GsonBuilder.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;

import java.lang.reflect.Type;
import java.sql.Timestamp;
import java.text.DateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Date;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import com.google.gson.internal.$Gson$Preconditions;
import com.google.gson.internal.Excluder;
import com.google.gson.internal.bind.TreeTypeAdapter;
import com.google.gson.internal.bind.TypeAdapters;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;

import static com.google.gson.Gson.DEFAULT_COMPLEX_MAP_KEYS;
import static com.google.gson.Gson.DEFAULT_ESCAPE_HTML;
import static com.google.gson.Gson.DEFAULT_JSON_NON_EXECUTABLE;
import static com.google.gson.Gson.DEFAULT_LENIENT;
import static com.google.gson.Gson.DEFAULT_PRETTY_PRINT;
import static com.google.gson.Gson.DEFAULT_SERIALIZE_NULLS;
import static com.google.gson.Gson.DEFAULT_SPECIALIZE_FLOAT_VALUES;
import static com.google.gson.Gson.DEFAULT_USE_JDK_UNSAFE;

/**
 * <p>Use this builder to construct a {@link Gson} instance when you need to set configuration
 * options other than the default. For {@link Gson} with default configuration, it is simpler to
 * use {@code new Gson()}. {@code GsonBuilder} is best used by creating it, and then invoking its
 * various configuration methods, and finally calling create.</p>
 *
 * <p>The following is an example shows how to use the {@code GsonBuilder} to construct a Gson
 * instance:
 *
 * <pre>
 * Gson gson = new GsonBuilder()
 *     .registerTypeAdapter(Id.class, new IdTypeAdapter())
 *     .enableComplexMapKeySerialization()
 *     .serializeNulls()
 *     .setDateFormat(DateFormat.LONG)
 *     .setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE)
 *     .setPrettyPrinting()
 *     .setVersion(1.0)
 *     .create();
 * </pre></p>
 *
 * <p>NOTES:
 * <ul>
 * <li> the order of invocation of configuration methods does not matter.</li>
 * <li> The default serialization of {@link Date} and its subclasses in Gson does
 *  not contain time-zone information. So, if you are using date/time instances,
 *  use {@code GsonBuilder} and its {@code setDateFormat} methods.</li>
 *  </ul>
 * </p>
 *
 * @author Inderjeet Singh
 * @author Joel Leitch
 * @author Jesse Wilson
 */
public final class GsonBuilder {
  private Excluder excluder = Excluder.DEFAULT;
  private LongSerializationPolicy longSerializationPolicy = LongSerializationPolicy.DEFAULT;
  private FieldNamingStrategy fieldNamingPolicy = FieldNamingPolicy.IDENTITY;
  private final Map<Type, InstanceCreator<?>> instanceCreators
      = new HashMap<Type, InstanceCreator<?>>();
  private final List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>();
  /** tree-style hierarchy factories. These come after factories for backwards compatibility. */
  private final List<TypeAdapterFactory> hierarchyFactories = new ArrayList<TypeAdapterFactory>();
  private boolean serializeNulls = DEFAULT_SERIALIZE_NULLS;
  private String datePattern;
  private int dateStyle = DateFormat.DEFAULT;
  private int timeStyle = DateFormat.DEFAULT;
  private boolean complexMapKeySerialization = DEFAULT_COMPLEX_MAP_KEYS;
  private boolean serializeSpecialFloatingPointValues = DEFAULT_SPECIALIZE_FLOAT_VALUES;
  private boolean escapeHtmlChars = DEFAULT_ESCAPE_HTML;
  private boolean prettyPrinting = DEFAULT_PRETTY_PRINT;
  private boolean generateNonExecutableJson = DEFAULT_JSON_NON_EXECUTABLE;
  private boolean lenient = DEFAULT_LENIENT;
  private boolean useJdkUnsafe = DEFAULT_USE_JDK_UNSAFE;

  /**
   * Creates a GsonBuilder instance that can be used to build Gson with various configuration
   * settings. GsonBuilder follows the builder pattern, and it is typically used by first
   * invoking various configuration methods to set desired options, and finally calling
   * {@link #create()}.
   */
  public GsonBuilder() {
  }

  /**
   * Constructs a GsonBuilder instance from a Gson instance. The newly constructed GsonBuilder
   * has the same configuration as the previously built Gson instance.
   *
   * @param gson the gson instance whose configuration should by applied to a new GsonBuilder.
   */
  GsonBuilder(Gson gson) {
    this.excluder = gson.excluder;
    this.fieldNamingPolicy = gson.fieldNamingStrategy;
    this.instanceCreators.putAll(gson.instanceCreators);
    this.serializeNulls = gson.serializeNulls;
    this.complexMapKeySerialization = gson.complexMapKeySerialization;
    this.generateNonExecutableJson = gson.generateNonExecutableJson;
    this.escapeHtmlChars = gson.htmlSafe;
    this.prettyPrinting = gson.prettyPrinting;
    this.lenient = gson.lenient;
    this.serializeSpecialFloatingPointValues = gson.serializeSpecialFloatingPointValues;
    this.longSerializationPolicy = gson.longSerializationPolicy;
    this.datePattern = gson.datePattern;
    this.dateStyle = gson.dateStyle;
    this.timeStyle = gson.timeStyle;
    this.factories.addAll(gson.builderFactories);
    this.hierarchyFactories.addAll(gson.builderHierarchyFactories);
  }

  /**
   * Configures Gson to enable versioning support.
   *
   * @param ignoreVersionsAfter any field or type marked with a version higher than this value
   * are ignored during serialization or deserialization.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  public GsonBuilder setVersion(double ignoreVersionsAfter) {
    excluder = excluder.withVersion(ignoreVersionsAfter);
    return this;
  }

  /**
   * Configures Gson to excludes all class fields that have the specified modifiers. By default,
   * Gson will exclude all fields marked transient or static. This method will override that
   * behavior.
   *
   * @param modifiers the field modifiers. You must use the modifiers specified in the
   * {@link java.lang.reflect.Modifier} class. For example,
   * {@link java.lang.reflect.Modifier#TRANSIENT},
   * {@link java.lang.reflect.Modifier#STATIC}.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  public GsonBuilder excludeFieldsWithModifiers(int... modifiers) {
    excluder = excluder.withModifiers(modifiers);
    return this;
  }

  /**
   * Makes the output JSON non-executable in Javascript by prefixing the generated JSON with some
   * special text. This prevents attacks from third-party sites through script sourcing. See
   * <a href="http://code.google.com/p/google-gson/issues/detail?id=42">Gson Issue 42</a>
   * for details.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.3
   */
  public GsonBuilder generateNonExecutableJson() {
    this.generateNonExecutableJson = true;
    return this;
  }

  /**
   * Configures Gson to exclude all fields from consideration for serialization or deserialization
   * that do not have the {@link com.google.gson.annotations.Expose} annotation.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  public GsonBuilder excludeFieldsWithoutExposeAnnotation() {
    excluder = excluder.excludeFieldsWithoutExposeAnnotation();
    return this;
  }

  /**
   * Configure Gson to serialize null fields. By default, Gson omits all fields that are null
   * during serialization.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.2
   */
  public GsonBuilder serializeNulls() {
    this.serializeNulls = true;
    return this;
  }

  /**
   * Enabling this feature will only change the serialized form if the map key is
   * a complex type (i.e. non-primitive) in its <strong>serialized</strong> JSON
   * form. The default implementation of map serialization uses {@code toString()}
   * on the key; however, when this is called then one of the following cases
   * apply:
   *
   * <h3>Maps as JSON objects</h3>
   * For this case, assume that a type adapter is registered to serialize and
   * deserialize some {@code Point} class, which contains an x and y coordinate,
   * to/from the JSON Primitive string value {@code "(x,y)"}. The Java map would
   * then be serialized as a {@link JsonObject}.
   *
   * <p>Below is an example:
   * <pre>  {@code
   *   Gson gson = new GsonBuilder()
   *       .register(Point.class, new MyPointTypeAdapter())
   *       .enableComplexMapKeySerialization()
   *       .create();
   *
   *   Map<Point, String> original = new LinkedHashMap<Point, String>();
   *   original.put(new Point(5, 6), "a");
   *   original.put(new Point(8, 8), "b");
   *   System.out.println(gson.toJson(original, type));
   * }</pre>
   * The above code prints this JSON object:<pre>  {@code
   *   {
   *     "(5,6)": "a",
   *     "(8,8)": "b"
   *   }
   * }</pre>
   *
   * <h3>Maps as JSON arrays</h3>
   * For this case, assume that a type adapter was NOT registered for some
   * {@code Point} class, but rather the default Gson serialization is applied.
   * In this case, some {@code new Point(2,3)} would serialize as {@code
   * {"x":2,"y":5}}.
   *
   * <p>Given the assumption above, a {@code Map<Point, String>} will be
   * serialize as an array of arrays (can be viewed as an entry set of pairs).
   *
   * <p>Below is an example of serializing complex types as JSON arrays:
   * <pre> {@code
   *   Gson gson = new GsonBuilder()
   *       .enableComplexMapKeySerialization()
   *       .create();
   *
   *   Map<Point, String> original = new LinkedHashMap<Point, String>();
   *   original.put(new Point(5, 6), "a");
   *   original.put(new Point(8, 8), "b");
   *   System.out.println(gson.toJson(original, type));
   * }
   *
   * The JSON output would look as follows:
   * <pre>   {@code
   *   [
   *     [
   *       {
   *         "x": 5,
   *         "y": 6
   *       },
   *       "a"
   *     ],
   *     [
   *       {
   *         "x": 8,
   *         "y": 8
   *       },
   *       "b"
   *     ]
   *   ]
   * }</pre>
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.7
   */
  public GsonBuilder enableComplexMapKeySerialization() {
    complexMapKeySerialization = true;
    return this;
  }

  /**
   * Configures Gson to exclude inner classes during serialization.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.3
   */
  public GsonBuilder disableInnerClassSerialization() {
    excluder = excluder.disableInnerClassSerialization();
    return this;
  }

  /**
   * Configures Gson to apply a specific serialization policy for {@code Long} and {@code long}
   * objects.
   *
   * @param serializationPolicy the particular policy to use for serializing longs.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.3
   */
  public GsonBuilder setLongSerializationPolicy(LongSerializationPolicy serializationPolicy) {
    this.longSerializationPolicy = serializationPolicy;
    return this;
  }

  /**
   * Configures Gson to apply a specific naming policy to an object's field during serialization
   * and deserialization.
   *
   * @param namingConvention the JSON field naming convention to use for serialization and
   * deserialization.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  public GsonBuilder setFieldNamingPolicy(FieldNamingPolicy namingConvention) {
    this.fieldNamingPolicy = namingConvention;
    return this;
  }

  /**
   * Configures Gson to apply a specific naming policy strategy to an object's field during
   * serialization and deserialization.
   *
   * @param fieldNamingStrategy the actual naming strategy to apply to the fields
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.3
   */
  public GsonBuilder setFieldNamingStrategy(FieldNamingStrategy fieldNamingStrategy) {
    this.fieldNamingPolicy = fieldNamingStrategy;
    return this;
  }

  /**
   * Configures Gson to apply a set of exclusion strategies during both serialization and
   * deserialization. Each of the {@code strategies} will be applied as a disjunction rule.
   * This means that if one of the {@code strategies} suggests that a field (or class) should be
   * skipped then that field (or object) is skipped during serialization/deserialization.
   *
   * @param strategies the set of strategy object to apply during object (de)serialization.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.4
   */
  public GsonBuilder setExclusionStrategies(ExclusionStrategy... strategies) {
    for (ExclusionStrategy strategy : strategies) {
      excluder = excluder.withExclusionStrategy(strategy, true, true);
    }
    return this;
  }

  /**
   * Configures Gson to apply the passed in exclusion strategy during serialization.
   * If this method is invoked numerous times with different exclusion strategy objects
   * then the exclusion strategies that were added will be applied as a disjunction rule.
   * This means that if one of the added exclusion strategies suggests that a field (or
   * class) should be skipped then that field (or object) is skipped during its
   * serialization.
   *
   * @param strategy an exclusion strategy to apply during serialization.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.7
   */
  public GsonBuilder addSerializationExclusionStrategy(ExclusionStrategy strategy) {
    excluder = excluder.withExclusionStrategy(strategy, true, false);
    return this;
  }

  /**
   * Configures Gson to apply the passed in exclusion strategy during deserialization.
   * If this method is invoked numerous times with different exclusion strategy objects
   * then the exclusion strategies that were added will be applied as a disjunction rule.
   * This means that if one of the added exclusion strategies suggests that a field (or
   * class) should be skipped then that field (or object) is skipped during its
   * deserialization.
   *
   * @param strategy an exclusion strategy to apply during deserialization.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.7
   */
  public GsonBuilder addDeserializationExclusionStrategy(ExclusionStrategy strategy) {
    excluder = excluder.withExclusionStrategy(strategy, false, true);
    return this;
  }

  /**
   * Configures Gson to output Json that fits in a page for pretty printing. This option only
   * affects Json serialization.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  public GsonBuilder setPrettyPrinting() {
    prettyPrinting = true;
    return this;
  }

  /**
   * By default, Gson is strict and only accepts JSON as specified by
   * <a href="http://www.ietf.org/rfc/rfc4627.txt">RFC 4627</a>. This option makes the parser
   * liberal in what it accepts.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @see JsonReader#setLenient(boolean)
   */
  public GsonBuilder setLenient() {
    lenient = true;
    return this;
  }

  /**
   * By default, Gson escapes HTML characters such as &lt; &gt; etc. Use this option to configure
   * Gson to pass-through HTML characters as is.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.3
   */
  public GsonBuilder disableHtmlEscaping() {
    this.escapeHtmlChars = false;
    return this;
  }

  /**
   * Configures Gson to serialize {@code Date} objects according to the pattern provided. You can
   * call this method or {@link #setDateFormat(int)} multiple times, but only the last invocation
   * will be used to decide the serialization format.
   *
   * <p>The date format will be used to serialize and deserialize {@link java.util.Date}, {@link
   * java.sql.Timestamp} and {@link java.sql.Date}.
   *
   * <p>Note that this pattern must abide by the convention provided by {@code SimpleDateFormat}
   * class. See the documentation in {@link java.text.SimpleDateFormat} for more information on
   * valid date and time patterns.</p>
   *
   * @param pattern the pattern that dates will be serialized/deserialized to/from
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.2
   */
  public GsonBuilder setDateFormat(String pattern) {
    // TODO(Joel): Make this fail fast if it is an invalid date format
    this.datePattern = pattern;
    return this;
  }

  /**
   * Configures Gson to to serialize {@code Date} objects according to the style value provided.
   * You can call this method or {@link #setDateFormat(String)} multiple times, but only the last
   * invocation will be used to decide the serialization format.
   *
   * <p>Note that this style value should be one of the predefined constants in the
   * {@code DateFormat} class. See the documentation in {@link java.text.DateFormat} for more
   * information on the valid style constants.</p>
   *
   * @param style the predefined date style that date objects will be serialized/deserialized
   * to/from
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.2
   */
  public GsonBuilder setDateFormat(int style) {
    this.dateStyle = style;
    this.datePattern = null;
    return this;
  }

  /**
   * Configures Gson to to serialize {@code Date} objects according to the style value provided.
   * You can call this method or {@link #setDateFormat(String)} multiple times, but only the last
   * invocation will be used to decide the serialization format.
   *
   * <p>Note that this style value should be one of the predefined constants in the
   * {@code DateFormat} class. See the documentation in {@link java.text.DateFormat} for more
   * information on the valid style constants.</p>
   *
   * @param dateStyle the predefined date style that date objects will be serialized/deserialized
   * to/from
   * @param timeStyle the predefined style for the time portion of the date objects
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.2
   */
  public GsonBuilder setDateFormat(int dateStyle, int timeStyle) {
    this.dateStyle = dateStyle;
    this.timeStyle = timeStyle;
    this.datePattern = null;
    return this;
  }

  /**
   * Configures Gson for custom serialization or deserialization. This method combines the
   * registration of an {@link TypeAdapter}, {@link InstanceCreator}, {@link JsonSerializer}, and a
   * {@link JsonDeserializer}. It is best used when a single object {@code typeAdapter} implements
   * all the required interfaces for custom serialization with Gson. If a type adapter was
   * previously registered for the specified {@code type}, it is overwritten.
   *
   * <p>This registers the type specified and no other types: you must manually register related
   * types! For example, applications registering {@code boolean.class} should also register {@code
   * Boolean.class}.
   *
   * @param type the type definition for the type adapter being registered
   * @param typeAdapter This object must implement at least one of the {@link TypeAdapter},
   * {@link InstanceCreator}, {@link JsonSerializer}, and a {@link JsonDeserializer} interfaces.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  @SuppressWarnings({"unchecked", "rawtypes"})
  public GsonBuilder registerTypeAdapter(Type type, Object typeAdapter) {
    $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
        || typeAdapter instanceof JsonDeserializer<?>
        || typeAdapter instanceof InstanceCreator<?>
        || typeAdapter instanceof TypeAdapter<?>);
    if (typeAdapter instanceof InstanceCreator<?>) {
      instanceCreators.put(type, (InstanceCreator) typeAdapter);
    }
    if (typeAdapter instanceof JsonSerializer<?> || typeAdapter instanceof JsonDeserializer<?>) {
      TypeToken<?> typeToken = TypeToken.get(type);
      factories.add(TreeTypeAdapter.newFactoryWithMatchRawType(typeToken, typeAdapter));
    }
    if (typeAdapter instanceof TypeAdapter<?>) {
      factories.add(TypeAdapters.newFactory(TypeToken.get(type), (TypeAdapter)typeAdapter));
    }
    return this;
  }

  /**
   * Register a factory for type adapters. Registering a factory is useful when the type
   * adapter needs to be configured based on the type of the field being processed. Gson
   * is designed to handle a large number of factories, so you should consider registering
   * them to be at par with registering an individual type adapter.
   *
   * @since 2.1
   */
  public GsonBuilder registerTypeAdapterFactory(TypeAdapterFactory factory) {
    factories.add(factory);
    return this;
  }

  /**
   * Configures Gson for custom serialization or deserialization for an inheritance type hierarchy.
   * This method combines the registration of a {@link TypeAdapter}, {@link JsonSerializer} and
   * a {@link JsonDeserializer}. If a type adapter was previously registered for the specified
   * type hierarchy, it is overridden. If a type adapter is registered for a specific type in
   * the type hierarchy, it will be invoked instead of the one registered for the type hierarchy.
   *
   * @param baseType the class definition for the type adapter being registered for the base class
   *        or interface
   * @param typeAdapter This object must implement at least one of {@link TypeAdapter},
   *        {@link JsonSerializer} or {@link JsonDeserializer} interfaces.
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.7
   */
  @SuppressWarnings({"unchecked", "rawtypes"})
  public GsonBuilder registerTypeHierarchyAdapter(Class<?> baseType, Object typeAdapter) {
    $Gson$Preconditions.checkArgument(typeAdapter instanceof JsonSerializer<?>
        || typeAdapter instanceof JsonDeserializer<?>
        || typeAdapter instanceof TypeAdapter<?>);
    if (typeAdapter instanceof JsonDeserializer || typeAdapter instanceof JsonSerializer) {
      hierarchyFactories.add(TreeTypeAdapter.newTypeHierarchyFactory(baseType, typeAdapter));
    }
    if (typeAdapter instanceof TypeAdapter<?>) {
      factories.add(TypeAdapters.newTypeHierarchyFactory(baseType, (TypeAdapter)typeAdapter));
    }
    return this;
  }

  /**
   * Section 2.4 of <a href="http://www.ietf.org/rfc/rfc4627.txt">JSON specification</a> disallows
   * special double values (NaN, Infinity, -Infinity). However,
   * <a href="http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf">Javascript
   * specification</a> (see section 4.3.20, 4.3.22, 4.3.23) allows these values as valid Javascript
   * values. Moreover, most JavaScript engines will accept these special values in JSON without
   * problem. So, at a practical level, it makes sense to accept these values as valid JSON even
   * though JSON specification disallows them.
   *
   * <p>Gson always accepts these special values during deserialization. However, it outputs
   * strictly compliant JSON. Hence, if it encounters a float value {@link Float#NaN},
   * {@link Float#POSITIVE_INFINITY}, {@link Float#NEGATIVE_INFINITY}, or a double value
   * {@link Double#NaN}, {@link Double#POSITIVE_INFINITY}, {@link Double#NEGATIVE_INFINITY}, it
   * will throw an {@link IllegalArgumentException}. This method provides a way to override the
   * default behavior when you know that the JSON receiver will be able to handle these special
   * values.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   * @since 1.3
   */
  public GsonBuilder serializeSpecialFloatingPointValues() {
    this.serializeSpecialFloatingPointValues = true;
    return this;
  }

  /**
   * Disables usage of JDK's {@code sun.misc.Unsafe}.
   *
   * <p>By default Gson uses {@code Unsafe} to create instances of classes which don't have
   * a no-args constructor. However, {@code Unsafe} might not be available for all Java
   * runtimes. For example Android does not provide {@code Unsafe}, or only with limited
   * functionality.<br>
   * Therefore, to get reliable behavior regardless of which runtime is used, and to detect
   * classes which cannot be deserialized in an early stage of development, this method allows
   * disabling usage of {@code Unsafe}.
   *
   * @return a reference to this {@code GsonBuilder} object to fulfill the "Builder" pattern
   */
  public GsonBuilder disableJdkUnsafe() {
    this.useJdkUnsafe = false;
    return this;
  }

  /**
   * Creates a {@link Gson} instance based on the current configuration. This method is free of
   * side-effects to this {@code GsonBuilder} instance and hence can be called multiple times.
   *
   * @return an instance of Gson configured with the options currently set in this builder
   */
  public Gson create() {
    List<TypeAdapterFactory> factories = new ArrayList<TypeAdapterFactory>(this.factories.size() + this.hierarchyFactories.size() + 3);
    factories.addAll(this.factories);
    Collections.reverse(factories);

    List<TypeAdapterFactory> hierarchyFactories = new ArrayList<TypeAdapterFactory>(this.hierarchyFactories);
    Collections.reverse(hierarchyFactories);
    factories.addAll(hierarchyFactories);

    addTypeAdaptersForDate(datePattern, dateStyle, timeStyle, factories);

    return new Gson(excluder, fieldNamingPolicy, instanceCreators,
        serializeNulls, complexMapKeySerialization,
        generateNonExecutableJson, escapeHtmlChars, prettyPrinting, lenient,
        serializeSpecialFloatingPointValues, useJdkUnsafe, longSerializationPolicy,
        datePattern, dateStyle, timeStyle,
        this.factories, this.hierarchyFactories, factories);
  }

  @SuppressWarnings("unchecked")
  private void addTypeAdaptersForDate(String datePattern, int dateStyle, int timeStyle,
      List<TypeAdapterFactory> factories) {
    DefaultDateTypeAdapter dateTypeAdapter;
    TypeAdapter<Timestamp> timestampTypeAdapter;
    TypeAdapter<java.sql.Date> javaSqlDateTypeAdapter;
    if (datePattern != null && !"".equals(datePattern.trim())) {
      dateTypeAdapter = new DefaultDateTypeAdapter(Date.class, datePattern);
      timestampTypeAdapter = (TypeAdapter) new DefaultDateTypeAdapter(Timestamp.class, datePattern);
      javaSqlDateTypeAdapter = (TypeAdapter) new DefaultDateTypeAdapter(java.sql.Date.class, datePattern);
    } else if (dateStyle != DateFormat.DEFAULT && timeStyle != DateFormat.DEFAULT) {
      dateTypeAdapter = new DefaultDateTypeAdapter(Date.class, dateStyle, timeStyle);
      timestampTypeAdapter = (TypeAdapter) new DefaultDateTypeAdapter(Timestamp.class, dateStyle, timeStyle);
      javaSqlDateTypeAdapter = (TypeAdapter) new DefaultDateTypeAdapter(java.sql.Date.class, dateStyle, timeStyle);
    } else {
      return;
    }

    factories.add(TypeAdapters.newFactory(Date.class, dateTypeAdapter));
    factories.add(TypeAdapters.newFactory(Timestamp.class, timestampTypeAdapter));
    factories.add(TypeAdapters.newFactory(java.sql.Date.class, javaSqlDateTypeAdapter));
  }
}