我正在创建一个管理一些客户信息的应用程序。但是，如果知道每次重新启动代码时，数据库中的数据都会完全丢失，代码就可以正常工作。我刚输入了springboot代码。
重启控制器.java

package crudapp.controller;

import java.util.Arrays;
import java.util.List;
import java.util.Optional;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.PutMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import crudapp.model.Customer;
import crudapp.model.Message;
import crudapp.service.CustomerServices;

@RestController
@RequestMapping("/api/customer")
public class RestAPIController {

    @Autowired
    CustomerServices customerServices;

    @PostMapping("/create")
    public ResponseEntity<Message> addNewCustomer(@RequestBody Customer customer) {
        try {
            Customer returnedCustomer = customerServices.saveCustomer(customer);

            return new ResponseEntity<Message>(new Message("Upload Successfully!", 
                                            Arrays.asList(returnedCustomer), ""), HttpStatus.OK);
        }catch(Exception e) {
            return new ResponseEntity<Message>(new Message("Fail to post a new Customer!", 
                                            null, e.getMessage()), HttpStatus.INTERNAL_SERVER_ERROR);           
        }
    }

    @GetMapping("/retrieveinfos")
    public ResponseEntity<Message> retrieveCustomerInfo() {

        try {
            List<Customer> customerInfos = customerServices.getCustomerInfos();

            return new ResponseEntity<Message>(new Message("Get Customers' Infos!", 
                                                customerInfos, ""), HttpStatus.OK);
        }catch(Exception e) {
            return new ResponseEntity<Message>(new Message("Fail!",
                                                null, e.getMessage()), HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }

    @GetMapping("/findone/{id}")
    public ResponseEntity<Message> getCustomerById(@PathVariable long id) {
        try {
            Optional<Customer> optCustomer = customerServices.getCustomerById(id);

            if(optCustomer.isPresent()) {
                return new ResponseEntity<Message>(new Message("Successfully! Retrieve a customer by id = " + id,
                                                            Arrays.asList(optCustomer.get()), ""), HttpStatus.OK);
            } else {
                return new ResponseEntity<Message>(new Message("Failure -> NOT Found a customer by id = " + id,
                        null, ""), HttpStatus.NOT_FOUND);
            }
        }catch(Exception e) {
            return new ResponseEntity<Message>(new Message("Failure",
                    null, e.getMessage()), HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }

    @PutMapping("/updatebyid/{id}")
    public ResponseEntity<Message> updateCustomerById(@RequestBody Customer _customer, 
                                                                    @PathVariable long id) {
        try {
            if(customerServices.checkExistedCustomer(id)) {
                Customer customer = customerServices.getCustomerById(id).get();

                //set new values for customer
                customer.setFirstname(_customer.getFirstname());
                customer.setLastname(_customer.getLastname());
                customer.setAddress(customer.getAddress());
                customer.setAge(_customer.getAge());

                // save the change to database
                customerServices.updateCustomer(customer);

                return new ResponseEntity<Message>(new Message("Successfully! Updated a Customer "
                                                                        + "with id = " + id,
                                                                    Arrays.asList(customer), ""), HttpStatus.OK);
            }else {
                return new ResponseEntity<Message>(new Message("Failer! Can NOT Found a Customer "
                        + "with id = " + id,
                    null, ""), HttpStatus.NOT_FOUND);
            }
        }catch(Exception e) {
            return new ResponseEntity<Message>(new Message("Failure",
                    null, e.getMessage()), HttpStatus.INTERNAL_SERVER_ERROR);           
        }
    }

    @DeleteMapping("/deletebyid/{id}")
    public ResponseEntity<Message> deleteCustomerById(@PathVariable long id) {
        try {
            // checking the existed of a Customer with id
            if(customerServices.checkExistedCustomer(id)) {
                customerServices.deleteCustomerById(id);

                return new ResponseEntity<Message> (new Message("Successfully! Delete a Customer with id = " + id, 
                                                        null, ""), HttpStatus.OK);
            }else {
                return new ResponseEntity<Message>(new Message("Failer! Can NOT Found a Customer "
                                                        + "with id = " + id, null, ""), HttpStatus.NOT_FOUND);
            }
        }catch(Exception e) {
            return new ResponseEntity<Message>(new Message("Failure",
                    null, e.getMessage()), HttpStatus.INTERNAL_SERVER_ERROR);
        }
    }
}

客户.java

package crudapp.model;

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

@Entity
@Table(name="customer")
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;

    @Column
    private String firstname;

    @Column
    private String lastname;

    @Column
    private String address;

    @Column
    private int age;

    public void setId(long id) {
        this.id = id;
    }

    public long getId() {
        return this.id;
    }

    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }

    public String getFirstname() {
        return this.firstname;
    }

    public void setLastname(String lastname) {
        this.lastname = lastname;
    }

    public String getLastname() {
        return this.lastname;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public String getAddress() {
        return this.address;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public int getAge() {
        return this.age;
    }

    protected Customer() {}

    public Customer(String firstname, String lastname, String address, int age) {
        this.firstname = firstname;
        this.lastname = lastname;
        this.address = address;
        this.age = age;
    }

    public String toString() {
        return String.format("id=%d, firstname='%s', lastname'%s', address=%s, age=%d", 
                                id, firstname, lastname, address, age); 
    }
}

customerrepository.java文件

package crudapp.repository;

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

import crudapp.model.Customer;

@Repository
public interface CustomerRepository extends JpaRepository<Customer, Long>{
}

客户服务.java

package crudapp.service;

import java.util.List;
import java.util.Optional;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

import crudapp.model.Customer;
import crudapp.repository.CustomerRepository;

@Service
public class CustomerServices {

    @Autowired CustomerRepository repository;

    public Customer saveCustomer(Customer customer) {
        return repository.save(customer);
    }

    public List<Customer> getCustomerInfos(){
        return repository.findAll();
    }

    public Optional<Customer> getCustomerById(long id) {
        return repository.findById(id);
    }

    public boolean checkExistedCustomer(long id) {
        if(repository.existsById((long) id)) {
            return true;
        }
        return false;
    }

    public Customer updateCustomer(Customer customer) {
        return repository.save(customer);       
    }

    public void deleteCustomerById(long id) {
        repository.deleteById(id);
    }
}

应用程序属性

spring.datasource.url=jdbc:mysql://localhost:3306/customerdb
spring.datasource.username=root
spring.datasource.password=12345
spring.jpa.generate-ddl=true

spring.jpa.hibernate.ddl-auto=create