Hi,
I am using the openapi-generator-maven-plugin with these versions: 3.3.1-SNAPSHOT (thats master ).
I came across this bug. Please see code snippet below of the pom.xml alongwith my suggestions for the fix.. Let me know if I should raise a PR.

I have also noted another smaller issue unrelated to this.
Thanks.
R
Please note value of asynch: false.

`

            <plugin>
                          <groupId>org.openapitools</groupId>
			<artifactId>openapi-generator-maven-plugin</artifactId>
			<version>${openapi.codegen.version}</version>
			<executions>
				<execution>
					<id>common</id>
					<goals>
						<goal>generate</goal>
					</goals>
					<configuration>
                                              <inputSpec>${project.basedir}/src/main/resources/api.yaml</inputSpec>
						<modelPackage>com.example.api.models</modelPackage>
						<apiPackage>com.example.api</apiPackage>
						<output>${project.basedir}</output>					
						<language>spring</language>
						<invokerPackage>com.example.api</invokerPackage>
						<basePackage>com.example.api</basePackage>
						<withXml>true</withXml>
						<configOptions>
						
							<artifactId>foo</artifactId>
							<artifactDescription>Test API</artifactDescription>
							<title>Test API</title>
							<artifactUrl>https://api.example.com</artifactUrl>
							<groupId>com.example.api</groupId>
							<artifactVersion>1</artifactVersion>								
							<sourceFolder>src/gen/java/main</sourceFolder>
							<serializableModel>true</serializableModel>
							<dateLibrary>java8</dateLibrary>
							<java8>true</java8>
							<async>false</async>
							<library>spring-boot</library>		
							<delegatePattern>true</delegatePattern>
							<useBeanValidation>true</useBeanValidation>
							<useOptional>true</useOptional>	
							<hideGenerationTimestamp>true</hideGenerationTimestamp>								
						</configOptions>							
					</configuration>
				</execution>
			</executions>
		</plugin>

`

For the above configuration in openapi-generator\modules\openapi-generator\src\main\resources\JavaSpring\methodBody.mustache an expression of
{{#async}}CompletableFuture.completedFuture({{/async}} should resolve to empty. Unfortunately : In openapi-generator\modules\openapi-generator\src\main\java\org\openapitools\codegen\DefaultGenerator.java the value of async in this.config.additionalProperties() is a string and not a boolean.

This causes wrong code generation.

I can think of a few solutions of which to my mind the most effective one is this:
In openapi-generator\modules\openapi-generator\src\main\java\org\openapitools\codegen\DefaultGenerator.java in method public Generator opts(ClientOptInput opts) right after this line
"this.config = opts.getConfig();"
invoke
'adjustToBoolean("async");'

//new method

`
public void adjustToBoolean(String propertyName)
{
Object object = this.config.additionalProperties().get(propertyName);
Boolean val=Boolean.FALSE;
if(object!=null)
{
if(object instanceof String)
{
String string=(String) object;
if(string.equalsIgnoreCase("true"))
{
val=true;
}
else
{
val=Boolean.FALSE;
}
}
else if(object instanceof Boolean)
{
val=(Boolean) object;
}
else
{
val=Boolean.TRUE;
}
}
else
{
val=Boolean.FALSE;
}
this.config.additionalProperties().put(propertyName, val);

} 

`
Note: In addition to the above problem and unrelated to this there is a generated code in the generated OpenAPI2SpringBoot.java

@bean
public WebMvcConfigurer webConfigurer() {
return new WebMvcConfigurer() {
/* @OverRide
public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/**")
.allowedOrigins("")
.allowedMethods("")
.allowedHeaders("Content-Type");
} */
};
}

This code is not fully implemented and seems mostly commented.
Maven based compilation causes compilation error here.
This can be fixed by changing in openapi-generator\modules\openapi-generator\src\main\resources\JavaSpring\libraries\spring-boot\openapi2SpringBoot.mustache
the following lines:

@bean
public Web{{^reactive}}Mvc{{/reactive}}{{#reactive}}Flux{{/reactive}}Configurer webConfigurer() {
return new Web{{^reactive}}Mvc{{/reactive}}
{{#reactive}}Flux{{/reactive}}Configurer{{^java8}}Adapter{{/java8}}() {
/* @OverRide
public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/**")
.allowedOrigins("")
.allowedMethods("")
.allowedHeaders("Content-Type");
} */
{{^useSpringfox}}

        @Override
        public void addResourceHandlers(ResourceHandlerRegistry registry) {
            registry.addResourceHandler("/swagger-ui/**").addResourceLocations("classpath:/META-INF/resources/webjars/swagger-ui/3.14.2/");
        }
      {{/useSpringfox}}
    };
}

TO

/* @bean
public Web{{^reactive}}Mvc{{/reactive}}{{#reactive}}Flux{{/reactive}}Configurer webConfigurer() {
return new Web{{^reactive}}Mvc{{/reactive}}{{#reactive}}Flux{{/reactive}}Configurer{{^java8}}Adapter{{/java8}}() {
@OverRide
public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/**")
.allowedOrigins("")
.allowedMethods("")
.allowedHeaders("Content-Type");
}
{{^useSpringfox}}

        @Override
        public void addResourceHandlers(ResourceHandlerRegistry registry) {
            registry.addResourceHandler("/swagger-ui/**").addResourceLocations("classpath:/META-INF/resources/webjars/swagger-ui/3.14.2/");
        }

{{/useSpringfox}}
};
}
*/

I can include this change also in the PR
Note: I have not yet learnt how to show the code blocks properly in this issue editor. I hope everything is clear in spite of that.