---- On Fri, 25 Nov 2016 23:02:00 +0930 Péter Gergely Horváth wrote ----
>Hi All,
>
>I am wondering why calling javax.servlet.Servlet#getServletConfig() is
>thread safe: if you check the implementation in
> javax.servlet.GenericServlet from servlet API 3.0.1, you see the following:
>
>package javax.servlet;
>
>// lines omitted
>
>public abstract class GenericServlet
> implements Servlet, ServletConfig, java.io.Serializable
>{
> // lines omitted
>
> private transient ServletConfig config;
>
> // lines omitted
>
> public void init(ServletConfig config) throws ServletException {
> this.config = config;
> this.init();
> }
>
> // lines omitted
>
> public ServletConfig getServletConfig() {
> return config;
> }
> // lines omitted
>}
>
>
>The field config is written in init(ServletConfig) without any
>synchronization, locking or config being volatile, while getServletConfig()
>could be called anytime from any later worker thread of the servlet
>container. I took a quick look into Tomcat/Catalina code base, however I
>see no indication of the servlet container performing any magic, which
>would guarantee thread safe access to field config through
>getServletConfig() from e.g.
>javax.servlet.GenericServlet#service(ServletRequest, ServletResponse) and
>the corresponding methods in javax.servlet.http.HttpServlet.
>
>Am I missing something here? What will guarantee that if Thread A is used
>to init(ServletConfig) then Thread B calling getServletConfig() will see
>the correct state of the field config (Java "happens-before"), and not e.g.
>null?
>
>I am asking this because I have seen some legacy code, where a servlet
>stored something into a field inside the init() method, which was then used
>from within a javax.servlet.http.HttpServlet#doGet or
>javax.servlet.http.HttpServlet#doPost method, without synchronization of
>any kind like this:
>
>public class FoobarServlet extends HttpServlet {
>
> private FoobarService foobarService;
>
> @Override
> public void init() throws ServletException {
>this.foobarService = // ... acquire foobarService
> }
> protected void doGet(HttpServletRequest request, HttpServletResponse
>response) throws ServletException, IOException {
> List<Foobar> foobars = this.foobarService.getFoobars(); // read the
>field WITHOUT synchronization
> // ...
> }
> // ...
>Is this approach (having no synchronization, locking or the field being
>volatile) correct? I assumed it is not, seeing something like that in the
>servlet API is quite confusing.
>
>
>What do you think?
>
>Thanks,
>Peter
A Servlet will process requests only if it is fully initialized, i.e. init has
been executed. The init method gets called only once and the servlet config
won't change afterwards. I don't think there is need for synchronization. The
same is probably valid for your own objects. Problems arise when individual
requests change the state of these objects.
Andy
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