# Use a Java SDK generated by API Gateway for a REST API
<a name="how-to-call-apigateway-generated-java-sdk"></a>

In this section, we outline the steps to use a Java SDK generated by API Gateway for a REST API, by using the [Simple Calculator](simple-calc-lambda-api-swagger-definition.md) API as an example. Before proceeding, you must complete the steps in [Generate SDKs for REST APIs in API Gateway](how-to-generate-sdk.md). 

**To install and use a Java SDK generated by API Gateway**

1. Extract the contents of the API Gateway-generated .zip file that you downloaded earlier.

1. Download and install [Apache Maven](https://maven.apache.org/) (must be version 3.5 or later).

1. Download and install [JDK 8](https://docs.oracle.com/javase/8/docs/technotes/guides/install/install_overview.html).

1. Set the `JAVA_HOME` environment variable.

1.  Go to the unzipped SDK folder where the pom.xml file is located. This folder is `generated-code` by default. Run the **mvn install** command to install the compiled artifact files to your local Maven repository. This creates a `target` folder containing the compiled SDK library. 

1.  Type the following command in an empty directory to create a client project stub to call the API using the installed SDK library. 

   ```
   mvn -B archetype:generate \
       -DarchetypeGroupdId=org.apache.maven.archetypes \
       -DgroupId=examples.aws.apig.simpleCalc.sdk.app \
       -DartifactId=SimpleCalc-sdkClient
   ```
**Note**  
 The separator `\` in the preceding command is included for readability. The whole command should be on a single line without the separator. 

    This command creates an application stub. The application stub contains a `pom.xml` file and an `src` folder under the project's root directory (*SimpleCalc-sdkClient* in the preceding command). Initially, there are two source files: `src/main/java/{package-path}/App.java` and `src/test/java/{package-path}/AppTest.java`. In this example, *\$1package-path\$1* is `examples/aws/apig/simpleCalc/sdk/app`. This package path is derived from the `DarchetypeGroupdId` value. You can use the `App.java` file as a template for your client application, and you can add others in the same folder if needed. You can use the `AppTest.java` file as a unit test template for your application, and you can add other test code files to the same test folder as needed. 

1. Update the package dependencies in the generated `pom.xml` file to the following, substituting your project's `groupId`, `artifactId`, `version`, and `name` properties, if necessary:

   ```
   <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
     <modelVersion>4.0.0</modelVersion>
     <groupId>examples.aws.apig.simpleCalc.sdk.app</groupId>
     <artifactId>SimpleCalc-sdkClient</artifactId>
     <packaging>jar</packaging>
     <version>1.0-SNAPSHOT</version>
     <name>SimpleCalc-sdkClient</name>
     <url>http://maven.apache.org</url>
   
      <dependencies>
         <dependency>
             <groupId>com.amazonaws</groupId>
             <artifactId>aws-java-sdk-core</artifactId>
             <version>1.11.94</version>
         </dependency>
         <dependency>
             <groupId>my-apig-api-examples</groupId>
             <artifactId>simple-calc-sdk</artifactId>
             <version>1.0.0</version>
         </dependency>
         
       <dependency>
         <groupId>junit</groupId>
         <artifactId>junit</artifactId>
         <version>4.12</version>
         <scope>test</scope>
       </dependency>
   
       <dependency>
           <groupId>commons-io</groupId>
           <artifactId>commons-io</artifactId>
           <version>2.5</version>
       </dependency>    
     </dependencies>
   
     <build>
       <plugins>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.5.1</version>
           <configuration>
             <source>1.8</source>
             <target>1.8</target>
           </configuration>
         </plugin>
       </plugins>
     </build>
   </project>
   ```
**Note**  
 When a newer version of dependent artifact of `aws-java-sdk-core` is incompatible with the version specified above (`1.11.94`), you must update the `<version>` tag to the new version.

1.  Next, we show how to call the API using the SDK by calling the `getABOp(GetABOpRequest req)`, `getApiRoot(GetApiRootRequest req)`, and `postApiRoot(PostApiRootRequest req)` methods of the SDK. These methods correspond to the `GET /{a}/{b}/{op}`, `GET /?a={x}&b={y}&op={operator}`, and `POST /` methods, with a payload of `{"a": x, "b": y, "op": "operator"}` API requests, respectively. 

    Update the `App.java` file as follows: 

   ```
   package examples.aws.apig.simpleCalc.sdk.app;
   
   import java.io.IOException;
   
   import com.amazonaws.opensdk.config.ConnectionConfiguration;
   import com.amazonaws.opensdk.config.TimeoutConfiguration;
   
   import examples.aws.apig.simpleCalc.sdk.*;
   import examples.aws.apig.simpleCalc.sdk.model.*;
   import examples.aws.apig.simpleCalc.sdk.SimpleCalcSdk.*;
   
   public class App 
   {
       SimpleCalcSdk sdkClient;
   
       public App() {
           initSdk();
       }
   
       // The configuration settings are for illustration purposes and may not be a recommended best practice.
       private void initSdk() {
           sdkClient = SimpleCalcSdk.builder()
                 .connectionConfiguration(
                     new ConnectionConfiguration()
                           .maxConnections(100)
                           .connectionMaxIdleMillis(1000))
                 .timeoutConfiguration(
                     new TimeoutConfiguration()
                           .httpRequestTimeout(3000)
                           .totalExecutionTimeout(10000)
                           .socketTimeout(2000))
           .build();
   
       }
       // Calling shutdown is not necessary unless you want to exert explicit control of this resource.
       public void shutdown() {
           sdkClient.shutdown();
       }
        
       // GetABOpResult getABOp(GetABOpRequest getABOpRequest)
       public Output getResultWithPathParameters(String x, String y, String operator) {
       	operator = operator.equals("+") ? "add" : operator;
       	operator = operator.equals("/") ? "div" : operator; 
   
           GetABOpResult abopResult = sdkClient.getABOp(new GetABOpRequest().a(x).b(y).op(operator));
           return abopResult.getResult().getOutput();
       }
   
       public Output getResultWithQueryParameters(String a, String b, String op) {
           GetApiRootResult rootResult = sdkClient.getApiRoot(new GetApiRootRequest().a(a).b(b).op(op));
           return rootResult.getResult().getOutput();
       }
   
       public Output getResultByPostInputBody(Double x, Double y, String o) {
       	PostApiRootResult postResult = sdkClient.postApiRoot(
       		new PostApiRootRequest().input(new Input().a(x).b(y).op(o)));
       	return postResult.getResult().getOutput();
       }
   
       public static void main( String[] args )
       {
           System.out.println( "Simple calc" );
           // to begin
           App calc = new App();
           
           // call the SimpleCalc API
           Output res = calc.getResultWithPathParameters("1", "2", "-");
           System.out.printf("GET /1/2/-: %s\n", res.getC());
   
           // Use the type query parameter
           res = calc.getResultWithQueryParameters("1", "2", "+");
           System.out.printf("GET /?a=1&b=2&op=+: %s\n", res.getC());
   
           // Call POST with an Input body.
           res = calc.getResultByPostInputBody(1.0, 2.0, "*");
           System.out.printf("PUT /\n\n{\"a\":1, \"b\":2,\"op\":\"*\"}\n %s\n", res.getC());
   
           
       }
   }
   ```

    In the preceding example, the configuration settings used to instantiate the SDK client are for illustration purposes and are not necessarily recommended best practice. Also, calling `sdkClient.shutdown()` is optional, especially if you need precise control on when to free up resources. 

 We have shown the essential patterns to call an API using a Java SDK. You can extend the instructions to calling other API methods.