Refactoring in Git

09 Jan 2011

To me, when using SVN, the most important reason for using an IDE plugin was the refactoring support: SVN doesn't notice when you rename a file, you have to explicitly call svn mv.

I thought this would be a major problem with Git, as a Java refactoring changes the content and the filename in one go. As the content changes the SHA1-checksum also changes and you'd run into problems. Fortunately, that's not the case.

With Git, you don't need a special operation: It detects renames with minor changes automatically.

Time for a test. Suppose you have a simple Java class like this:

public class TestClass {

    public static void main(String [] args) {
        System.out.println("Hello Git");
    }

}

Commit it to the Git repository:

flo@hank:~/git-netbeans$ git add src/TestClass.java
flo@hank:~/git-netbeans$ git commit -m "added test class"
[master 9269c2f] added test class
 1 files changed, 7 insertions(+), 0 deletions(-)
 create mode 100644 src/TestClass.java

Rename the class (either by using an IDE or by executing a manual refactoring by changing the file name and the class name):

public class TestClassWithNewName {

    public static void main(String [] args) {
        System.out.println("Hello Git");
    }

}

git status will tell you something like this:

flo@hank:~/git-netbeans$ git status
# On branch master
# Changed but not updated:
#   (use "git add/rm ..." to update what will be committed)
#   (use "git checkout -- ..." to discard changes in working directory)
#
#       deleted:    src/TestClass.java
#
# Untracked files:
#   (use "git add ..." to include in what will be committed)
#
#       src/TestClassWithNewName.java
no changes added to commit (use "git add" and/or "git commit -a")

Doesn't look that good yet. It detects an added and a removed file. Next, stage the changes and have another look at the status:

flo@hank:~/git-netbeans$ git rm src/TestClass.java
rm 'src/TestClass.java'
flo@hank:~/git-netbeans$ git add src/TestClassWithNewName.java
flo@hank:~/git-netbeans$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
#       renamed:    src/TestClass.java -> src/TestClassWithNewName.java
#

Neat, Git detected a rename. Let's commit and see the log:

flo@hank:~/git-netbeans$ git commit -m "refactored class"
[master 4acd7f1] refactored class
 1 files changed, 1 insertions(+), 1 deletions(-)
 rename src/{TestClass.java => TestClassWithNewName.java} (72%)
flo@hank:~/git-netbeans$ git log src/TestClassWithNewName.java
commit 4acd7f19ccd6cc02816ee7f1293ea5a69d7a4ca7
Author: Florian Hopf 
Date:   Sun Jan 9 14:27:59 2011 +0100

    refactored class

Hmmm, only the last commit? Looks like we have to tell that we want to follow renames:

flo@hank:~/git-netbeans$ git log --follow src/TestClassWithNewName.java
commit 4acd7f19ccd6cc02816ee7f1293ea5a69d7a4ca7
Author: Florian Hopf 
Date:   Sun Jan 9 14:27:59 2011 +0100

    refactored class

commit 9269c2fd194b2bd2b93a18ab88f21fb2180c5870
Author: Florian Hopf 
Date:   Sun Jan 9 13:48:35 2011 +0100

    added test class 

What do I take from this experiment? I guess I won't use the Netbeans Git plugin for now. I still have to get acquainted to the command line and its better to learn the basics first.