How to convert data into json format by using spring rest api in eclipse

You can use @RestController or @ReponseBody annotation.
Official Tutorial is here.And the code snippet is like

@RestController
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping("/greeting")
    public Greeting greeting(@RequestParam(value="name", defaultValue="World") String name) {
        return new Greeting(counter.incrementAndGet(),
                            String.format(template, name));
    }
}

@RestController annotation can be used when all the controller handler should return a JSON string.However if your need is that some method may return JSON string,just use @ResponseBody above that method.And return a Object.The Spring framework will do the serialize work for you.The code snippet is like:

@Controller
public class GreetingController {

    private static final String template = "Hello, %s!";
    private final AtomicLong counter = new AtomicLong();

    @RequestMapping("/greeting")
    @ResponseBody
    public Greeting greeting(@RequestParam(value="name", defaultValue="World") String name) {
        return new Greeting(counter.incrementAndGet(),
                            String.format(template, name));
    }
}

you can use the jackson library

Jackson library

Use @ResponseBody annotation on your method which is returning data...Also if you can show the code it will be more helpful

Spring boot uses Jackson to deserialize JSON into Java instances and serialize Java objects back to JSON payloads. You may check the sample project written by me here [1]. Literally you don't need to manipulate any JSON. You work using Java Objects. Let Jackson take care of the transformation between Java Objects and JSON payloads. You just need to follow up few rules. For an example your Java class should have same field names as JSON payload and compatible data types, then Jackson will bind them on your behalf. Hope this helps. Happy Coding.

[1] https://github.com/ravindraranwala/SpringBootRxJava/

In addition to CALTyang answer , we can also use ResponseEntity to return the response.

code snippet :

        @RestController
        public class PersonController {

            @Autowired
            private PersonRepository personRepository;

            @RequestMapping(value = "/persons/{id}", method = RequestMethod.GET,produces={MediaType.APPLICATION_XML_VALUE},headers = "Accept=application/xml")
            public ResponseEntity<?> getPersonDetails(@PathVariable Long id, final HttpServletRequest request)throws Exception {
                ConnectionManager cm=new ConnectionManager();
                Person personResponse=cm.getDetails();
                return ResponseEntity.ok(personResponse);
            }

        }

@user7271107 - here is sample prototype code and response format.I am fetching the data from DB.

=====================================================

Response

{
    "records":
    [
        {
            "hr": 1,
            "km": 20
        },
        {
            "hr": 2,
            "km": 23
        },
        {
            "hr": 3,
            "km": 29
        },
        {
            "hr": 4,
            "km": 50
        },
        {
            "hr": 5,
            "km": 55
        },
        {
            "hr": 6,
            "km": 60
        }
    ]
}

            package com.subu;

            import org.springframework.boot.SpringApplication;
            import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
            import org.springframework.boot.autoconfigure.SpringBootApplication;
            import org.springframework.boot.builder.SpringApplicationBuilder;
            import org.springframework.boot.context.web.SpringBootServletInitializer;
            import org.springframework.context.annotation.ComponentScan;
            import org.springframework.context.annotation.Configuration;
            import org.springframework.scheduling.annotation.EnableScheduling;


            @SpringBootApplication
            @Configuration
            @ComponentScan
            @EnableAutoConfiguration
            @EnableScheduling
            public class Application extends SpringBootServletInitializer{



               public static void main(String[] args) {
                  SpringApplication.run(Application.class, args);
               }

               @Override
               protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
                   return application.sources(Application.class);
               }

               private static Class<Application> applicationClass = Application.class;

            }

            @RestController
            public class PersonController {

                @Autowired
                private RecordRepository  recordRepository;


                @RequestMapping(value = "/records/", method = RequestMethod.GET,produces={MediaType.APPLICATION_JSON_VALUE},headers = "Accept=application/json")
                public ResponseEntity<?> getRecords(final HttpServletRequest request)throws Exception {
                    DBRecordArray dr= new DBRecordArray();
                    List<DBRecord> list=recordRepository.findAll();
                    dr.setRecords(list);
                    return ResponseEntity.ok(dr);
                }

            }

            package com.subu;

            import java.io.Serializable;

            import javax.persistence.Entity;
            import javax.persistence.GeneratedValue;
            import javax.persistence.Id;
            import javax.persistence.Table;


            @Entity
            @Table(name="record")
            public class DBRecord implements Serializable{

                private static final long serialVersionUID = 1L;
                @Id
                @GeneratedValue
                private Long id;

                private int hr;
                private int km;
                public int getHr() {
                    return hr;
                }
                public void setHr(int hr) {
                    this.hr = hr;
                }
                public int getKm() {
                    return km;
                }
                public void setKm(int km) {
                    this.km = km;
                }

            }

            package com.subu;

            import java.util.List;

            public class DBRecordArray {

                private List<DBRecord> records;

                public List<DBRecord> getRecords() {
                    return records;
                }

                public void setRecords(List<DBRecord> records) {
                    this.records = records;
                }
            }

            package com.subu;


            import java.util.List;

            import org.springframework.data.jpa.repository.JpaRepository;

            public interface RecordRepository extends JpaRepository<DBRecord, Long> {

                List<DBRecord> findAll();
            }