TypeToken.java

/*
 * Copyright (C) 2008 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.gson.reflect;

import com.google.gson.internal.$Gson$Types;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;

/**
 * Represents a generic type {@code T}. Java doesn't yet provide a way to
 * represent generic types, so this class does. Forces clients to create a
 * subclass of this class which enables retrieval the type information even at
 * runtime.
 *
 * <p>For example, to create a type literal for {@code List<String>}, you can
 * create an empty anonymous class:
 *
 * <p>
 * {@code TypeToken<List<String>> list = new TypeToken<List<String>>() {};}
 *
 * <p>Capturing a type variable as type argument of a {@code TypeToken} should
 * be avoided. Due to type erasure the runtime type of a type variable is not
 * available to Gson and therefore it cannot provide the functionality one
 * might expect, which gives a false sense of type-safety at compilation time
 * and can lead to an unexpected {@code ClassCastException} at runtime.
 *
 * <p>If the type arguments of the parameterized type are only available at
 * runtime, for example when you want to create a {@code List<E>} based on
 * a {@code Class<E>} representing the element type, the method
 * {@link #getParameterized(Type, Type...)} can be used.
 *
 * @author Bob Lee
 * @author Sven Mawson
 * @author Jesse Wilson
 */
public class TypeToken<T> {
  private final Class<? super T> rawType;
  private final Type type;
  private final int hashCode;

  /**
   * Constructs a new type literal. Derives represented class from type
   * parameter.
   *
   * <p>Clients create an empty anonymous subclass. Doing so embeds the type
   * parameter in the anonymous class's type hierarchy so we can reconstitute it
   * at runtime despite erasure.
   */
  @SuppressWarnings("unchecked")
  protected TypeToken() {
    this.type = getTypeTokenTypeArgument();
    this.rawType = (Class<? super T>) $Gson$Types.getRawType(type);
    this.hashCode = type.hashCode();
  }

  /**
   * Unsafe. Constructs a type literal manually.
   */
  @SuppressWarnings("unchecked")
  private TypeToken(Type type) {
    this.type = $Gson$Types.canonicalize(Objects.requireNonNull(type));
    this.rawType = (Class<? super T>) $Gson$Types.getRawType(this.type);
    this.hashCode = this.type.hashCode();
  }

  /**
   * Verifies that {@code this} is an instance of a direct subclass of TypeToken and
   * returns the type argument for {@code T} in {@link $Gson$Types#canonicalize
   * canonical form}.
   */
  private Type getTypeTokenTypeArgument() {
    Type superclass = getClass().getGenericSuperclass();
    if (superclass instanceof ParameterizedType) {
      ParameterizedType parameterized = (ParameterizedType) superclass;
      if (parameterized.getRawType() == TypeToken.class) {
        return $Gson$Types.canonicalize(parameterized.getActualTypeArguments()[0]);
      }
    }
    // Check for raw TypeToken as superclass
    else if (superclass == TypeToken.class) {
      throw new IllegalStateException("TypeToken must be created with a type argument: new TypeToken<...>() {}; "
          + "When using code shrinkers (ProGuard, R8, ...) make sure that generic signatures are preserved.");
    }

    // User created subclass of subclass of TypeToken
    throw new IllegalStateException("Must only create direct subclasses of TypeToken");
  }

  /**
   * Returns the raw (non-generic) type for this type.
   */
  public final Class<? super T> getRawType() {
    return rawType;
  }

  /**
   * Gets underlying {@code Type} instance.
   */
  public final Type getType() {
    return type;
  }

  /**
   * Check if this type is assignable from the given class object.
   *
   * @deprecated this implementation may be inconsistent with javac for types
   *     with wildcards.
   */
  @Deprecated
  public boolean isAssignableFrom(Class<?> cls) {
    return isAssignableFrom((Type) cls);
  }

  /**
   * Check if this type is assignable from the given Type.
   *
   * @deprecated this implementation may be inconsistent with javac for types
   *     with wildcards.
   */
  @Deprecated
  public boolean isAssignableFrom(Type from) {
    if (from == null) {
      return false;
    }

    if (type.equals(from)) {
      return true;
    }

    if (type instanceof Class<?>) {
      return rawType.isAssignableFrom($Gson$Types.getRawType(from));
    } else if (type instanceof ParameterizedType) {
      return isAssignableFrom(from, (ParameterizedType) type,
          new HashMap<String, Type>());
    } else if (type instanceof GenericArrayType) {
      return rawType.isAssignableFrom($Gson$Types.getRawType(from))
          && isAssignableFrom(from, (GenericArrayType) type);
    } else {
      throw buildUnexpectedTypeError(
          type, Class.class, ParameterizedType.class, GenericArrayType.class);
    }
  }

  /**
   * Check if this type is assignable from the given type token.
   *
   * @deprecated this implementation may be inconsistent with javac for types
   *     with wildcards.
   */
  @Deprecated
  public boolean isAssignableFrom(TypeToken<?> token) {
    return isAssignableFrom(token.getType());
  }

  /**
   * Private helper function that performs some assignability checks for
   * the provided GenericArrayType.
   */
  private static boolean isAssignableFrom(Type from, GenericArrayType to) {
    Type toGenericComponentType = to.getGenericComponentType();
    if (toGenericComponentType instanceof ParameterizedType) {
      Type t = from;
      if (from instanceof GenericArrayType) {
        t = ((GenericArrayType) from).getGenericComponentType();
      } else if (from instanceof Class<?>) {
        Class<?> classType = (Class<?>) from;
        while (classType.isArray()) {
          classType = classType.getComponentType();
        }
        t = classType;
      }
      return isAssignableFrom(t, (ParameterizedType) toGenericComponentType,
          new HashMap<String, Type>());
    }
    // No generic defined on "to"; therefore, return true and let other
    // checks determine assignability
    return true;
  }

  /**
   * Private recursive helper function to actually do the type-safe checking
   * of assignability.
   */
  private static boolean isAssignableFrom(Type from, ParameterizedType to,
      Map<String, Type> typeVarMap) {

    if (from == null) {
      return false;
    }

    if (to.equals(from)) {
      return true;
    }

    // First figure out the class and any type information.
    Class<?> clazz = $Gson$Types.getRawType(from);
    ParameterizedType ptype = null;
    if (from instanceof ParameterizedType) {
      ptype = (ParameterizedType) from;
    }

    // Load up parameterized variable info if it was parameterized.
    if (ptype != null) {
      Type[] tArgs = ptype.getActualTypeArguments();
      TypeVariable<?>[] tParams = clazz.getTypeParameters();
      for (int i = 0; i < tArgs.length; i++) {
        Type arg = tArgs[i];
        TypeVariable<?> var = tParams[i];
        while (arg instanceof TypeVariable<?>) {
          TypeVariable<?> v = (TypeVariable<?>) arg;
          arg = typeVarMap.get(v.getName());
        }
        typeVarMap.put(var.getName(), arg);
      }

      // check if they are equivalent under our current mapping.
      if (typeEquals(ptype, to, typeVarMap)) {
        return true;
      }
    }

    for (Type itype : clazz.getGenericInterfaces()) {
      if (isAssignableFrom(itype, to, new HashMap<>(typeVarMap))) {
        return true;
      }
    }

    // Interfaces didn't work, try the superclass.
    Type sType = clazz.getGenericSuperclass();
    return isAssignableFrom(sType, to, new HashMap<>(typeVarMap));
  }

  /**
   * Checks if two parameterized types are exactly equal, under the variable
   * replacement described in the typeVarMap.
   */
  private static boolean typeEquals(ParameterizedType from,
      ParameterizedType to, Map<String, Type> typeVarMap) {
    if (from.getRawType().equals(to.getRawType())) {
      Type[] fromArgs = from.getActualTypeArguments();
      Type[] toArgs = to.getActualTypeArguments();
      for (int i = 0; i < fromArgs.length; i++) {
        if (!matches(fromArgs[i], toArgs[i], typeVarMap)) {
          return false;
        }
      }
      return true;
    }
    return false;
  }

  private static AssertionError buildUnexpectedTypeError(
      Type token, Class<?>... expected) {

    // Build exception message
    StringBuilder exceptionMessage =
        new StringBuilder("Unexpected type. Expected one of: ");
    for (Class<?> clazz : expected) {
      exceptionMessage.append(clazz.getName()).append(", ");
    }
    exceptionMessage.append("but got: ").append(token.getClass().getName())
        .append(", for type token: ").append(token.toString()).append('.');

    return new AssertionError(exceptionMessage.toString());
  }

  /**
   * Checks if two types are the same or are equivalent under a variable mapping
   * given in the type map that was provided.
   */
  private static boolean matches(Type from, Type to, Map<String, Type> typeMap) {
    return to.equals(from)
        || (from instanceof TypeVariable
        && to.equals(typeMap.get(((TypeVariable<?>) from).getName())));

  }

  @Override public final int hashCode() {
    return this.hashCode;
  }

  @Override public final boolean equals(Object o) {
    return o instanceof TypeToken<?>
        && $Gson$Types.equals(type, ((TypeToken<?>) o).type);
  }

  @Override public final String toString() {
    return $Gson$Types.typeToString(type);
  }

  /**
   * Gets type literal for the given {@code Type} instance.
   */
  public static TypeToken<?> get(Type type) {
    return new TypeToken<>(type);
  }

  /**
   * Gets type literal for the given {@code Class} instance.
   */
  public static <T> TypeToken<T> get(Class<T> type) {
    return new TypeToken<>(type);
  }

  /**
   * Gets a type literal for the parameterized type represented by applying {@code typeArguments} to
   * {@code rawType}. This is mainly intended for situations where the type arguments are not
   * available at compile time. The following example shows how a type token for {@code Map<K, V>}
   * can be created:
   * <pre>{@code
   * Class<K> keyClass = ...;
   * Class<V> valueClass = ...;
   * TypeToken<?> mapTypeToken = TypeToken.getParameterized(Map.class, keyClass, valueClass);
   * }</pre>
   * As seen here the result is a {@code TypeToken<?>}; this method cannot provide any type safety,
   * and care must be taken to pass in the correct number of type arguments.
   *
   * @throws IllegalArgumentException
   *   If {@code rawType} is not of type {@code Class}, or if the type arguments are invalid for
   *   the raw type
   */
  public static TypeToken<?> getParameterized(Type rawType, Type... typeArguments) {
    Objects.requireNonNull(rawType);
    Objects.requireNonNull(typeArguments);

    // Perform basic validation here because this is the only public API where users
    // can create malformed parameterized types
    if (!(rawType instanceof Class)) {
      // See also https://bugs.openjdk.org/browse/JDK-8250659
      throw new IllegalArgumentException("rawType must be of type Class, but was " + rawType);
    }
    Class<?> rawClass = (Class<?>) rawType;
    TypeVariable<?>[] typeVariables = rawClass.getTypeParameters();

    int expectedArgsCount = typeVariables.length;
    int actualArgsCount = typeArguments.length;
    if (actualArgsCount != expectedArgsCount) {
      throw new IllegalArgumentException(rawClass.getName() + " requires " + expectedArgsCount +
          " type arguments, but got " + actualArgsCount);
    }

    for (int i = 0; i < expectedArgsCount; i++) {
      Type typeArgument = typeArguments[i];
      Class<?> rawTypeArgument = $Gson$Types.getRawType(typeArgument);
      TypeVariable<?> typeVariable = typeVariables[i];

      for (Type bound : typeVariable.getBounds()) {
        Class<?> rawBound = $Gson$Types.getRawType(bound);

        if (!rawBound.isAssignableFrom(rawTypeArgument)) {
          throw new IllegalArgumentException("Type argument " + typeArgument + " does not satisfy bounds "
              + "for type variable " + typeVariable + " declared by " + rawType);
        }
      }
    }

    return new TypeToken<>($Gson$Types.newParameterizedTypeWithOwner(null, rawType, typeArguments));
  }

  /**
   * Gets type literal for the array type whose elements are all instances of {@code componentType}.
   */
  public static TypeToken<?> getArray(Type componentType) {
    return new TypeToken<>($Gson$Types.arrayOf(componentType));
  }
}