FastJson用法

1.导入依赖

<dependency>
            <groupId>com.jwebmp.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>0.66.0.1</version>
        </dependency>
         <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.8.3</version>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-annotations</artifactId>
            <version>2.11.2</version>
        </dependency>
        <dependency>

2. java对象转json

 Person person = new Person();
        person.setAge(13);
        person.setEmail("129duckflew@gmamil.com");
        person.setName("张三");
        ObjectMapper objectMapper=new ObjectMapper();
        /**
         * 转换方法
         * writeValue(arg1,obj)
         *   参数1:
         *          File  :将obj转换为json字符串  并白村到指定的文件
         *          Writer ：将obj对象转换到json 字符串  并且将json数据填充到输出流中
         *          OutputStream 将obj对象转换到json 字符串  并且将json数据填充到字节输出流中
         */
        String json = objectMapper.writeValueAsString(person);
        objectMapper.writeValue(new File("F://personJson.txt"),person);
        objectMapper.writeValue(new FileWriter(new File("F://personJson.txt")),person);
        System.out.println(json);

输出如下
{"email":"129duckflew@gmamil.com","name":"张三","age":13}
F盘下生成了personJson.txt 里面的内容就是上面那句

注解的使用

@JsonIgnore 忽略某个属性 不转化成json

public class Person {
    String email;
    String name;
    int age;
    @JsonIgnore
    Date birthday;
}

@JsonFormat(pattern = "yyyy-MM-dd") 格式化某个属性
格式化后的json如下
{"email":"129duckflew@gmamil.com","name":"张三","age":13,"birthday":"2020-09-19"}

复杂类型的转换

  Person person = new Person();
        person.setAge(13);
        person.setEmail("129duckflew@gmamil.com");
        person.setName("张三");
        person.setBirthday(new Date());
        Person person1 = new Person();
        person1.setAge(13);
        person1.setEmail("129duckflew@gmamil.com");
        person1.setName("李四");
        person1.setBirthday(new Date());
        Person person2n = new Person();
        person2n.setAge(13);
        person2n.setEmail("129dflew@ssssgmamil.com");
        person2n.setName("王五");
        person2n.setBirthday(new Date());
        List<Person> personList=new ArrayList<>();
        personList.add(person);
        personList.add(person1);
        personList.add(person2n);
        /**
         * 转换
         */
        ObjectMapper objectMapper=new ObjectMapper();
        String json = objectMapper.writeValueAsString(personList);
        System.out.println(json);

输出结果如下

[{"email":"129duckflew@gmamil.com","name":"张三","age":13,"birthday":"2020-09-19"},{"email":"129duckflew@gmamil.com","name":"李四","age":13,"birthday":"2020-09-19"},{"email":"129dflew@ssssgmamil.com","name":"王五","age":13,"birthday":"2020-09-19"}]
另外 如果要这样转换 则 detabind必须为下面这个版本 可能会爆如下错误
NoSuchMethodError：com.fasterxml.jackson.core.JsonGenerator.writeStartArray

 <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.8.3</version>
 </dependency>

如果仍然无法解决的话 可能是springboot内置的fastJosn冲突了 那就需要在spring boot web starter中排除掉这个部分 依赖如下

 <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <exclusions>
                <exclusion>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-starter-json</artifactId>
                </exclusion>
            </exclusions>
        </dependency>

案例--验证注册的用户名是否可用

前端代码--在输入框失去焦点的时候发送ajax请求

还是要先引入Jquery 老生常谈

$(function () {
            $("#username").blur(function () {
                let username = $(this).val();
                $.get("findExistUser",{username:username},function (data) {
                    let msg =$("#msg_username");
                    if(data.userExist==true)
                    {
                        /**
                         * 用户名存在
                         */
                       msg.css("color","red");
                    }
                    else
                    {
                        /**
                         * 用户名不存在
                         */
                        msg.css("color","green");
                    }
                    msg.html(data.msg);
                },"json")
            })
        })

<form>
        <label for="username">用户名</label><input type="text" id="username" name="username" >
        <span id="msg_username"></span><br>
        <label>
        密码         <input type="password" name="username" >
        </label><br>
        <input type="submit" value="注册">
    </form>

发送到findExistUser这个控制器

代码如下

WebServlet("/findExistUser")
public class FindExistUserServlet extends HttpServlet {
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        /**
         * 调用service层  判断用户是否存在
         */
        //这里假设数据库中有tom这个数据
        Map<String,Object>msg=new HashMap<>();
        resp.setContentType("text/html;charset=utf-8");
        if("tom".equals(username))
        {
            msg.put("userExist",true);
            msg.put("msg","此用户名太受欢迎");
        }
        else
        {
            msg.put("userExist",false);
            msg.put("msg","此用户名可用");
        }
        ObjectMapper objectMapper=new ObjectMapper();
        objectMapper.writeValue(resp.getWriter(),msg);
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        this.doPost(req, resp);
    }
}

这里有两个小问题 第一个防止乱码

resp.setContentType("text/html;charset=utf-8");

此外 还可以在这里把text/html这个mime-type换成json的application/json mime-type

这样的话就不用在 前端的js $.get()的最后一个参数里面输入接收的格式为json了