private static ClassLoader getClassLoader(ClassLoader classLoader) {
        if (classLoader == null) {
            return ClassLoader.getSystemClassLoader();
        }
        return classLoader;
    }
@author Jongho Moon
@author jaehong.kim
agent-module/profiler-test/src/main/java/com/navercorp/pinpoint/profiler/test/TestProfilerPluginClassInjector.java