dataflow/src/main/java/org/checkerframework/dataflow/cfg/builder/TryCatchFrame.java

package org.checkerframework.dataflow.cfg.builder;

import java.util.List;
import java.util.Set;
import java.util.StringJoiner;
import javax.lang.model.type.DeclaredType;
import javax.lang.model.type.TypeKind;
import javax.lang.model.type.TypeMirror;
import javax.lang.model.type.TypeVariable;
import javax.lang.model.type.UnionType;
import javax.lang.model.util.Types;
import org.checkerframework.javacutil.Pair;

/**
 * A TryCatchFrame contains an ordered list of catch labels that apply to exceptions with specific
 * types.
 */
class TryCatchFrame implements TryFrame {
  /** The Types utilities. */
  protected final Types types;

  /** An ordered list of pairs because catch blocks are ordered. */
  protected final List<Pair<TypeMirror, Label>> catchLabels;

  /**
   * Construct a TryCatchFrame.
   *
   * @param types the Types utilities
   * @param catchLabels the catch labels
   */
  public TryCatchFrame(Types types, List<Pair<TypeMirror, Label>> catchLabels) {
    this.types = types;
    this.catchLabels = catchLabels;
  }

  @Override
  public String toString() {
    if (this.catchLabels.isEmpty()) {
      return "TryCatchFrame: no catch labels.";
    } else {
      StringJoiner sb = new StringJoiner(System.lineSeparator(), "TryCatchFrame: ", "");
      for (Pair<TypeMirror, Label> ptml : this.catchLabels) {
        sb.add(ptml.first.toString() + " -> " + ptml.second.toString());
      }
      return sb.toString();
    }
  }

  /**
   * Given a type of thrown exception, add the set of possible control flow successor {@link Label}s
   * to the argument set. Return true if the exception is known to be caught by one of those labels
   * and false if it may propagate still further.
   */
  @Override
  public boolean possibleLabels(TypeMirror thrown, Set<Label> labels) {
    // A conservative approach would be to say that every catch block might execute for any thrown
    // exception, but we try to do better.
    //
    // We rely on several assumptions that seem to hold as of Java 7.
    // 1) An exception parameter in a catch block must be either a declared type or a union composed
    //    of declared types, all of which are subtypes of Throwable.
    // 2) A thrown type must either be a declared type or a variable that extends a declared type,
    //    which is a subtype of Throwable.
    //
    // Under those assumptions, if the thrown type (or its bound) is a subtype of the caught type
    // (or one of its alternatives), then the catch block must apply and none of the later ones can
    // apply.
    // Otherwise, if the thrown type (or its bound) is a supertype of the caught type (or one of its
    // alternatives), then the catch block may apply, but so may later ones.
    // Otherwise, the thrown type and the caught type are unrelated declared types, so they do not
    // overlap on any non-null value.

    while (!(thrown instanceof DeclaredType)) {
      assert thrown instanceof TypeVariable : "thrown type must be a variable or a declared type";
      thrown = ((TypeVariable) thrown).getUpperBound();
    }
    DeclaredType declaredThrown = (DeclaredType) thrown;
    assert thrown != null : "thrown type must be bounded by a declared type";

    for (Pair<TypeMirror, Label> pair : catchLabels) {
      TypeMirror caught = pair.first;
      boolean canApply = false;

      if (caught.getKind() == TypeKind.DECLARED) {
        DeclaredType declaredCaught = (DeclaredType) caught;
        if (types.isSubtype(declaredThrown, declaredCaught)) {
          // No later catch blocks can apply.
          labels.add(pair.second);
          return true;
        } else if (types.isSubtype(declaredCaught, declaredThrown)) {
          canApply = true;
        }
      } else {
        assert caught.getKind() == TypeKind.UNION
            : "caught type must be a union or a declared type";
        UnionType caughtUnion = (UnionType) caught;
        for (TypeMirror alternative : caughtUnion.getAlternatives()) {
          assert alternative.getKind() == TypeKind.DECLARED
              : "alternatives of an caught union type must be declared types";
          DeclaredType declaredAlt = (DeclaredType) alternative;
          if (types.isSubtype(declaredThrown, declaredAlt)) {
            // No later catch blocks can apply.
            labels.add(pair.second);
            return true;
          } else if (types.isSubtype(declaredAlt, declaredThrown)) {
            canApply = true;
          }
        }
      }

      if (canApply) {
        labels.add(pair.second);
      }
    }

    return false;
  }
}