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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.framework.source;

import com.sun.source.tree.ClassTree;
import com.sun.source.tree.CompilationUnitTree;
import com.sun.source.tree.MethodTree;
import com.sun.source.tree.Tree;
import com.sun.source.tree.VariableTree;
import com.sun.source.util.TreePath;
import com.sun.source.util.TreePathScanner;
import com.sun.source.util.Trees;

import org.checkerframework.javacutil.BugInCF;
import org.checkerframework.javacutil.TreeUtils;

import java.util.ArrayList;
import java.util.List;

import javax.annotation.processing.ProcessingEnvironment;
import javax.lang.model.element.Element;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;

/**
 * An AST visitor that provides a variety of compiler utilities and interfaces to facilitate
 * type-checking.
 */
public abstract class SourceVisitor extends TreePathScanner {

    /** The {@link Trees} instance to use for scanning. */
    protected final Trees trees;

    /** The {@link Elements} helper to use when scanning. */
    protected final Elements elements;

    /** The {@link Types} helper to use when scanning. */
    protected final Types types;

    /**
     * The root of the AST that this {@link SourceVisitor} will scan.
     *
     * Is set by {@link #setRoot}.
     */
    protected CompilationUnitTree root;

    /** The trees that are annotated with {@code @SuppressWarnings}. */
    public final List treesWithSuppressWarnings;

    /** Whether or not a warning should be issued for unneeded warning suppressions. */
    private final boolean warnUnneededSuppressions;

    /**
     * Creates a {@link SourceVisitor} to use for scanning a source tree.
     *
     * @param checker the checker to invoke on the input source tree
     */
    protected SourceVisitor(SourceChecker checker) {
        // Use the checker's processing environment to get the helpers we need.
        ProcessingEnvironment env = checker.getProcessingEnvironment();

        this.trees = Trees.instance(env);
        this.elements = env.getElementUtils();
        this.types = env.getTypeUtils();

        this.treesWithSuppressWarnings = new ArrayList<>();
        this.warnUnneededSuppressions = checker.hasOption("warnUnneededSuppressions");
    }

    /**
     * Set the CompilationUnitTree to be used during any visits. For any later calls of {@code
     * com.sun.source.util.TreePathScanner.scan(TreePath, P)}, the CompilationUnitTree of the
     * TreePath has to be equal to {@code root}.
     */
    public void setRoot(CompilationUnitTree root) {
        this.root = root;
    }

    /**
     * Store the last Tree visited by the SourceVisitor. This is necessary because the finally
     * blocks in {@link com.sun.source.util.TreePathScanner#scan(TreePath, Object)} and {@link
     * com.sun.source.util.TreePathScanner#scan(Tree, Object)} set the visited Path to null. This
     * field is used to report a rough location for the error in {@link
     * org.checkerframework.framework.source.SourceChecker#logBugInCF(BugInCF)}.
     */
    /*package-private*/ Tree lastVisited;

    /** Entry point for a type processor: the TreePath leaf is a top-level type tree within root. */
    public void visit(TreePath path) {
        lastVisited = path.getLeaf();
        this.scan(path, null);
    }

    @Override
    public R scan(Tree tree, P p) {
        lastVisited = tree;
        return super.scan(tree, p);
    }

    @Override
    public R visitClass(ClassTree classTree, P p) {
        storeSuppressWarningsAnno(classTree);
        return super.visitClass(classTree, p);
    }

    @Override
    public R visitVariable(VariableTree variableTree, P p) {
        storeSuppressWarningsAnno(variableTree);
        return super.visitVariable(variableTree, p);
    }

    @Override
    public R visitMethod(MethodTree tree, P p) {
        storeSuppressWarningsAnno(tree);
        return super.visitMethod(tree, p);
    }

    /**
     * If {@code tree} has a {@code @SuppressWarnings} add it to treesWithSuppressWarnings.
     *
     * @param tree a declaration on which a {@code @SuppressWarnings} annotation may be placed
     */
    private void storeSuppressWarningsAnno(Tree tree) {
        if (!warnUnneededSuppressions) {
            return;
        }
        Element elt = TreeUtils.elementFromTree(tree);
        if (elt.getAnnotation(SuppressWarnings.class) != null) {
            treesWithSuppressWarnings.add(tree);
        }
    }
}