Native libraries are platform-specific library files, including .dll, .so,
or *SRVPGM objects, that can be configured within shared libraries.
Native libraries are visible to an application class loader whenever
the shared library is associated with an application. Similarly, native
libraries are visible to an application server class loader whenever
the shared library is associated with an application server.
Before you begin
When designing a shared library, consider the following conditions
regarding Java native library support:
- The Java virtual machine (JVM) allows only one class loader to
load a particular native library.
- There is no application programming interface (API) to unload
a native library from a class loader.
Native libraries are unloaded
by the JVM when the class loader that found the library is collected
from the heap during garbage collection.
- Application server class loaders, unlike the native JVM class
loader, only load native shared libraries that use the default operating
system extension for the current platform. For example, on AIX, native
shared libraries must end in .a when loaded by
application server class loaders. The JVM class loader loads files
ending in .a or .so.
The Java Web Start can load native shared libraries
with the .so file extension only. Rename any native shared libraries
so that they have the .so file extension before packaging them in
a Java archive (JAR) file for Java Web Start deployment.
- Application server class loaders persist for the duration of the
application server.
- Application class loaders persist until an application is stopped
or dynamically reloaded.
If a shared library that is configured
with a native library path is associated with an application, whenever
the application is restarted or dynamically reloaded the application
might fail with an UnsatisfiedLinkError indicating that the library
is already loaded. The error occurs because, when the application
restarts, it invokes the shared library class to reload the native
library. The native library, however, is still loaded in memory because
the application class loader which previously loaded the native library
has not yet been garbage collected.
- Only the JVM class loader can load a dependent native library.
For
example, if NativeLib1 is dependent on NativeLib2, then NativeLib2 must
be visible to the JVM class loader. The path containing NativeLib2 must
be specified on Java library path defined by the LIBPATH environment
variable.
If a native
library configured in a shared library is dependent on other native
libraries, the dependent libraries must be configured on the LIBPATH
of the JVM hosting the application server in order for that library
to load successfully.
About this task
When configuring a shared library on a
shared library settings page,
if you specify a value for
Native library path,
the native libraries on this path are not located by the
WebSphere® Application Server application or
shared library class loaders unless the class which loads the native
library was itself loaded by the same class loader.
Because a native
library cannot be loaded more than once by a class loader, it is preferable
for native libraries to be loaded within shared libraries associated
with the class loader of an application server, because these class
loaders persist for the lifetime of the server.
Procedure
- Implement a static method in the class that loads the native
library.
In the class that loads the native library,
call System.loadLibrary(native_library) in
a static block. For example:
static {System.loadLibrary("native_library");
native_library loads
during the static initialization of the class, which occurs exactly
once when the class loads.
- On the shared library
settings page, set values for Classpath and Native
library path that enable the shared library to load the
native library.
If you want to associate your shared
library with an application or module, also select Use
an isolated class loader for this shared library. If you
do not enable this setting, associate the shared library with an application
server.
- Associate the shared library.
- If you did not enable Use an isolated class loader
for this shared library, associate the shared
library with an application server.
Associating a shared
library with the class loader of an application server, rather than
with an application, ensures that the shared library is loaded exactly
once by the application server class loader, even though applications
on the server are restarted or dynamically reloaded. Because the native
library is loaded within a static block, the native library is never
loaded more than once.
- If you enabled Use an isolated class loader for
this shared library, associate the shared library with an application or module.
Associating
an isolated shared library file with an application or module loads
the classes represented by the shared library in a separate class
loader created for that shared library. Do not associate an isolated
shared library file with a server if you want a separate class loader
for a shared library. If you associate the shared library with a server,
the product ignores the isolation setting and still adds files in
the shared library to the application server class loader. That is,
associating an isolated shared library file with a server associates
the file with all applications on the server.
The class loader
created for an isolated shared library does not reload and, like a
server class loader, exists for the lifetime of a server. For shared
native libraries, you can use an isolated shared library to avoid
errors resulting from reloading of native libraries.
What to do next
To verify that an application can use a shared library, test
the application or examine the class loader in the Class loader viewer.
Click . The classpath of
the application module class loader lists the classes used by the
shared library.