[Deep thinking] Custom date format, why @JSONField takes effect, and @JsonFormat does not take effect?

1. Preface

Recently, when I was testing the interface, I found a problem: the field type defines Date, but the return value of the interface is a timestamp (1744959978674).

Instead of the expected 2025-04-18 15:06:18.

private Date useTime;

{
    "code": "200",
    "message": "",
    "result": [
        {
            "id": 93817601,
            "useTime": 1744959978674
        }
    ]
}

This kind of return value cannot be quickly known which time it is. If you want to know whether the time is right, you have to find a timestamp conversion tool to down-convert it, which is very inconvenient.

Therefore, I want the interface to directly return to the expected 2025-04-18 15:06:18 format.

At the beginning, I added it to the field@JsonFormatThe annotation is found to be ineffective, and the time stamp is returned:

import ;

@JsonFormat(pattern="yyyy-MM-dd HH:mm:ss")
private Date useTime;

Then, it was changed to@JSONFieldThe annotation was found to take effect and achieved the expected results:

import ;

@JSONField(format = "yyyy-MM-dd HH:mm:ss")
private Date useTime;

{
    "code": "200",
    "message": "",
    "result": [
        {
            "id": 93817601,
            "useTime": "2025-04-18 15:06:18"
        }
    ]
}

Then the question is,Why does @JSONField take effect, and @JsonFormat not take effect?

2. Cause analysis

By default, the JSON message converter used by Spring Boot is Jackson'sMappingJackson2HttpMessageConverter, the core dependencies are:

<dependency>
    <groupId></groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.8.11.3</version>
</dependency>

Now use Jackson's@JsonFormatThe annotation does not take effect, which means Spring Boot does not use the default one.MappingJackson2HttpMessageConverter。

Using fastjson@JSONFieldThe annotation takes effect, indicating that Spring Boot uses the JSON message converter under fastjson, that is

FastJsonHttpMessageConverter, dependencies are:

<dependency>
    <groupId></groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.83</version>
</dependency>

So how do you find where the code is configured?

The first step is to search globally in the projectFastJsonHttpMessageConverter(Windows shortcut key: Ctrl+Shift+F), but it is likely that you can't search because

Projects in the company are generally inherited from the company's public xxx-spring-boot-starter.

Step 2: Press Shift key twice to searchFastJsonHttpMessageConverter, and then look for a reference or subclass of the class (the subclass is most likely written in the company's underlying framework).

Then, it is very likely that you will find a code like the following:

@Configuration
public class FastJsonMessageConverterConfig {
    @Bean
    public HttpMessageConverters customConverters() {
        FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
        return new HttpMessageConverters(new HttpMessageConverter[]{fastJsonHttpMessageConverter});
    }
}

The above code explicitly registers oneFastJsonHttpMessageConverter, and passHttpMessageConvertersOverrides the default HTTP message converter

(Jackson'sMappingJackson2HttpMessageConverter), so Spring MVC will only handle JSON serialization/deserialization using fastjson.

This is also@JSONFieldTake effect,@JsonFormatThe root cause of ineffectiveness.

3. Default behavior and global configuration

fastjson 1.2.36 and above, the date is serialized as a timestamp by default (such as 1744959978674). If you want to serialize the date to yyyy-MM-dd HH:mm:ss

(For example, 2025-04-18 15:06:18), it needs to be enabledWriteDateUseDateFormatcharacteristic:

@Configuration
 public class FastJsonMessageConverterConfig {
     @Bean
     public HttpMessageConverters customConverters() {
         FastJsonConfig fastJsonConfig = new FastJsonConfig();
         // Enable date formatting feature (disable timestamps)
   		 ();
         // Set the date format (when not specified, the default is yyyy-MM-dd HH:mm:ss, but it is recommended to specify it clearly even if it is consistent with the default value)
	     ("yyyy-MM-dd HH:mm:ss");
        
         FastJsonHttpMessageConverter fastJsonHttpMessageConverter = new FastJsonHttpMessageConverter();
         (fastJsonConfig);
         return new HttpMessageConverters(new HttpMessageConverter[]{fastJsonHttpMessageConverter});
     }
 }

If a date field has special serialization requirements, you can use@JSONFieldFlexible configuration of annotations (this annotation will override global configuration):

import ;

@JSONField(format = "yyyy-MM-dd")
private Date anotherUseTime;

Notes:

You need to be careful when modifying the global configuration. If an old project returns a time stamp when the original date type returns, and suddenly all of them are changed to return a string, it may cause the caller to report an error.

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