Java example source code file (VMState.java)

This example Java source code file (VMState.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

cache, caching, iterator, jdi, jdwpexception, level, list, packetstream, util, virtualmachineimpl, vmaction, vmlistener, vmstate, weakreference

The VMState.java Java example source code

/*
 * 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

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 listeners = new ArrayList>(); // 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 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 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;
    }

}