[ jEdit-devel ] Important - Do not use "instanceof" to check the type of EditBus messages of other plugins in handleMessage()

[Shlomy R. <sre...@gm...>]

Hi,

Before I dive into details, please avoid checking the types of edit
bus messages of other plugins using "instanceof", such as:

  void handleMessage(EBMessage message)
  {
    if (message instanceof ProjectUpdate)
      ...
  }

Instead, use something like:

  void handleMessage(EBMessage message)
  {
    if (message.getClass().getName().endsWith("ProjectUpdate"))
      ...
  }

Why? The "instanceof" version can cause the other plugin to be loaded
- i.e. if the plugin which defines the message type (ProjectViewer in
this case) is not loaded, the JARClassLoader will be invoked to load
it. And one of the first things that JARClassLoader does is activate
the plugin. All this can happen just because the type of the edit bus
message was checked - not because the plugin really had to be loaded
(or activated).
The problem is a lot worse than just wasting resources. In the above
example, ProjectViewer will be loaded (if it wasn't loaded before) and
activated. Part of the activation is instantiating the plugin and
calling its "start" method. In the case of ProjectViewer, this will
also cause an edit bus message (ProjectUpdate) to be sent. Then, the
above 'handleMessage' will be called again, recursively, and will
check again if the message is "instanceof ProjectUpdate". Since the
JARClassLoader that was invoked to load the ProjectViewer hasn't
returned yet, the ProjectViewer is considered as currently being
loaded, but not loaded yet. So this recursive invocation of
"handleMessage" will generate a "ClassCircularityError" exception, and
I am not sure what the consequences are.

Instead of using "instanceof", which might load and activate the
plugin, it should be safe to use "message.getClass().getName()" and
check if the name is the type we expect. This will not cause the
plugin to be loaded. Perhaps at some point we'll add something to the
JARClassLoader to take care of this (if possible), but until then,
please avoid using "instanceof" with classes of other plugins.

Note that the danger is only for plugins that might not yet be loaded
when this edit bus message is sent. If you are sure that the other
plugin is already loaded, there is no problem with using "instanceof".

Thanks,
Shlomy