netbeans.apache.org/src/content/wiki/DevFaqModuleCCE.asciidoc

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= DevFaqModuleCCE
:jbake-type: wiki
:jbake-tags: wiki, devfaq, needsreview
:jbake-status: published
:keywords: Apache NetBeans wiki DevFaqModuleCCE
:description: Apache NetBeans wiki DevFaqModuleCCE
:toc: left
:toc-title:
:syntax: true

== Why am I getting a `ClassCastException` when the class is clearly of the right type?

Q: I am getting an exception when I run my module, such as

[source,java]
----

java.lang.ClassCastException: Implementation cannot be cast to Interface
        at Factory.newInstance (Factory.java:123)
----

But `Implementation` implements `Interface`, so why is this a CCE?

A: Usually this is because the `Interface` that `Implementation` actually implements was loaded from a different class loader than what `Factory` sees.

That is very likely the immediate cause of the CCE. The root cause is not always obvious, but it is likely to be that some code uses `Thread.currentThread().getContextClassLoader()` to load a class by name. In NB by default the context class loader just looks around in loaded modules (lacking any better information), so it is possible for the following situation to arise:

[source,java]
----

module A: Interface, Factory
module B > A: Implementation implements Interface
module C: Interface, Factory
module D > A: Implementation implements Interface
----

where `Factory` does something like:

[source,java]
----

Interface i = (Interface) Class.forName("Implementation", true,
    Thread.currentThread().getContextClassLoader()).newInstance();
----

(This is a common design pattern for XML parsers, etc.)

Now if A's `Factory` happens to get D's `Implementation` (or C's gets B's) then you get a CCE at runtime.

An especially silly variant of this problem, known to occur at least in Xerces (), is that `Implementation` actually resides in the same JAR as `Interface` and `Factory`, and is the standard impl almost everyone uses unless overridden somehow - yet `Factory` loads it by name from the CCL rather than simply loading it directly using e.g. `new Implementation()`.

The usual workaround is to wrap the problematic call(s) in a dynamic block:

[source,java]
----

ClassLoader orig = Thread.currentThread().getContextClassLoader();
Thread.currentThread().setContextClassLoader(SomeReferenceClass.class.getClassLoader());
try {
  Factory.load(...);
} finally {
  Thread.currentThread().setContextClassLoader(orig);
}
----

Note: NetBeans forbids ambiguous delegations. ()
If a class could be loaded from two (or more) places, it will not be loaded at all.
This does not solve your problem but it at least ensures it gets reported more reliably
and with a descriptive message rather than an odd `ClassCastException`.

<hr/>
Applies to: NetBeans 6.5 and later

=== Apache Migration Information

The content in this page was kindly donated by Oracle Corp. to the
Apache Software Foundation.

This page was exported from link:http://wiki.netbeans.org/DevFaqModuleCCE[http://wiki.netbeans.org/DevFaqModuleCCE] , 
that was last modified by NetBeans user Jglick 
on 2009-12-03T14:11:30Z.


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