我试图构建一个简单的web服务，现在我只有3个类，一个实体类，一个dao类和一个tester类。
我的实体类：

@Entity
@Table(name="sales")
public class Sale implements Serializable{

    @Id
    @Column(name = "idsale_id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int idsale_id;

    @Column(name = "grand_total", nullable = false)
    private double grand_total;

    public Sale() {
    }

    public Sale(double grand_total) {
        this.grand_total = grand_total;
    }

我的数据库操作类

@ApplicationScoped
public class DatabaseOperations {

    @PersistenceContext(unitName = "owlJPA")
    EntityManager em;

    @Transactional
    public String createSale(double grand_total) {
        Sale sale = new Sale(grand_total);
        em.persist(sale);
        em.flush();

        return "Successfully added new entry in DB";
    }
}

休息处理代码

@RequestScoped
@Path("/hello-world")
public class HelloResource {

    @Inject
    DatabaseOperations databaseOperations;

    @POST
    @Produces("text/plain")
    @Consumes(MediaType.APPLICATION_JSON)
    public String POSTrecieved(JsonObject jsonRaw) {
        DatabaseOperations databaseOperations = new DatabaseOperations();

        try {
            String tempStr = jsonRaw.getJsonObject("newSale").getString("grand_total");
            double grand_total = Double.parseDouble(tempStr);
            String x = databaseOperations.createSale(grand_total);
            return "SUCESSFULLY ADDED NEW SALE, with grand total of: "+x;
        }
        catch(Exception error){
            return error.toString();
        }
    }

每当我尝试通过调用createsale方法来运行事务时，sale对象都会被很好地创建，但是当entitymanager em为null时，我会得到一个nullpointerexception错误。但是我的entitymanager em不应该像@applicationscoped那样被示例化吗？