ANTLR parse multiple files to produce one AST

How do i parse multiple source file and end up with just one AST to perform analysis and code generation from? Typically, I find example usage of ANTLR in the form of

public void process(String source) 
{
    ANTLRStringStream Input = new ANTLRStringStream(input);
    TLexer lex = new TLexer(Input);
    CommonTokenStream tokens = new CommonTokenStream(lex);
    TParser parser = new TParser(tokens);
    var tree = parser.parse().Tree; 
} 

but neither the lexer nor the parser seems to be able to take additional files. Am I supposed to create a lexer and parser pr. inputfile and use tree.Add() to add trees from the other files to the tree of the first file?