ServletContext

A)Which class implements ServletContext interface. Why is the concrete class not present in API?(src:Tomcat servlet API)

B)The only handle to ServletContext is via ServletConfig.i.e. getServletConfig().getServletContext()
getServletConfig():API doc says that this method invokes abive method to fetch ServletContext.
Why is ServletContext coupled with ServletConfig?

C)thread-safety of context attributes(src:HFSJ pg no:197)
It mentions that ServletContext should be syncrhonized in every Servlet.
Does it mean that there is seperate instance of ServletContext for every Servlet?
But its said that there is a single ServletContext for every web-app(lets not talk abt distributed).What does the preivous statement signify.

Which class implements ServletContext interface. Why is the concrete class not present in API?

It's up to container which package and classname to give to that class. It will also contain many container-specific methods, which should not be used by application developers.

This is common way of designing an API. JDBC works the same way - it consists of a number of interfaces (Connection, Statement, ResultSet, ...), and it's up to the driver implementation to create objects that implement these.

The only handle to ServletContext is via ServletConfig.i.e. getServletConfig().getServletContext()
getServletConfig():API doc says that this method invokes abive method to fetch ServletContext. Why is ServletContext coupled with ServletConfig?

It's not. The GenericServlet class -which just about all servlets extend via the HttpServlet class- also has a getServletContext method, so that's another way to get at it.

thread-safety of context attributes(src:HFSJ pg no:197)
It mentions that ServletContext should be syncrhonized in every Servlet.
Does it mean that there is seperate instance of ServletContext for every Servlet?
But its said that there is a single ServletContext for every web-app(lets not talk abt distributed). What does the preivous statement signify.

There is one ServletContext per web app. But servlets are multi-threaded, so whenever they access a mutable shared resource (like ServletContext), access to it must be protected, e.g. by synchronization.

Even if servlets were not multi-threaded, access to it would still need to be protected, because different servlets could use it at the same time.

Since ServletContext is the shared object in an web application and should be available for all servlet instances running, it should be synchronized.

Otherwise, the purpose of having such an entity will not be saved.