Sometimes a library you are writing may have optional dependencies. E.g. “if apache http client is on the classpath, use it; otherwise – fallback to HttpURLConnection”.

Why would you do that? For various reasons – when distributing a library and you may not want to force a big dependency footprint. On the other hand, a more advanced library may have performance benefits, so whoever needs these, may include it. Or you may want to allow easily pluggable implementations of some functionality – e.g. json serialization. Your library doesn’t care whether it’s Jackson, gson or native android json serialization – so you may provide implementations using all of these, and pick the one whose dependency is found.

One way to achieve this is to explicitly specify/pass the library to use. When the user of your library/framework instantiates its main class, they can pass a booleanuseApacheClient=true, or an enum value JsonSerializer.JACKSON. That is not a bad option, as it forces the user to be aware of what dependency they are using (and is a de-facto dependency injection)

Another option, used by spring among others, is to dynamically check is the dependency is available on the classpath. E.g.

private static final boolean apacheClientPresent = isApacheHttpClientPresent();
private static boolean isApacheHttpClientPresent() {
  try {
    Class.forName("org.apache.http.client.HttpClient");
    logger.info("Apache HTTP detected, using it for HTTP communication.);
    return true;
  } catch (ClassNotFoundException ex) {
    logger.info("Apache HTTP client not found, using HttpURLConnection.");
    return false;
  }
}

and then, whenever you need to make HTTP requests (where ApacheHttpClient and HttpURLConnectionClient are your custom implementations of your own HttpClient interface):

HttpClient client = null;
if (apacheClientPresent) {
   client = new ApacheHttpClient();
} else {
   client = new HttpURLConnectionClient();
}

Note that it’s important to guard any code that may try to load classes from the dependency with the “isXPresent” boolean. Otherwise class loading exceptions may fly. E.g. in spring, they wrapped the Jackson dependencies in a MappingJackson2HttpMessageConverter

if (jackson2Present) {
    this.messageConverters.add(new MappingJackson2HttpMessageConverter());
}

That way, if Jackson is not present, the class is not instantiated and loading of Jackson classes is not attempted at all.

Whether to prefer the automatic detection, or require explicit configuration of what underlying dependency to use, is a hard question. Because automatic detection may leave the user of your library unaware of the mechanism, and when they add a dependency for a different purpose, it may get picked by your library and behaviour may change (though it shouldn’t, tiny differences are always there). You should document that, of course, and even log messages (as above), but that may not be enough to avoid (un)pleasant surprises. So I can’t answer when to use which, and it should be decided case-by-case.

This approach is applicable also to internal dependencies – your core module may look for a more specific module to be present in order to use it, and otherwise fallback to a default. E.g. you provide a default implementation of “elapsed time” using System.nano(), but when using Android you’d better rely on SystemClock for that – so you may want to detect whether your elapsed time android implementation is present. This looks like logical coupling, so in this scenario it’s maybe wiser to prefer to explicit approach, though.

Overall, this is a nice technique to use optional dependencies, with a basic fallback; or one of many possible options without a fallback. And it’s good to know that you can do it, and have it in your “toolkit” of possible solutions to a problem. But you shouldn’t always use it over the explicit (dependency injection) option.