Developing RESTful Clients in Java: A Comprehensive Overview

Introduction

REST (Representational State Transfer) has become a dominant architectural style for web services, and Java developers have multiple options for building clients to interact with these services. Unlike SOAP, which has standardized clients like Apache Axis, REST clients in Java vary widely in implementation. This article surveys the modern libraries available for creating REST clients in Java, drawing from community insights and official documentation.

Overview of REST Client Libraries

Based on the Stack Overflow answer from 2008 updated to 2020, here are the key REST client options in Java:

• Apache CXF: Offers three different REST client APIs.
• Jersey: A JAX-RS implementation with a fluent client API.
• Spring RestTemplate (deprecated) and Spring WebClient: Part of the Spring Framework, with RestTemplate being synchronous and WebClient reactive.
• Apache HTTP Components: A low-level HTTP client with a fluent adapter.
• OkHttp: A modern HTTP client supporting SPDY and HTTP/2.
• Feign: A declarative REST client that auto-generates implementations from interfaces.
• Retrofit: Similar to Feign, built on OkHttp.
• And others like Volley, Google HTTP Client, Unirest, Resteasy, jcabi-http, restlet, rest-assured.

It's crucial to consider your existing framework stack when choosing a client to avoid threading issues.

Code Examples

Let's look at some practical examples. First, using the standard Java SE API with HttpURLConnection:

// Example code for updating a customer via PUT request
try {
    URL url = new URL("http://www.example.com/customers");
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setDoOutput(true);
    connection.setRequestMethod("PUT");
    connection.setRequestProperty("Content-Type", "application/xml");
    // Marshal object to XML and write to output stream
    // Assume jaxbContext is available
    jaxbContext.createMarshaller().marshal(customer, connection.getOutputStream());
    connection.getResponseCode();
    connection.disconnect();
} catch (Exception e) {
    throw new RuntimeException(e);
}

This approach is low-level and requires manual handling. For a higher-level option, Jersey provides a fluent API:

// Jersey client example
Client client = Client.create();
WebResource resource = client.resource("http://localhost:8080");
String response = resource.path("foo").accept("application/xml").get(String.class);

Spring's RestClient, as described in the reference article, offers a modern alternative:

// Spring RestClient example for a GET request
RestClient restClient = RestClient.create();
String result = restClient.get()
    .uri("https://example.com")
    .retrieve()
    .body(String.class);
System.out.println(result);

Best Practices

When selecting a REST client, evaluate factors such as performance, ease of use, integration with your stack, and support for asynchronous operations. For instance, if using Spring, WebClient is recommended for reactive applications. Always handle errors appropriately, as most clients throw exceptions for 4xx and 5xx status codes by default.

Conclusion

Java developers have a rich ecosystem of REST client libraries. Choosing the right one depends on specific needs like simplicity, performance, or framework alignment. This article has highlighted key options and provided examples to guide implementation.