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DeadClasses.adoc

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How To Find Dead Classes

To find dead classes in your application there are mainly two classes in clazzfish-monitor:

  • ClasspathMonitor

  • ClazzStatistic

Tip
 See also the talk Dead Classes on the Java Forum Stuttgart 2025

Snapshot

While running your program the ClasspathMonitor allows you to see

  • all classes of the program’s classpath,

  • the loaded classes,

  • the unused classes

  • and more…​

These informations are also accessible via JMX so you start e.g. a JMX console (like jconsole) to look at it.

The unused classes are candiates for dead classes. But remember - it is only a snapshot. Perhaps some of these classes are needed for other branches of your program.

What can you do? ClasspathMonitor can dump unused classes at program end during the shutdown phase. All you have to do is to collect the different dumps and search for the intersection of all unused classes. This is the point where ClazzStatistic comes into play (see next section).

Activation

To activate the dump of loaded and unused classes you have to add the following lines into your source code:

    static {
        ClasspathMonitor.getInstance().addMeAsShutdownHook();
    }

Be sure you start your application with clazzfish-agent as Java agent, e.g.

java -javaagent:/Users/ob/.m2/repository/de/aosd/clazzfish/clazzfish-agent/2.5.0/clazzfish-agent-2.5.0.jar ...

The ClasspathMonitor and ClassStatistic (see below) will work also without the Java agent. But then classes with only static methods will be detected as "dead classes".

ClazzStatistic

ClazzStatistic records for each class in the classpath

  • if class was loaded and

  • to which JAR or directory the class belongs.

Added as Agent

Since v3.x the ClasspathAgent is able to use the ClazzStatistic class to record loaded classes. This allows you use the agent standalone without adding the ClazzFish libraries as dependency.

The configuration is limited to environment variables or system properties (see next section). And you can only use files to store the statistic.

Added as Library

You have the most options if you add one of the ClazzFish libraries (at least clazzfish-monitor). All you have to do to activate the export of ClazzStatistic.csv at the end of your application is encapsulated in the Starter class:

    static {
        Starter.record();
    }

See the static initializer in the Bank class to see how it is done and what is possible. In contradiction to the ClasspathMonitor it imports the statistic from the last run and merge it with the actual dates before it dumps it at shutdown of your application:

Classpath Classname Count

file:/ClazzFish/sample/target/classes

clazzfish.sample.Bank

6

file:/ClazzFish/sample/target/classes

clazzfish.sample.DeadClass

0

file:/ClazzFish/sample/target/classes

clazzfish.sample.jdbc.Account

6

file:/ClazzFish/sample/target/classes

clazzfish.sample.jdbc.BankRepository

3

file:/ClazzFish/sample/target/classes

clazzfish.sample.jdbc.User

6

This is an extract of ClazzStatistic.csv which will be exported to the temp directory (we will later see how to change it). In this example the sample application of ClazzFish is started several times. Some classes are loaded each times, some classes only sometimes and one class (DeadClass) is never loaded (count = 0). This class is a probably a dead class which can be deleted.

How more often you run you application how more you can be sure that a class with count = 0 is a dead class. When can you be really be sure that this class is really dead? Never. But after a year or so it is very probably that you have found a dead class but that depends on your business. If you have special classes to handle leap years you have to wait at least 4 years. In other words: it depends…​

Evaluating ClazzStatistic.csv

Collecting

If you start your application on different computers you have the problem that each computer has its own temp directory, where the resulting ClazzStatistic.csv is stored. You can collect all these CSV files, filter out the loaded classes which you want to observe and merge them into a new one:

grep "clazzfish.sample" stat*.csv | grep -v ";0" > ClazzStatistic.csv

What happen if you put this new ClazzStatistic.csv to the temp directory? It will be merged with the actual statistic. The missing headline and double entries in your ClazzStatistic are no problem. Double entries are ignored, i.e. your count statistic will be to low. But since you are only interested in classes with count = 0 this is irrelevant.

How to find the location of ClazzStatistic.csv? It is placed in the a directory named after the application name or the start class:

08:12:39  INFO [main|clazzfish.sample.Bank      ] Good bye - shutting down...
08:13:35  INFO [ad-2|monitor.stat.ClazzStatistic] Statistics exported to '/tmp/ClazzFish/clazzfish.sample.Bank/ClazzStatistic.csv'.

In this example the starter class is clazzfish.sample.bank and the temp directory is /tmp (it depends on your environment and OS).

Global Dump Dir

If you have hundreds of users which uses your application it is a little be cumbersome to collect all CSV files from the different hosts. In this case look for a global directory which can be written from all your clients and set the system property clazzfish.dump.dir

java -Dclazzfish.dump.dir=/path/to/global/dir MyApp.jar

Each application will write its ClazzStatistic.csv in this directory.

Since 2.5. ClazzFish listen also to the system property clazzfish.dump.dir

java -Dclazzfish.dump.uri=file://path/to/global/dir MyApp.jar

This is same directory location as before but as you guess you can use here also other protocols like (S)FTP, HTTP or GIT. As default ClazzFish only supports the file protocol. If you need another protocol you have to implement the CsvXPorter and CsvXPorterProvider interface and register it as service.

For more info see SPI.

Trouble Shooting

Missing Log

If you don’t see the log message with the name of the exported file it may be that no logging bridge for SLF4J is in the classpath. If you use Log4J2 add log4j-slf4j-impl to your dependencies (e.g. to your pom.xml if you use maven):

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-slf4j-impl</artifactId>
    <version>2.25.1</version>
</dependency>

To see more log messages you set the log level for clazzfish to DEBUG or TRACE.

Dead Static Classes

If you start your application without the Java agent clazzfish-agent static and abstract classes will be reported as dead classes in older versions of ClazzFish (before 2.5). The reason for it is the use of the garbage collector (GC) as fallback. But the GC only collect object instances not classes.

Also this problem is fixed with v2.5 it is not guaranteed that it works with all kind of Java VMs. In this case start your application with clazzfish-agent:

java -javaagent:/Users/ob/.m2/repository/de/aosd/clazzfish/clazzfish-agent/2.4/clazzfish-agent-2.4.jar ...