（废弃）Jackson 处理json（ObjectMapper）-马育民老师

[Jackson介绍和目录](https://www.malaoshi.top/show_1EF61Bp7UvwW.html "Jackson介绍和目录")

# 废弃

见 [Jackson 处理json-准备（ObjectMapper）](https://www.malaoshi.top/show_1IX34L64qmuf.html "Jackson 处理json-准备（ObjectMapper）")

# ObjectMapper

功能强大，本文介绍：

1. 使用 ObjectMapper 类将POJO对象序列化成JSON字符串
2. 将JSON字符串反序列化成POJO对象
3. 通过response输出json

# 依赖

### 添加jar包 - 普通工程

依赖 jackson jar包，将 javabean 转换成 json：

- jackson-annotations-2.3.1.jar
- jackson-core-2.3.1.jar
- jackson-databind-2.3.1.jar

### 修改pom.xml - maven工程

```
<dependency> 
    <groupId>com.fasterxml.jackson.core</groupId> 
    <artifactId>jackson-databind</artifactId> 
    <version>2.9.1</version> 
</dependency>
```
jackson-databind 依赖 jackson-core 和 jackson-annotations，当添加 jackson-databind 之后， jackson-core 和 jackson-annotations 也随之添加到 Java 项目工程中。在添加相关依赖包之后，就可以使用 Jackson。

# 将java对象转成json

### 注意

javabean至少有一个成员变量有 `getter` 方法，如果没有成员变量，或者没有 `getter` 方法，会 **报错如下**：

```
com.fasterxml.jackson.databind.JsonMappingException: No serializer found for class vo.JsonResult and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) )
```

### 将javabean转成json

代码：

```
public class JsonResult {
    private int code;
    private String msg;

public JsonResult(int code, String msg) {
        this.code = code;
        this.msg = msg;
    }

public int getCode() {
        return code;
    }

public void setCode(int code) {
        this.code = code;
    }

public String getMsg() {
        return msg;
    }

public void setMsg(String msg) {
        this.msg = msg;
    }
}
```

转换代码：
```
JsonResult result=new JsonResult();
result.setCode(10000);
result.setMsg("请登录！");
ObjectMapper om=new ObjectMapper();
String json=om.writeValueAsString(result);
```

### List转换json

```
List list = new ArrayList();
list.add(1);
list.add(2);
list.add(3);
ObjectMapper mapper = new ObjectMapper();
try {
    String json = mapper.writeValueAsString(list);
    System.out.println(json);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
```
执行结果：
```
[1,2,3]
```

复杂：
```
Map map1=new HashMap();
map1.put("name","root");
map1.put("pwd","123456");

Map map2=new HashMap();
map2.put("name","user");
map2.put("pwd","user");

List list = new ArrayList();
list.add(map1);
list.add(map2);
ObjectMapper mapper = new ObjectMapper();
try {
    String json = mapper.writeValueAsString(list);
    System.out.println(json);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
```
执行结果：
```
[{"name":"root","pwd":"123456"},{"name":"user","pwd":"user"}]
```

### Map转成json

```
Map<String, String> map = new HashMap<>();
map.put("username", "root");
map.put("password", "123456");
ObjectMapper mapper = new ObjectMapper();
try {
    String json = mapper.writeValueAsString(map);
    System.out.println(json);
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
```
执行结果：
```
{"password":"123456","username":"root"}
```

# json转成java对象

### json转javabean
```
String json = "{\"code\":10000,\"msg\":\"请登录！\"}";
ObjectMapper mapper = new ObjectMapper();
JsonResult result = mapper.readValue(json, JsonResult.class);
```

### json转list

##### 简单：

```
String json="[1,2,3]";
ObjectMapper mapper = new ObjectMapper();
try {
    List<Integer> list=mapper.readValue(json,List.class);
    for(Integer item:list){
        System.out.println(item);
    }
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
```
执行结果：
```
1
2
3
```

##### 复杂：

先要先反序列化复杂类型
使用ObjectMapper的`getTypeFactory().constructParametricType(Class<?> parametrized, Class<?>... parameterClasses)`方法
```
String json="[{\"name\":\"root\",\"pwd\":\"123456\"},{\"name\":\"user\",\"pwd\":\"111111\"}]";
ObjectMapper mapper = new ObjectMapper();
JavaType javaType = mapper.getTypeFactory().constructParametricType(List.class, Student.class);
try {
    List<Student> list=mapper.readValue(json,javaType);
    for(Student std:list){
        System.out.println(String.format("%s--%s", std.getName(),std.getPwd()));
    }
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
```
Student：
```
class Student{

private String name;
    private String pwd;

public String getName() {
        return name;
    }

public void setName(String name) {
        this.name = name;
    }

public String getPwd() {
        return pwd;
    }

public void setPwd(String pwd) {
        this.pwd = pwd;
    }

}
```
执行结果：
```
root--123456
user--111111
```

### json转Map

```
String json="{\"password\":\"123456\",\"username\":\"root\"}";
ObjectMapper mapper = new ObjectMapper();
try {
    Map<String,String> map=mapper.readValue(json,Map.class);
    for(Map.Entry<String,String> entry:map.entrySet()){
        System.out.println(entry.getKey()+"--"+entry.getValue());
    }
} catch (JsonProcessingException e) {
    e.printStackTrace();
}
```

# response输出json

通过response输出json
```
Map<String,String> map=new HashMap<>();
map.put("error",e.getMessage());

response.setCharacterEncoding("utf-8");
response.setContentType("application/json; charset=utf-8");
try {
    //使用Jackson输出json
    new ObjectMapper().writeValue(response.getWriter(),map);
    response.getWriter().flush();
} catch (IOException ioe) {
    ioe.printStackTrace();
}
```

原文出处：http://www.malaoshi.top/show_1EF5rdjyd8U3.html