GWT+GAE TodoMVCServerCommandExecutor.java
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package com.todomvc.server.command;

import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import java.util.Map;

import javax.inject.Inject;
import javax.inject.Singleton;

import com.google.common.collect.Maps;
import com.todomvc.shared.command.Command;
import com.todomvc.shared.command.CommandExecutor;
import com.todomvc.shared.command.todo.ToDoCommandExecutor;
import com.todomvc.shared.command.todo.ToDoListCommandExecutor;
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Server-side implementation of a delegating CommandExecutor. It delegates execution of commands to an appropriate executor based on the type of the command passed to execute(Command). See for example the server-side ServerToDoCommandExecutor.

There is a corresponding client-side implementation.

@Singleton
@SuppressWarnings("rawtypes")
public class ServerCommandExecutor implements CommandExecutor {

    private final Map<String, CommandExecutor> executors = Maps.newHashMap();

    @Inject
    public ServerCommandExecutor(ToDoListCommandExecutor toDoListExecutor, ToDoCommandExecutor toDoExecutor) {
        executors.put(ToDoListCommandExecutor.TYPE, checkNotNull(toDoListExecutor));
        executors.put(ToDoCommandExecutor.TYPE, checkNotNull(toDoExecutor));
    }

    @SuppressWarnings("unchecked")
    @Override
    public Command execute(Command command) {
    	CommandExecutor executor = executors.get(command.getCommandType());
        checkState(executor != null,
                "there is no executor for command of type " + command.getCommandType());
        return executor.execute(command);
    }
}