Java NMI

• What is a Native Method (NM)?

a method implemented in a programming language other than Java
 
 

• Why use NM?
 
1. access to special functionalities that are not yet implemented in Java

e.g.  interfacing to "new' peripheral device/plug-in cards, accessing special operating system features
2. software reuse

i.e.  may not want to rewrite existing codes in Java
3. speed

current Java programs do not run as fast as optimized C programs on some tasks
(intepretation overhead of bytecode?)
 
 
• Disadvantages of NM:
 
1. loss of portability -- native library needs to be reimplemented and recompiled for each platform
2. cannot currently use NM in an applet due to security restrictions
 
 
• What is Java Native Interface (JNI)?
 
Before JDK 1.1, implementation of native method interface varied across JVMs (e.g. Netscape's Java Runtime Interface, Microsoft's Raw Native Interface), which required a separate NM binary for each JVM.  JNI is a new feature of JDK 1.1 that defines an unified interface for incorporating the native methods.  JNI allows native methods to manipulate Java objects, invoke Java methods, handle exceptions, load classes, and perform runtime type checking as well as enabling non-Java applications to load and access JVM.
 

 


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• 6 steps to NMI
 
1. write the Java code
2. compile the Java code
3. create the JNI-style header file
4. implement the Native Method
5. create the shared library
6. run the Java program
 

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• NMI with arguments
 
• Java code
• header file
• Native Method using C
• Native Method using C++
• NMI using arrays
 
 

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• Accessing Java Methods from Native Methods
 
• Java code
• header file
• Native Method using C++
 

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• Invoking the Java Virtual Machine (excerps from Sun's JNI documentation )
 
In JDK1.1, the Java Virtual Machine is shipped as a shared library (or dynamic link library on Win32). You can embed the Java VM into your native application by linking the native application with the shared library. The JNI supports an Invocation API that allows you to load, initialize, and invoke the Java Virtual Machine. Indeed, the normal way of starting the Java interpreter, java, is no more than a simple C program that parses the command line arguments and invoke the Java Virtual Machine through the Invocation API.
 
 

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• Making a system call to execute external programs
 
NMI allows Java programs and Native programs to interact (i.e. call each other's methods and pass parameters back and forth), but its current API expects Native programs to be in C or C++. One way to expand the limitations of NMI is to use exec method of the Runtime class,which allow you to run external programs from your Java application. The instance of Process class returned by exec allows you to communicate with the external program (i.e. send and receive data via input and output stream).

Example:
Java Code: syscall.java
Perl Code: hello.pl

Programs are in /afs/cs.unc.edu/project/courses/190-25s98/members/yang/java.

 
 

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References

References useful for this discussion include the following::

• Sun's JNI documentation
• Introduction to O-O Programming in Java
• Java Overview
• Sun's Java Development Kit (JDK)
• Sun's Java API
• Java API Class Hierarchy