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Suseendran P
Suseendran P

Reputation: 557

How native mixed Java code is platform independent?

In the code of many Java library classes I can see native methods. Even in the Object class.

If Java is platform independent when Java code is converted into byte codes, then what about native code? Is it also converted into byte code?

Does this native code call go to the OS or is it coming from downloaded or installation of Java itself?

Upvotes: 0

Views: 295

Answers (2)

Stephen C
Stephen C

Reputation: 718816

There are different flavours of native method:

  • The native methods in the standard Java libraries will all be implemented (by Oracle and / or the vendor of your Java implementation) for the platform that you are running on. Doing this is part of the process of developing Java for the platform. By the time you get to use Java (on that platform) the porting work has been done. (The methods are implemented the JVM and its associated native code libraries / dlls.)

  • Native methods in your code or in 3rd party libraries are a different matter. The native code that implements these methods does indeed represent a portability impediment, because it need to (at least) be recompiled for each platform. And in a lot of cases, the porting process may even extent to a complete rewrite of the (native) code.

If Java is platform independent when Java code is converted into byte codes, then what about native code? Is it also converted into byte code?

No1. Native methods are implemented in some other programming language; e.g. C or C++.

(If the native methods could be translated to bytecodes, there would be no need for them to be written as "native" in the first place!)

Does this native code call go to the OS or is it coming from downloaded or installation of Java itself?

It is unlikely that a Java native method will map directly to an system call or a call to one of the standard OS provided libraries. Native methods are usually implemented either by the JVM implementation, or by customer or 3rd party native libraries. See above.

1 - Actually, there is one exception to this. On the JNode platform, most methods in the Java core libraries that are marked as native do in fact map back to Java code. But that is because, almost the entire JNode operating system is implemented in Java. JNode's native code compiler implements some "clever tricks" to allow this to happen.

Upvotes: 1

Saurabh
Saurabh

Reputation: 7964

Java library code does make native calls. Now these calls are fulfilled by JVM. If you notice then each system has OS specific JVM, so all the system-dependent native calls are ultimately served by the system dependent JVM implementations.

Upvotes: 3

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