JDBC新手技术问询：如何从HQL查询返回的Query对象中获取数据？

Hey there! Since you're new to working with HQL and JPA's Query interface, let's walk through exactly how to extract your query results clearly and safely.

Option 1: Retrieve Results as an Array of Objects (Quick & Simple) #

Your HQL query selects two specific fields (appkey and type), so the Query will return a List<Object[]> where each array represents a single row of results. Each element in the array maps to the fields in your query order.

Here's how to use it:

// Execute the query and get the result list
List<Object[]> results = query.getResultList();

// Iterate through each row
for (Object[] row : results) {
    // Cast elements to the correct data types (match your entity's field types)
    String appkey = (String) row[0];
    String type = (String) row[1]; // Adjust this if your `type` is an Integer or another type

    // Do something with the data (e.g., print, store in a collection, etc.)
    System.out.printf("Appkey: %s | Type: %s%n", appkey, type);
}

Option 2: Use a DTO for Type-Safe Results (Cleaner & Maintainable) #

For more readable and type-safe code, create a simple Data Transfer Object (DTO) to hold your query results. This avoids casting Object[] and makes your code easier to follow.

1. First, create the DTO class:

// Replace com.yourpackage with your actual package path
public class AppInfoDTO {
    private String appkey;
    private String type;

    // Constructor must match the order of fields in your HQL query
    public AppInfoDTO(String appkey, String type) {
        this.appkey = appkey;
        this.type = type;
    }

    // Add getters (and setters if needed)
    public String getAppkey() {
        return appkey;
    }

    public String getType() {
        return type;
    }
}

2. Update your HQL query to use the DTO:

// Use the fully qualified class name of your DTO in the HQL
final String hql = "select new com.yourpackage.AppInfoDTO(app.appkey, app.type) from " + getClassName() + " app where app.appkey<>'no-appkey' group by app.type";
Query query = getEntityManager().createQuery(hql);

// Now you can get a list of your DTO directly
List<AppInfoDTO> appInfoList = query.getResultList();

for (AppInfoDTO appInfo : appInfoList) {
    System.out.printf("Appkey: %s | Type: %s%n", appInfo.getAppkey(), appInfo.getType());
}

Bonus: Getting a Single Result #

If you expect only one row from your query, you can use getSingleResult() instead of getResultList(). Just be aware this will throw an exception if there are 0 or more than 1 results, so it's safe to pair it with setMaxResults(1):

query.setMaxResults(1);
try {
    Object[] singleResult = (Object[]) query.getSingleResult();
    String appkey = (String) singleResult[0];
    String type = (String) singleResult[1];
    // Handle the single result
} catch (NoResultException | NonUniqueResultException e) {
    // Handle cases where there's no result or too many results
    e.printStackTrace();
}

内容的提问来源于stack exchange，提问作者Shuitian Wei