How JVM call the native method in host environment

Question

These day I have been reading about Java Native Interface.So by the way I do have doubt.Let say for a instance,If we need to do a I/O operation in Java program, we exercise the Java API for I/O operations.Moreover,at low level, it should be mapped to OS level I/O handling.The doubt is how Java API interact with native I/O methods in host operating system.

Short and Sweet, I heard some of the methods in JDK are implemented natively.How those native methods are called by Java API.I guess it would be JNI(Java Native Interface).

Could somebody clarify my doubts.

Thanks

Nuwan Arambage

Answer 1

Methods in Java can be marked native to indicate that their implementation is not written in Java or in bytecode, but rather in something platform-dependent. For example, Java I/O operations are almost always implemented as native methods so that they can take advantage of the underlying hardware or OS interface on the machine.

There is no guarantee whatsoever about how native methods are actually implemented. In Sun's (now Oracle's) implementation of the JVM, you can write implementations for native methods by using JNI to define specially-named C functions that interact with custom libraries in order to interface with Java code. However, another JVM could implement native methods in a totally different way. In fact, right now I'm working on a project to implement a JVM in JavaScript, and so all the native methods are implemented in JavaScript rather than C.

In short, there's no "one way" in which native methods are implemented. The whole point is to give maximum flexibility to the JVM and Java library implementations, and so the less specified the behavior is the better.

Answer 2

After a native library has been loaded, the methods are bound (integrated) into the JVM. The Java VM Spec uses the term binding rather then linking to avoid confusion. But that's just wording.

The rest is simple. There are some bytecode operands that are used to invoke a method. And if that message is declared native, then the associated native code is invoked. Parameters and results are converted so that it doesn't make a difference if we call a native or a non-native method.

Have a look at the spec of invokevirtual, the bulleted list covers the native case.