Java native util java native library

Java Language Java Native Interface Loading native libraries

The common idiom for loading shared library files in Java is the following :

public class ClassWithNativeMethods < static < System.loadLibrary("Example"); >public native void someNativeMethod(String arg); . 

Calls to System.loadLibrary are almost always static so as to occur during class loading, ensuring that no native method can execute before the shared library has been loaded. However the following is possible :

public class ClassWithNativeMethods < // Call this before using any native method public static void prepareNativeMethods() < System.loadLibrary("Example"); >. 

This allows to defer shared library loading until necessary, but requires extra care to avoid java.lang.UnsatisfiedLinkError s.

Target file lookup

Shared library files are searched for in the paths defined by the java.library.path system property, which can be overriden using the -Djava.library.path= JVM argument at runtime :

java -Djava.library.path=path/to/lib/:path/to/other/lib MainClassWithNativeMethods 

Watch out for system path separators : for example, Windows uses ; instead of : .

Note that System.loadLibrary resolves library filenames in a platform-dependent manner : the code snippet above expects a file named libExample.so on Linux, and Example.dll on Windows.

An alternative to System.loadLibrary is System.load(String) , which takes the full path to a shared library file, circumventing the java.library.path lookup :

public class ClassWithNativeMethods < static < System.load("/path/to/lib/libExample.so"); >. 

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A simple library class which helps with loading dynamic JNI libraries stored in the JAR archive

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adamheinrich/native-utils

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The tested method throws IllegalArgumentException instead of NullPointerException when called with a null argument.

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README.md

A simple library class which helps with loading dynamic libraries stored in the JAR archive. These libraries usualy contain implementation of some methods in native code (using JNI — Java Native Interface).

• The temporary file is stored into temp directory specified by java.io.tmpdir (by default it’s the operating system’s temporary directory). It should be automatically deleted when the application exits.
• Although the code has some try-finally section (to be sure that streams are closed properly in case an exception is thrown), it does not catch exceptions. The exception has to be handled by the application. I belive this approach is cleaner and has some benefits.

To load the dynamic library, just make sure it is packed inside the JAR archive and call method loadLibraryFromJar:

import cz.adamh.NativeUtils; public class HelloJNI < static < try < NativeUtils.loadLibraryFromJar("/resources/libHelloJNI.so"); >catch (IOException e) < // This is probably not the best way to handle exception :-) e.printStackTrace(); >> public native void hello(); > 

More information can be found in accompanying blog post.

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A simple library class which helps with loading dynamic JNI libraries stored in the JAR archive

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