001/*
002 * Copyright (C) 2009 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.reflect;
016
017import static com.google.common.base.Preconditions.checkArgument;
018import static com.google.common.base.Preconditions.checkNotNull;
019import static com.google.common.base.Preconditions.checkState;
020import static java.util.Arrays.asList;
021
022import com.google.common.annotations.Beta;
023import com.google.common.base.Joiner;
024import com.google.common.base.Objects;
025import com.google.common.collect.ImmutableMap;
026import com.google.common.collect.Maps;
027import java.lang.reflect.GenericArrayType;
028import java.lang.reflect.ParameterizedType;
029import java.lang.reflect.Type;
030import java.lang.reflect.TypeVariable;
031import java.lang.reflect.WildcardType;
032import java.util.Arrays;
033import java.util.LinkedHashSet;
034import java.util.Map;
035import java.util.Map.Entry;
036import java.util.Set;
037import java.util.concurrent.atomic.AtomicInteger;
038import org.checkerframework.checker.nullness.qual.Nullable;
039
040/**
041 * An object of this class encapsulates type mappings from type variables. Mappings are established
042 * with {@link #where} and types are resolved using {@link #resolveType}.
043 *
044 * <p>Note that usually type mappings are already implied by the static type hierarchy (for example,
045 * the {@code E} type variable declared by class {@code List} naturally maps to {@code String} in
046 * the context of {@code class MyStringList implements List<String>}. In such case, prefer to use
047 * {@link TypeToken#resolveType} since it's simpler and more type safe. This class should only be
048 * used when the type mapping isn't implied by the static type hierarchy, but provided through other
049 * means such as an annotation or external configuration file.
050 *
051 * @author Ben Yu
052 * @since 15.0
053 */
054@Beta
055public final class TypeResolver {
056
057  private final TypeTable typeTable;
058
059  public TypeResolver() {
060    this.typeTable = new TypeTable();
061  }
062
063  private TypeResolver(TypeTable typeTable) {
064    this.typeTable = typeTable;
065  }
066
067  /**
068   * Returns a resolver that resolves types "covariantly".
069   *
070   * <p>For example, when resolving {@code List<T>} in the context of {@code ArrayList<?>}, {@code
071   * <T>} is covariantly resolved to {@code <?>} such that return type of {@code List::get} is
072   * {@code <?>}.
073   */
074  static TypeResolver covariantly(Type contextType) {
075    return new TypeResolver().where(TypeMappingIntrospector.getTypeMappings(contextType));
076  }
077
078  /**
079   * Returns a resolver that resolves types "invariantly".
080   *
081   * <p>For example, when resolving {@code List<T>} in the context of {@code ArrayList<?>}, {@code
082   * <T>} cannot be invariantly resolved to {@code <?>} because otherwise the parameter type of
083   * {@code List::set} will be {@code <?>} and it'll falsely say any object can be passed into
084   * {@code ArrayList<?>::set}.
085   *
086   * <p>Instead, {@code <?>} will be resolved to a capture in the form of a type variable {@code
087   * <capture-of-? extends Object>}, effectively preventing {@code set} from accepting any type.
088   */
089  static TypeResolver invariantly(Type contextType) {
090    Type invariantContext = WildcardCapturer.INSTANCE.capture(contextType);
091    return new TypeResolver().where(TypeMappingIntrospector.getTypeMappings(invariantContext));
092  }
093
094  /**
095   * Returns a new {@code TypeResolver} with type variables in {@code formal} mapping to types in
096   * {@code actual}.
097   *
098   * <p>For example, if {@code formal} is a {@code TypeVariable T}, and {@code actual} is {@code
099   * String.class}, then {@code new TypeResolver().where(formal, actual)} will {@linkplain
100   * #resolveType resolve} {@code ParameterizedType List<T>} to {@code List<String>}, and resolve
101   * {@code Map<T, Something>} to {@code Map<String, Something>} etc. Similarly, {@code formal} and
102   * {@code actual} can be {@code Map<K, V>} and {@code Map<String, Integer>} respectively, or they
103   * can be {@code E[]} and {@code String[]} respectively, or even any arbitrary combination
104   * thereof.
105   *
106   * @param formal The type whose type variables or itself is mapped to other type(s). It's almost
107   *     always a bug if {@code formal} isn't a type variable and contains no type variable. Make
108   *     sure you are passing the two parameters in the right order.
109   * @param actual The type that the formal type variable(s) are mapped to. It can be or contain yet
110   *     other type variables, in which case these type variables will be further resolved if
111   *     corresponding mappings exist in the current {@code TypeResolver} instance.
112   */
113  public TypeResolver where(Type formal, Type actual) {
114    Map<TypeVariableKey, Type> mappings = Maps.newHashMap();
115    populateTypeMappings(mappings, checkNotNull(formal), checkNotNull(actual));
116    return where(mappings);
117  }
118
119  /** Returns a new {@code TypeResolver} with {@code variable} mapping to {@code type}. */
120  TypeResolver where(Map<TypeVariableKey, ? extends Type> mappings) {
121    return new TypeResolver(typeTable.where(mappings));
122  }
123
124  private static void populateTypeMappings(
125      final Map<TypeVariableKey, Type> mappings, final Type from, final Type to) {
126    if (from.equals(to)) {
127      return;
128    }
129    new TypeVisitor() {
130      @Override
131      void visitTypeVariable(TypeVariable<?> typeVariable) {
132        mappings.put(new TypeVariableKey(typeVariable), to);
133      }
134
135      @Override
136      void visitWildcardType(WildcardType fromWildcardType) {
137        if (!(to instanceof WildcardType)) {
138          return; // okay to say <?> is anything
139        }
140        WildcardType toWildcardType = (WildcardType) to;
141        Type[] fromUpperBounds = fromWildcardType.getUpperBounds();
142        Type[] toUpperBounds = toWildcardType.getUpperBounds();
143        Type[] fromLowerBounds = fromWildcardType.getLowerBounds();
144        Type[] toLowerBounds = toWildcardType.getLowerBounds();
145        checkArgument(
146            fromUpperBounds.length == toUpperBounds.length
147                && fromLowerBounds.length == toLowerBounds.length,
148            "Incompatible type: %s vs. %s",
149            fromWildcardType,
150            to);
151        for (int i = 0; i < fromUpperBounds.length; i++) {
152          populateTypeMappings(mappings, fromUpperBounds[i], toUpperBounds[i]);
153        }
154        for (int i = 0; i < fromLowerBounds.length; i++) {
155          populateTypeMappings(mappings, fromLowerBounds[i], toLowerBounds[i]);
156        }
157      }
158
159      @Override
160      void visitParameterizedType(ParameterizedType fromParameterizedType) {
161        if (to instanceof WildcardType) {
162          return; // Okay to say Foo<A> is <?>
163        }
164        ParameterizedType toParameterizedType = expectArgument(ParameterizedType.class, to);
165        if (fromParameterizedType.getOwnerType() != null
166            && toParameterizedType.getOwnerType() != null) {
167          populateTypeMappings(
168              mappings, fromParameterizedType.getOwnerType(), toParameterizedType.getOwnerType());
169        }
170        checkArgument(
171            fromParameterizedType.getRawType().equals(toParameterizedType.getRawType()),
172            "Inconsistent raw type: %s vs. %s",
173            fromParameterizedType,
174            to);
175        Type[] fromArgs = fromParameterizedType.getActualTypeArguments();
176        Type[] toArgs = toParameterizedType.getActualTypeArguments();
177        checkArgument(
178            fromArgs.length == toArgs.length,
179            "%s not compatible with %s",
180            fromParameterizedType,
181            toParameterizedType);
182        for (int i = 0; i < fromArgs.length; i++) {
183          populateTypeMappings(mappings, fromArgs[i], toArgs[i]);
184        }
185      }
186
187      @Override
188      void visitGenericArrayType(GenericArrayType fromArrayType) {
189        if (to instanceof WildcardType) {
190          return; // Okay to say A[] is <?>
191        }
192        Type componentType = Types.getComponentType(to);
193        checkArgument(componentType != null, "%s is not an array type.", to);
194        populateTypeMappings(mappings, fromArrayType.getGenericComponentType(), componentType);
195      }
196
197      @Override
198      void visitClass(Class<?> fromClass) {
199        if (to instanceof WildcardType) {
200          return; // Okay to say Foo is <?>
201        }
202        // Can't map from a raw class to anything other than itself or a wildcard.
203        // You can't say "assuming String is Integer".
204        // And we don't support "assuming String is T"; user has to say "assuming T is String".
205        throw new IllegalArgumentException("No type mapping from " + fromClass + " to " + to);
206      }
207    }.visit(from);
208  }
209
210  /**
211   * Resolves all type variables in {@code type} and all downstream types and returns a
212   * corresponding type with type variables resolved.
213   */
214  public Type resolveType(Type type) {
215    checkNotNull(type);
216    if (type instanceof TypeVariable) {
217      return typeTable.resolve((TypeVariable<?>) type);
218    } else if (type instanceof ParameterizedType) {
219      return resolveParameterizedType((ParameterizedType) type);
220    } else if (type instanceof GenericArrayType) {
221      return resolveGenericArrayType((GenericArrayType) type);
222    } else if (type instanceof WildcardType) {
223      return resolveWildcardType((WildcardType) type);
224    } else {
225      // if Class<?>, no resolution needed, we are done.
226      return type;
227    }
228  }
229
230  Type[] resolveTypesInPlace(Type[] types) {
231    for (int i = 0; i < types.length; i++) {
232      types[i] = resolveType(types[i]);
233    }
234    return types;
235  }
236
237  private Type[] resolveTypes(Type[] types) {
238    Type[] result = new Type[types.length];
239    for (int i = 0; i < types.length; i++) {
240      result[i] = resolveType(types[i]);
241    }
242    return result;
243  }
244
245  private WildcardType resolveWildcardType(WildcardType type) {
246    Type[] lowerBounds = type.getLowerBounds();
247    Type[] upperBounds = type.getUpperBounds();
248    return new Types.WildcardTypeImpl(resolveTypes(lowerBounds), resolveTypes(upperBounds));
249  }
250
251  private Type resolveGenericArrayType(GenericArrayType type) {
252    Type componentType = type.getGenericComponentType();
253    Type resolvedComponentType = resolveType(componentType);
254    return Types.newArrayType(resolvedComponentType);
255  }
256
257  private ParameterizedType resolveParameterizedType(ParameterizedType type) {
258    Type owner = type.getOwnerType();
259    Type resolvedOwner = (owner == null) ? null : resolveType(owner);
260    Type resolvedRawType = resolveType(type.getRawType());
261
262    Type[] args = type.getActualTypeArguments();
263    Type[] resolvedArgs = resolveTypes(args);
264    return Types.newParameterizedTypeWithOwner(
265        resolvedOwner, (Class<?>) resolvedRawType, resolvedArgs);
266  }
267
268  private static <T> T expectArgument(Class<T> type, Object arg) {
269    try {
270      return type.cast(arg);
271    } catch (ClassCastException e) {
272      throw new IllegalArgumentException(arg + " is not a " + type.getSimpleName());
273    }
274  }
275
276  /** A TypeTable maintains mapping from {@link TypeVariable} to types. */
277  private static class TypeTable {
278    private final ImmutableMap<TypeVariableKey, Type> map;
279
280    TypeTable() {
281      this.map = ImmutableMap.of();
282    }
283
284    private TypeTable(ImmutableMap<TypeVariableKey, Type> map) {
285      this.map = map;
286    }
287
288    /** Returns a new {@code TypeResolver} with {@code variable} mapping to {@code type}. */
289    final TypeTable where(Map<TypeVariableKey, ? extends Type> mappings) {
290      ImmutableMap.Builder<TypeVariableKey, Type> builder = ImmutableMap.builder();
291      builder.putAll(map);
292      for (Entry<TypeVariableKey, ? extends Type> mapping : mappings.entrySet()) {
293        TypeVariableKey variable = mapping.getKey();
294        Type type = mapping.getValue();
295        checkArgument(!variable.equalsType(type), "Type variable %s bound to itself", variable);
296        builder.put(variable, type);
297      }
298      return new TypeTable(builder.build());
299    }
300
301    final Type resolve(final TypeVariable<?> var) {
302      final TypeTable unguarded = this;
303      TypeTable guarded =
304          new TypeTable() {
305            @Override
306            public Type resolveInternal(TypeVariable<?> intermediateVar, TypeTable forDependent) {
307              if (intermediateVar.getGenericDeclaration().equals(var.getGenericDeclaration())) {
308                return intermediateVar;
309              }
310              return unguarded.resolveInternal(intermediateVar, forDependent);
311            }
312          };
313      return resolveInternal(var, guarded);
314    }
315
316    /**
317     * Resolves {@code var} using the encapsulated type mapping. If it maps to yet another
318     * non-reified type or has bounds, {@code forDependants} is used to do further resolution, which
319     * doesn't try to resolve any type variable on generic declarations that are already being
320     * resolved.
321     *
322     * <p>Should only be called and overridden by {@link #resolve(TypeVariable)}.
323     */
324    Type resolveInternal(TypeVariable<?> var, TypeTable forDependants) {
325      Type type = map.get(new TypeVariableKey(var));
326      if (type == null) {
327        Type[] bounds = var.getBounds();
328        if (bounds.length == 0) {
329          return var;
330        }
331        Type[] resolvedBounds = new TypeResolver(forDependants).resolveTypes(bounds);
332        /*
333         * We'd like to simply create our own TypeVariable with the newly resolved bounds. There's
334         * just one problem: Starting with JDK 7u51, the JDK TypeVariable's equals() method doesn't
335         * recognize instances of our TypeVariable implementation. This is a problem because users
336         * compare TypeVariables from the JDK against TypeVariables returned by TypeResolver. To
337         * work with all JDK versions, TypeResolver must return the appropriate TypeVariable
338         * implementation in each of the three possible cases:
339         *
340         * 1. Prior to JDK 7u51, the JDK TypeVariable implementation interoperates with ours.
341         * Therefore, we can always create our own TypeVariable.
342         *
343         * 2. Starting with JDK 7u51, the JDK TypeVariable implementations does not interoperate
344         * with ours. Therefore, we have to be careful about whether we create our own TypeVariable:
345         *
346         * 2a. If the resolved types are identical to the original types, then we can return the
347         * original, identical JDK TypeVariable. By doing so, we sidestep the problem entirely.
348         *
349         * 2b. If the resolved types are different from the original types, things are trickier. The
350         * only way to get a TypeVariable instance for the resolved types is to create our own. The
351         * created TypeVariable will not interoperate with any JDK TypeVariable. But this is OK: We
352         * don't _want_ our new TypeVariable to be equal to the JDK TypeVariable because it has
353         * _different bounds_ than the JDK TypeVariable. And it wouldn't make sense for our new
354         * TypeVariable to be equal to any _other_ JDK TypeVariable, either, because any other JDK
355         * TypeVariable must have a different declaration or name. The only TypeVariable that our
356         * new TypeVariable _will_ be equal to is an equivalent TypeVariable that was also created
357         * by us. And that equality is guaranteed to hold because it doesn't involve the JDK
358         * TypeVariable implementation at all.
359         */
360        if (Types.NativeTypeVariableEquals.NATIVE_TYPE_VARIABLE_ONLY
361            && Arrays.equals(bounds, resolvedBounds)) {
362          return var;
363        }
364        return Types.newArtificialTypeVariable(
365            var.getGenericDeclaration(), var.getName(), resolvedBounds);
366      }
367      // in case the type is yet another type variable.
368      return new TypeResolver(forDependants).resolveType(type);
369    }
370  }
371
372  private static final class TypeMappingIntrospector extends TypeVisitor {
373
374    private final Map<TypeVariableKey, Type> mappings = Maps.newHashMap();
375
376    /**
377     * Returns type mappings using type parameters and type arguments found in the generic
378     * superclass and the super interfaces of {@code contextClass}.
379     */
380    static ImmutableMap<TypeVariableKey, Type> getTypeMappings(Type contextType) {
381      checkNotNull(contextType);
382      TypeMappingIntrospector introspector = new TypeMappingIntrospector();
383      introspector.visit(contextType);
384      return ImmutableMap.copyOf(introspector.mappings);
385    }
386
387    @Override
388    void visitClass(Class<?> clazz) {
389      visit(clazz.getGenericSuperclass());
390      visit(clazz.getGenericInterfaces());
391    }
392
393    @Override
394    void visitParameterizedType(ParameterizedType parameterizedType) {
395      Class<?> rawClass = (Class<?>) parameterizedType.getRawType();
396      TypeVariable<?>[] vars = rawClass.getTypeParameters();
397      Type[] typeArgs = parameterizedType.getActualTypeArguments();
398      checkState(vars.length == typeArgs.length);
399      for (int i = 0; i < vars.length; i++) {
400        map(new TypeVariableKey(vars[i]), typeArgs[i]);
401      }
402      visit(rawClass);
403      visit(parameterizedType.getOwnerType());
404    }
405
406    @Override
407    void visitTypeVariable(TypeVariable<?> t) {
408      visit(t.getBounds());
409    }
410
411    @Override
412    void visitWildcardType(WildcardType t) {
413      visit(t.getUpperBounds());
414    }
415
416    private void map(final TypeVariableKey var, final Type arg) {
417      if (mappings.containsKey(var)) {
418        // Mapping already established
419        // This is possible when following both superClass -> enclosingClass
420        // and enclosingclass -> superClass paths.
421        // Since we follow the path of superclass first, enclosing second,
422        // superclass mapping should take precedence.
423        return;
424      }
425      // First, check whether var -> arg forms a cycle
426      for (Type t = arg; t != null; t = mappings.get(TypeVariableKey.forLookup(t))) {
427        if (var.equalsType(t)) {
428          // cycle detected, remove the entire cycle from the mapping so that
429          // each type variable resolves deterministically to itself.
430          // Otherwise, a F -> T cycle will end up resolving both F and T
431          // nondeterministically to either F or T.
432          for (Type x = arg; x != null; x = mappings.remove(TypeVariableKey.forLookup(x))) {}
433          return;
434        }
435      }
436      mappings.put(var, arg);
437    }
438  }
439
440  // This is needed when resolving types against a context with wildcards
441  // For example:
442  // class Holder<T> {
443  //   void set(T data) {...}
444  // }
445  // Holder<List<?>> should *not* resolve the set() method to set(List<?> data).
446  // Instead, it should create a capture of the wildcard so that set() rejects any List<T>.
447  private static class WildcardCapturer {
448
449    static final WildcardCapturer INSTANCE = new WildcardCapturer();
450
451    private final AtomicInteger id;
452
453    private WildcardCapturer() {
454      this(new AtomicInteger());
455    }
456
457    private WildcardCapturer(AtomicInteger id) {
458      this.id = id;
459    }
460
461    final Type capture(Type type) {
462      checkNotNull(type);
463      if (type instanceof Class) {
464        return type;
465      }
466      if (type instanceof TypeVariable) {
467        return type;
468      }
469      if (type instanceof GenericArrayType) {
470        GenericArrayType arrayType = (GenericArrayType) type;
471        return Types.newArrayType(
472            notForTypeVariable().capture(arrayType.getGenericComponentType()));
473      }
474      if (type instanceof ParameterizedType) {
475        ParameterizedType parameterizedType = (ParameterizedType) type;
476        Class<?> rawType = (Class<?>) parameterizedType.getRawType();
477        TypeVariable<?>[] typeVars = rawType.getTypeParameters();
478        Type[] typeArgs = parameterizedType.getActualTypeArguments();
479        for (int i = 0; i < typeArgs.length; i++) {
480          typeArgs[i] = forTypeVariable(typeVars[i]).capture(typeArgs[i]);
481        }
482        return Types.newParameterizedTypeWithOwner(
483            notForTypeVariable().captureNullable(parameterizedType.getOwnerType()),
484            rawType,
485            typeArgs);
486      }
487      if (type instanceof WildcardType) {
488        WildcardType wildcardType = (WildcardType) type;
489        Type[] lowerBounds = wildcardType.getLowerBounds();
490        if (lowerBounds.length == 0) { // ? extends something changes to capture-of
491          return captureAsTypeVariable(wildcardType.getUpperBounds());
492        } else {
493          // TODO(benyu): handle ? super T somehow.
494          return type;
495        }
496      }
497      throw new AssertionError("must have been one of the known types");
498    }
499
500    TypeVariable<?> captureAsTypeVariable(Type[] upperBounds) {
501      String name =
502          "capture#" + id.incrementAndGet() + "-of ? extends " + Joiner.on('&').join(upperBounds);
503      return Types.newArtificialTypeVariable(WildcardCapturer.class, name, upperBounds);
504    }
505
506    private WildcardCapturer forTypeVariable(final TypeVariable<?> typeParam) {
507      return new WildcardCapturer(id) {
508        @Override
509        TypeVariable<?> captureAsTypeVariable(Type[] upperBounds) {
510          Set<Type> combined = new LinkedHashSet<>(asList(upperBounds));
511          // Since this is an artifically generated type variable, we don't bother checking
512          // subtyping between declared type bound and actual type bound. So it's possible that we
513          // may generate something like <capture#1-of ? extends Foo&SubFoo>.
514          // Checking subtype between declared and actual type bounds
515          // adds recursive isSubtypeOf() call and feels complicated.
516          // There is no contract one way or another as long as isSubtypeOf() works as expected.
517          combined.addAll(asList(typeParam.getBounds()));
518          if (combined.size() > 1) { // Object is implicit and only useful if it's the only bound.
519            combined.remove(Object.class);
520          }
521          return super.captureAsTypeVariable(combined.toArray(new Type[0]));
522        }
523      };
524    }
525
526    private WildcardCapturer notForTypeVariable() {
527      return new WildcardCapturer(id);
528    }
529
530    private Type captureNullable(@Nullable Type type) {
531      if (type == null) {
532        return null;
533      }
534      return capture(type);
535    }
536  }
537
538  /**
539   * Wraps around {@code TypeVariable<?>} to ensure that any two type variables are equal as long as
540   * they are declared by the same {@link java.lang.reflect.GenericDeclaration} and have the same
541   * name, even if their bounds differ.
542   *
543   * <p>While resolving a type variable from a {@code var -> type} map, we don't care whether the
544   * type variable's bound has been partially resolved. As long as the type variable "identity"
545   * matches.
546   *
547   * <p>On the other hand, if for example we are resolving {@code List<A extends B>} to {@code
548   * List<A extends String>}, we need to compare that {@code <A extends B>} is unequal to {@code <A
549   * extends String>} in order to decide to use the transformed type instead of the original type.
550   */
551  static final class TypeVariableKey {
552    private final TypeVariable<?> var;
553
554    TypeVariableKey(TypeVariable<?> var) {
555      this.var = checkNotNull(var);
556    }
557
558    @Override
559    public int hashCode() {
560      return Objects.hashCode(var.getGenericDeclaration(), var.getName());
561    }
562
563    @Override
564    public boolean equals(Object obj) {
565      if (obj instanceof TypeVariableKey) {
566        TypeVariableKey that = (TypeVariableKey) obj;
567        return equalsTypeVariable(that.var);
568      } else {
569        return false;
570      }
571    }
572
573    @Override
574    public String toString() {
575      return var.toString();
576    }
577
578    /** Wraps {@code t} in a {@code TypeVariableKey} if it's a type variable. */
579    static TypeVariableKey forLookup(Type t) {
580      if (t instanceof TypeVariable) {
581        return new TypeVariableKey((TypeVariable<?>) t);
582      } else {
583        return null;
584      }
585    }
586
587    /**
588     * Returns true if {@code type} is a {@code TypeVariable} with the same name and declared by the
589     * same {@code GenericDeclaration}.
590     */
591    boolean equalsType(Type type) {
592      if (type instanceof TypeVariable) {
593        return equalsTypeVariable((TypeVariable<?>) type);
594      } else {
595        return false;
596      }
597    }
598
599    private boolean equalsTypeVariable(TypeVariable<?> that) {
600      return var.getGenericDeclaration().equals(that.getGenericDeclaration())
601          && var.getName().equals(that.getName());
602    }
603  }
604}