github.com/keysonZZZ/kmg@v0.0.0-20151121023212-05317bfd7d39/kmgRpc/kmgRpcJava/java/src/com/google/gson/InstanceCreator.java (about)

     1  /*
     2   * Copyright (C) 2008 Google Inc.
     3   *
     4   * Licensed under the Apache License, Version 2.0 (the "License");
     5   * you may not use this file except in compliance with the License.
     6   * You may obtain a copy of the License at
     7   *
     8   * http://www.apache.org/licenses/LICENSE-2.0
     9   *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS,
    12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   * See the License for the specific language governing permissions and
    14   * limitations under the License.
    15   */
    16  
    17  package com.google.gson;
    18  
    19  import java.lang.reflect.Type;
    20  
    21  /**
    22   * This interface is implemented to create instances of a class that does not define a no-args
    23   * constructor. If you can modify the class, you should instead add a private, or public
    24   * no-args constructor. However, that is not possible for library classes, such as JDK classes, or
    25   * a third-party library that you do not have source-code of. In such cases, you should define an
    26   * instance creator for the class. Implementations of this interface should be registered with
    27   * {@link GsonBuilder#registerTypeAdapter(Type, Object)} method before Gson will be able to use
    28   * them.
    29   * <p>Let us look at an example where defining an InstanceCreator might be useful. The
    30   * {@code Id} class defined below does not have a default no-args constructor.</p>
    31   *
    32   * <pre>
    33   * public class Id&lt;T&gt; {
    34   *   private final Class&lt;T&gt; clazz;
    35   *   private final long value;
    36   *   public Id(Class&lt;T&gt; clazz, long value) {
    37   *     this.clazz = clazz;
    38   *     this.value = value;
    39   *   }
    40   * }
    41   * </pre>
    42   *
    43   * <p>If Gson encounters an object of type {@code Id} during deserialization, it will throw an
    44   * exception. The easiest way to solve this problem will be to add a (public or private) no-args
    45   * constructor as follows:</p>
    46   *
    47   * <pre>
    48   * private Id() {
    49   *   this(Object.class, 0L);
    50   * }
    51   * </pre>
    52   *
    53   * <p>However, let us assume that the developer does not have access to the source-code of the
    54   * {@code Id} class, or does not want to define a no-args constructor for it. The developer
    55   * can solve this problem by defining an {@code InstanceCreator} for {@code Id}:</p>
    56   *
    57   * <pre>
    58   * class IdInstanceCreator implements InstanceCreator&lt;Id&gt; {
    59   *   public Id createInstance(Type type) {
    60   *     return new Id(Object.class, 0L);
    61   *   }
    62   * }
    63   * </pre>
    64   *
    65   * <p>Note that it does not matter what the fields of the created instance contain since Gson will
    66   * overwrite them with the deserialized values specified in Json. You should also ensure that a
    67   * <i>new</i> object is returned, not a common object since its fields will be overwritten.
    68   * The developer will need to register {@code IdInstanceCreator} with Gson as follows:</p>
    69   *
    70   * <pre>
    71   * Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdInstanceCreator()).create();
    72   * </pre>
    73   *
    74   * @param <T> the type of object that will be created by this implementation.
    75   *
    76   * @author Inderjeet Singh
    77   * @author Joel Leitch
    78   */
    79  public interface InstanceCreator<T> {
    80  
    81    /**
    82     * Gson invokes this call-back method during deserialization to create an instance of the
    83     * specified type. The fields of the returned instance are overwritten with the data present
    84     * in the Json. Since the prior contents of the object are destroyed and overwritten, do not
    85     * return an instance that is useful elsewhere. In particular, do not return a common instance,
    86     * always use {@code new} to create a new instance.
    87     *
    88     * @param type the parameterized T represented as a {@link Type}.
    89     * @return a default object instance of type T.
    90     */
    91    public T createInstance(Type type);
    92  }