jackson 学习笔记

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Jackson以优异的解析性能赢得了好评，今天就看看Jackson的一些简单的用法。
Jackson使用之前先要下载，这里一共有三个jar包，想要获得完美的Jackson体验，这三个jar包都不可或缺。

Java–>json

1.将一个类以json字符串的形式输出：

    //将一个类以json字符串的形式输出    @Test    public void test1(){        ObjectMapper mapper = new ObjectMapper();        User user = new User();        user.setMoney(1000);        user.setUsername("张三");        user.setPassword("123");        try {            System.out.println(mapper.writeValueAsString(user));        } catch (IOException e) {            e.printStackTrace();        }    }

User.java

import java.io.Serializable;public class User implements Serializable{    private String username;    private String password;    //添加了transient属性的字段不会被存储    private int money;    public int getMoney() {        return money;    }    public void setMoney(int money) {        this.money = money;    }    public User() {    }    public User(String username, String password, int money) {        this.username = username;        this.password = password;        this.money = money;    }    public String getUsername() {        return username;    }    public void setUsername(String username) {        this.username = username;    }    public String getPassword() {        return password;    }    public void setPassword(String password) {        this.password = password;    }

输出：
这里写图片描述

2.以json字符串的形式输出一个稍微复杂的类：

Book.java

public class Book {    private int id;    private String name;    private int price;    private String author;    private Detail detail;    private Attribute attribute;    public Attribute getAttribute() {        return attribute;    }    public void setAttribute(Attribute attribute) {        this.attribute = attribute;    }    public Detail getDetail() {        return detail;    }    public void setDetail(Detail detail) {        this.detail = detail;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getPrice() {        return price;    }    public void setPrice(int price) {        this.price = price;    }    public String getAuthor() {        return author;    }    public void setAuthor(String author) {        this.author = author;    }}

Detail.java

public class Detail {    private String pressTime;    private String storyTime;    public String getPressTime() {        return pressTime;    }    public void setPressTime(String pressTime) {        this.pressTime = pressTime;    }    public String getStoryTime() {        return storyTime;    }    public void setStoryTime(String storyTime) {        this.storyTime = storyTime;    }}

Attribute.java

public class Attribute {    private String category;    private String edition;    public String getCategory() {        return category;    }    public void setCategory(String category) {        this.category = category;    }    public String getEdition() {        return edition;    }    public void setEdition(String edition) {        this.edition = edition;    }}

输出为：

{"id":1,"name":"三国演义","price":20,"author":"罗贯中","detail":{"pressTime":"2001-01-01","storyTime":"196-05-06"},"attribute":{"category":"小说","edition":"9"}}

3.以json字符串输出一个List集合：

    @Test    public void test2(){        ObjectMapper mapper = new ObjectMapper();        List<User> list = new ArrayList<User>();        User u = new User("张三", "123", 1000);        list.add(u);        u = new User("李四", "456", 2000);        list.add(u);        u = new User("王五", "789", 3000);        list.add(u);        u = new User("赵六", "555", 4000);        list.add(u);        try {            System.out.println(mapper.writeValueAsString(list));        } catch (JsonProcessingException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }

输出结果：

[{"username":"张三","password":"123","money":1000},{"username":"李四","password":"456","money":2000},{"username":"王五","password":"789","money":3000},{"username":"赵六","password":"555","money":4000}]

4.将一个Map以json字符串的形式输出：

    @Test    public void test3(){        ObjectMapper mapper = new ObjectMapper();        Map<String, String> map = new HashMap<String, String>();        map.put("username", "张三");        map.put("password", "123456");        try {            System.out.println(mapper.writeValueAsString(map));        } catch (JsonProcessingException e) {            e.printStackTrace();        }    }

结果为：

这里写图片描述

5.如果想把List集合中的map以json字符串格式输出，又该如何？和前文List一样。

    @Test    public void test4(){        ObjectMapper mapper = new ObjectMapper();        List<Map<String, String>> list = new ArrayList<Map<String,String>>();        Map<String, String> map = new HashMap<String, String>();        map.put("username", "张三");        map.put("password", "123456");        list.add(map);        map = new HashMap<String, String>();        map.put("username", "李四");        map.put("password", "888888");        list.add(map);        try {            System.out.println(mapper.writeValueAsString(list));        } catch (JsonProcessingException e) {            e.printStackTrace();        }    }

6.在看看一个Map中有Book.java,Book.java中又有其他类：

    @Test    public void test1(){        ObjectMapper mapper = new ObjectMapper();        Detail detail = new Detail();        detail.setPressTime("2001-01-01");        detail.setStoryTime("196-05-06");        Attribute attr = new Attribute();        attr.setCategory("小说");        attr.setEdition("9");        Book book = new Book();        book.setAttribute(attr);        book.setAuthor("罗贯中");        book.setDetail(detail);        book.setId(1);        book.setName("三国演义");        book.setPrice(20);        Map<String, Object> map = new HashMap<String, Object>();        map.put("namespace", "books");        map.put("book", book);        try {            System.out.println(mapper.writeValueAsString(map));        } catch (IOException e) {            e.printStackTrace();        }    }

输出结果：

{"book":{"id":1,"name":"三国演义","price":20,"author":"罗贯中","detail":{"pressTime":"2001-01-01","storyTime":"196-05-06"},"attribute":{"category":"小说","edition":"9"}},"namespace":"books"}

Json–>java

1.json字符串转为javaBean：

    @Test    public void test5(){        String str = "{\"id\":1,\"name\":\"三国演义\",\"price\":20,\"author\":\"罗贯中\",\"detail\":{\"pressTime\":\"2001-01-01\",\"storyTime\":\"196-05-06\"},\"attribute\":{\"category\":\"小说\",\"edition\":\"9\"}}";        ObjectMapper mapper = new ObjectMapper();        try {            Book book = mapper.readValue(str, Book.class);            System.out.println(book.getAuthor()+","+book.getAttribute().getCategory());        } catch (IOException e) {            e.printStackTrace();        }    }

2.json字符串转为List

    //json-->List    @Test    public void test6(){        String str = "[{\"username\":\"张三\",\"password\":\"123\",\"money\":1000},{\"username\":\"李四\",\"password\":\"456\",\"money\":2000},{\"username\":\"王五\",\"password\":\"789\",\"money\":3000},{\"username\":\"赵六\",\"password\":\"555\",\"money\":4000}]";        ObjectMapper mapper = new ObjectMapper();        try {            List<User> us = mapper.readValue(str, new TypeReference<ArrayList<User>>() {});            for (User user : us) {                System.out.println(user.getUsername()+","+user.getMoney());            }        } catch (IOException e) {            e.printStackTrace();        }    }

3.json字符串转为Map：

    //json-->map    @Test    public void test7(){        String str = "{\"password\":\"888888\",\"username\":\"李四\"}";        ObjectMapper mapper = new ObjectMapper();        try {            Map<String, String> map = mapper.readValue(str, new TypeReference<Map<String, String>>() {});            for (String key : map.keySet()) {                System.out.println(key+","+map.get(key));            }        } catch (IOException e) {            e.printStackTrace();        }    }

唉，仔细一琢磨，这个Jackson真的好简单，以前一直以为好难，想起来小学的课文《小马过河》，看来还是要多实践。

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