rest api

1. For simplicity, our example application only sends JSON back and forth, but the application should be architectured in such a way that you can easily change the format of the data, to tailor for different clients or user preferences.
2.  

android rest

1. https://code.tutsplus.com/tutorials/android-from-scratch-using-rest-apis--cms-27117
2. https://code.tutsplus.com/tutorials/a-beginners-guide-to-http-and-rest--net-16340
3. https://dzone.com/articles/creating-a-simple-android-rest-client-using-http-r
4. 
   

spring boot

1. externalized configuration
2. 
   

java features latest

1. optional
2. 
   

spring exception handling

1. https://www.journaldev.com/2651/spring-mvc-exception-handling-controlleradvice-exceptionhandler-handlerexceptionresolver
2. https://dzone.com/articles/global-exception-handling-with-controlleradvice
3. 
   

mockito

1. https://medium.com/@gustavo.ponce.ch/spring-boot-restful-junit-mockito-hamcrest-eclemma-5add7f725d4e 
2. can we use same mock in two different test methods? what would be its implications ie. pros and cons?
3. what layer is best suitable for being tested through mockito? controller or service? or both?
4. https://dzone.com/articles/spring-boot-unit-testing-and-mocking-with-mockito
5. @Mock vs @InjectMocks?
6. 
   

error-selection-does-not-contain-a-main-type

1. https://stackoverflow.com/questions/31250359/git-repository-how-to-recover-deleted-files
2. check the folder structure
3. do you have src/main/java structure and inside it do you have Main class?
4. 
   

github cloning a repository from a specific commit

1. clone a repository as usual ie 
1. git clone repository url
2. user name and password if it is private
2. git reset --hard a0f64dcd987669aa15d59a39a51f583b3cqd05k2
3. in the above command the value after hard flag is the SHA of commit at that particular instant of repository
4. import as maven project in sts
5. 
   

github recovering a deleted file from previous commit

1. click on total number of commits
2. here you can view all the commits
3. now go to a commit where a file existed and click on < > symbol and if you find the file just download it 
4. 
   

redirecting request on port 80 to 8080 on ubuntu

1. sudo iptables -A PREROUTING -t nat -p tcp --dport 80 -j REDIRECT --to-ports 8080

   the above command will redirect the requests coming on port 80 to port 8080

spring boot file upload to amazon s3 on ubuntu

1. https://medium.com/oril/uploading-files-to-aws-s3-bucket-using-spring-boot-483fcb6f8646 
2. first sign up or login to your amazon account and create bucket in s3. then create a user and give permission ie programmatic access 
3. @Configuration
   public class S3Config {
   
       @Value("${userid.aws.access_key_id}")
       private String awsId;
   
       @Value("${userid.aws.secret_access_key}")
       private String awsKey;
      
       @Value("${userid.s3.region}")
       private String region;
   
       @Bean
       public AmazonS3 xyz(){
           System.out.println("Inside s3Client of S3Config ");
           BasicAWSCredentials awsCreds = new BasicAWSCredentials(awsId, awsKey);
           AmazonS3 s3Client = AmazonS3ClientBuilder.standard()
                                   .withRegion(Regions.fromName(region))
                                   .withCredentials(new AWSStaticCredentialsProvider(awsCreds))
                                   .build();
           return s3Client;
         }
   
   
   }
4. @Controller
   public class AController {
      
       @Autowired
       private S3Service s3service;
     
             @PostMapping("/uploadFile")
            
           
             public String uploadFile(@RequestPart(value = "xyz") MultipartFile multipartFile,Model map,HttpServletRequest request) {
                
                 System.out.println("Inside upload file of bucket controller ");
   
                 try
                 {
                     String filePath = request.getServletContext().getRealPath("/");
                                    
                     System.out.println("filePath is "+filePath);
                     File f1=new File(filePath+"/"+multipartFile.getOriginalFilename());
                     multipartFile.transferTo(f1);
                     System.out.println("f1 is "+f1);                 
                    
                     String fileName=f1.getName();
   
                  String fileUrl=s3service.uploadFile(fileName, f1);
                
                 /**
                  * we should be sure that the file has indeed been uploaded 
                  */
                   map.addAttribute("fileUrl", fileUrl);                         
                  f1.delete();
                 
                /* for rest request
                  return fileUrl;
                  */              
                 
                  return "uploadSuccess";
                 
                 }
                 catch(IOException e)
                 {
                     e.printStackTrace();
                     return e.toString();
                 }
                
           }
5. @Service
   public class S3ServicesImpl implements S3Service {
       private Logger logger = LoggerFactory.getLogger(S3ServicesImpl.class);
       /* (non-Javadoc)
        * @see com.epilen.androidPatient.service.aws.s3.S3Service#downloadFile(java.lang.String)
        */
      
   
      
       @Autowired
       private AmazonS3 s3client;
   
       @Value("${userid.s3.bucket}")
       private String bucketName;
      
       @Value("${s3.endpointUrl}")
       private String endpointUrl;
      
      
       /**
        * convert multipart to file
        * @param file
        * @return
        * @throws IOException
        */
      
      
       @Override
       public void downloadFile(String keyName) {
          
           try {
              
              System.out.println("Downloading an object");
              S3Object s3object = s3client.getObject(new GetObjectRequest(bucketName, keyName));
              System.out.println("Content-Type: "  + s3object.getObjectMetadata().getContentType());
              Utility.displayText(s3object.getObjectContent());
              logger.info("===================== Import File - Done! =====================");
              
           } catch (AmazonServiceException ase) {
               logger.info("Caught an AmazonServiceException from GET requests, rejected reasons:");
               logger.info("Error Message:    " + ase.getMessage());
               logger.info("HTTP Status Code: " + ase.getStatusCode());
               logger.info("AWS Error Code:   " + ase.getErrorCode());
               logger.info("Error Type:       " + ase.getErrorType());
               logger.info("Request ID:       " + ase.getRequestId());
           } catch (AmazonClientException ace) {
               logger.info("Caught an AmazonClientException: ");
               logger.info("Error Message: " + ace.getMessage());
           } catch (IOException ioe) {
               logger.info("IOE Error Message: " + ioe.getMessage());
           }
       }
   
       /**
        * this method will return the fileurl
        */
       @Override
       public String uploadFile(String fileName, File file) {
           System.out.println("Inside upload file of S3ServiceImpl");
           try {
                  
               s3client.putObject(new PutObjectRequest(bucketName, fileName, file));
               logger.info("===================== Upload File - Done! =====================");
              
               String fileUrl = endpointUrl + "/" + bucketName + "/" + fileName;
               return fileUrl;
              
           } catch (AmazonServiceException ase) {
               logger.info("Caught an AmazonServiceException from PUT requests, rejected reasons:");
               logger.info("Error Message:    " + ase.getMessage());
               logger.info("HTTP Status Code: " + ase.getStatusCode());
               logger.info("AWS Error Code:   " + ase.getErrorCode());
               logger.info("Error Type:       " + ase.getErrorType());
               logger.info("Request ID:       " + ase.getRequestId());
               return ase.toString();
              
           } catch (AmazonClientException ace) {
               logger.info("Caught an AmazonClientException: ");
               logger.info("Error Message: " + ace.getMessage());
               return ace.toString();
           }
       }
   
      
   }
6. public interface S3Service {
           public String uploadFile(String fileName, File file);
   }
7. 
   pom.xml entry->.   
       <dependencyManagement>
     <dependencies>
       <dependency>
         <groupId>com.amazonaws</groupId>
         <artifactId>aws-java-sdk-bom</artifactId>
         <version>1.11.327</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>
     </dependencies>
   </dependencyManagement>
   
     <dependencies>
    
      <dependency>
       <groupId>com.amazonaws</groupId>
       <artifactId>aws-java-sdk-s3</artifactId>
     </dependency>
    
      <dependency>
      <groupId>org.apache.httpcomponents</groupId>
      <artifactId>httpclient</artifactId>
      <version>4.5.2</version> 
    </dependency>
    
     the above reduces the jar size from 90 mb to 35 mb and downloads only s3 dependencies
8. application.properties entries                                                                          userid.aws.access_key_id=  userid.aws.secret_access_key=                                                                             userid.s3.bucket=
   userid.s3.region=
   s3.endpointUrl=