JsonMappingException在阅读JSON文件，并映射它使用POJO jackson.fasterxml

Question

com.fasterxml.jackson.databind.JsonMappingException：无法反序列化的java.util.ArrayList实例出来START_OBJECT令牌

我使用fasterxml.jackson从json文件中提取数据并且想要在POJO中设置它们。
这是我的JSON文件：

{"studentsjson": 
    [ 
     { 
      "name": "Henry Lee", 
      "course": "reading", 
      "year": "2000", 
      "grade": "3" 
     }, 
     { 
      "name": "John Flex", 
      "course": "writing", 
      "year": "2001", 
      "grade": "4" 
     } 
    ] 
} 

和我的POJO类是：

import java.util.*; 
import com.fasterxml.jackson.annotation.*; 

@JsonRootName("studentsjson") 
public class Students{ 
    List<StudentInfo> studentInfo; 
    // getter and setter for above property 
    public Students(){ 
     System.out.println("in Students constructor"); 
    } 
} // end of students class 

import com.fasterxml.jackson.annotation.*; 

public class StudentInfo{ 
    String name; 
    String course; 
    int grade; 
    int yob; 
    // getters and setters  
    @Override 
    public String toString() { 
     return "Student [name=" + name + ", course=" + course + ", grade=" + grade + ",   yob=" + yob + "]"; 
    } 

} // end of student class 

这是我的主要方法类：

@JsonIgnoreProperties(ignoreUnknown = true) 
public class JsonReadJava{ 

    public static void main(String args[])throws JsonParseException, JsonMappingException, IOException 
    { 
    try{ 
     Path path = Paths.get("C:/Users/Radhika/Desktop/JavaJson/studentinfo.json.txt"); 
     byte[] fileasbytes = Files.readAllBytes(path); 
     String JsonAsString = new String(fileasbytes, "UTF-8"); 
     System.out.println("bytes[]asString: "+ JsonAsString); 


     ObjectMapper mapper = new ObjectMapper(); 
     mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); 
     mapper.configure(DeserializationConfig.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true); 

     JavaType type = mapper.getTypeFactory().constructCollectionType(List.class, Students.class); 
     List<Students> stuList = (ArrayList<Students>)mapper.readValue(JsonAsString, type); 

     System.out.println("student is:"+stuList); 

    } 
    catch(Exception e){ 
     e.printStackTrace(); 
    } 
    }//end of main 
}//end of class 

JsonReadJava.java:34: error: cannot find symbol 
    mapper.configure(DeserializationConfig.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY, tru 
    e); 
            ^
     symbol: variable Feature 
    location: class DeserializationConfig 
    Note: JsonReadJava.java uses unchecked or unsafe operations. 
    Note: Recompile with -Xlint:unchecked for details. 
    1 error 

    com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instanc 
    e of java.util.ArrayList out of START_OBJECT token 
    at [Source: {"studentsjson":[ 
    { "name":"Henry Lee", "course":"reading", "year":"2000", "grade":"3"}, 
    { "name":"John Flex", "course":"writing", "year":"2001", "grade":"4"} 
    ] 
    } 

    ; line: 1, column: 1] 
     at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingE 
    xception.java:164) 
     at com.fasterxml.jackson.databind.DeserializationContext.mappingExceptio 
    n(DeserializationContext.java:749) 
     at com.fasterxml.jackson.databind.DeserializationContext.mappingExceptio 
    n(DeserializationContext.java:745) 
     at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.handl 
    eNonArray(CollectionDeserializer.java:273) 
     at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deser 
    ialize(CollectionDeserializer.java:216) 
     at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deser 
    ialize(CollectionDeserializer.java:206) 
     at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deser 
    ialize(CollectionDeserializer.java:25) 
     at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMa 
    pper.java:3051) 
     at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.ja 
    va:2160) 
     at JsonReadJava.main(JsonReadJava.java:36) 

     C:\Users\Radhika\Desktop\JavaJson>javac *.java 
    JsonReadJava.java:34: error: cannot find symbol 
    mapper.configure(DeserializationConfig.Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY, tru 
    e); 
            ^
     symbol: variable Feature 
     location: class DeserializationConfig 
    Note: JsonReadJava.java uses unchecked or unsafe operations. 
    Note: Recompile with -Xlint:unchecked for details. 
    1 error 

Answer 1

以下是恕我直言反序列化的一个更简单的方法到Students：

public class Students { 

    @JsonProperty(value="studentsjson") 
    private List<StudentInfo> studentInfo; 
    ..... 
} 

ObjectMapper mapper = new ObjectMapper(); 
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); 

Students students = mapper.readValue(JsonAsString, Students.class); 
System.out.println("student is:"+students);