将Java双数组传递给Android Studio项目中的C ++

问题描述

我正在用Java开发Android Studio项目，该项目需要C ++才能实现某些信号处理功能。我需要能够将数组类型Double从Java传递给C ++。我尝试过分依赖于这样的示例，例如（Get java array from c++ via JNI），因为MainActivity.java文件已经作为jobject链接到我的native-lib.cpp文件中，我认为这让事情变得太复杂了。有没有更简单的方法来回传递变量？ （在下面的非常简单的示例中，我创建了一个名为“ getDouble”的方法来对其进行测试）

C ++代码：

SELECT 
  c.council_name,COUNT(*) AS transaction_count,SUM(amount_tendered) AS revenue
FROM councils c 
JOIN
(
  SELECT council_id,amount_tendered
  FROM market_levy  
  WHERE transaction_date >= CURDATE() 
  UNION ALL
  SELECT s.council_id,st.amount_tendered 
  FROM station_levy st    
  JOIN stations s ON s.station_id = st.station_id
  WHERE st.transaction_datetime >= CURDATE()
) totalCouncilRevenue USING (council_id)
GROUP BY council_id,c.council_name 
ORDER BY SUM(amount_tendered) DESC;

Java代码：

SELECT 
  c.council_name,COALESCE(t1.cnt,0) + COALESCE(t2.cnt,0) AS transaction_count,COALESCE(t1.total,0) + COALESCE(t2.total,0) AS revenue
FROM councils c 
LEFT JOIN
(
  SELECT council_id,SUM(amount_tendered) as total,COUNT(*) as cnt
  FROM market_levy  
  WHERE transaction_date >= CURDATE() 
  GROUP BY council_id
) t1 USING (council_id)
LEFT JOIN
(
  SELECT s.council_id,SUM(st.amount_tendered) as total,COUNT(*) as cnt
  FROM station_levy st    
  JOIN stations s ON s.station_id = st.station_id
  WHERE st.transaction_datetime >= CURDATE()
  GROUP BY s.council_id
) t2 USING (council_id)
ORDER BY revenue DESC;

解决方法

不，您的方法非常正确。但是，该实现有些偏离：

• 您应该使用MainActivity获得对env->GetObjectClass(obj)类的引用，其中obj是函数的第二个参数。
• getDouble的签名为()[D。使用javap -s查看正确的签名而不是猜测。

如果数组很大，您可能需要考虑使用GetDoubleArrayRegion来获取double*，您可以直接阅读，或者使用GetDoubleArrayElements / ReleaseDoubleArrayElements进行操作。