I want to know how #RequestBody map works for json to object? Which mapper it use?
When Spring goes to generate an argument for a #RequestBody parameter, it loops through the HttpMessageConverter instances, checks if that instance HttpMessageConverter#canRead the content type given in the request and can generate an instance of the parameter type. If it can, Spring will use that HttpMessageConverter to produce an argument. If it can't, Spring will skip it and try the next instance, until it runs out. At which point, it will throw an exception.
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The Default Message Converters
By default, the following HttpMessageConverters instances are pre-enabled:
ByteArrayHttpMessageConverter – converts byte arrays
StringHttpMessageConverter – converts Strings
ResourceHttpMessageConverter – converts org.springframework.core.io.Resource for any type of octet stream
SourceHttpMessageConverter – converts javax.xml.transform.Source
FormHttpMessageConverter – converts form data to/from a
MultiValueMap.
Jaxb2RootElementHttpMessageConverter – converts Java objects to/from XML (added only if JAXB2 is present on the classpath)
MappingJackson2HttpMessageConverter – converts JSON (added only if Jackson 2 is present on the classpath)
MappingJacksonHttpMessageConverter – converts JSON (added only if Jackson is present on the classpath)
AtomFeedHttpMessageConverter – converts Atom feeds (added only if Rome is present on the classpath)
RssChannelHttpMessageConverter – converts RSS feeds (added only if Rome is present on the classpath)
Related
I'm having an issue with downloading a pdf into a viewer.
The errors states that there is no converter
org.springframework.http.converter.HttpMessageNotWritableException: No converter for [class com.....controller.rest.DocumentController$$Lambda$894/964587173] with preset Content-Type 'application/pdf'
I'm using Spring boot and I assumed(and read that) these converters were enabled by default
ByteArrayHttpMessageConverter – converts byte arrays
StringHttpMessageConverter – converts Strings
ResourceHttpMessageConverter – converts org.springframework.core.io.Resource for any type of octet stream
SourceHttpMessageConverter – converts javax.xml.transform.Source
FormHttpMessageConverter – converts form data to/from a MultiValueMap<String, String>.
Jaxb2RootElementHttpMessageConverter – converts Java objects to/from XML (added only if JAXB2 is present on the classpath)
MappingJackson2HttpMessageConverter – converts JSON (added only if Jackson 2 is present on the classpath)
MappingJacksonHttpMessageConverter – converts JSON (added only if Jackson is present on the classpath)
AtomFeedHttpMessageConverter – converts Atom feeds (added only if Rome is present on the classpath)
RssChannelHttpMessageConverter – converts RSS feeds (added only if Rome is present on the classpath)
When i check the actuator I only see the String and Jackson Message Converters..
I do have a WebMvcConfigurer that only overrides addViewControllers
Does anyone have any idea why the preset converters are not present?
I am new in Spring MVC. My question is, why do we need jackson databind? Because We can receive the Request Params by #ModelAttribute and requests through http PUT or POST by #RequestBody. I can't find a reason why we need jackson databind to convert json/xml to POJO or vice versa.
Thanks.
Why do we need jackson databind?
Because representing structured data is much easier using XML (or JSON) than using simple name-value pairs.
Because it is more convenient to send and receive JSON from the client side when you are doing AJAX.
Because once you have to deal with sending and receiving JSON or XML in the server side Java app, it is more convenient to deal with structured data as POJOs.
None of the above points mean you have to use a binding. There are other ways of dealing with each of the above. But many Java developers think that data bindings the better way to go: more efficient in terms of developer time, and more reliable. Especially if you are implementing services with a complex APIs. That's why they are popular.
And as other answers/comments point out, if you are using #RequestBody, then that is using a binding library under the hood to give you the POJOs. In the case of Spring, it is Jackson that is being used.
By default, when an endpoint expects a JSON document as input and a given controller method argument is annotated with #RequestBody, Spring will use Jackson databind features to map the incoming JSON document to a Java object. You don't need to use the Jackson's ObjectMapper directly, as Spring does it for you.
For example purposes, consider the following HTTP request to create a comment:
POST /comments HTTP/1.1
Host: example.org
Content-Type: application/json
{
"content": "Lorem ipsum"
}
And the following class which represents a comment:
#Data
public class Comment {
private String content;
}
A #RestController to handle such request would be like:
#RestController
#RequestMapping("/comments")
public class CommentController {
#PostMapping(consumes = MediaType.APPLICATION_JSON_VALUE)
public ResponseEntity<Foo> createComment(#RequestBody Comment comment) {
// By default, Spring will rely on Jackson databind to map the incoming
// JSON document to the comment argument annotated with #RequestBody
...
}
}
If you are interested in the Spring component that maps the incoming JSON document to a Java object, have a look at the MappingJackson2HttpMessageConverter class:
Implementation of HttpMessageConverter that can read and write JSON using Jackson 2.x's ObjectMapper.
This converter can be used to bind to typed beans, or untyped HashMap instances.
By default, this converter supports application/json and application/*+json with UTF-8 character set. [...]
If you are creating a HTTP API and exposing resources that can be manipulated with JSON representations, it's unlikely you'll use #ModelAtribute. Such annotation is particularly useful when you are dealing with web views.
When you get some request in some data types like json/xml, the Spring MVC platform will try to deserialize this request attributes in some model object of your project.
But the platform itself don't provide a des-serialize implementation out of the box. So it will try to use some des-serializer provider in the classpath like jackson, jersey, gson, etc.
As you said - is possible to use #ModelAttribute - but this annotation is a better option to a request from a form view in the front-end. In cases rest json/xml requests, the #ModelAttribute won't be able to convert correctly the received data to a business class of your program.
What is the difference between object mapper and Jackson tester. According to my knowledge object mapper maps POJO to JSON and vice versa whereas Jackson tester does the same during assertj while writing unit tests of rest controllers.
But I want to know their difference in more detail
Is there an easy way to convert a JSON payload to a Java object using a custom ObjectMapper (Jackson) or do I have to provide a custom type converter. I know that I could use a processor, but somehow it would be nice to use input and output types of the stream definition.
In the second case: Am I even able to provide a custom type converter for application/json to Java?
The documentation states: "The customMessageConverters are added after the standard converters in the order defined. So it is generally easier to add converters for new media types than to replace existing converters."
I bet that there is an existing "application/json" converter - but at a first glance I could not find further information if it is even possible to replace existing converters.
Thanks!
Peter
If you look at streams.xml You can see the relevant configuration. The configured lists are used to construct a CompositeMessageConverter which visits every MessageConverter in list order until it finds one that can do the conversion and returns a non-null result. A CompositeConverter instance is created for each module instance that is configured for conversion (i.e., defines an inputType or outputType value) by filtering the list of candidate message converters, which all inherit AbstractFromMessageConverter. The list is paired down to those which respond true to public boolean supportsTargetMimeType(MimeType mimeType) (where mimeType is the value of the input/outputType). The CompositeMessageConverter is injected into the corresponding MessageChannel and converts the payload.
There are a couple of things you can do. You can override the xd.messageConverters bean definition. For example, you can replace JsonToPojoMessageConverter and PojoToJsonMessageConverter with your own subclasses. You can also insert your own implementations in the list before the above converters and have your implementation match only specific domain objects for which you need a custom JSON mapper.
Another possibility is to define your own mime type and provide converters for that mime type as customMessageConverters. In any case, follow these guidelines forextending Spring XD
I use jersey and have a method in my Resource class which has multiple parameters. These parameters are filled using #FormParam but the problem is, the type of the parameters are custom java types, not some primitives or String. I want to convert the value of parameters from json to custom java types. If I use #Cosume(MediaType.APPLICATION_JSON), then I cannot use multiple parameters and if I remove it, parameters cannot be converted from json to their java instances.
#POST #Path("/add")
#Consumes(MediaType.APPLICATION_FORM_URLENCODED)
#Produces(MediaType.APPLICATION_JSON)
public String add(#FormParam("source") BookEntity source, #FormParam("author") AuthorEntity a) throws JsonGenerationException, JsonMappingException, IOException, TransformationException
{
...
}
If I change the parameter types to String and then use Jackson deserialization, I can deserialize json parameters to java instances but I want to do it for other methods too and get it done automatically.
I tried to use the approach used in Custom Java type for consuming request parameters but I cannot make it work.
You can use a custom type mapper.
See this answer
Anyway by default Jersey tries to map received json object representation using JAXB. Obiously you must annotate your objects.