Android NDK，保持实时 C++ 对象

问题描述

我遇到了以下问题.我想编写一个使用我的遗留 C++ 类的 Android 应用程序.我必须在整个应用程序生命周期内保持 C++ 对象处于活动状态.

I've got a following problem. I want to write an Android application, which uses my legacy C++ classes. I have to keep a C++ object alive during the whole application lifetime.

我用 C# 编写了一个类似的应用程序，并通过将指向 C++ 类的指针传递给 C# 并使用 IntPtr 将其存储在那里来解决了这个问题.然后，当我想调用该对象的方法时，我只需将该指针再次传递给 C++，转换为类指针并调用它的方法.

I wrote a similar application in C# and solved the problem by passing a pointer to a C++ class to C# and storing it there using IntPtr. Then, when I wanted to call a method on that object, I simply passed that pointer again to C++, converted to a class pointer and called a method on it.

如何在 Java 和 Android NDK 中获得类似的结果?Java 是否支持存储指针?

How can I achieve similar result in Java and Android NDK? Does Java support storing pointers?

推荐答案

是的，您可以执行与在 C# 中所做的完全相同的操作.

Yes, you can do the exact same than what you did in C#.

创建新的 C++ 对象:

To create your new C++ Object:

jlong
Java_package_name_new(JNIEnv *, jobject) {
  return (long)(new CPP_Object()); 
}

您可以将此方法的返回值存储在 Java ptr 变量中，并将其传递给所有需要它的 NDK 方法:

You can store the return value of this method in a Java ptr variable, and pass it to all NDK methods that need it:

void
Java_package_name_doSomething(JNIEnv *, jobject, jlong ptr) {
  CPP_Object *obj = (CPP_Object *)ptr;
  // do whatever you want with the object
}

最后用类似这样的方式删除它:

And finally delete it with something like:

void
Java_package_name_delete(JNIEnv *, jobject, jlong ptr) {
  delete (CPP_Object *)(ptr);
}

除了将 ptr 传递给所有需要它的方法，您还可以使用 SetLongField 和 GetLongField<直接从 NDK 部分获取并设置它/code> 方法:这允许仅从代码的 NDK 部分管理 Java ptr 变量，我发现它更安全且更易于管理.