Feign series (01) using the simplest gesture

Feign series (01) using the simplest gesture

Spring Cloud Series catalog ( https://www.cnblogs.com/binarylei/p/11563952.html#feign )

For more use cases see Feign Github official website

1. introduced maven dependent

<dependencies>
    <dependency>
        <groupId>io.github.openfeign</groupId>
        <artifactId>feign-core</artifactId>
        <version>10.4.0</version>
    </dependency>
    <dependency>
        <groupId>io.github.openfeign</groupId>
        <artifactId>feign-gson</artifactId>
        <version>10.4.0</version>
    </dependency>
</dependencies>

2. Basic Usage

interface GitHub {
  @RequestLine("GET /repos/{owner}/{repo}/contributors")
  List<Contributor> contributors(@Param("owner") String owner, @Param("repo") String repo);

  @RequestLine("POST /repos/{owner}/{repo}/issues")
  void createIssue(Issue issue, @Param("owner") String owner, @Param("repo") String repo);

}

public static class Contributor {
  String login;
  int contributions;
}

public static class Issue {
  String title;
  String body;
  List<String> assignees;
  int milestone;
  List<String> labels;
}

public class MyApp {
  public static void main(String... args) {
    GitHub github = Feign.builder()
                         .decoder(new GsonDecoder())
                         .target(GitHub.class, "https://api.github.com");
  
    // Fetch and print a list of the contributors to this library.
    List<Contributor> contributors = github.contributors("OpenFeign", "feign");
    for (Contributor contributor : contributors) {
      System.out.println(contributor.login + " (" + contributor.contributions + ")");
    }
  }
}

Summary: Feign.target() actually creating a dynamic agent of GitHub.

3. Feign declarative annotations

Feign via Contractinterface marked on the annotated method resolves MethodMetadata, eventually resolve to the parameter request line Http request, the request line, the request body, and then send the request using HttpClient.

Annotation	Interface Target	Usage
@RequestLine	Method	HttpMethod definitions and UriTemplate. UriTemplate used {}wrapped expression, may be used by automatic injector @Param method parameters on
@Param	Parameter	Defined template variables, the value of the variable template way can be used to resolve names using a template injection
@Headers	Method, Type	Definition header template variables, use @Param annotation provides injection parameter values. If the annotation is added to the interface classes, all requests will Header carries information corresponding to; if on the method, it will only add to the request on a corresponding method
@QueryMap	Parameter	Map or define a POJO, the parameter value will be converted to the query string in the URL
@HeaderMap	Parameter	Map ->Http Headers
@Body	Method	Defines a Template, similar to a UriTemplate and HeaderTemplate, that uses @Param annotated values to resolve the corresponding Expressions.

The basic method of using annotations as follows:

public interface FeignService {
  // @Headers
  @RequestLine("GET /api/documents/{contentType}")
  @Headers("Accept: {contentType}")
  String getDocumentByType(@Param("contentType") String type);
  
  // @QueryMap: Map or POJO
  @RequestLine("GET /find")
  V find(@QueryMap Map<String, Object> queryMap);
  @RequestLine("GET /find")
  V find(@QueryMap CustomPojo customPojo);
  
  // @HeaderMap: Map
  @RequestLine("POST /")
  void post(@HeaderMap Map<String, Object> headerMap);
    
  // @Body
  @RequestLine("POST /")
  @Headers("Content-Type: application/xml")
  @Body("<login \"user_name\"=\"{user_name}\" \"password\"=\"{password}\"/>")
  void xml(@Param("user_name") String user, @Param("password") String password);

  @RequestLine("POST /")
  @Headers("Content-Type: application/json")
  @Body("%7B\"user_name\": \"{user_name}\", \"password\": \"{password}\"%7D")
  void json(@Param("user_name") String user, @Param("password") String password);
}

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