com.fasterxml.jackson---03_如何使用ObjectMapper序列化以及反序列化一个对象
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2024-02-04 13:44:58
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本文的例子以jackson-databind-2.9.4为例。
创建Person类如下:
public class Person {
private String name;
private String address;
private String mobile;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public String getMobile() {
return mobile;
}
public void setMobile(String mobile) {
this.mobile = mobile;
}
}
序列化一个Person对象:
public static void main(String[] args) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
Person person = new Person();
person.setName("davenkin");
person.setAddress("");
System.out.println(objectMapper.writeValueAsString(person));
}
返回结果:
{"name":"davenkin","address":"","mobile":null}
在默认情况下,ObjectMapper在序列化时,将所有的字段一一序列化,无论这些字段是否有值,或者为null。
另外,序列化依赖于getter方法,如果某个字段没有getter方法,那么该字段是不会被序列化的。由此可见,在序列化时,OjbectMapper是通过反射机制找到了对应的getter,然后将getter方法对应的字段序列化到Json中。请注意,此时ObjectMapper并不真正地检查getter对应的属性是否存在于Person对象上,而是通过getter的命名规约进行调用,比如对于getAbc()方法:
public String getAbc(){
return "this is abc";
}
即便Person上没有abc属性,abc也会被序列化:
{"name":"davenkin","address":"","mobile":null,"abc":"this is abc"}
反序列化一个Json字符串,其中少了一个mobile字段:
public static void main(String[] args) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
Person person = objectMapper.readValue("{\"name\":\"davenkin\",\"address\":\"\"}", Person.class);
System.out.println("name: " + person.getName());
System.out.println("address: " + person.getAddress());
System.out.println("mobile: " + person.getMobile());
}
输出:
name: davenkin
address:
mobile: null
可以看出,少了mobile字段,程序依然正常工作,只是mobile的值为null。
另外,如果我们向Json中增加一个Person中没有的字段:
public static void main(String[] args) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
Person person = objectMapper.readValue("{\"name\":\"davenkin\",\"address\":\"\",\"mobile\":null,\"extra\":\"extra-value\"}", Person.class);
System.out.println("name: " + person.getName());
System.out.println("address: " + person.getAddress());
System.out.println("mobile: " + person.getMobile());
}
此时运行程序将报错:
Exception in thread "main" com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException:
Unrecognized field "extra" (class com.shell.b2b.factory.model.Person), not marked as ignorable (3 known properties: "mobile", "name", "address"])
表示在Person对象上找不到对应的extra属性,但是如果我们在Person上增加一个空的setter:
public void setExtra(String extra) {
}
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