HTTP Client Library

最近因為團隊使用的 Apache HttpClient 3.x 已經是 2007.12 就已經停止維護的項目了，過了十年，也該是尋找下一個工具的時候了，所以我就開始了 Java HTTP Client Library 的研究心得，一開始當然要先了解各自的特色，然後針對每種 Library 寫最單純的 Example Code，最後就是比較表。

Introduction

Apache HttpClient

• Standards based, pure Java, implementation of HTTP versions 1.0 and 1.1
• Full implementation of all HTTP methods (GET, POST, PUT, DELETE, HEAD, OPTIONS, and TRACE) in an extensible OO framework.
• Basic, Digest, NTLMv1, NTLMv2, NTLM2 Session, SNPNEGO, Kerberos authentication schemes.
• Plug-in mechanism for custom authentication schemes.
• Connection management support for use in multi-threaded applications. Supports setting the maximum total connections as well as the maximum connections per host. Detects and closes stale connections.
• Response input streams to efficiently read the response body by streaming directly from the socket to the server.
• The ability to set connection timeouts.
• Support for HTTP/1.1 response caching.

Square OkHttp

• HTTP/2 support allows all requests to the same host to share a socket.
• Connection pooling reduces request latency (if HTTP/2 isn’t available).
• Transparent GZIP shrinks download sizes.
• Response caching avoids the network completely for repeat requests.
• Supports both synchronous blocking calls and async calls with callbacks.

OkHttp supports Java, the minimum requirement is 1.7.

Image Source

Apache CXF

• JAX-WS Support
• Spring Integration
• Aegis Databinding
• RESTful Service
• WS-* Support

Square Retrofit2

• RESTful Service
• Support synchronous, asynchronous request

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Example Code

為了比較好相互比較，以下盡量會以相同寫法呈現，大致為四個步驟：

1. New Client
2. New Method/Request
3. Call/Execute
4. Output status code, body, headers

AHC 4.x

HttpClient client = HttpClientBuilder.create().build(); 

HttpGet get = new HttpGet(url);

HttpResponse response = client.execute(get);

System.out.println(response.getStatusLine().getStatusCode());
System.out.println(EntityUtils.toString(response.getEntity()));
System.out.println(response.getAllHeaders());

AHC 3.x

HttpClient client = new HttpClient();

GetMethod method = new GetMethod(url);

try {

    int statusCode = client.executeMethod(method);

    System.out.println(statusCode);
    System.out.println(new String(method.getResponseBody()));
    System.out.println(method.getResponseHeaders());

} finally {
    // Release the connection.
    method.releaseConnection();
}

Okhttp

OkHttpClient client = new OkHttpClient();

Request request = new Request.Builder().url(url).build();

Response response = client.newCall(request).execute();

System.out.println(response.code());
System.out.println(response.body().string());
System.out.println(response.headers().toMultimap());

CXF

UserService userService = JAXRSClientFactory.create(baseAddress, UserService.class, Collections.singletonList(new JacksonJsonProvider()));

Client client = WebClient.client(userService).accept(MediaType.APPLICATION_JSON_TYPE);

Response result = userService.list(0, 10);

System.out.println(client.getResponse().getStatus());
System.out.println(result);
System.out.println(client.getResponse().getHeaders());

Retrofit2

Retrofit retrofit = new Retrofit.Builder().baseUrl(baseUrl).addConverterFactory(GsonConverterFactory.create()).build();

UserService service = retrofit.create(UserService.class);

retrofit2.Response> response = service.list(0, 10).execute();

System.out.println(response.code());
System.out.println(response.body());
System.out.println(response.headers().toMultimap());

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Comparison table

HTTP Transport Layer

	HttpClient 3.x	HttpClient 4.x	OkHttp
RESTful Support	Basic HTTP	Basic HTTP	Retrofit
Performance	1	3	2
Easy Use	3	2	1

RESTFul API Layer

• Encapsulation HTTP Connection Layer
• REST API Interface Proxy
• Response Converter

	CXF	Retrofit
RESTful Support	JAX-RS	Retrofit Annotation
Performance	X	O
Easy Use	O	O+

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Conclusion

其實這只是一小部分比較常用的，其他還有很多 Client Library 是沒有時間去了解的，不過經過這次研究，更清楚不同層級 Library 的差異，以及各自專注在不同的職責；從最後的比較表來看，目前在各方面比較好的應該是使用 Okhttp + Retrofit2 吧。

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Appendix

Retrofit 2.0: The biggest update yet on the best HTTP Client Library

official Converter modules

Gson: com.squareup.retrofit:converter-gson
Jackson: com.squareup.retrofit:converter-jackson
Moshi: com.squareup.retrofit:converter-moshi
Protobuf: com.squareup.retrofit:converter-protobuf
Wire: com.squareup.retrofit:converter-wire
Simple XML: com.squareup.retrofit:converter-simplexml

Builder Pattern

Java REST Client Example

Why Retrofit custom annotations instead of JAX-RS (JSR-311) ones?

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