/** Copyright (c) 1996-1999 Bill Venners. All Rights Reserved.** This Java source file is part of the Interactive Illustrations Web* Site, which is delivered in the applets directory of the CD-ROM* that accompanies the book "Inside the Java 2 Virtual Machine" by Bill* Venners, published by McGraw-Hill, 1999, ISBN: 0-07-135093-4. This* source file is provided for evaluation purposes only, but you can* redistribute it under certain conditions, described in the full* copyright notice below.** Full Copyright Notice:** All the web pages and Java applets delivered in the applets* directory of the CD-ROM, consisting of ".html," ".gif," ".class,"* and ".java" files, are copyrighted (c) 1996-1999 by Bill* Venners, and all rights are reserved.  This material may be copied* and placed on any commercial or non-commercial web server on any* network (including the internet) provided that the following* guidelines are followed:** a. All the web pages and Java Applets (".html," ".gif," ".class,"* and ".java" files), including the source code, that are delivered* in the applets directory of the CD-ROM that* accompanies the book must be published together on the same web* site.** b. All the web pages and Java Applets (".html," ".gif," ".class,"* and ".java" files) must be published "as is" and may not be altered* in any way.** c. All use and access to this web site must be free, and no fees* can be charged to view these materials, unless express written* permission is obtained from Bill Venners.** d. The web pages and Java Applets may not be distributed on any* media, other than a web server on a network, and may not accompany* any book or publication.** BILL VENNERS MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE* SUITABILITY OF THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING* BUT NOT LIMITED TO THE IMPLIED WARRANTIES OF MERCHANTABILITY,* FITNESS FOR PARTICULAR PURPOSE, OR NON-INFRINGEMENT.  BILL VENNERS* SHALL NOT BE LIABLE FOR ANY DAMAGES SUFFERED BY A LICENSEE AS A* RESULT OF USING, MODIFYING OR DISTRIBUTING THIS SOFTWARE OR ITS* DERIVATIVES.*/package COM.artima.jvmsim;import java.awt.*;import java.applet.*;/*** An applet that simulates the Java Virtual Machine executing a* sequence of bytecodes.** @author      Bill Venners*/public abstract class JVMSimulator extends Applet implements Runnable {    ColoredLabel explanationLabel = new ColoredLabel("",        Label.CENTER, Color.lightGray);    GrayButton stepButton = new GrayButton(StringTable.step);    GrayButton resetButton = new GrayButton(StringTable.reset);    GrayButton runButton = new GrayButton(StringTable.run);    GrayButton stopButton = new GrayButton(StringTable.stop);    MethodAreaPanel methodAreaPanel;    LocalVarsPanel localVarsPanel;    OperandStackPanel operandStackPanel;    int pcRegister;    int optopRegister;    StackFrame currentFrame;    Method currentMethod;    int bytecodeViewSize;    int[] theProgram;    String[] bytecodeMnemonics;    int maxStack;    int maxLocals;	ExceptionTableEntry[] exceptionTable;	ConstantPoolEntry[] constantPool;	boolean stoppedThreadWhenLeftPage = false;    // If the "run" button is pushed, a separate thread will be invoked that    // will cause the JVM to execute until the "stop" button is pressed.    Thread runner;    final int millisecondDelayBetweenSteps = 250;    public void init() {        theProgram = getTheProgram();        bytecodeMnemonics = getBytecodeMnemonics();        bytecodeViewSize = getBytecodeViewSize();        maxStack = getMaxStack();        maxLocals = getMaxLocals();		exceptionTable = getExceptionTable();		constantPool = getConstantPool();        methodAreaPanel = new MethodAreaPanel(bytecodeViewSize,            theProgram, bytecodeMnemonics);        localVarsPanel = new LocalVarsPanel(maxLocals);        operandStackPanel = new OperandStackPanel(maxStack);        // Set background and layout for the applet panel        setBackground(getAppletBackgroundColor());        setLayout(new BorderLayout(5, 5));        // Initialize the title bar        ColoredLabel title = new ColoredLabel(getAppletTitle(),            Label.CENTER, getTitleColor());        title.setFont(new Font("Helvetica", Font.BOLD, 12));        // Initialize the control panel        Panel controlPanel = new Panel();        controlPanel.setLayout(new BorderLayout(5, 5));        // Build the panel of four buttons        Panel leftButtonPanel = new Panel();        leftButtonPanel.setLayout(new GridLayout(2,2,5,5));        leftButtonPanel.add(stepButton);        resetButton.disable();        leftButtonPanel.add(runButton);        leftButtonPanel.add(resetButton);        leftButtonPanel.add(stopButton);        stopButton.disable();        // Initialize the explanation label        explanationLabel.setBackground(getExplanationLabelColor());        Font plainFont = new Font("TimesRoman", Font.ITALIC, 12);        explanationLabel.setFont(plainFont);        // Place the four button panel and explanation label onto        // the control panel        controlPanel.add("West", leftButtonPanel);        controlPanel.add("Center", explanationLabel);        // Initialize the panel that holds both the stack and        // the method area (the twoParts panel)        Panel twoParts = new Panel();        twoParts.setLayout(new GridLayout(1, 2, 5, 5));        Panel stackPanel = new Panel();        stackPanel.setLayout(new GridLayout(2, 1, 5, 5));        stackPanel.add(localVarsPanel);        stackPanel.add(operandStackPanel);        stackPanel.setBackground(getStackAreaColor());        methodAreaPanel.setBackground(getMethodAreaColor());        // Add stack and methodarea panels to the twoParts panel        twoParts.add(stackPanel);        twoParts.add(methodAreaPanel);        // Place the title, controlPanel, and twoParts onto the applet        // panel.        add("North", title);        add("South", controlPanel);        add("Center", twoParts);        // Place the bytecodes into a byte array.        byte[] ba = new byte[theProgram.length];        for (int i = 0; i < ba.length; ++i) {            ba[i] = (byte) theProgram[i];        }        // Pass byte array to new StackFrame        currentFrame = new StackFrame(maxStack, maxLocals);        currentMethod = new Method(currentFrame, ba, exceptionTable, constantPool);        resetState();        updateStateDisplay();    }    // If they leave the page, stop a Run button press, then restart    // automatically if they come back. In effect, leaving the page is like clicking    // the Stop button. Returning to the page is like clicking the Run button.    public void start() {        if (runner == null && stoppedThreadWhenLeftPage) {            stopButton.enable();            runButton.disable();            stepButton.disable();            resetButton.disable();            stoppedThreadWhenLeftPage = false;            runner = new Thread(this);            runner.start();        }    }    public void stop() {        runButton.enable();        stepButton.enable();        resetButton.enable();        stopButton.disable();        if (runner != null) {            // Synchronize on currentMethod to allow the thread            // to complete execution of the current instruction before            // killing it.            synchronized (currentMethod) {                runner.stop();            }            runner = null;            stoppedThreadWhenLeftPage = true;        }    }    // Pushing the Reset button will cause resetState() to be executed, which will    // reset all the data to its initial values.    void resetState() {        pcRegister = 0;        optopRegister = 0;        currentMethod.resetState();    }    // updateStateDisplay writes the current state of the JVM to the UI.    void updateStateDisplay() {        methodAreaPanel.updateProgramCounter(pcRegister);        updateExplanationText();        localVarsPanel.updateView(currentFrame.getLocalVars());        operandStackPanel.updateView(currentFrame.getOptop(),            currentFrame.getOperandStack());    }    public void run() {        while (true) {            try {                pcRegister = currentMethod.executeNextInstruction();            }            catch (BreakpointException be) {                // On a breakpoint, kill the thread.                runButton.enable();                stepButton.enable();                resetButton.enable();                stopButton.disable();                if (runner != null) {                    Thread runnerAboutToStop = runner;                    runner = null;                    runnerAboutToStop.stop();                }            }            updateStateDisplay();            try {                Thread.sleep(millisecondDelayBetweenSteps);            }            catch (InterruptedException e) {            }        }    }    // updateExplanationText() prints out a line of text to the bottom of the    // applet that describes the next opcode to execute.    private void updateExplanationText() {        int nextOpCode = theProgram[pcRegister];        switch (nextOpCode) {        case OpCode.AALOAD:            explanationLabel.setLabelText(StringTable.aaloadText);            break;        case OpCode.ALOAD_0:            explanationLabel.setLabelText(StringTable.aload_0Text);            break;        case OpCode.ALOAD_1:            explanationLabel.setLabelText(StringTable.aload_1Text);            break;        case OpCode.ALOAD_2:            explanationLabel.setLabelText(StringTable.aload_2Text);            break;        case OpCode.ALOAD_3:            explanationLabel.setLabelText(StringTable.aload_3Text);            break;        case OpCode.ASTORE:            explanationLabel.setLabelText(StringTable.astoreText);            break;        case OpCode.ASTORE_0:            explanationLabel.setLabelText(StringTable.astore_0Text);            break;        case OpCode.ASTORE_1:            explanationLabel.setLabelText(StringTable.astore_1Text);            break;        case OpCode.ASTORE_2:            explanationLabel.setLabelText(StringTable.astore_2Text);            break;        case OpCode.ASTORE_3:            explanationLabel.setLabelText(StringTable.astore_3Text);            break;        case OpCode.BIPUSH:            explanationLabel.setLabelText(StringTable.bipushText);            break;        case OpCode.BREAKPOINT:            explanationLabel.setLabelText(StringTable.breakpointText);            break;        case OpCode.DCMPG:            explanationLabel.setLabelText(StringTable.dcmpgText);            break;        case OpCode.DCONST_0:            explanationLabel.setLabelText(StringTable.dconst_0Text);            break;        case OpCode.DCONST_1:            explanationLabel.setLabelText(StringTable.dconst_1Text);            break;        case OpCode.DDIV:            explanationLabel.setLabelText(StringTable.ddivText);            break;        case OpCode.DLOAD:            explanationLabel.setLabelText(StringTable.dloadText);            break;        case OpCode.DLOAD_0:            explanationLabel.setLabelText(StringTable.dload_0Text);            break;        case OpCode.DLOAD_2:            explanationLabel.setLabelText(StringTable.dload_2Text);            break;