org.eclipse.jdt.debug/jdi/org/eclipse/jdi/hcr/VirtualMachine.java

/*******************************************************************************
 * Copyright (c) 2000, 2003 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials 
 * are made available under the terms of the Common Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/cpl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.jdi.hcr;


/**
 * Hot code replacement extension to <code>com.sun.jdi.VirtualMachine</code>.
 */
public interface VirtualMachine {
	/** All the given type were reloaded */
	public static final int RELOAD_SUCCESS = 0;
	/** The VM is inconsistent after the reload operation */
	public static final int RELOAD_FAILURE = 1;
	/** The reload operation was ignored */
	public static final int RELOAD_IGNORED = 2;

	/**     
	 * Determines if this implementation supports the early return
	 * of the top stack frame of a thread.
	 *
	 * @return <code>true</code> if the feature is supported, 
	 * <code>false</code> otherwise.
	 */
	public boolean canDoReturn();
	
	/**     
	 * Determines if this implementation supports the retrieval
	 * of a class file version.
	 *
	 * @return <code>true</code> if the feature is supported, 
	 * <code>false</code> otherwise.
	 */
	public boolean canGetClassFileVersion();
	
	/**     
	 * Determines if this implementation supports the reenter stepping.
	 *
	 * @return <code>true</code> if the feature is supported, 
	 * <code>false</code> otherwise.
	 */
	public boolean canReenterOnExit();
	
	/**     
	 * Determines if this implementation supports the replacement
	 * of classes on the fly.
	 *
	 * @return <code>true</code> if the feature is supported, 
	 * <code>false</code> otherwise.
	 */
	public boolean canReloadClasses();
	
	/**
	 * Notifies the VM that the class file base that it is running from has changed. 
	 * Classes are given by their names.
	 * <p>
	 * The class file base is the collection of class files available on the various VM's class paths 
	 * consulted by the class loaders that are integral to the system.  In JDK 1.2, these would 
	 * include all files on the boot class path (used by the bootstrap class loader), the extension 
	 * directory (used by the extension class loader), and the regular class path (used by the 
	 * application class loader).  The notion is important because only those classes that the VM 
	 * knows to be in the class file base will be eligible for hot code replacement. Classes that 
	 * are actually loaded by non-standard class loaders cannot be replaced on the fly (because the 
	 * VM has no way of asking non-standard class loaders to reload them). Classes loaded from the 
	 * class file base by cooperating class loaders are said to be HCR-eligible.
	 * <p>
	 * The VM is expected to:
	 * <ol>
	 *   <li>Suspend all running threads.
	 *   <li>For a given JNI signature, try to find the definition of the corresponding class.
	 *     <ul>
	 *       <li>If the class definition can be found then it replaces the previous definition 
	 *           for that class.
	 *       <li>If a definition for the class is not found, then it is unloaded.
	 *     <ul>
	 *   <li>This operation returns only when the classes have been reloaded and/or deleted.
	 *   <li>If the suspend policy of the class unload event is not to suspend the VM, then the 
	 *       VM resumes all the threads that it has suspended.
	 *   <li>Finally for each class that has been reloaded, the VM is expected to 
	 *     <ul>
	 *        <li>send a class unload event, 
	 *     	  <li>note the VM is already suspended if the suspend policy of class unload event
	 *            said so,
	 *        <li>when the frontend resumes the VM, send a class prepare event,
	 *        <li>suspend the VM according to the suspend policy of the class prepare event
	 *            request.
	 *     </ul>
	 *   <li>For each class that has been unloaded, the VM is expected to 
	 *     <ul>
	 *        <li>send a class unload event,
	 *        <li>suspend the VM if it was requested by the class unload event request.
	 *     </ul>
	 * </ol>
	 * <p>
	 * Subsequent references to classes will work with the new class definition. Note the existing
	 * <code>com.sun.jdi.ReferenceType</code>, <code>com.sun.jdi.Method</code> and 
	 * <code>com.sun.jdi.Field</code> still refer to the old class definition. So they
	 * should be discarded when the class unload event come in.
	 * <p>
	 * The VM does not discard stack frames automatically:
	 * <ul>
	 *   <li>methods on the stack are not affected, and could therefore be referencing 
	 *       obsolete code
	 *   <li>replacing a class does not affect anything on the stack
	 *   <li>subsequent class and method lookups find the replacements
	 * </ul>
	 * <p>
	 * Installed breakpoints are not automatically carried over to the reloaded class:
	 * <ul>
	 *   <li>breakpoints are resolved to particular locations in particular classes and methods
	 *   <li>the VM must clear breakpoints to methods in classes that have been reloaded or
	 *       unloaded (the debugger will reinstall them when it gets the class prepare event.)
	 * </ul>
	 * <p>
	 * A change notice encompasses changes to the content of a class file in the base, the addition 
	 * of a class files to the base, and the removal of a class file from the base. 
	 * <p>
	 * Change notices apply to all classes that are HCR-eligible (i.e., loaded by one of the 
	 * cooperative system class loaders); other classes are never affected.
	 * <p>
	 * Returns whether the operation could be completed as specified above, whether it was
	 * ignored (for example if the VM doesn't support this kind of replacement), or whether the
	 * operation failed and the VM should be restarted.
	 *
	 */
	public int classesHaveChanged(String[] arg1);
}