When you create an AsyncRestTemplate, Spring uses SimpleClientHttpRequestFactory by default which depends on default configuration of HttpURLConnection. This implementation doesn’t have connection pooling.

Apache HttpComponents provides the HttpComponentsAsyncClientHttpRequestFactory with connection pooling support and using NIO (Non-blocking Input/Output) asynchronous http client, called HttpAsyncClient.

The problem

In a recent project, we decided to change the the Spring’s default SimpleClientHttpRequestFactory to the Apache Components factory HttpComponentsAsyncClientHttpRequestFactory by the advantages of connection pooling. The solution seemed easy, you should only add the dependency:

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<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpasyncclient</artifactId>
</dependency>

and change the Spring Bean’s definition from this:

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@Bean
public AsyncRestTemplate asyncRestTemplate() {
    return new AsyncRestTemplate();
}

to this:

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@Bean
public AsyncRestTemplate asyncRestTemplate() {
    return new AsyncRestTemplate(new HttpComponentsAsyncClientHttpRequestFactory());
}

But when we did a request from Postman to our controller propagating the received headers to a new call using for example https://httpstat.us/200 (GET), we received this error:

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Async GET request for "https://httpstat.us/200" resulted in 404 (Site Not Found); invoking error handler

The same request with SimpleClientHttpRequestFactory returned “200 OK”.

The solution

After a time trying to resolve the error we found that Postman includes a header called host that provokes a 404 with the Apache Factory implementation.

host=localhost:8080

So, the solution was to delete this header from the request. You could be a example of this error in [1]. In this example we make four calls with and without headers and using the two factories. The log result:

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===== Execution without host header =====
Factory=[SimpleClientHttpRequestFactory] ResponseBody=["200 OK"]
Factory=[HttpComponentsAsyncClientHttpRequestFactory] ResponseBody=["200 OK"]

===== Execution with host header =====
Factory=[SimpleClientHttpRequestFactory] ResponseBody=["200 OK"]
Factory=[HttpComponentsAsyncClientHttpRequestFactory] Exception=[org.springframework.web.client.HttpClientErrorException: 404 Site Not Found]

References

[1] Link to the project in Github