http://en.wikipedia.org/wiki/JNI

http://java.sun.com/docs/books/jni/html/intro.html#1811



The Java Native Interface (JNI) is a programming framework that allows Java code running in a Java Virtual Machine (JVM) to call and to be called[1]hardware and operating system platform) and libraries written in other languages, such as C, C++ and assembly.


JNI allows one to write native methods to handle situations when an application cannot be written entirely in the Java programming language e.g. when the standard Java class library does not support the platform-specific features or program library. It is also used to modify an existing application, written in another programming language, to be accessible to Java applications. Many of the standard library classes depend on JNI to provide functionality to the developer and the user, e.g. I/O file reading and sound capabilities. Including performance- and platform-sensitive API implementations in the standard library allows all Java applications to access this functionality in a safe and platform-independent manner. Before resorting to using JNI, developers should make sure the functionality is not already provided in the standard libraries.

The JNI framework lets a native method utilize Java objects in the same way that Java code uses these objects. A native method can create Java objects and then inspect and use these objects to perform its tasks. A native method can also inspect and use objects created by Java application code.

JNI is sometimes referred to as the "escape hatch" for Java developers because it allows them to add functionality to their Java application that the standard Java APIs cannot otherwise provide. It can be used to interface with code written in other languages, such as C and C++. It is also used for time-critical calculations or operations like solving complicated mathematical equations, since native code can be faster than JVM code.



The JNI is designed to handle situations where you need to combine Java applications with native code. As a two-way interface, the JNI can support two types of native code: native libraries and native applications.

  • You can use the JNI to write native methods that allow Java applications to call functions implemented in native libraries. Java applications call native methods in the same way that they call methods implemented in the Java programming language. Behind the scenes, however, native methods are implemented in another language and reside in native libraries.
  • The JNI supports an invocation interface that allows you to embed a Java virtual machine implementation into native applications. Native applications can link with a native library that implements the Java virtual machine, and then use the invocation interface to execute software components written in the Java programming language. For example, a web browser written in C can execute downloaded applets in an embedded Java virtual machine implemention.


Remember that once an application uses the JNI, it risks losing two benefits of the Java platform.

First, Java applications that depend on the JNI can no longer readily run on multiple host environments. Even though the part of an application written in the Java programming language is portable to multiple host environments, it will be necessary to recompile the part of the application written in native programming languages.

Second, while the Java programming language is type-safe and secure, native languages such as C or C++ are not. As a result, you must use extra care when writing applications using the JNI. A misbehaving native method can corrupt the entire application. For this reason Java applications are subject to security checks before invoking JNI features.

As a general rule, you should architect the application so that native methods are defined in as few classes as possible. This entails a cleaner isolation between native code and the rest of the application.