Question

I have several classes that use the same JNI library, called “jni”. In each of those classes, I have a System.loadLibrary() call in the class’s static initializer:

Class class1 {
    static{
        System.loadLibrary("jni");
    }
    ...
}

Class class2 {
    static{
        System.loadLibrary("jni");
    }
    ...
}

The only problem is, I only want System.loadLibrary() to be called once, since it can take a while. Is there an easy and clean way to prevent multiple loadings of the same library?

Answer 1

If the library has already be loaded any subsequent calls to load the same library will be ignored.

System.loadLibrary:

Loads the system library specified by
the libname argument. The manner in
which a library name is mapped to the
actual system library is system
dependent.

The call System.loadLibrary(name) is
effectively equivalent to the call

 Runtime.getRuntime().loadLibrary(name)

Runtime.loadLibrary:

Loads the dynamic library with the
specified library name. A file
containing native code is loaded from
the local file system from a place
where library files are conventionally
obtained. The details of this process
are implementation-dependent. The
mapping from a library name to a
specific filename is done in a
system-specific manner.

First, if there is a security manager,
its checkLink method is called with
the libname as its argument. This may
result in a security exception.

The method System.loadLibrary(String)
is the conventional and convenient
means of invoking this method. If
native methods are to be used in the
implementation of a class, a standard
strategy is to put the native code in
a library file (call it LibFile) and
then to put a static initializer:

 static { System.loadLibrary("LibFile"); }

within the class declaration. When the
class is loaded and initialized, the
necessary native code implementation
for the native methods will then be
loaded as well.

If this method is called more than
once with the same library name, the
second and subsequent calls are
ignored.