Develop an Apache HttpClient client for Android to a JAX-RS web service

Learn to create an Apache HttpClient Android client

Summary:  Create JAX-RS web service access with the Apache HttpClient
library. Jersey, a reference implementation for JAX-RS, simplifies development of RESTful web services in the Java™ environment. Android is a popular smartphone and this article shows you how to create a JAX-RS client for Android. You'll create an Apache HttpClient
library client to a JAX-RS web service.




Introduction

Explore Spring Android

Spring Android is another option. Learn to access a RESTful web service with the Spring Android REST client in this article, also by Deepak:

Develop a Spring client for Android to a JAX-RS web service

The REST software architecture is based on transferring representation of resources. RESTful web services offer some advantages: They are simple, lightweight, and fast. A RESTful web service exposes a set of resources identified by URIs. Resources respond to
the HTTP methods GET, POST, PUT, and DELETE. Resources may be accessed in various formats, such as HTML, plain text, XML, PDF, JPEG, or JSON. The Java API for RESTful web services (JAX-RS) is defined in JSR 311. Jersey is a reference implementation for JAX-RS
that simplifies development of RESTful web services in Java.

In this article, use the Apache HttpClient library to create a JAX-RS client for Android—the popular smartphone platform. You candownload the sample
code used in this article.

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Setting up the environment

Frequently used abbreviations

API: Application programming interface

HTML: HyperText Markup Language

HTTP: HyperText Transfer Protocol

IDE: Integrated Development Environment

JSON: JavaScript Object Notation

MIME: Multipurpose Internet Mail Extensions

POJO: Plain Old Java Object

REST: Representational State Transfer

SDK: Software Development Kit

UI: User Interface

URI: Uniform Resource Identifier

URL: Uniform Resource Locator

XML: Extensible Markup Language

Before you can create a client for a JAX-RS web service, you need to set up the environment. See Resources for links.

Install Eclipse.

Install the Android Development Tools (ADT) plug-in for Eclipse, which provides a set of extensions to develop Android applications in Eclipse.

Install the SDK Platform for Android 2.2.The Android SDK provides tools for developing Android applications.

Create an Android Virtual Device (AVD), which is an emulator for Android, in Eclipse.

Download the Jersey archive, jersey-archive-1.4.zip, containing the Jersey JARs and core dependencies. Download the Jersey bundle JAR jersey-bundle-1.4.jar.

Jersey is built using JDK 6.0, so you also need to install JDK 6.0.

Install a web server such as Tomcat, or an application server such as WebSphere®, or WebLogic server. Add the Jersey JAR files in Listing 1 to
the runtime 

CLASSPATH

 of the application/web server. 

Listing 1. Jersey JAR Files

C:\Jersey\jersey-bundle-1.4.jar;C:\Jersey\jersey-archive-1.4\lib\asm-3.1.jar; C:\Jersey\jersey-archive-1.4\lib\jsr311-api-1.1.1.jar

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Creating an Eclipse project

In this section, you will create a web project and add the JAX-RS facet to the project. Use the following steps to create an Eclipse project.

Select File > New, and in the New window, select Web > Dynamic Web Project. Click Next.

Specify a Project name (for example, AndroidJAX-RS) and click New Runtime, to configure a new target runtime for your WebSphere, Tomcat, or WebLogic server. Figure
1 shows the completed Dynamic Web Project window. 

Figure 1. Configure a new runtime


 

In the New Server Runtime Environment window, select a server, such as the Tomcat server, the WebSphere server, or the WebLogic server. Click Next, as in Figure
2. 

Figure 2. Select an application or web server


 

In the New IBM WebSphere v6.0 Runtime window, configure a JRE and the IBM WebSphere Installation Directory. ClickNext in the Dynamic Web Project dialog. Select the default Java settings for Source folder and Output folder, and clickNext.

Specify Context root as 

AndroidJAX-RS

, select the default Content Directory, and click Finish.

A Dynamic Web Project is created and gets added to Project Explorer. Right-click on the project node and selectProperties.

Select Project Facets, then select the JAX-RS (REST Web Services) 1.1 project facet. Click Further configuration required, as in Figure
3. 

Figure 3. Configure the JAX-RS Project Facet


 

In the JAX-RS Capabilities window, specify a Servlet name (JAX-RS Servlet) and configure a JAX-RS Implementation library. Select Type as User Library and click Manage.

In the User Libraries window, click New. In the New User Library dialog, specify a User library name and click OK.

A user library is added. Click Add JARs to add Jersey JARs to the user library. As in Figure 4, add the following Jersey JARs:

jersey-bundle-1.4.jar

C:\Jersey\jersey-archive-1.4\lib\asm-3.1.jar

C:\Jersey\jersey-archive-1.4\lib\jsr311-api-1.1.1.jar

Click OK. 

Figure 4. Add Jersey JAR files


 

In the JAX-RS Capabilities window, specify the JAX-RS servlet class name as

com.sun.jersey.spi.container.servlet.ServletContainer

, as in Figure
5. Click OK. 

Figure 5. Specify the JAX-RS servlet class


 

In the Project Facets window, click Apply as in Figure 6, then click OK. 

Figure 6. Apply the JAX-RS Project Facet


 

The targeted runtimes are configured with the JAX-RS project facet. Click OK in the Properties dialog.

The JAX-RS User libraries get added to the project, and the JAX-RS servlet and servlet mapping get configured in web.xml. You need to add 

init-param

 elements for the 

com.sun.jersey.config.property.resourceConfigClass

 and
the

com.sun.jersey.config.property.packages

 init parameters. Listing 2 shows the 

web.xml

.

Listing 2. web.xml

<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/ xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5"> <servlet> <description>JAX-RS Tools Generated - Do not modify</description> <servlet-name>JAX-RS Servlet</servlet-name> <servlet-class>com.sun.jersey.spi.container.servlet.ServletContainer</servlet-class> <init-param> <param-name>com.sun.jersey.config.property.resourceConfigClass</param-name> <param-value>com.sun.jersey.api.core.PackagesResourceConfig</param-value> </init-param> <init-param> <param-name>com.sun.jersey.config.property.packages</param-name> <param-value>jaxrs</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>JAX-RS Servlet</servlet-name> <url-pattern>/jaxrs/*</url-pattern> </servlet-mapping> </web-app>

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Creating and running a resource class

The next step is to create a RESTful web service resource using a root resource class. A root resource class is a POJO annotated with the 

@PATH

 annotation. It consists of at least one method annotated with the 

@PATH

 annotation
or 

@GET

, 

@PUT

, 

@POST

 or

@DELETE

.

Select File > New > Other. In the New dialog, select Java > Class, and click Finish.

In the New Java Class window, as in Figure 7, specify:

A Source folder: 

AndroidJAX-RS/src

Package: 

jaxrs

Class name: 

HelloWorldResource

Click Finish.

Figure 7. Create the resource class


 

Annotate the Java class with the 

@PATH

 annotation. The code in Listing 3 specifies the URI path
on which the Java class shall be hosted as 

/helloworld

.

Listing 3. Annotate resource class with @Path

@Path("/helloworld") public class HelloWorldResource { ... }

To add resource methods to produce three different MIME types, add the 

getClichedMessage()

, 

getXMLMessage()

, and

getHTMLMessage()

 methods.
Annotate each method with the 

@GET

 annotation, which indicates that the methods shall process HTTP GET requests. Specify 

String

 as the return type for each of the
methods. Annotate each method with the 

@PRODUCES

annotation, and specify a different MIME type for each method.

Now you want to output a "Hello JAX-RS" method using the MIME types 

text/plain

, 

text/xml

, and 

text/html

. The 

getXMLMessage

method
is annotated with the 

@Produces

 (

"text/xml"

) annotation that produces an XML message. Uncomment only one of the methods annotated with the 

@GET

 method.
If no other distinguishing path component is specified, the 

@GET

 request is routed to the method annotated with 

@GET

. If multiple methods match a request URI, the
JAX-RS selection algorithm is used to select the resource method. For example, you can specify multiple methods with the 

@GET

 annotation by using a different 

path

 id
for the methods annotated with 

@GET

. Listing 4 shows the root resource class.

Listing 4. HelloWorldResource.java

package jaxrs; import javax.ws.rs.GET; import javax.ws.rs.Produces; import javax.ws.rs.Path; import javax.ws.rs.core.MediaType; // The Java class will be hosted at the URI path //"/helloworld" @Path("/helloworld") public class HelloWorldResource { // The Java method will process HTTP GET requests @GET // The Java method will produce content identified by the MIME Media // type "text/plain" @Produces("text/plain") public String getClichedMessage() { // Return some cliched textual content return "Hello Android"; } // @GET // @Produces("text/xml") // public String getXMLMessage() { // return "<?xml version=\"1.0\"?>" + "<hello> Hello Android" + "</hello>"; // } // @GET //@Produces("text/html") //public String getHTMLMessage() { //return "<html> " + "<title>" + "Hello Android" + "</title>" // + "<body><h1>" + "Hello Android" + "</body></h1>" + "</html> "; // } }

Run the resource class to produce different types of output. Comment out the methods that are not to be tested, and keep one method uncommented for each of the test runs. First, test the 

text/xml

 MIME type as output.
Start the application/web server if not already started. Right-click on the resource class and select Run As > Run on Server, as in Figure
8.

Figure 8. Run the resource class


 

On the server, the init parameter 

com.sun.jersey.config.property.resourceConfigClass

 is initiated as

com.sun.jersey.api.core.PackagesResourceConfig

 and the init parameter 

com.sun.jersey.config.property.packages

 is
initiated as 

jaxrs

, as specified in web.xml. The root resource class 

jaxrs.HelloWorldResource

 is found. The Jersey application v1.4 is initiated, and the AndroidJAX-RS
module deploys on the server.

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Creating an Android client project

In this section you'll create an Android project in which you create the JAX-RS client for Android.

In the Eclipse IDE, select File > New. In the New dialog, select Android > Android Project, then click Next.

Complete the fields in the New Android Project window, as in Figure 9.

Project name: 

AndroidJAXRSClient

Build Target: Android Platform 2.2 API 8

Properties: Application name 

AndroidJAXRSClient

 and Package name 

android.jaxrs

Select Create Activity, and specify Activity class 

AndroidJAXRSClient

.

An activity represents a user interaction, and the class extending the Activity class creates a window for a UI.

Minimum SDK Version: 

8

Click Next

Figure 9. Create the JAX-RS client class


 

The files in the Android project are:

An activity class (

AndroidJAXRSClient

), which extends the 

Activity

 class

The res/layout/main.xml file to specify the layout of the Android application

The AndroidManifest.xml file, which contains application configuration information such as the package name, application components, processes, permissions, and minimum API level for the Android system

In the res/layout/main.xml file, specify the layout of the Android UI components. Create a 

LinearLayout

 with

android:orientation="vertical"

. You'll create a UI in which
the response from the web service is displayed as a text message. Add a 

TextView

 element with id 

jaxrs

 to display the JAX-WS web service response for a method call
to one of the 

get

 methods. The method invocation gets a Hello message as a response in either XML, HTML, or text. Listing
5 shows the main.xml file:

Listing 5. main.xml

<?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent"> <TextView android:id="@+id/jaxrs" android:layout_width="fill_parent" android:layout_height="wrap_content" /> </LinearLayout>

To access the JAX-RS web service from an Android device, enable the 

android.permission.INTERNET

 permission, in AndroidManifest.xml, which allows applications to open network sockets. Add the 

uses-permission

 element
in Listing 6.

Listing 6. Set INTERNET permission

<uses-permission android:name="android.permission.INTERNET"></uses-permission>

Specify the minimum Android version with the 

uses-sdk

 element. The 

AndroidJAXRSClient

 activity, the 

intent-filter

, and 

action

are
specified with the 

activity

 element and sub-elements. Listing 7 shows the AndroidManifest.xml file.

Listing 7. AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" package="android.jaxrs" android:versionCode="1" android:versionName="1.0"> <uses-sdk android:minSdkVersion="8" /> <application android:icon="@drawable/icon" android:label="@string/app_name"> <activity android:name=".AndroidJAXRSClient" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application> <uses-sdk android:minSdkVersion="8" /> <uses-permission android:name="android.permission.INTERNET"></uses-permission> </manifest>

The Android SDK includes the Apache HttpClient library. Import the classes in Listing 8 into 

AndroidJAXRSClient

.

Listing 8. Apache HttpClient library

org.apache.http.HttpEntity; org.apache.http.HttpResponse; org.apache.http.client.ClientProtocolException; org.apache.http.client.HttpClient; org.apache.http.client.methods.HttpGet; org.apache.http.impl.client.DefaultHttpClient;

The 

AndroidJAXRSClient

 class extends the 

Activity

 class. The 

onCreate(Bundle savedInstanceState)

 method is invoked when
the activity is first called. Define the user interface using the 

setContentView

 method and the layout resource, as in Listing
9.

Listing 9. Define the UI

setContentView(R.layout.main);

Listing 10 shows how to create an Android widget 

TextView

 object, using the 

findViewById

 method
on the 

TextView

 element, with the id 

jaxrs

 that was defined in main.xml.

Listing 10. Create an Android widget

TextView jaxrs = (TextView) findViewById(R.id.jaxrs);

The default implementation of 

HttpClient

 is 

DefaultHttpClient

. Create a 

DefaultHttpClient

 object, as Listing
11.

Listing 11. Create an HttpClient

HttpClient httpclient = new DefaultHttpClient();

Create an 

HttpGet

 object to retrieve information from the server, as in Listing 12. Specify the
URL to the resource hosted on the URI path 

/helloworld

. Specify the IP address for host instead of localhost. The client runs on the Android device, and localhost for the Android device is not the host on which
the JAX-RS web service runs (unless the JAX-RS web service is also hosted on the Android device, which is not the case in this example).

Listing 12. Create an HttpGet object

HttpGet request = new HttpGet("http://192.168.1.68:7001/AndroidJAX-RS/jaxrs/helloworld");

Specify acceptable media types using the 

Accept

 header. Set only one of the media types, which corresponds to the media type produced in the JAX-RS web service, in the 

Accept

 header.
In the first run, set the 

Accept

 header to 

text/xml

 to output the

text/xml

 response, as in Listing
13.

Listing 13. Set the Accept Header

request.addHeader("Accept", "text/xml"); //request.addHeader("Accept", "text/html"); //request.addHeader("Accept", "text/plain");

Test the output of each of the response types (plain text, html, and XML). The accepted response type should match the MIME type produced in the resource class. The MIME type produced by the resource class should match an acceptable MIME type. If the produced
MIME type and the acceptable MIME type do not match, a 

com.sun.jersey.api.client.UniformInterfaceException

is generated. For example, set the acceptable MIME type to 

text/xml

 and
the produced MIME type to 

application/xml

. The

UniformInterfaceException

 is generated. As in Listing
14, invoke the 

execute()

 method of the 

HttpClient

, with the 

HttpGet

method as an argument, to retrieve the 

HttpResponse

 object.

Listing 14. Get the HttpResponse

HttpResponse response = httpclient.execute(request);

From the 

HttpResponse

 obtain the 

HttpEntity

 using the 

getEntity

 method (Listing
15).

Listing 15. Get the HttpEntity

HttpEntity entity = response.getEntity();

Get the content as an 

InputStream

 from the 

HttpGet

 using the 

getContent()

 method (Listing
16).

Listing 16. Create an 

InputStream

 from the 

HttpEntity

InputStream instream = entity.getContent();

Create a 

StringBuilder

 for the message returned from the JAX-RS web service (Listing 17).

Listing 17. Create a StringBuilder

StringBuilder sb = new StringBuilder();

Create a 

BufferedReader

 from the 

InputStream

 (Listing
18).

Listing 18. Create a BufferedReader

BufferedReader r = new BufferedReader(new InputStreamReader(instream));

Read each line from the 

BufferedReader

 and add it to the 

StringBuilder

 (Listing
19).

Listing 19. Read the BufferedReader

for (String line = r.readLine(); line != null; line = r.readLine()) { sb.append(line); }

Get the 

String

 message from the 

StringBuilder

 and close the 

InputStream

 (Listing
20).

Listing 20. Get the StringBuilder message

String jaxrsmessage = sb.toString(); instream.close();

Set the 

String

 message on the 

TextView

 UI component (Listing
21).

Listing 21. Set the StringBuilder message

jaxrs.setText(jaxrsmessage);

Listing 22 shows the 

AndroidJAXRSClient

 class.

Listing 22. AndroidJAXRSClient.java

package android.jaxrs; import android.app.Activity; import android.os.Bundle; import android.widget.TextView; import java.io.BufferedReader; import java.io.InputStream; import java.io.InputStreamReader; import java.io.IOException; import org.apache.http.HttpEntity; import org.apache.http.HttpResponse; import org.apache.http.client.ClientProtocolException; import org.apache.http.client.HttpClient; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; public class AndroidJAXRSClient extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); TextView jaxrs = (TextView) findViewById(R.id.jaxrs);try { HttpClient httpclient = new DefaultHttpClient();HttpGet request = new HttpGet( "http://192.168.1.68:7001/AndroidJAX-RS/jaxrs/helloworld"); //request.addHeader("Accept", "text/html");// request.addHeader("Accept", "text/xml"); request.addHeader("Accept", "text/plain");HttpResponse response = httpclient.execute(request); HttpEntity entity = response.getEntity();InputStream instream = entity.getContent();String jaxrsmessage = read(instream); jaxrs.setText(jaxrsmessage); } catch (ClientProtocolException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } private static String read(InputStream instream) { StringBuilder sb = null; try { sb = new StringBuilder(); BufferedReader r = new BufferedReader(new InputStreamReader( instream)); for (String line = r.readLine(); line != null; line = r.readLine()) { sb.append(line); } instream.close(); } catch (IOException e) { } return sb.toString(); } }

Figure 10 shows the directory structure of the Android client application. (View a larger
version of Figure 10.)

Figure 10. Directory structure of the Android application


 

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Running the Android client

Now you're ready to run the Android client to invoke the JAX-RS web service and output the XML message. Right-click on theAndroidJAXRSClient project and select Run As > Android Application, as in Figure
11.

Figure 11. Run the Android JAX-RS client



The Android AVD starts and the JAX-RS client application installs on the Android device, as in Figure 12.

Figure 12. Android JAX-RS client installed on Android


 

The 

AndroidJAXRSClient

 activity starts, and the XML message produced by the JAX-RS web service resource is output to the Android device, as in Figure
13.

Figure 13. Output XML message to Android device



Similarly, the HTML message is the output if the method that produces the 

text/html

 media type is uncommented in the resource class and the 

Accept

 header is set to
receive the same media type. For example, in the resource class, uncomment the method in Listing 23.

Listing 23. Produce HTML in resource class

@GET @Produces("text/html") public String getHTMLMessage() { return "<html> " + "<title>" + "Hello Android" + "</title>" + "<body><h1>" + "Hello Android" + "</body></h1>" + "</html> "; }

In the client class, uncomment the 

addHeader

 invocation in Listing 24.

Listing 24. Set media type to Accept

request.addHeader("Accept", "text/html");

Rerun the 

AndroidJAXRSClient

 application to get the HTML response, as in Figure 14.

Figure 14. HTML output to Android device


 

To get the text response, uncomment the method in the resource class in Listing 25.

Listing 25. Produce text media type in the resource class

@GET @Produces("text/plain") public String getClichedMessage() { return "Hello Android"; }

In the client class, uncomment the following 

Accept

 header media type setting in Listing 26.

Listing 26. Set Accept Header

request.addHeader("Accept", "text/plain");

Rerun the 

AndroidJAXRSClient

 application to get the text message output, as in Figure 15.

Figure 15. Output text message to Android device


 

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Summary

In this article, you learned how to create a JAX-RS web service and invoke the web service from an Android client. You sent XML, HTML, and text output to the Android device.

原文地址：http://www.ibm.com/developerworks/library/x-android-jax-rs/#resources

PS：最近对REST比较感兴趣，特别是通过Android去调用REST的Web服务，于是找到这篇文章，获益匪浅。
按着例子做，最后是可以成功的，但是前面Eclipse环境和Jersey的配置需要自己额外操作，
我的Android项目部署在Eclipse SDK4.2.2，而Web服务部署在Eclipse Java EE IDE for Web Developers+Tomcat6.0
如果你对这块配置不熟悉，可以留言！