Java example source code file (TypeToken.java)

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Java - Java tags/keywords

class, deprecated, genericarraytype, gson, hashmap, missing, override, parameterizedtype, reflection, stringbuilder, suppresswarnings, type, typetoken, typevariable, unexpected, util

The TypeToken.java Java example source code

/*
 * 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 com.google.gson.internal.$Gson$Preconditions;
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;

/**
 * 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}, you can
 * create an empty anonymous inner class:
 *
 * <p>
 * {@code TypeToken<List list = new TypeToken>() {};}
 *
 * <p>This syntax cannot be used to create type literals that have wildcard
 * parameters, such as {@code Class<?>} or {@code List}.
 *
 * @author Bob Lee
 * @author Sven Mawson
 * @author Jesse Wilson
 */
public class TypeToken<T> {
  final Class<? super T> rawType;
  final Type type;
  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 = getSuperclassTypeParameter(getClass());
    this.rawType = (Class<? super T>) $Gson$Types.getRawType(type);
    this.hashCode = type.hashCode();
  }

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

  /**
   * Returns the type from super class's type parameter in {@link $Gson$Types#canonicalize
   * canonical form}.
   */
  static Type getSuperclassTypeParameter(Class<?> subclass) {
    Type superclass = subclass.getGenericSuperclass();
    if (superclass instanceof Class) {
      throw new RuntimeException("Missing type parameter.");
    }
    ParameterizedType parameterized = (ParameterizedType) superclass;
    return $Gson$Types.canonicalize(parameterized.getActualTypeArguments()[0]);
  }

  /**
   * 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<String, Type>(typeVarMap))) {
        return true;
      }
    }

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

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