By default, grammar files should be in the ${basedir}/src/main/antlr directory.
The order is based on the grammar definition. For instance, with the following configuration <grammars>grammar1.g,grammar2.g</grammars>, antlr is called twice, first time with grammar1.g, second time with grammar2.g.
The order is important when grammar files use import vocabulary
Use the antlr3-maven-plugin.