Java Native Interface

Java Native Interface

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Java Native Interface

The Java Native Interface (JNI) is a programming framework that allows Java code running in the Java virtual machine (JVM) to call and be called^[1] by native applications (programs specific to a hardware and operating system platform) and libraries written in other languages, such as C, C++ and assembly.

Purpose and features
Pitfalls
How the JNI works

Mapping types
JNIEnv*

Advanced uses

Native AWT painting
Access to assembly code

Microsoft's RNI
See also
References
External links

Purpose and features

The JNI is used to write native methods to handle situations when an application cannot be written entirely in the Java programming language such as 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 the 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 the 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 valve" for Java developers because it allows them to add functionality to their Java Application that the Java API can't provide. It can be used to interface with code written in other languages, like 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.

Pitfalls

The JNI is not trivial and requires a considerable effort to learn, and some people recommend that only advanced programmers should use the JNI. However, the capability for Java to communicate with C++ and assembly removes any limitations on what function Java programs can perform. Programmers considering using the JNI should be aware that

as mentioned before, the JNI is not an easy API to learn;
subtle errors in the use of JNI can destabilize the entire JVM in ways that are very difficult to reproduce and debug;
only applications and signed applets can invoke the JNI;
an application that relies on JNI loses the platform portability Java offers (a workaround is to write a separate implementation of the JNI code for each platform and have Java detect the Operating System and load the correct one at runtime);
there is no garbage collection for the JNI side (JNI code must do explicit deallocation);
error checking is a MUST or it has the potential to crash the JNI side and the JVM.

Java automated garbage collection is quite different from C malloc/free because it can move objects after they are allocated. It is thus vital that pointers to Java objects are obtained and locked correctly. C programmers often do not understand this which can lead to very obscure and unreproduceable errors.

JNI needs great caution and is generally avoided by Java developers. For example, most Java JDBC databases communicate directly with a socket rather than use existing C side APIs.

How the JNI works

In JNI, native functions are implemented in a separate .c or .cpp files. (C++ provides a slightly cleaner interface with JNI.) When the JVM invokes the function, it passes a JNIEnv pointer, a jobject pointer, and any Java arguments declared by the Java method. A JNI function may look like this:

The env pointer is a structure that contains the interface to the JVM. It includes all of the functions necessary to interact with the JVM and to work with Java objects. Example JNI functions are converting native arrays to/from Java arrays, converting native strings to/from Java strings, instantiating objects, throwing exceptions, etc. Basically, anything that Java code can do can be done using JNIEnv, albeit with considerably less ease.

For example, the following converts a Java string to a native string:

Note that C++ JNI code is syntactically slightly cleaner than C JNI code because like Java, C++ uses object method invocation semantics. That means that in C, the env parameter is dereferenced using (*env)-> and env has to be explicitly passed to JNIEnv methods. In C++, the env parameter is dereferenced using env-> and the env parameter is implicitly passed as part of the object method invocation semantics.

Native data types can be mapped to/from Java data types. For compound types such as objects, arrays and strings the native code must explicitly convert the data by calling methods in the JNIEnv.

Mapping types

The following table shows the mapping of types between Java and native code.

Native Type	Java Language Type	Description	Type signature
unsigned char	jboolean	unsigned 8 bits	Z
signed char	jbyte	signed 8 bits	B
unsigned short	jchar	unsigned 16 bits	C
short	jshort	signed 16 bits	S
int	jint	signed 32 bits	I
long long __int64	jlong	signed 64 bits	J
float	jfloat	32 bits	F
double	jdouble	64 bits	D

In addition, the signature "L fully-qualified-class ;" would mean the class uniquely specified by that name; e.g., the signature "Ljava/lang/String;" refers to the class java.lang.String. Also, prefixing [ to the signature makes the array of that type; for example, [I means the int array type.

Here, these types are interchangeable. You can use jint where you normally use an int, and vice-versa, without any typecasting required.

However, mapping between Java Strings and arrays to native strings and arrays is different. If you use a jstring in where a char * would be, your code could crash the JVM.

This is similar with Java arrays, as illustrated in the example below that takes the sum of all the elements in an array.

Of course, there is much more to it than this. Look for links below for more information.

JNIEnv*

In each native call JNIEnv argument is only valid during the call. To use the argument outside the call you need to use AttachCurrentThread and DetachCurrentThread, like so:

JNIEnv *env;
(*g_vm)->AttachCurrentThread (g_vm, (void **) &env, NULL);
// do stuff
(*g_vm)->DetachCurrentThread (g_vm);

Advanced uses

Native AWT painting

Not only can native code interface with Java, it can also draw on a Java , which is possible with the Java AWT Native Interface. The process is almost the same, with just a few changes. The Java AWT Native Interface is only available since J2SE 1.3.

Access to assembly code

JNI also allows direct access to assembly code, without even going through a C bridge^[2]. Accessing Java applications from assembly is also possible in the same way^[3].

Microsoft's RNI

Microsoft's proprietary implementation of a Java Virtual Machine (Visual J++) had a similar mechanism for calling native Windows code from Java, called the Raw Native Interface (RNI). However, following the Sun - Microsoft litigation about this implementation, Visual J++ is no longer maintained.