How to determine which jar a class is loaded from?

When the method that contains this sniplet is called, it will print out something like:

file://path/to/deployment/lib/detected.jar

Will the codeSource be null? The answer is yes. When a jar file is loaded by the system class loader, it’s codeSource will be null. What jars will be loaded by the system class loader? the rule of thumb is that all the jars in the class path (not the ones you package in your application) will be loaded by the system class loader. Obviously the above code can’t used to find out which jar a class is loaded from, if the jar is loaded by the system class loader. You can use the “verbose” java command line argument when you start the application:

java -verbose app

it will print out every class in each jar the system class loader loads, ie:

[Opened C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.Object from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.io.Serializable from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.Comparable from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.CharSequence from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.String from C:\dev\bea\JDK160~1\jre\lib\rt.jar]
[Loaded java.lang.reflect.GenericDeclaration from C:\dev\bea\JDK160~1\jre\lib\rt.jar]

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