标签：ring dHash cti ast sse com mars gate commit
注意的地方就是实体类一定要有无参的构造方法，否则会报异常
//com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.jackjson.UnmarshallCollectionOrArray$User` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator)
package com.example.jackjson; import com.fasterxml.jackson.core.type.TypeReference; import com.fasterxml.jackson.databind.ObjectMapper; import com.fasterxml.jackson.databind.type.CollectionType; import org.assertj.core.util.Lists; import org.junit.Assert; import org.junit.Test; import java.io.IOException; import java.util.ArrayList; import java.util.LinkedHashMap; import java.util.List; /** * @author: GuanBin * @date: Created in 下午2:33 2019/8/31 */ public class UnmarshallCollectionOrArray { @Test public void unmarshallToArray() throws IOException { ObjectMapper mapper = new ObjectMapper(); ArrayList<User> users = Lists.newArrayList(new User("tom", 10), new User("sam", 11)); String str = mapper.writeValueAsString(users); System.out.println("user json:" + str); //若user没无参构造方法会报错 //com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.jackjson.UnmarshallCollectionOrArray$User` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator) // at [Source: (String)"[{"name":"tom","age":10},{"name":"sam","age":11}]"; line: 1, column: 3] (through reference chain: java.lang.Object[][0]) User[] userArray = mapper.readValue(str, User[].class); Assert.assertTrue(userArray[0] instanceof User); } static class User { public User() { } public User(String name, int age) { this.name = name; this.age = age; } private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } } }
1）如果直接使用mapper.readValue(str, List.class); 虽然不会异常，但是list中的每个元素都是LinkedHashMap，而强转为User会报错；故如果转化为List<User> 用此方法是不行的
@Test public void unmarshallToList() throws IOException { ObjectMapper mapper = new ObjectMapper(); ArrayList<User> users = Lists.newArrayList(new User("tom", 10), new User("sam", 11)); String str = mapper.writeValueAsString(users); System.out.println("user json:" + str); //若user没无参构造方法会报错 //com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.jackjson.UnmarshallCollectionOrArray$User` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator) // at [Source: (String)"[{"name":"tom","age":10},{"name":"sam","age":11}]"; line: 1, column: 3] (through reference chain: java.lang.Object[][0]) List list = mapper.readValue(str, List.class); Assert.assertTrue(list.get(0) instanceof LinkedHashMap); }
2） 正确转化为list的有两种方式
1. 使用TypeReference
List<User> list = mapper.readValue(str, new TypeReference<List<User>>() { });
@Test public void unmarshallToListOneWay() throws IOException { ObjectMapper mapper = new ObjectMapper(); ArrayList<User> users = Lists.newArrayList(new User("tom", 10), new User("sam", 11)); String str = mapper.writeValueAsString(users); System.out.println("user json:" + str); //若user没无参构造方法会报错 //com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.jackjson.UnmarshallCollectionOrArray$User` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator) // at [Source: (String)"[{"name":"tom","age":10},{"name":"sam","age":11}]"; line: 1, column: 3] (through reference chain: java.lang.Object[][0]) List<User> list = mapper.readValue( str, new TypeReference<List<User>>() { }); Assert.assertTrue(list.get(0) instanceof User); }
2. 获取CollectionType
CollectionType javaType = mapper.getTypeFactory().constructCollectionType(List.class, User.class);
List<User> list = mapper.readValue(str, javaType);
@Test public void unmarshallToListTwoWay() throws IOException { ObjectMapper mapper = new ObjectMapper(); ArrayList<User> users = Lists.newArrayList(new User("tom", 10), new User("sam", 11)); String str = mapper.writeValueAsString(users); System.out.println("user json:" + str); //若user没无参构造方法会报错 //com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.example.jackjson.UnmarshallCollectionOrArray$User` (no Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator) // at [Source: (String)"[{"name":"tom","age":10},{"name":"sam","age":11}]"; line: 1, column: 3] (through reference chain: java.lang.Object[][0]) CollectionType javaType = mapper.getTypeFactory().constructCollectionType(List.class, User.class); List<User> list = mapper.readValue(str, javaType); Assert.assertTrue(list.get(0) instanceof User); }
注意：以上转化过程都要求实体有无参的构造方法，否则会报异常
demo test：https://github.com/BinbinGuan/rabbitMq-test/commit/2c27f547ff4fdb249cd0d64294e3855d00993a96
标签：ring dHash cti ast sse com mars gate commit
原文地址：https://www.cnblogs.com/guanbin-529/p/11441805.html
