Sunday, November 14, 2004

A handy JNI trick for package names

One thing I find myself doing a lot is writing "package/Class" strings in calls to C++ JNI. A shortcut— define them as static members of a struct hierarchy mirroring the Java package hierarchy. To illustrate, take java.lang.Exception. In some header the declaration:

extern struct java
{
  struct lang
  {
    const char* Exception;
  } lang;
} java;

And elsewhere in the matching definition:

const char*
java::lang::Exception = "java/lang/Exception";

(Remember that JNI uses slashes instead of dots to separate package names.)

Now you can write code like this:

inline void
throwJavaException(JNIEnv* env, const std::string& msg)
{
  env->ThrowNew(env->FindClass(java.lang.Exception), msg.c_str());
}

I find using C++ program text that looks like Java program text is easier for me to read and conceptualize. I more interesting problems to occupy my mind than mentally translating strings to class names.

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