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Java example source code file (ConstructorConstructor.java)

This example Java source code file (ConstructorConstructor.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

class, constructorconstructor, enumset, failed, instancecreator, jsonioexception, object, objectconstructor, override, parameterizedtype, reflection, runtimeexception, suppresswarnings, threading, threads, type, util

The ConstructorConstructor.java Java example source code

/*
 * Copyright (C) 2011 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.internal;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Collection;
import java.util.EnumSet;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.Map;
import java.util.Queue;
import java.util.Set;
import java.util.SortedMap;
import java.util.SortedSet;
import java.util.TreeMap;
import java.util.TreeSet;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ConcurrentNavigableMap;
import java.util.concurrent.ConcurrentSkipListMap;

import com.google.gson.InstanceCreator;
import com.google.gson.JsonIOException;
import com.google.gson.reflect.TypeToken;

/**
 * Returns a function that can construct an instance of a requested type.
 */
public final class ConstructorConstructor {
  private final Map<Type, InstanceCreator instanceCreators;

  public ConstructorConstructor(Map<Type, InstanceCreator instanceCreators) {
    this.instanceCreators = instanceCreators;
  }

  public <T> ObjectConstructor get(TypeToken typeToken) {
    final Type type = typeToken.getType();
    final Class<? super T> rawType = typeToken.getRawType();

    // first try an instance creator

    @SuppressWarnings("unchecked") // types must agree
    final InstanceCreator<T> typeCreator = (InstanceCreator) instanceCreators.get(type);
    if (typeCreator != null) {
      return new ObjectConstructor<T>() {
        @Override public T construct() {
          return typeCreator.createInstance(type);
        }
      };
    }

    // Next try raw type match for instance creators
    @SuppressWarnings("unchecked") // types must agree
    final InstanceCreator<T> rawTypeCreator =
        (InstanceCreator<T>) instanceCreators.get(rawType);
    if (rawTypeCreator != null) {
      return new ObjectConstructor<T>() {
        @Override public T construct() {
          return rawTypeCreator.createInstance(type);
        }
      };
    }

    ObjectConstructor<T> defaultConstructor = newDefaultConstructor(rawType);
    if (defaultConstructor != null) {
      return defaultConstructor;
    }

    ObjectConstructor<T> defaultImplementation = newDefaultImplementationConstructor(type, rawType);
    if (defaultImplementation != null) {
      return defaultImplementation;
    }

    // finally try unsafe
    return newUnsafeAllocator(type, rawType);
  }

  private <T> ObjectConstructor newDefaultConstructor(Class rawType) {
    try {
      final Constructor<? super T> constructor = rawType.getDeclaredConstructor();
      if (!constructor.isAccessible()) {
        constructor.setAccessible(true);
      }
      return new ObjectConstructor<T>() {
        @SuppressWarnings("unchecked") // T is the same raw type as is requested
        @Override public T construct() {
          try {
            Object[] args = null;
            return (T) constructor.newInstance(args);
          } catch (InstantiationException e) {
            // TODO: JsonParseException ?
            throw new RuntimeException("Failed to invoke " + constructor + " with no args", e);
          } catch (InvocationTargetException e) {
            // TODO: don't wrap if cause is unchecked!
            // TODO: JsonParseException ?
            throw new RuntimeException("Failed to invoke " + constructor + " with no args",
                e.getTargetException());
          } catch (IllegalAccessException e) {
            throw new AssertionError(e);
          }
        }
      };
    } catch (NoSuchMethodException e) {
      return null;
    }
  }

  /**
   * Constructors for common interface types like Map and List and their
   * subtypes.
   */
  @SuppressWarnings("unchecked") // use runtime checks to guarantee that 'T' is what it is
  private <T> ObjectConstructor newDefaultImplementationConstructor(
      final Type type, Class<? super T> rawType) {
    if (Collection.class.isAssignableFrom(rawType)) {
      if (SortedSet.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new TreeSet<Object>();
          }
        };
      } else if (EnumSet.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @SuppressWarnings("rawtypes")
          @Override public T construct() {
            if (type instanceof ParameterizedType) {
              Type elementType = ((ParameterizedType) type).getActualTypeArguments()[0];
              if (elementType instanceof Class) {
                return (T) EnumSet.noneOf((Class)elementType);
              } else {
                throw new JsonIOException("Invalid EnumSet type: " + type.toString());
              }
            } else {
              throw new JsonIOException("Invalid EnumSet type: " + type.toString());
            }
          }
        };
      } else if (Set.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new LinkedHashSet<Object>();
          }
        };
      } else if (Queue.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new ArrayDeque<Object>();
          }
        };
      } else {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new ArrayList<Object>();
          }
        };
      }
    }

    if (Map.class.isAssignableFrom(rawType)) {
      if (ConcurrentNavigableMap.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new ConcurrentSkipListMap<Object, Object>();
          }
        };
      } else if (ConcurrentMap.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new ConcurrentHashMap<Object, Object>();
          }
        };
      } else if (SortedMap.class.isAssignableFrom(rawType)) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new TreeMap<Object, Object>();
          }
        };
      } else if (type instanceof ParameterizedType && !(String.class.isAssignableFrom(
          TypeToken.get(((ParameterizedType) type).getActualTypeArguments()[0]).getRawType()))) {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new LinkedHashMap<Object, Object>();
          }
        };
      } else {
        return new ObjectConstructor<T>() {
          @Override public T construct() {
            return (T) new LinkedTreeMap<String, Object>();
          }
        };
      }
    }

    return null;
  }

  private <T> ObjectConstructor newUnsafeAllocator(
      final Type type, final Class<? super T> rawType) {
    return new ObjectConstructor<T>() {
      private final UnsafeAllocator unsafeAllocator = UnsafeAllocator.create();
      @SuppressWarnings("unchecked")
      @Override public T construct() {
        try {
          Object newInstance = unsafeAllocator.newInstance(rawType);
          return (T) newInstance;
        } catch (Exception e) {
          throw new RuntimeException(("Unable to invoke no-args constructor for " + type + ". "
              + "Register an InstanceCreator with Gson for this type may fix this problem."), e);
        }
      }
    };
  }

  @Override public String toString() {
    return instanceCreators.toString();
  }
}

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