;

class VMState {
    private final VirtualMachineImpl vm;

    // Listeners
    private final List<WeakReference<VMListener>> listeners = new ArrayList<WeakReference<VMListener>>(); // synchronized (this)
    private boolean notifyingListeners = false;  // synchronized (this)

    /*
     * Certain information can be cached only when the entire VM is
     * suspended and there are no pending resumes. The fields below
     * are used to track whether there are pending resumes. (There
     * is an assumption that JDWP command ids are increasing over time.)
     */
    private int lastCompletedCommandId = 0;   // synchronized (this)
    private int lastResumeCommandId = 0;      // synchronized (this)

    // This is cached only while the VM is suspended
    private static class Cache {
        List<ThreadGroupReference> groups = null;  // cached Top Level ThreadGroups
        List<ThreadReference> threads = null; // cached Threads
    }

    private Cache cache = null;               // synchronized (this)
    private static final Cache markerCache = new Cache();

    private void disableCache() {
        synchronized (this) {
            cache = null;
        }
    }

    private void enableCache() {
        synchronized (this) {
            cache = markerCache;
        }
    }

    private Cache getCache() {
        synchronized (this) {
            if (cache == markerCache) {
                cache = new Cache();
            }
            return cache;
        }
    }

    VMState(VirtualMachineImpl vm) {
        this.vm = vm;
    }

    /**
     * Is the VM currently suspended, for the purpose of caching?
     * Must be called synchronized on vm.state()
     */
    boolean isSuspended() {
        return cache != null;
    }

    /*
     * A JDWP command has been completed (reply has been received).
     * Update data that tracks pending resume commands.
     */
    synchronized void notifyCommandComplete(int id) {
        lastCompletedCommandId = id;
    }

    synchronized void freeze() {
        if (cache == null && (lastCompletedCommandId >= lastResumeCommandId)) {
            /*
             * No pending resumes to worry about. The VM is suspended
             * and additional state can be cached. Notify all
             * interested listeners.
             */
            processVMAction(new VMAction(vm, VMAction.VM_SUSPENDED));
            enableCache();
        }
    }

    synchronized PacketStream thawCommand(CommandSender sender) {
        PacketStream stream = sender.send();
        lastResumeCommandId = stream.id();
        thaw();
        return stream;
    }

    /**
     * All threads are resuming
     */
    void thaw() {
        thaw(null);
    }

    /**
     * Tell listeners to invalidate suspend-sensitive caches.
     * If resumingThread != null, then only that thread is being
     * resumed.
     */
    synchronized void thaw(ThreadReference resumingThread) {
        if (cache != null) {
            if ((vm.traceFlags & VirtualMachine.TRACE_OBJREFS) != 0) {
                vm.printTrace("Clearing VM suspended cache");
            }
            disableCache();
        }
        processVMAction(new VMAction(vm, resumingThread, VMAction.VM_NOT_SUSPENDED));
    }

    private synchronized void processVMAction(VMAction action) {
        if (!notifyingListeners) {
            // Prevent recursion
            notifyingListeners = true;

            Iterator<WeakReference<VMListener>> iter = listeners.iterator();
            while (iter.hasNext()) {
                WeakReference<VMListener> ref = iter.next();
                VMListener listener = ref.get();
                if (listener != null) {
                    boolean keep = true;
                    switch (action.id()) {
                        case VMAction.VM_SUSPENDED:
                            keep = listener.vmSuspended(action);
                            break;
                        case VMAction.VM_NOT_SUSPENDED:
                            keep = listener.vmNotSuspended(action);
                            break;
                    }
                    if (!keep) {
                        iter.remove();
                    }
                } else {
                    // Listener is unreachable; clean up
                    iter.remove();
                }
            }

            notifyingListeners = false;
        }
    }

    synchronized void addListener(VMListener listener) {
        listeners.add(new WeakReference<VMListener>(listener));
    }

    synchronized boolean hasListener(VMListener listener) {
        return listeners.contains(listener);
    }

    synchronized void removeListener(VMListener listener) {
        Iterator<WeakReference<VMListener>> iter = listeners.iterator();
        while (iter.hasNext()) {
            WeakReference<VMListener> ref = iter.next();
            if (listener.equals(ref.get())) {
                iter.remove();
                break;
            }
        }
    }

    List<ThreadReference> allThreads() {
        List<ThreadReference> threads = null;
        try {
            Cache local = getCache();

            if (local != null) {
                // may be stale when returned, but not provably so
                threads = local.threads;
            }
            if (threads == null) {
                threads = Arrays.asList((ThreadReference[])JDWP.VirtualMachine.AllThreads.
                                        process(vm).threads);
                if (local != null) {
                    local.threads = threads;
                    if ((vm.traceFlags & VirtualMachine.TRACE_OBJREFS) != 0) {
                        vm.printTrace("Caching all threads (count = " +
                                      threads.size() + ") while VM suspended");
                    }
                }
            }
        } catch (JDWPException exc) {
            throw exc.toJDIException();
        }
        return threads;
    }


    List<ThreadGroupReference> topLevelThreadGroups() {
        List<ThreadGroupReference> groups = null;
        try {
            Cache local = getCache();

            if (local != null) {
                groups = local.groups;
            }
            if (groups == null) {
                groups = Arrays.asList(
                                (ThreadGroupReference[])JDWP.VirtualMachine.TopLevelThreadGroups.
                                       process(vm).groups);
                if (local != null) {
                    local.groups = groups;
                    if ((vm.traceFlags & VirtualMachine.TRACE_OBJREFS) != 0) {
                        vm.printTrace(
                          "Caching top level thread groups (count = " +
                          groups.size() + ") while VM suspended");
                    }
                }
            }
        } catch (JDWPException exc) {
            throw exc.toJDIException();
        }
        return groups;
    }

}