java.lang.UnsatisfiedLinkError no *****.dll in java.library.path

Question

How can I load a custom dll file in my web application? I've tried the following:

• Copied all required dlls in system32 folder and tried to load one of them in Servlet constructor System.loadLibrary
• Copied required dlls into tomcat_home/shared/lib and tomcat_home/common/lib

All these dlls are in WEB-INF/lib of the web-application

Answer 1

In order for System.loadLibrary() to work, the library (on Windows, a DLL) must be in a directory somewhere on your PATH or on a path listed in the java.library.path system property (so you can launch Java like java -Djava.library.path=/path/to/dir).

Additionally, for loadLibrary(), you specify the base name of the library, without the .dll at the end. So, for /path/to/something.dll, you would just use System.loadLibrary("something").

You also need to look at the exact UnsatisfiedLinkError that you are getting. If it says something like:

Exception in thread "main" java.lang.UnsatisfiedLinkError: no foo in java.library.path 

then it can't find the foo library (foo.dll) in your PATH or java.library.path. If it says something like:

Exception in thread "main" java.lang.UnsatisfiedLinkError: com.example.program.ClassName.foo()V 

then something is wrong with the library itself in the sense that Java is not able to map a native Java function in your application to its actual native counterpart.

To start with, I would put some logging around your System.loadLibrary() call to see if that executes properly. If it throws an exception or is not in a code path that is actually executed, then you will always get the latter type of UnsatisfiedLinkError explained above.

As a sidenote, most people put their loadLibrary() calls into a static initializer block in the class with the native methods, to ensure that it is always executed exactly once:

class Foo {      static {         System.loadLibrary('foo');     }      public Foo() {     }  } 

Answer 2

Changing 'java.library.path' variable at runtime is not enough because it is read only once by JVM. You have to reset it like:

System.setProperty("java.library.path", path); //set sys_paths to null final Field sysPathsField = ClassLoader.class.getDeclaredField("sys_paths"); sysPathsField.setAccessible(true); sysPathsField.set(null, null); 

Please, take a loot at: Changing Java Library Path at Runtime.