jackson简单使用，对象转json，json转对象，json转list

た入场券 2022-07-16 07:44 372阅读 0赞

**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);