Recently I was facing a problem where a Java application should persist a part of its state when it was closed. This was already implemented but seemed to work just unreliably and it was not clearly reproducible when it worked properly and when it failed. After analyzing the existing source code I figured out that finalizers were used to implement the functionality by delegating the call of the persist logic to the garbage collector. This seems like a very good idea at first but lacks in reliability. As a short note up front don’t use finalizers for important things.
A finalizer is a method which represents the opposite of a constructor. It is a method with the purpose to do some cleanup work, free up some memory and so on which gets called by the garbage collector before the object gets collected. A simple example would look like this
[java]
protected void finalize() {
logger.info(“start important work before GC”);
fooBar.persist(this.getDataMap());
}
[/java] …
When generating Excel Sheets it is sometimes necessary to remove rows which match a given criteria afterwards. The first idea for this would be to use removeRow(Row row) but this has the problem that it deletes all the row contents and leaves the empty row in your sheet which is probably not what you want. To remove the rows as a whole the shiftRows(int startRow, int endRow, int n) is needed.
Lets say we have a xls file with column A being the title which is always filled but we want to remove every row in which the data column B is not filled. For this case the following snippet would be appropriate …
This is a book I wish I had read when starting my career as a professional developer because I would have had a head start about many of the things everybody takes for granted but at as a student nobody tells you about.
Don’t get me wrong I don’t say that I couldn’t get anything out of the book because I have already known it all but as it tries to cover nearly everything that is important for a professional developer there are naturally some things I already was aware about.
This book is not about telling you which patterns to use or how to solve specific problems but covers more the everyday life of a developer and how to become better in getting the right behavior, communicating with colleagues and customers, important general concepts in programming, a bit of tooling hints, basics of agile development and how to learn and what to read.
When you create Excel documents with the popular Apache POI library it is possible that you encounter a problem where Excel tells you there are “Too many different cell formats”. A possible source for this is that you probably have a loop structure in which you define a new cell style for every cell you create.
Excel is limited in the amount of cell styles it can handle in a single document (4000 per document). Therefore the above code would break. The easy fix would be to define the cell style outside of your loop and reuse it.
But sometimes you want to create different cell styles depending on a line number for example. In this case POI comes with a handy utility class named CellUtil that allows you to modify existing cell styles without creating a new one. Internally it has to create a new cell style and saves it in a map. If you make the same modification at any other place in your code again it will iterate that map, find the previously created style and reuses it. Therefore the same style is only created once. The usage is quite simple and would adapt to the sample above as follows
The code above will add a border to the bottom of every second cell created but because it is every time the same modification CellUtil will only create one additional style and apply this every time again.
Using a remote subversion repository with git locally requires some adoptions to the local git usage compared to working in a pure git environment. The reason for this is that the remote subversion don’t know about many of the concepts the local git supports (local branches or multiple local commits before checkin for example). Because of this it is essential to make it look as close to a normal svn checkin as possible when using git svn.
As I now work since quite a while with such a setup I defined a workflow for me that don’t causes any problems with the svn and gives me many of the benefits I have with git as there are the possibility to work with the version history offline, making local branches to develop new features or making multiple commits when sub tasks are done.
Before starting to develop a new feature I need to make sure everything is up to date. In contrast to a git remote repository no pull option is available here but I have to rebase the svn repository. I call the following from the master branch:
git svn rebase
This gets any new code changes from the svn and rebases all local work done by me which is not checked in yet on top of it. This is essential because svn history goes in a straight line and would not know what to do if you push a local commit that is dated before the actual newest check in.
After having done that I can create a local feature branch using
git checkout -b newFeature
Here everything works as normal. You can make commits, branch out further, go back to a former version and so on. When you want to get the newest code from the remote repository merged into your branch I prefer pulling it in the master first and rebasing it from there in my featureBranch (it is important not to merge it)
git checkout master
git svn rebase
git checkout featureBranch
git rebase master
When the new feature is ready comes the essential part where the git svn mix breaks sometimes if you don’t do it right. As I mentioned it is essential to make the commit as equal to a svn commit as possible. When using svn you don’t have the possibility to make local commits you just push everything at once to the server where it gets a version id and thats it. To achieve that behaviour with git you change back to the master, checkout the newest code changes and merge it back to the feature branch as explained above. After that we can merge the feature branch back to master:
git checkout master
git merge featureBranch
When you look at the version history now you will see that every commit done in the feature branch is now at the top of the version history. This is because we rebased the master in the feature branch and not merged it. This gives us now the possibility to pack all our local commits to one commit which will then look like the normal subversion workflow to the repository. If we had done 5 commits in our local feature branch for example we achieve this with
git rebase --interactive master~5
This will guide you through two steps. In the first you have to select which of the five commits you want to squash together. Here you should squash all subsequent commits into the first. After that a new dialogue will open which needs you to define the new commit message. Here you see all commit messages you entered in the single commits and can copy the contents to a new single message. When you are done and saved the new message git will rebase everything to one commit which lies on top in the version history. The only thing left to do is push it to svn. Once again the usual git push won’t work here. Instead we call
git svn dcommit
That is all. Everything should work fine and your new feature is checked in to subversion. You can now delete the feature branch or keep to track back the single steps you made to develop the feature.
Update:
Just to clarify, the idea is not to work weeks long on a feature and try to integrate it with a big bang all at once. This was just meant for small features that require just a few hours or maybe a day of work. If you implement a larger feature you obviously should commit smaller portions of your code changes to svn to make your progress clear and reproducible and avoid large merge conflicts with other team mates changes. In the beginning I tried to dcommit all my single commits to svn which worked to a point but had one nearly fatal break. Since then I try to avoid that and use the described workflow.
