jackson簡單使用，對象轉json，json轉對象，json轉list

添加jackson依賴：

// 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還以為找錯依賴，還以為和com.alibaba:fastjson這個有啥聯系，還以為是一個叫做jack的人寫的。為啥有三個依賴，當發現大多數的框架都依賴於jackson來處理json轉換的時候就自然而然的當做理所當然了。

 

 

POJO序列化為json字符串：

准備一個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) 是為了反序列化的時候，如果遇到不認識的filed，忽略之

• 無參構造函數是為了在反序列化的時候，jackson可以創建POJO實例
• getter方法是為了序列化的時候，jackson可以獲取filed值
• toString是方便我自己debug看顯示
• 至於Serializable，習慣的給實體增加一個持久化的能力。

 

通過write來轉化成jason字符串：

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

通過read來parse json字符串為POJO對象：

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

 

 

jsonArray轉換成Array數組：

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轉換成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默認將對象轉換為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);