jackson简单使用，对象转json，json转对象，json转list

添加jackson依赖：

// https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core
compile group: 'com.fasterxml.jackson.core', name: 'jackson-core', version: '2.8.2'
// https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind
compile group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.8.2'
// https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-annotations
compile group: 'com.fasterxml.jackson.core', name: 'jackson-annotations', version: '2.8.2'

看到fasterxml还以为找错依赖，还以为和com.alibaba:fastjson这个有啥联系，还以为是一个叫做jack的人写的。为啥有三个依赖，当发现大多数的框架都依赖于jackson来处理json转换的时候就自然而然的当做理所当然了。

POJO序列化为json字符串：

准备一个POJO：

@JsonIgnoreProperties(ignoreUnknown = true)
class User implements Serializable {
    private static final long serialVersionUID = -5952920972581467417L;
    private String name;

    public User() {
    }

    public User(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "User{" +
                "name=" + name +
                '}';
    }
}

@JsonIgnoreProperties(ignoreUnknown = true) 是为了反序列化的时候，如果遇到不认识的filed，忽略之
无参构造函数是为了在反序列化的时候，jackson可以创建POJO实例
getter方法是为了序列化的时候，jackson可以获取filed值
toString是方便我自己debug看显示
至于Serializable，习惯的给实体增加一个持久化的能力。

通过write来转化成jason字符串：

String expected = "{"name":"Test"}";
String test = mapper.writeValueAsString(new User("Test"));
Assert.assertEquals(expected, test);

通过read来parse json字符串为POJO对象：

User user = mapper.readValue(expected, User.class);
Assert.assertEquals("Test", user.getName());

jsonArray转换成Array数组：

String expected = "[{"name":"Ryan"},{"name":"Test"},{"name":"Leslie"}]";
ArrayType arrayType = mapper.getTypeFactory().constructArrayType(User.class);
User[] users = mapper.readValue(expected, arrayType);
Assert.assertEquals("Ryan", users[0].getName());

jsonArray转换成List<>泛型：

expected="[{"a":12},{"b":23},{"name":"Ryan"}]";
CollectionType listType = mapper.getTypeFactory().constructCollectionType(ArrayList.class, User.class);
//the sieze of the list is dependon the str json length although the json content is not the POJO type maybe
List<User> userList = mapper.readValue(expected, listType);
Assert.assertEquals(3, userList.size());
Assert.assertNull(userList.get(0).getName());
Assert.assertEquals("Ryan",userList.get(2).getName());

jackson默认将对象转换为LinkedHashMap：

String expected = "[{"name":"Ryan"},{"name":"Test"},{"name":"Leslie"}]";
ArrayList arrayList = mapper.readValue(expected, ArrayList.class);
Object o = arrayList.get(0);
Assert.assertTrue(o instanceof LinkedHashMap);