什么是本地对象?我发现Java有对接本地对象的同级类接口?
Java程序可以使用JNI来访问在本地代码(编译成机器代码的所有内容)中实现的函数。与面向对象的本地代码进行交互需要一个Java类,该类使用JNI将方法调用从Java转发到本地类的实例。这个类是本地类的Java对等体。
例如:我们有一个C++中的print_hello类,我们需要在Java程序中使用它,为此我们需要定义它在Java中的对等体。
本地类
class print_hello{
public:
void do_stuff(){std::cout<<"hello"<<std::endl;}
}
Java 中的 peer 类
class PrintHello{
//Address of the native instance (the native object)
long pointer;
//ctor. calls native method to create
//instance of print_hello
PrintHello(){pointer = newNative();}
////////////////////////////
//This whole class is for the following method
//which provides access to the functionality
//of the native class
public void doStuff(){do_stuff(pointer);}
//Calls a jni wrapper for print_hello.do_stuff()
//has to pass the address of the instance.
//the native keyword keeps the compiler from
//complaining about the missing method body
private native void do_stuff(long p);
//
//Methods for management of native resources.
//
//Native instance creation/destruction
private native long newNative();
private native deleteNative(long p);
//Method for manual disposal of native resources
public void dispose(){deleteNative(pointer);pointer = 0;}
}
JNI代码(不完整)
所有声明为native
的方法都需要一个本地JNI实现。以下仅实现了上面声明的其中一个本地方法。
//the method name is generated by the javah tool
//and is required for jni to identify it.
void JNIEXPORT Java_PrintHello_do_stuff(JNIEnv* e,jobject i, jlong pointer){
print_hello* printer = (print_hello*)pointer;
printer->do_stuff();
}
如果Java对象有一些用C语言编写的本地方法,那么它就有一个对等的/本地对象。