How do I get the type of the expression in a MemberSelectTree from a javac plugin?

Question

I am trying to write an annotation processor in the JSR 269 format which uses javac's Compiler Tree API to do some source code analysis. I am interested in member select expressions, such as method calls.

I can easily get the name of the method (or field, etc.) being selected. But I want to know what type the member is being selected from, and I cannot seem to find a straightforward way to do this. Trees.getTypeMirror returns null for everything I try calling it on (and the Javadoc gives no hints).

I suppose I could exhaustively analyze every kind of expression on the left side of the member select and determine the static type of the expression by recursive analysis: NewClassTree, TypeCastTree, MethodInvocationTree, ArrayAccessTree, and many others. But this seems like a lot of error-prone work, and clearly javac already knows the static type of the expression since it needs this information for many purposes. But how do I get access to this type information?

What I have so far:

import com.sun.source.tree.MemberSelectTree;
import com.sun.source.tree.MethodInvocationTree;
import com.sun.source.util.TreePath;
import com.sun.source.util.TreePathScanner;
import com.sun.source.util.Trees;
import java.util.Set;
import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.TypeElement;
@SupportedAnnotationTypes("*")
@SupportedSourceVersion(SourceVersion.RELEASE_6)
public class PublicProcessor extends AbstractProcessor {
    public @Override boolean process(Set<? extends TypeElement> annotations, RoundEnvironment roundEnv) {
        for (Element e : roundEnv.getRootElements()) {
            final Trees trees = Trees.instance(processingEnv);
            final TreePath root = trees.getPath(e);
            new TreePathScanner<Void,Void>() {
                public @Override Void visitMethodInvocation(MethodInvocationTree node, Void p) {
                    System.err.println("visiting method invocation: " + node + " of kind: " + node.getMethodSelect().getKind());
                    TreePath expr = TreePath.getPath(root, node);
                    System.err.println("  of type: " + trees.getTypeMirror(expr));
                    return super.visitMethodInvocation(node, p);
                }
                public @Override Void visitMemberSelect(MemberSelectTree node, Void p) {
                    System.err.println("accessing member: " + node.getIdentifier());
                    System.err.println("  from: " + getCurrentPath().getCompilationUnit().getSourceFile().toUri());
                    TreePath expr = TreePath.getPath(root, node.getExpression());
                    System.err.println("  in expr: " + expr.getLeaf());
                    System.err.println("  of type: " + trees.getTypeMirror(expr));
                    return super.visitMemberSelect(node, p);
                }
            }.scan(root, null);
        }
        return true;
    }
}

and what it prints when run on some simple code making method calls:

visiting method invocation: new Class().method() of kind: MEMBER_SELECT
  of type: null
accessing member: method
  from: .../Whatever.java
  in expr: new Class()
  of type: null

Answer 1

Discover the class of a methodinvocation in the Annotation Processor for java

seems to be addressing a very similar question, so I will try to use the advice given there. Unfortunately it does not look straightforward, and use of the com.sun.tools.javac package appears to be required.