ObjectMapper 的一些坑

相信做过Java 开发对这个类应该不陌生,没错,这个类是jackson提供的，主要是用来把对象转换成为一个json字符串返回到前端,

现在大部分数据交换都是以json来传输的,所以这个很重要,那你到底又对这个类有着有多少了解呢，下面我说一下我遇到的一些坑

首先，先把我要说的几个坑需要设置的属性贴出来先

ObjectMapper objectMapper = new ObjectMapper();
		
		//序列化的时候序列对象的所有属性
		objectMapper.setSerializationInclusion(Include.ALWAYS);
		
		//反序列化的时候如果多了其他属性,不抛出异常
		objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
		
		//如果是空对象的时候,不抛异常
		objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
		
		//取消时间的转化格式,默认是时间戳,可以取消,同时需要设置要表现的时间格式
		objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
		objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"))

简单说一下这个类的基本用法，以下采用代码块加截图的形式来说明和部分文字件数

package com.shiro.test;

import java.text.SimpleDateFormat;
import java.util.Date;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

public class Main2 {
	public static void main(String[] args) throws Exception{
		ObjectMapper objectMapper = new ObjectMapper();
		//序列化的时候序列对象的所有属性
		objectMapper.setSerializationInclusion(Include.ALWAYS);
		//取消时间的转化格式,默认是时间戳,可以取消,同时需要设置要表现的时间格式
		objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
		objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
		
		Person person = new Person(1, "zxc", new Date());
		//这是最简单的一个例子,把一个对象转换为json字符串
		String personJson = objectMapper.writeValueAsString(person);
		System.out.println(personJson);
		
		//默认为true,会显示时间戳
		objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, true);
		personJson = objectMapper.writeValueAsString(person);
		System.out.println(personJson);
	}
}

输出的信息如下

objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false)的作用

package com.shiro.test;

import java.text.SimpleDateFormat;
import java.util.Date;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;

public class Main2 {
	public static void main(String[] args) throws Exception{
		ObjectMapper objectMapper = new ObjectMapper();
		//序列化的时候序列对象的所有属性
		objectMapper.setSerializationInclusion(Include.ALWAYS);
		//如果是空对象的时候,不抛异常,也就是对应的属性没有get方法
		objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
		
		Person person = new Person(1, "zxc", new Date());

		String personJson = objectMapper.writeValueAsString(person);
		System.out.println(personJson);
		
		//默认是true,即会抛异常
		objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, true);
		personJson = objectMapper.writeValueAsString(person);
		System.out.println(personJson);
		
		
	}
}

对应的person类此时为

package com.shiro.test;

import java.util.Date;

public class Person {

	private Integer id;
	private String name;
	private Date birthDate;
//	public Integer getId() {
//		return id;
//	}
//	public void setId(Integer id) {
//		this.id = id;
//	}
//	public String getName() {
//		return name;
//	}
//	public void setName(String name) {
//		this.name = name;
//	}
//	public Date getBirthDate() {
//		return birthDate;
//	}
//	public void setBirthDate(Date birthDate) {
//		this.birthDate = birthDate;
//	}
	@Override
	public String toString() {
		return "Person [id=" + id + ", name=" + name + ", birthDate=" + birthDate + "]";
	}
	public Person(Integer id, String name, Date birthDate) {
		super();
		this.id = id;
		this.name = name;
		this.birthDate = birthDate;
	}
	
	public Person() {
		// TODO Auto-generated constructor stub
	}
}

结果如下

package com.shiro.test;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Main2 {
	public static void main(String[] args) throws Exception{
		ObjectMapper objectMapper = new ObjectMapper();
		//序列化的时候序列对象的所有属性
		objectMapper.setSerializationInclusion(Include.ALWAYS);
		//反序列化的时候如果多了其他属性,不抛出异常
		objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
		
//		Person person = new Person(1, "zxc", new Date());

//		String personJson = objectMapper.writeValueAsString(person);
//		System.out.println(personJson);
		
		//注意,age属性是不存在在person对象中的
		String personStr = "{\"id\":1,\"name\":\"zxc\",\"age\":\"zxc\"}";
		
		Person person = objectMapper.readValue(personStr, Person.class);
		System.out.println(person);
		
		//默认为true
		objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, true);
		person = objectMapper.readValue(personStr, Person.class);
		System.out.println(person);
		
	}
}

执行后的结果如下

这些便是这几个属性的作用所以，由于第一个比较简单我就这样说一下吧

Include.ALWAYS  是序列化对像所有属性

Include.NON_NULL 只有不为null的字段才被序列化

Include.NON_EMPTY 如果为null或者 空字符串和空集合都不会被序列化

然后再说一下如何把一个对象集合转换为一个 Java里面的数组

package com.shiro.test;

import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Main2 {
	public static void main(String[] args) throws Exception{
		ObjectMapper objectMapper = new ObjectMapper();
		//序列化的时候序列对象的所有属性
		objectMapper.setSerializationInclusion(Include.NON_DEFAULT);
		
		Person person1 = new Person(1, "zxc", new Date());
		Person person2 = new Person(2, "ldh", new Date());
		
		List persons = new ArrayList<>();
		persons.add(person1);
		persons.add(person2);
		
		//先转换为json字符串
		String personStr = objectMapper.writeValueAsString(persons);
		
		//反序列化为List 集合,1需要通过 TypeReference 来具体传递值
		List persons2 = objectMapper.readValue(personStr, new TypeReference>() {});
		
		for(Person person : persons2) {
			System.out.println(person);
		}
		
		//2,通过 JavaType 来进行处理返回
		JavaType javaType = objectMapper.getTypeFactory().constructParametricType(List.class, Person.class);
		List persons3 = objectMapper.readValue(personStr, javaType);
		
		for(Person person : persons3) {
			System.out.println(person);
		}
	}
}

以上，是个人在项目遇到的部分坑,希望对大家有所帮助