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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.common.returnsreceiver;

import com.sun.source.tree.AnnotationTree;
import com.sun.source.tree.MethodTree;
import com.sun.source.tree.ModifiersTree;
import com.sun.source.tree.Tree;
import com.sun.source.tree.TypeCastTree;
import com.sun.source.tree.VariableTree;
import com.sun.source.util.TreePath;
import javax.lang.model.element.AnnotationMirror;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.common.basetype.BaseTypeVisitor;
import org.checkerframework.javacutil.AnnotationUtils;
import org.checkerframework.javacutil.ElementUtils;
import org.checkerframework.javacutil.TreeUtils;

/** The visitor for the Returns Receiver Checker. */
public class ReturnsReceiverVisitor extends BaseTypeVisitor {

  /**
   * Create a new {@code ReturnsReceiverVisitor}.
   *
   * @param checker the type-checker associated with this visitor
   */
  public ReturnsReceiverVisitor(BaseTypeChecker checker) {
    super(checker);
  }

  @Override
  public Void visitAnnotation(AnnotationTree tree, Void p) {
    AnnotationMirror annot = TreeUtils.annotationFromAnnotationTree(tree);
    // Warn if a @This annotation is in an illegal location.
    if (AnnotationUtils.areSame(annot, getTypeFactory().THIS_ANNOTATION)) {
      TreePath parentPath = getCurrentPath().getParentPath();
      Tree parent = parentPath.getLeaf();
      Tree grandparent = parentPath.getParentPath().getLeaf();
      Tree greatGrandparent = parentPath.getParentPath().getParentPath().getLeaf();
      boolean isReturnAnnot =
          grandparent instanceof MethodTree
              && (parent.equals(((MethodTree) grandparent).getReturnType())
                  || parent instanceof ModifiersTree);
      boolean isReceiverAnnot =
          greatGrandparent instanceof MethodTree
              && grandparent.equals(((MethodTree) greatGrandparent).getReceiverParameter())
              && parent.equals(((VariableTree) grandparent).getModifiers());
      boolean isCastAnnot =
          grandparent instanceof TypeCastTree
              && parent.equals(((TypeCastTree) grandparent).getType());
      if (!(isReturnAnnot || isReceiverAnnot || isCastAnnot)) {
        checker.reportError(tree, "this.location");
      }
      if (isReturnAnnot
          && ElementUtils.isStatic(TreeUtils.elementFromDeclaration((MethodTree) grandparent))) {
        checker.reportError(tree, "this.location");
      }
    }
    return super.visitAnnotation(tree, p);
  }
}