关于fasterxml-jackson发生Can not deserialize instance of异常原因验证

关于fasterxml-jackson发生Can not deserialize instance of异常原因验证

这两天线上有大量的java.lang.IllegalArgumentException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

 at [Source: N/A; line: -1, column: -1]错误发生。

有经验的人一看，就知道是对象属性转换发生异常了。为了把这个错误的根本原因找到。只能上代码模拟了。

/**
 * Created by changle on 17/1/9.
 */
@Slf4j
public class JSONTest {
    public static void main(String[] args) {
        testAtoB();
        //testAtoB() 发生:Can not deserialize instance of com.test.JSONTest$Hobby out of START_ARRAY token

        testBtoA();
        //testBtoA() 发生:Can not deserialize instance of java.util.ArrayList out of START_OBJECT token
    }

    public static void testAtoB(){
        List hobbies = new ArrayList<>();
        Random random = new Random();
        for(int i=0;i<3;i++){
            Hobby hobby = new Hobby(random.nextInt(),"测试名称","测试类型",random.nextInt(100));
            hobbies.add(hobby);
        }
        StudentA studentA = new StudentA();
        studentA.setAge(23);
        studentA.setFromCity(true);
        studentA.setMoney(3000);
        studentA.setName("张三");
        studentA.setHobbies(hobbies);
        try {
            String str = JSON.json(studentA);
            log.info("str={}",str);
            //list转换单个projo
            StudentB studentB = JsonUtil.jsonObject(str, StudentB.class);
            log.info("studentB.name:{}",studentB.getName());
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    public static void testBtoA(){
        Random random = new Random();
        Hobby hobby = new Hobby(random.nextInt(), "测试名称", "测试类型", random.nextInt(100));
        StudentB studentB2 = new StudentB();
        studentB2.setAge(23);
        studentB2.setFromCity(true);
        studentB2.setMoney(3000);
        studentB2.setName("张三");
        studentB2.setHobbies(hobby);
        String str2 = null;
        try {
            str2 = JSON.json(studentB2);
            //单个projo转换list
            StudentA studentA2 = JsonUtil.jsonObject(str2, StudentA.class);
            log.info("studentB.name:{}", studentA2 == null ? "" : studentA2.getName());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Data
    public static class StudentA{
        private String name;
        private int age;
        private long money;
        private boolean isFromCity;
        private List hobbies;
    }

    @Data
    public static class StudentB{
        private String name;
        private int age;
        private long money;
        private boolean isFromCity;
        private Hobby hobbies;
    }

    @Data
    public static class Hobby{
        private long hId;
        private String hName;
        private String type;
        private int score;

        public Hobby(){}

        public Hobby(long hId,String hName,String type,int score){
            this.hId = hId;
            this.hName = hName;
            this.type = type;
            this.score = score;
        }
    }
}

结论：

Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

该错误是因为目标类属性keyX需要ArrayList类型的，待转换的json串里属性名keyX对应的，不是一个ArrayList集合，而是单个 POJO。

Can not deserialize instance of com.test.JSONTest$Hobby out of START_ARRAY token

该错误是因为目标类属性keyX需要JSONTest$Hobby类型的，待转换的json串里属性名keyX对应的，不是一个POJO对象，而是ArrayList集合。