A native method has the same syntax as an abstract method, but where is it implemented?
解决方案
What are native methods in Java and where should they be used?
Once you see a small example, it becomes clear:
Main.java:
public class Main {
public native int intMethod(int i);
public static void main(String[] args) {
System.loadLibrary("Main");
System.out.println(new Main().intMethod(2));
}
}
Main.c:
#include
#include "Main.h"
JNIEXPORT jint JNICALL Java_Main_intMethod(
JNIEnv *env, jobject obj, jint i) {
return i * i;
}
Compile and run:
javac Main.java
javah -jni Main
gcc -shared -fpic -o libMain.so -I${JAVA_HOME}/include \
-I${JAVA_HOME}/include/linux Main.c
java -Djava.library.path=. Main
Output:
4
Tested on Ubuntu 14.04 with Oracle JDK 1.8.0_45.
So it is clear that it allows you to:
call a compiled dynamically loaded library (here written in C) with arbitrary assembly code from Java
and get results back into Java
This could be used to:
write faster code on a critical section with better CPU assembly instructions (not CPU portable)
make direct system calls (not OS portable)
with the tradeoff of lower portability.
It is also possible for you to call Java from C, but you must first create a JVM in C: How to call Java functions from C++?
Example on GitHub for you to play with.
