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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.
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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */

package freemarker.debug;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.InetAddress;
import java.net.Socket;
import java.rmi.RemoteException;
import java.rmi.server.RemoteObject;
import java.security.MessageDigest;
import java.util.Collection;
import java.util.List;

import freemarker.debug.impl.RmiDebuggerListenerImpl;
import freemarker.template.utility.UndeclaredThrowableException;

/**
 * A utility class that allows you to connect to the FreeMarker debugger service
 * running on a specific host and port. 
 */
public class DebuggerClient {
    private DebuggerClient() {
    }
    
    /**
     * Connects to the FreeMarker debugger service running on a specific host
     * and port. The Java VM to which the connection is made must have defined
     * the system property freemarker.debug.password in order to enable
     * the debugger service. Additionally, the freemarker.debug.port
     * system property can be set to specify the port where the debugger service
     * is listening. When not specified, it defaults to 
     * {@link Debugger#DEFAULT_PORT}.
     * @param host the host address of the machine where the debugger service is
     * running.
     * @param port the port of the debugger service
     * @param password the password required to connect to the debugger service
     * @return Debugger a debugger object. null is returned in case incorrect
     * password was supplied.
     * @throws IOException if an exception occurs.
     */
    public static Debugger getDebugger(InetAddress host, int port, String password)
    throws IOException {
        try {
            Socket s = new Socket(host, port);
            try {
                ObjectOutputStream out = new ObjectOutputStream(s.getOutputStream());
                ObjectInputStream in = new ObjectInputStream(s.getInputStream());
                int protocolVersion = in.readInt();
                if (protocolVersion > 220) {
                    throw new IOException(
                        "Incompatible protocol version " + protocolVersion + 
                        ". At most 220 was expected.");
                }
                byte[] challenge = (byte[]) in.readObject();
                MessageDigest md = MessageDigest.getInstance("SHA");
                md.update(password.getBytes("UTF-8"));
                md.update(challenge);
                out.writeObject(md.digest());
                return new LocalDebuggerProxy((Debugger) in.readObject());
                //return (Debugger)in.readObject();
            } finally {
                s.close();
            }
        } catch (IOException e) {
            throw e;
        } catch (Exception e) {
            throw new UndeclaredThrowableException(e); 
        }
    }
    
    private static class LocalDebuggerProxy implements Debugger {
        private final Debugger remoteDebugger;

        LocalDebuggerProxy(Debugger remoteDebugger) {
            this.remoteDebugger = remoteDebugger;
        }

        public void addBreakpoint(Breakpoint breakpoint) throws RemoteException {
            remoteDebugger.addBreakpoint(breakpoint);
        }

        public Object addDebuggerListener(DebuggerListener listener) 
        throws RemoteException {
            if (listener instanceof RemoteObject) {
                return remoteDebugger.addDebuggerListener(listener);
            } else {
                RmiDebuggerListenerImpl remotableListener = 
                    new RmiDebuggerListenerImpl(listener);
                return remoteDebugger.addDebuggerListener(remotableListener);
            }
        }

        public List getBreakpoints() throws RemoteException {
            return remoteDebugger.getBreakpoints();
        }

        public List getBreakpoints(String templateName) throws RemoteException {
            return remoteDebugger.getBreakpoints(templateName);
        }

        public Collection getSuspendedEnvironments() throws RemoteException {
            return remoteDebugger.getSuspendedEnvironments();
        }

        public void removeBreakpoint(Breakpoint breakpoint) throws RemoteException {
            remoteDebugger.removeBreakpoint(breakpoint);
        }

        public void removeBreakpoints(String templateName) throws RemoteException {
            remoteDebugger.removeBreakpoints(templateName);
        }

        public void removeBreakpoints() throws RemoteException {
            remoteDebugger.removeBreakpoints();
        }

        public void removeDebuggerListener(Object id) throws RemoteException {
            remoteDebugger.removeDebuggerListener(id);
        }
    }
}