Tag: Eclipse

Hot-Code Replace With The Breakpoint Properties Page in Eclipse

This is a convenient exploitation of how the Breakpoint Properties Page works.

A lot of people are aware of hot-code replacement functionality in the Eclipse JDT (Java Development Tools). This allows one to modify source code while an application is being debugged and have the generated class files be used in the current runtime.

This is great but in the past I have run into issues where this functionality, for some reason fails. This also depends on having editable source files, which doesn’t seem like such a big deal, except that it’s very possible to have source artifacts (fetched by Maven) or source bundles (in the PDE Target Platform) that show up as class files and can’t be (immediately) edited.

So what should you do when you’d like to do a hot-code replace on a class for which you have read-only sources ?

Enter the JDT Breakpoint Properties Page. Generally, this page is meant to help set the conditions on which a breakpoint causes the thread to suspend. For example, you may have some kind of loop that iterates through many values, but only want to investigate what happens on a particular value. Rather than setting a breakpoint in the loop, seeing it suspend on the line, checking the value, hitting resume, over and over until you get the desired value, you could set a condition to suspend when your value is set.


So the suspend can be triggered on some condition that will be evaluated prior to executing the line. But there’s nothing stopping us from making the “condition” a code snippet that always returns “false”. The result is a breakpoint that never causes a suspend, but still evaluates the entire code snippet.

One of the simplest things for which this can be used is to insert “printf” statements without actually having to litter them throughout your codebase. Much easier to track and clean :

return false;

With this simple pattern, we can basically emulate something like the CDT’s dynamic printfs .Of course, since this merely inserts code there are some limitations to what can be done versus an actual hot-code replace, but this comes pretty close.

Common Mylyn Data Across Workspaces

I used to have a lot of projects in a single Eclipse workspace (~500), and after a certain point it became really difficult managing all of them at once. I already had them sorted into working sets but even having to constantly open/close them was a bit too much. As a result I decided to split these various projects over a few workspaces.

Now I had a new problem. All of my Mylyn Tasks (Bug Repositories/Queries) were configured for my original workspace, and that metadata was all under that workspace’s main directory (this is the default). I didn’t want to be constantly switching to my main workspace just to get bug updates, or make changes, so it’s nice that Mylyn’s data location is configurable (Window -> Preferences -> Mylyn Tasks -> Advanced).


Simply copy over your task data from $WORKSPACE/.metadata/.mylyn to some common location, and set that to be your data directory for each of your workspace preferences.

Changing the Behaviour of Eclipse’s Update Manager

If you’ve developed plugins for the Eclipse environment, you’re moderately aware that Eclipse’s update manager can behave in strange ways from a user perspective. Things have gotten better with the p2 Remediation Support in Kepler (4.3.0) but what about dependency resolution done by Maven plugins, like Tycho, at build-time ? You get to specify a list of repositories, their content is aggregated, and if your request is satisfiable, it will be satisfied. Of course there’s some criteria p2 will attempt to optimize. For example, preferring highest version with fewest dependencies (minimize transitive closure) from a set of identically named units.

However with the increasing usage of things like software collections, I’m starting to care much more about where and how dependencies are being resolved during a build.

To understand how to even go about changing p2’s behaviour, we first need to know what is actually happening when you request to have some unit installed.  There’s a few different things happening :

  1. Installable Units are collected from the user input (units to install, and repositories provided)
  2. Installable Units are made part of a Profile Change Request
  3. Planner takes the Profile Change Request and delegates solution finding to the Projector
  4. Projector transforms request into a boolean satisfiability problem and delegates to SAT4J library
  5. If solution is found, Projector passes result back to Planner, which creates a plan (set of operations on the profile) to achieve the new state
  6. The plan is then executed on the profile by the Engine

For more information on some of the finer details of (4) I would recommend reading http://www.cril.univ-artois.fr/spip/publications/lash2010.pdf .

As a demonstration we can define an OSGi bundle org.foo.root versioned 1.0.0 which has a Require-Bundle on org.foo.bar (unversioned). We then provide org.foo.bar at various versions (eg. 1.0.0, 5.0.0, 10.0.0, 20.0.0, 50.0.0, 99.0.0). Performing the installation of org.foo.root 10 times yields the following results :

$ for i in {1..10}; do eclipse -nosplash -application org.eclipse.equinox.p2.director -repository fedora:/tmp -installIU org.foo.root -destination $(pwd)/install_$i ; done;
Installing org.foo.root 1.0.0.
Operation completed in 742 ms.
$ (for f in `find ./install_*/plugins/ -type f`; do basename $f ; done;) | sort | uniq -c
     10 org.foo.bar_99.0.0.jar
     10 org.foo.root_1.0.0.jar

So in every case that we attempted to install org.foo.root, p2 decided to satisfy the version-less requirement org.foo.bar using the latest version available (99.0.0).

Note that the p2 Director will always use the latest version of a root, so to truly test the Projector and SAT4J it is necessary to define some root and see how its dependencies are satisfied, as we did above. Also, if that “fedora:” repository scheme seemed a little strange to you, it’s because we ship a special plugin in Fedora that allows treating filesystem locations as p2 repositories , without needing all that metadata/artifact repository data on disk.

To make p2 prefer certain installable units over others, we need to hook into the component that sets the constraints for the SAT4J Solver. The Projector seems like the place for this and a quick look reveals createOptimizationFunction is probably a good starting point. We’ll want to define our own optimization (instead of OptimizationFunction) function.

To favour certain locations over others, we could look at sets of units that have the same ID, and are not installed, or roots, and assign them weights based on their location. Since SAT4J will attempt to satisfy the contstraints while minimizing the objective function, we can assume that lower weighted units will be preferred over higher weighted ones. In fact, installed/root units have a weight of 1. All we need to do is subclass OptimizationFunction, override createOptimizationFunction to call its parent, and then just modify the weight for the units we care about as per the repository precedence before returning the final list of weighted units.

Once we’re done adding the necessary code, and rebuilding our modifications, we’re ready to test things out.

We now change the structure of our repository as follows :

  • /tmp/repo_priority/low_priority contains org.foo.bar versions 5.0.0, 20.0.0 and 50.0.0
  • /tmp/repo_priority/high_priority contains org.foo.bar version 10.0.0.
  • /tmp/repo_priority/ contains org.foo.bar versions 1.0.0 and 99.0.0
$ export JAVACONFDIRS='/tmp/repo_priority:/tmp/repo_priority/high_priority:/tmp/repo_priority/low_priority'
$ for i in {1..10}; do eclipse -nosplash -application org.eclipse.equinox.p2.director -repository fedora:/tmp -installIU org.foo.root -destination $(pwd)/install_$i -vmargs -Dfedora.p2.scl.order=/tmp/repo_priority/high_priority,/tmp/repo_priority,/tmp/repo_priority/low_priority ; done;
Installing org.foo.root 1.0.0.
Operation completed in 550 ms.
$ (for f in `find ./install_*/plugins/ -type f`; do basename $f ; done;) | sort | uniq -c
     10 org.foo.bar_10.0.0.jar
     10 org.foo.root_1.0.0.jar

So clearly we’ve made p2 favour units within /tmp/repo_priority/high_priority over others. This was a very basic example, and I didn’t really define what should happen when multiple units are in the same “preferred” repository but hopefully even a modification as basic as this shows the kind of things possible.