How to develop a simple REST Client using Swagger codegen?

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FYI a simple approach using command line:

There is a good tutorial at baeldung about it: how to create rest client with swagger codegen

E.g. Execute command:

java -jar swagger-codegen-cli.jar generate \
  -i http://mydomain/v2/swagger.json \
  --api-package com.mypackage.api \
  --model-package com.mypackage.model \
  --invoker-package com.mypackage.invoker \
  --group-id com.mygroup \
  --artifact-id spring-swagger-codegen-api-client \
  --artifact-version 0.0.1-SNAPSHOT \
  -l java \
  --library resttemplate \
  -o spring-swagger-codegen-api-client

Swagger Codegen supports the following Client implementations:

1. jersey1 + Jackson
2. Jersey2 + Jackson
3. Feign + Jackson
4. OkHttp + Gson
5. Retrofit2/OkHttp + Gson
6. Spring RestTemplate + Jackson
7. Resteasy + Jackson

P.S. As you can see, the rest client is generated from swagger spec definition and it is definided with “-i” argument.

Yes. You can use swagger-codegen-maven-plugin to generate a REST client. But before that , you need to describe the REST API in YAML or JSON in OpenAPI Specification mainly because swagger-codegen-maven-plugin only can generate a REST client from a file written in this specification.

Other answers assume that you need to write the specification manually while my solution take a step further to automatically generating the specification from the REST controller source codes.

^{The latest OpenAPI version is 3.0 .But based on the package of your imported swagger annotation , you are using version 2.0 (or before). So my solution assume you are using OpenAPI 2.0.}

Generating Open API Specification

First, you can use swagger-maven-plugin to generate an OpenAPI spec from the RestController source codes. It basically analyses the Swagger annotations annotated in the @RestController classes that specified in <locations> and dump the OpenAPI spec to /src/main/resources/swagger.json :

<plugin>
    <groupId>com.github.kongchen</groupId>
    <artifactId>swagger-maven-plugin</artifactId>
    <version>3.1.5</version>
    <configuration>
        <apiSources>
            <apiSource>
                <springmvc>true</springmvc>
                <locations>
                    <location>com.dgs.spring.springbootswagger.controller.EmployeeController</location>
                    <location>com.dgs.spring.springbootswagger.controller.FooController</location>
                </locations>
                <schemes>
                    <scheme>http</scheme>
                </schemes>
                <host>127.0.0.1:8080</host>
                <basePath>/</basePath>
                <info>
                    <title>My API</title>
                    <version>1.1.1</version>
                </info>
                <swaggerDirectory>${basedir}/src/main/resources/</swaggerDirectory>
            </apiSource>
        </apiSources>
    </configuration>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Execute the followings maven command to start generation:

mvn clean compile

Generating Rest Client

After swagger.json is generated, you can copy and paste it to your client project (e.g. /src/main/resources/swagger.json) . We can then use swagger-codegen-maven-plugin to generate a HTTP client .

By default , it will generate the whole maven project which includes test cases and other documentation stuff. But what I want is just the HttpClient 's source codes without other things. After several trial and error , I settle down to the following configuration :

<plugin>
    <groupId>io.swagger</groupId>
    <artifactId>swagger-codegen-maven-plugin</artifactId>
    <version>2.4.7</version>
    <executions>
        <execution>
            <goals>
                <goal>generate</goal>
            </goals>
            <configuration>
                <inputSpec>${basedir}/src/main/resources/swagger.json</inputSpec>
                <language>java</language>
                <library>resttemplate</library>
                <output>${project.basedir}/target/generated-sources/</output>

                <apiPackage>com.example.demo.restclient.api</apiPackage>
                <modelPackage>com.example.demo.restclient.model</modelPackage>
                <invokerPackage>com.example.demo.restclient</invokerPackage>

                <generateApiTests>false</generateApiTests>
                <generateModelTests>false</generateModelTests>
                <generateApiDocumentation>false</generateApiDocumentation>
                <generateModelDocumentation>false</generateModelDocumentation>
                <configOptions>
                    <dateLibrary>java8</dateLibrary>
                    <sourceFolder>restclient</sourceFolder>
                </configOptions>
            </configuration>
        </execution>
    </executions>
</plugin>

The generated HTTP client is based on RestTemplate and will be generated to the folder target/generated-sources/restclient. You may have to configure your IDE to import the generated client in order to use it . (In case of Eclipse, you can configure in Project Properties ➡️ Java Build Path ➡️ Add the folder of the generated rest client)

To start generating the client , just execute the maven command :

mvn clean compile

To use the generated HTTP client :

ApiClient apiClient = new ApiClient();

//Override the default API base path configured in Maven
apiClient.setBasePath("http://api.example.com/api");

EmployeeManagementSystemApi api = new EmployeeManagementSystemApi(apiClient);
api.getEmployeeById(1l);

---

Note :

• If you come across javax/xml/bind/annotation/XmlRootElement exception during generation when using java8+ , you may need to refer to this.