Add support for a command factory

This allows programs using Airline to have systems like Guice
or Dagger instantiate commands for use with Airline.

src/main/java/io/airlift/airline/Cli.java

@@ -99,6 +99,16 @@ public C parse(String... args)
     }
 
     public C parse(Iterable<String> args)
+    {
+        return parse(new DefaultCommandFactory<C>(), args);
+    }
+
+    public C parse(CommandFactory<C> commandFactory, String... args)
+    {
+        return parse(commandFactory, ImmutableList.copyOf(args));
+    }
+
+    public C parse(CommandFactory<C> commandFactory, Iterable<String> args)
     {
         Preconditions.checkNotNull(args, "args is null");
 
@@ -124,7 +134,8 @@ public C parse(Iterable<String> args)
                 command.getArguments(),
                 state.getParsedArguments(),
                 command.getMetadataInjections(),
-                ImmutableMap.<Class<?>, Object>of(GlobalMetadata.class, metadata));
+                ImmutableMap.<Class<?>, Object>of(GlobalMetadata.class, metadata),
+                commandFactory);
     }
 
     private void validate(ParseState state)
@@ -173,6 +184,7 @@ public static class CliBuilder<C>
         private Class<? extends C> defaultCommand;
         private final List<Class<? extends C>> defaultCommandGroupCommands = newArrayList();
         protected final Map<String, GroupBuilder<C>> groups = newHashMap();
+        protected CommandFactory<C> commandFactory;
 
         public CliBuilder(String name)
         {
@@ -189,6 +201,13 @@ public CliBuilder<C> withDescription(String description)
             return this;
         }
 
+        public CliBuilder<C> withCommandFactory(CommandFactory<C> commandFactory)
+        {
+            Preconditions.checkNotNull(commandFactory, "commandFactory is null");
+            this.commandFactory = commandFactory;
+            return this;
+        }
+
 //        public CliBuilder<C> withTypeConverter(TypeConverter typeConverter)
 //        {
 //            Preconditions.checkNotNull(typeConverter, "typeConverter is null");

src/main/java/io/airlift/airline/CommandFactory.java

@@ -0,0 +1,6 @@
+package io.airlift.airline;
+
+public interface CommandFactory<T>
+{
+    T createInstance(Class<?> type);
+}

src/main/java/io/airlift/airline/DefaultCommandFactory.java

@@ -0,0 +1,12 @@
+package io.airlift.airline;
+
+public class DefaultCommandFactory<T>
+        implements CommandFactory<T>
+{
+    @SuppressWarnings("unchecked")
+    @Override
+    public T createInstance(Class<?> type)
+    {
+        return (T) ParserUtil.createInstance(type);
+    }
+}

src/main/java/io/airlift/airline/ParserUtil.java

@@ -31,10 +31,11 @@ public static <T> T createInstance(Class<?> type,
             ArgumentsMetadata arguments,
             Iterable<Object> parsedArguments,
             Iterable<Accessor> metadataInjection,
-            Map<Class<?>, Object> bindings)
+            Map<Class<?>, Object> bindings,
+            CommandFactory<T> commandFactory)
     {
         // create the command instance
-        T commandInstance = (T) ParserUtil.createInstance(type);
+        T commandInstance = commandFactory.createInstance(type);
 
         // inject options
         for (OptionMetadata option : options) {

src/main/java/io/airlift/airline/SingleCommand.java

@@ -72,7 +72,8 @@ public C parse(Iterable<String> args)
                 command.getArguments(),
                 state.getParsedArguments(),
                 command.getMetadataInjections(),
-                ImmutableMap.<Class<?>, Object>of(CommandMetadata.class, commandMetadata));
+                ImmutableMap.<Class<?>, Object>of(CommandMetadata.class, commandMetadata),
+                new DefaultCommandFactory<C>());
     }
 
     private void validate(ParseState state)

src/main/java/io/airlift/airline/SuggestCommand.java

@@ -64,7 +64,8 @@ public Iterable<String> generateSuggestions()
                         null,
                         null,
                         suggesterMetadata.getMetadataInjections(),
-                        bindings.build());
+                        bindings.build(),
+                        new DefaultCommandFactory<Suggester>());
 
                 return suggester.suggest();
             }