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

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

=== When should I use RequestProcessor.getDefault() and when should I create my own RequestProcessor?

`RequestProcessor.getDefault()` is tempting to use, but it is also dangerous.  This FAQ item will tell you when *not* to use it.

One of the most common threading bugs in NetBeans happens like this:

* Someone needs to run a background operation, so they use [link:http://bits.netbeans.org/dev/javadoc/org-openide-util/org/openide/util/RequestProcessor.html#getDefault([http://bits.netbeans.org/dev/javadoc/org-openide-util/org/openide/util/RequestProcessor.html#getDefault(]) `RequestProcessor.getDefault()`, the global thread pool
* They don't realize that it is possible for either that code to be called repeatedly (by the user, in a loop or whatever)
* The background work can take some time to run
* A user notices that NetBeans is slow, takes a thread dump, and there are 40 background threads all doing the same thing at the same time

`RequestProcessor` has a constructor argument for its _throughput_.  That says how many threads this `RequestProcessor` is allowed to use at the same time. When you call `new RequestProcessor("Useful name for thread dump", 3)` you are creating a thread pool that can have 3 threads available to run things on simultaneously.

*The throughput of `RequestProcessor.getDefault()` is `Integer.MAX_VALUE`*.  Think about what that means:  it can potentially create thousands of threads;  but your OS cannot necessarily handle thousands of threads, and you probably don't have thousands of CPUs.  More threads than CPUs means the OS does extra work time-slicing between the threads and things get slower, not faster.

`RequestProcessor.getDefault()` is useful for _one-off_ operations - you have some situation that happens once in a great while, and, say, while constructing some object, you need to do some work in the background;  that work will probably never need to be done again for the life of the Java VM.  That's a perfect case for `RequestProcessor.getDefault()`.

Now here is the *anti-example*:  You are creating a `link:http://bits.netbeans.org/dev/javadoc/org-openide-nodes/org/openide/nodes/Node.html[Node]` that represents a file.  It needs to mark itself with an error badge and color its text in red if the file contains errors.  You can't read the file when you create the Node - that takes too long.  So when the node is created, it runs a background task to check its status, and updates its icon and display name after it has read the file.  Now imagine you did this with `RequestProcessor.getDefault()`.  What happens when the user expands a folder that contains 1000 of your files?  1000 threads get created, and the whole application gets very, very slow.  For that, you are much better off creating one new `RequestProcessor` and using it for all your nodes.  The FAQ entry about `link:DevFaqRequestProcessorTask.asciidoc[RequestProcessor.Task]` shows how to do this correctly.

If you create your own RequestProcessor, please always use a name. If you get a deadlock it makes debugging much easier.

=== 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/DevFaqRequestProcessor[http://wiki.netbeans.org/DevFaqRequestProcessor] , 
that was last modified by NetBeans user Braiam 
on 2010-03-03T22:04:22Z.


*NOTE:* This document was automatically converted to the AsciiDoc format on 2018-02-07, and needs to be reviewed.