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17450075
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2013-07-03T14:00:47.853
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2013-07-03T18:51:27.890
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2013-07-03T18:51:27.890
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text
Body
That's because it's required to create a new instance via reflection. <a href="http://www.docjar.com/html/api/java/util/ServiceLoader.java.html" rel="nofollow">I guess here's relevant code, line 362</a>: <pre><code>S p = service.cast(Class.forName(cn, true, loader).newInstance()); </code></pre> Thus it invokes "parameterless" constructor since <a href="http://docs.oracle.com/javase/tutorial/reflect/member/ctorInstance.html" rel="nofollow"><code>Class.newInstance()</code> can only invoke the zero-argument constructor</a>. Edit: Actually, it's not required to create a zero-argument constructor for <code>public class</code> unless you define another constructor. <a href="http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html" rel="nofollow">javadoc</a>: <blockquote> <blockquote> You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. </blockquote> </blockquote> So, the following code is perfectly correct: <pre><code>public final class MyServiceImpl implements MyService { @Override public long getTime() { return System.currentTimeMillis(); } } </code></pre> Default constructor for <code>MyServiceImpl</code> will be the same as: <pre><code>public MyServiceImpl() { } </code></pre> Since <code>MyServiceImpl</code> <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-8.html#jls-8.8.9" rel="nofollow">is a public class</a>: <blockquote> <blockquote> if the class is declared public, then the default constructor is implicitly given the access modifier public </blockquote> </blockquote> Thus constructor in <code>MyServiceImpl</code> is redundant. My guess is that author of the blog post just wanted to make sure that <code>ServiceLoader</code> will be able to instantiate service provider even when it contains some explicit constructors with arguments. <hr> Conclusion: Service provider should have explicit no-argument constructor only if it contains explicit constructors with arguments. Default constructor will suffice otherwise.
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