Android Json处理之Gson

概述：

项目地址:https://github.com/google/gson

Gson：java的json解析库。（其他类似的有json-lib,Jackson,fastson）

核心类：Gson或者GsonBuilder

使用jsonschema2pojo来创建POJO

1、通过网站http://www.jsonschema2pojo.org/在线创建: 选择源代码类型为Json，注解类型是Gson,然后点击preview.

2、通过命令行工具:jsonschema2pojo.bat -a GSON -T JSON --source jsonaddress --target java-gen

使用Gson

你可以使用new Gson()或者通过GsonBuilder来构建Gson对象。

(Serialization)
Gson gson = new Gson();
gson.toJson(1);            ==> prints 1
gson.toJson("abcd");       ==> prints "abcd"
gson.toJson(new Long(10)); ==> prints 10
int[] values = { 1 };
gson.toJson(values);       ==> prints [1]

(Deserialization)
int one = gson.fromJson("1", int.class);
Integer one = gson.fromJson("1", Integer.class);
Long one = gson.fromJson("1", Long.class);
Boolean false = gson.fromJson("false", Boolean.class);
String str = gson.fromJson("\"abc\"", String.class);
String anotherStr = gson.fromJson("[\"abc\"]", String.class);

class BagOfPrimitives {
  private int value1 = 1;
  private String value2 = "abc";
  private transient int value3 = 3;
  BagOfPrimitives() {
    // no-args constructor
  }
}

(Serialization)
BagOfPrimitives obj = new BagOfPrimitives();
Gson gson = new Gson();
String json = gson.toJson(obj);  
==> json is {"value1":1,"value2":"abc"}

Note that you can not serialize objects with circular references since that will result in infinite recursion. 

(Deserialization)
BagOfPrimitives obj2 = gson.fromJson(json, BagOfPrimitives.class);   
==> obj2 is just like obj

Nested Classes (including Inner Classes)

Gson可以很容易地序列化静态static的内部类（但是非静态的这不能）

Array Examples

Gson gson = new Gson();
int[] ints = {1, 2, 3, 4, 5};
String[] strings = {"abc", "def", "ghi"};

(Serialization)
gson.toJson(ints);     ==> prints [1,2,3,4,5]
gson.toJson(strings);  ==> prints ["abc", "def", "ghi"]

(Deserialization)
int[] ints2 = gson.fromJson("[1,2,3,4,5]", int[].class); 

Collections Examples

Gson gson = new Gson();
Collection<Integer> ints = Lists.immutableList(1,2,3,4,5);

(Serialization)
String json = gson.toJson(ints); ==> json is [1,2,3,4,5]

(Deserialization)
Type collectionType = new TypeToken<Collection<Integer>>(){}.getType();
Collection<Integer> ints2 = gson.fromJson(json, collectionType);

Serializing and Deserializing Generic Types

Type fooType = new TypeToken<Foo<Bar>>() {}.getType();
gson.toJson(foo, fooType);
gson.fromJson(json, fooType);

Serializing and Deserializing Collection with Objects of Arbitrary Types

Sometimes you are dealing with JSON array that contains mixed types. For example:

['hello',5,{name:'GREETINGS',source:'guest'}]

public class RawCollectionsExample {
  static class Event {
    private String name;
    private String source;
    private Event(String name, String source) {
      this.name = name;
      this.source = source;
    }
    @Override
    public String toString() {
      return String.format("(name=%s, source=%s)", name, source);
    }
  }

  @SuppressWarnings({ "unchecked", "rawtypes" })
  public static void main(String[] args) {
    Gson gson = new Gson();
    Collection collection = new ArrayList();
    collection.add("hello");
    collection.add(5);
    collection.add(new Event("GREETINGS", "guest"));
    String json = gson.toJson(collection);
    System.out.println("Using Gson.toJson() on a raw collection: " + json);
    JsonParser parser = new JsonParser();
    JsonArray array = parser.parse(json).getAsJsonArray();
    String message = gson.fromJson(array.get(0), String.class);
    int number = gson.fromJson(array.get(1), int.class);
    Event event = gson.fromJson(array.get(2), Event.class);
    System.out.printf("Using Gson.fromJson() to get: %s, %d, %s", message, number, event);
  }
}

Compact Vs. Pretty Printing for JSON Output Format

默认情况下Gson的json输出是没有空格与忽略null值得，所以不是很友好。我们可以采用pretty输出,同时输出null

Gson gson = new GsonBuilder().setPrettyPrinting().serialzeNulls().create();

NOTE: when serializing nulls with Gson, it will add a JsonNull element to the JsonElement structure. Therefore, this object can be used in custom serialization/deserialization.

Versioning Support

本文不做介绍

Excluding Fields From Serialization and Deserialization

默认情况下transient 和static field是被忽略的。但是如果你想包含某些transient域可以采取如下形式：

import java.lang.reflect.Modifier;

Gson gson = new GsonBuilder()
    .excludeFieldsWithModifiers(Modifier.STATIC)
    .create();

NOTE: you can use any number of the Modifier constants to "excludeFieldsWithModifiers" method.  For example:
Gson gson = new GsonBuilder()
    .excludeFieldsWithModifiers(Modifier.STATIC, Modifier.TRANSIENT, Modifier.VOLATILE)
    .create();

不过有更好的形式可以采取 @Expose注解：然后调用 new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create()创建Gson凡 是被@Expose注解的域都会被包括，没有被注解的都被忽略。

JSON Field Naming Support  @SerializedName

private class SomeObject {
  @SerializedName("custom_naming") private final String someField;
  private final String someOtherField;

  public SomeObject(String a, String b) {
    this.someField = a;
    this.someOtherField = b;
  }
}

SomeObject someObject = new SomeObject("first", "second");
Gson gson = new GsonBuilder().setFieldNamingPolicy(FieldNamingPolicy.UPPER_CAMEL_CASE).create();
String jsonRepresentation = gson.toJson(someObject);
System.out.println(jsonRepresentation);

======== OUTPUT ========
{"custom_naming":"first","SomeOtherField":"second"}

Gson2.3新的功能

New Methods in JsonArray

@TypeAdapter Annotation
JsonPath Support

参考：

https://sites.google.com/site/gson/gson-user-guide
http://www.studytrails.com/java/json/java-google-json-new-2.3.jsp