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 + '}'; } }
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@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);
唯有不断学习方能改变！ -- Ryan Miao
