From the ServletContext JavaDocs

There is one context per “web application” per Java Virtual Machine. (A “web application” is a collection of servlets and content installed under a specific subset of the server’s URL namespace such as /catalog and possibly installed via a .war file.)

I never really thought too deeply about how the ServletContext behaved until I saw it used differently at my new job. I am used to getting an instance of ServletContext and accessing it directly, which is fine in the case of web applications that consist of a single context (or .war file); however, now I am dealing with an application that spans across multiple application contexts (and .war files)… ServletContext can be used a bit differently under such circumstances.

Consider two web applications, Foo and Bar, each deployed on the same server. In the “Foo” application, if you call getServletContext() and then getContextPath(), it will be “/Foo”, and likewise on “Bar” the same code would yield “/Bar”. The interesting part of this is that if, in “Foo” set an attribute on the ServletContext, it will not be directly accessible on “Bar”; however, you can get it by getting a reference to the “Foo” ServletContext… this is the part I had never seen before.

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// on Foo
getServletContext().setAttribute("foo","I have foo");

// on Bar
getServletContext().getAttribute("foo") // is null
getServletContext("/Foo").getAttribute("foo"); // is "I have foo"

It makes sense when you think about it but it surprised me that after all the years I have been working with web applications, that I had never run across a multi-war application that needed to cross-communicate.

You can also, it seems, use “/”, but perhaps that is only if you have a ROOT context available… I will have to test that one a little more. Along those same lines, you can use the full path to resolve the context, such as a servlet path, though I would imagine that it does have to be a valid path. If you do try and use a context that does not exist, you will get an error.

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