Question

我编写了一个Java EE应用程序来转换货币。用户输入要转换的值，＆＃34;来自＆＃34;货币和＆＃34;到＆＃34;货币，从数据库表中检索转换率。

！ [Netbeans中表格的片段] https://s16.postimg.org/8m242gyyd/Screen_Shot_2016_12_03_at_23_45_23.png

我的货币控制器具有以下代码：

public double convertTo(double value, String fromCur, String toCur) {
    TypedQuery<Rate> query = em.createQuery(""
            + "SELECT c FROM Rate c WHERE c.fromCur = 'BRA' AND c.toCur = 'EUR'", Rate.class);

    query.setParameter("tocurrency", toCur);
    query.setParameter("fromcurrency", fromCur);

    Rate result = query.getSingleResult();

    if (result == null) {
        throw new EntityNotFoundException("Can't find rate");
    }

    return value * result.getValue();

}

我的目标是从＆＃34; BRA＆＃34;到＆＃34; EUR＆＃34;例如，计算转换后的金额并将其返回。当我运行应用程序时，我得到了异常：

getsingleresult（）没有检索任何实体

必须有一种更简单的方法从数据库中检索速率以使用实体管理器计算此转换，但我无法为此找到正确的查询方法或JPA解决方案。有什么建议吗？

java实体Rate.java的代码如下：

package currency.model;

import java.io.Serializable;
import java.math.BigDecimal;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

/**
 *
 * @author teowey
 */
@Entity
public class Rate implements ConversionDTO, Serializable {


@Id
@GeneratedValue
private Integer conversionid;

private double rate;
private BigDecimal value;

private String fromCur;
private String toCur;
/**
* Creates a new instance of Account
*/
public Rate() {
}

@Override
public Integer getConversionId() {
    return conversionid;
}

public void setConversionId(Integer conversionid) {
    this.conversionid = conversionid;
}
@Override
public BigDecimal getValue() {
    return value;
}

public void setRate(double rate) {
    this.rate = rate;
}

public double getRate(String conversion) {
    return rate;
}

public String getFromCur() {
    return fromCur;
}

public void setFromCur(String from) {
    this.fromCur = from;
}

public String getToCur() {
    return toCur;
}

public void setToCur(String to) {
    this.toCur = to;
}

}

Answer 1

试图查看代码中是否存在导致问题的小问题但未找到问题。

这是我在测试中使用一些注释和调试代码来协助的方法。

// changed input amount to big decimal to avoid floating point issues
public BigDecimal convertTo(String from, String to, BigDecimal value) {

    // changed to validate the currency code - note BRA will fail - need to change to BRL - remove this code if you dont need it.
    Currency fromCurr = Currency.getInstance(from);
    Currency toCurr = Currency.getInstance(to);

    // debugging code - to help troubleshoot (only use this on small table) - will print out the contents of the table so you can see if data is not correct
    List<Rate> rates = em.createQuery("SELECT c FROM Rate c", Rate.class).getResultList();
    if (rates == null || rates.isEmpty()) {
        throw new RuntimeException("rates table is empty");
    }

    for (Rate r : rates) {
        // push to sysout or whatever logger you are using
        System.out.println(String.format("Rate:fromCur[%s], toCur[%s]", r.getFromCur(), r.getToCur()));
    }
    // end debugging code - remove when no longer needed.

    // changed the query to use bind parameters - :fromCurrency, :toCurrency
    TypedQuery<Rate> query = em.createQuery(
            "SELECT c FROM Rate c WHERE c.fromCur = :fromCurrency AND c.toCur = :toCurrency", Rate.class);

    query.setParameter("fromCurrency", fromCurr.getCurrencyCode());
    query.setParameter("toCurrency", toCurr.getCurrencyCode());

    Rate rate = query.getSingleResult();
    if (rate == null) {
        throw new EntityNotFoundException("Can't find rate");
    }

    // changed to use big decimal, scale and rounding as well as avoid floating point issues
    BigDecimal converted =
            value.multiply(rate.getValue()).setScale(toCurr.getDefaultFractionDigits(), RoundingMode.HALF_DOWN);

    return converted;
}

我提供了一些处理货币的说明，以帮助您避免潜在的陷阱。希望这有助于你前进

Answer 2

使用JPQL从数据库中检索行数据或表数据只需要实现JPQl查询。简单地说，我做了这个。我的表名是农民，className是农民，我使用了Annotations for database。

public static void main(String[] args) {
    EntityManager entityMgr = getEntityManager();
    entityMgr.getTransaction().begin();
    Query q = entityMgr.createQuery("SELECT s FROM Farmer s");

    List<Farmer> list = q.getResultList();
    for(Farmer s:list) {
         System.out.println("Id : "+s.getId()+" Farmer NAME : "+s.getName()+" location : "+s.getVillage());
      }
    entityMgr.getTransaction().commit();
    entityMgr.clear();
}

使用JPQL从数据库中检索行数据

2 个答案: