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[From the technology and the way] Unified encapsulation API returns the result and the String returns the error file solution principle - hierarchical diagram and rule description of Spring message converter [Development problem log of artificial intellig

Popularity：900 ℃/2025-03-12 21:25:00
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           ▀■■■▀ AI2077 log fragment
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 [ERROR] | Quantum channel fluctuation abnormality!
         | StringConverter detected attempt to swallow ApiResult object
         | Start the two-way foil defense program...
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Dialogue record：
Product Manager: "I want the interface to return both JSON and plain text!" Artificial Intelligence Retardant 2077: "Are you going to observe the position and momentum of particles in three-dimensional space at the same time?" Product Manager: "Is it very difficult?" Artificial Intelligence Retardant 2077: "It's harder than having a cat in a state of life and death at the same time!"

The following is in SpringHttpMessageConverterThe hierarchy diagram of processing logic and the analysis of core process, and the modular structure is used to illustrate key nodes:

HTTP response processing flow diagram

┌───────────────────────────────────────────┐
 │ Controller method returns │
 │ (Return value type: Object/String/ApiResult, etc.) │
 └───────────────────┬───────────────────────┘
                     │
                     ▼
 ┌───────────────────────────────────────────┐
 │Travel over registered HttpMessageConverter │
 │ Call the canWrite() method in priority order to detect the match │
 └───────────────────┬───────────────────────┘
                     │
                     ▼
 ┌───────────────────────────────────────────┐
 │ Confirm the first one to support "return value type + response MediaType" │
 │Converter instance │
 └───────────────────┬───────────────────────┘
                     │
                     ▼
 ┌───────────────────────────────────────────┐
 │ Call the write() method to perform the actual serialization operation │
 │ (Generate byte streams of HTTP Response Body) │
 └───────────────────────────────────────────┘

Key Converter role and priority rules

1. Core Converter Type

Converter Type Processing data types Output format
MappingJackson2HttpMessageConverter POJO objects, collections, maps, etc. application/json
StringHttpMessageConverter String Type text/plain
ByteArrayHttpMessageConverter byte[] application/octet-stream
ResourceHttpMessageConverter Resource resource type Automatically judge based on resource type

2. Default priority order 

// Spring Boot default loading order (some key converters)
 [
   ByteArrayHttpMessageConverter,
   StringHttpMessageConverter, // The default priority is higher
   ResourceHttpMessageConverter,
   MappingJackson2HttpMessageConverter // Default is in the back position
 ]

3. Your order of adjustment code 

@Override
 public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
     // mention Jackson converter to 2nd position (only keep ByteArray in the first position)
     (mappingJackson2HttpMessageConverter);
     (1, mappingJackson2HttpMessageConverter);
 }

After adjustment, the order becomes:

[
   ByteArrayHttpMessageConverter, // Bit 0 (processing binary)
   MappingJackson2HttpMessageConverter, // Bit 1 (Preferred to object to JSON)
   StringHttpMessageConverter, // Bit 2 (Finishing the string)
   ...
 ]

Scenario Process Comparison

Scene 1: ReturnApiResultObject

@GetMapping("/data")
public ApiResult<User> getData() {
    return (());
}

Processing process:

Controller → canWrite(ApiResult) → JacksonConverter → JSON output

Scene 2: ReturnStringBut need to be sealed

@GetMapping("/message")
 public String getMessage() {
     return "Hello"; // Need to be packaged as ApiResult
 }

Processing process:

GlobalResponseWrapper packet is ApiResult → JacksonConverter → JSON output

Scene 3: Return to the originalString(No packet)

@IgnoreResultPackage
@GetMapping("/raw")
public String getRaw() {
    return "RawText"; 
}

Processing process:

String → StringHttpMessageConverter → text/plain output

Converter matching rule logic table

Condition combination Match results
The method return type is String Priority useStringHttpMessageConverter
Method returns POJO + request header Accept=json triggerMappingJackson2HttpMessageConverter
Method returns byte[] ByteArrayHttpMessageConverterPriority treatment

Why can adjust the order solve the double quote problem?

Original question:
 Controller returns String → Packet logic returns ApiResult<String>
 → StringHttpMessageConverter Try to serialize the ApiResult object
 → Trigger ClassCastException

 Adjusted logic:
 Controller returns String → Package as ApiResult
 → JacksonConverter has higher priority than StringConverter
 → Correctly serialized to JSON

This hierarchy and rule description allows you to clearly understand the collaboration mechanism of Spring Message Converter and the importance of prioritization.

The road is simple

In the dimensional war of message converters, we have touched the truth of software development—The eternal game between order and chaos. As the Tao Te Ching says: "The great way is wide and it can be left and right", an excellent packaging design should be like flowing water:

  1. Hardness and softness: Dialectical Unity of Forced Specification (ApiResult) and Free Export (@IgnoreResultPackage)
  2. Yin and Yang Generation: The priority game between StringConverter and JacksonConverter is just like the rise and fall of Tai Chi Liangyi
  3. The unity of man and nature: The developer's will is naturally revealed through the framework mechanism, reaching the realm of "not knowing that Converter is used for packets"

As quantum physicist Bohr said, "A great truth is a truth whose opposite is also a great truth." Our packaging scheme is the perfect embodiment of this philosophical view - finding the golden point between norms and flexibility.

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