An instance of this class represents a value that is provided per (thread)
context classloader.
Occasionally it is necessary to store data in "global" variables
(including uses of the Singleton pattern). In applications which have only
a single classloader such data can simply be stored as "static" members on
some class. When multiple classloaders are involved, however, this approach
can fail; in particular, this doesn't work when the code may be run within a
servlet container or a j2ee container, and the class on which the static
member is defined is loaded via a "shared" classloader that is visible to all
components running within the container. This class provides a mechanism for
associating data with a ClassLoader instance, which ensures that when the
code runs in such a container each component gets its own copy of the
"global" variable rather than unexpectedly sharing a single copy of the
variable with other components that happen to be running in the same
container at the same time (eg servlets or EJBs.)
This class is strongly patterned after the java.lang.ThreadLocal
class, which performs a similar task in allowing data to be associated
with a particular thread.
When code that uses this class is run as a "normal" application, ie
not within a container, the effect is identical to just using a static
member variable to store the data, because Thread.getContextClassLoader
always returns the same classloader (the system classloader).
Expected usage is as follows:
public class SomeClass {
private static final ContextClassLoaderLocal global
= new ContextClassLoaderLocal() {
protected Object initialValue() {
return new String("Initial value");
};
public void testGlobal() {
String s = (String) global.get();
System.out.println("global value:" + s);
buf.set("New Value");
}
Note: This class takes some care to ensure that when
a component which uses this class is "undeployed" by a container the
component-specific classloader and all its associated classes (and their
static variables) are garbage-collected. Unfortunately there is one
scenario in which this does not work correctly and there
is unfortunately no known workaround other than ensuring that the
component (or its container) calls the "unset" method on this class for
each instance of this class when the component is undeployed. The problem
occurs if:
- the class containing a static instance of this class was loaded via
a shared classloader, and
- the value stored in the instance is an object whose class was loaded
via the component-specific classloader (or any of the objects it refers
to were loaded via that classloader).
The result is that the map managed by this object still contains a strong
reference to the stored object, which contains a strong reference to the
classloader that loaded it, meaning that although the container has
"undeployed" the component the component-specific classloader and all the
related classes and static variables cannot be garbage-collected. This is
not expected to be an issue with the commons-beanutils library as the only
classes which use this class are BeanUtilsBean and ConvertUtilsBean and
there is no obvious reason for a user of the beanutils library to subclass
either of those classes.
Note: A WeakHashMap bug in several 1.3 JVMs results in
a memory leak for those JVMs.
Note: Of course all of this would be unnecessary if
containers required each component to load the full set of classes it
needs, ie avoided providing classes loaded via a "shared" classloader.
See Also: java.lang.Thread.getContextClassLoader See Also: author: Eric Pabst |