Servlet Init() method magic

A Common Mistake in Servlet Init() Methods:

Servlet init methods allows a servlet to perform one-time initialization prior to servicing requests. One common mistake when implementing init method is in this form:

public void init(ServletConfig config)
 throws ServletException {
 //do custom initialization ...
 System.out.println(" init(ServletConfig config) invoked.");
 ...
}

This is wrong because it doesn’t invoke super.init(ServletConfig). As a result, ServletConfig is not stored in the servlet instance, and subsequent calls to getServletConfig will return null.

Another variant of this mistake is to store the ServletConfig parameter in a class variable of the concrete servlet class:

private ServletConfig config;

public void init(ServletConfig config)
 throws ServletException {
 this.config = config;
 System.out.println(" init(ServletConfig config) invoked.");
 //do custom initialization ...
 ...
}

This is also wrong because config saved in the current servlet won’t be available to its superclass, usually GenericServlet or HttpServlet. When the default implementation of getServletConfig() method looks for ServletConfig in superclasses, it’s still null. The only way it can work is that you also override getServletConfig method to look in the concrete servlet class, which is unusual and unnecessary.

 

This is the error from Glassfish/JavaEE SDK 5, when running a servlet with those incorrect init methods:

java.lang.NullPointerException
test.HelloWorldServlet.doGet(HelloWorldServlet.java:14)
javax.servlet.http.HttpServlet.service(HttpServlet.java:707)
javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:73)
com.sun.enterprise.web.VirtualServerPipeline.invoke(VirtualServerPipeline.java:120)
org.apache.coyote.tomcat5.CoyoteAdapter.service(CoyoteAdapter.java:231)

To implement servlet init methods correctly, you have several options:

  • If you only need to save ServletConfig, do not override any init methods in servlet class. It’s already implemented in servlet superclass GenericServlet.
  • If you have custom initialization work to do, override the no-arg init() method, and forget about init(ServletConfig). Is it ok to call getServletConfig() method inside the no-arg init() method? Yes, an instance of ServletConfig has already been saved by superclass GenericServlet.
  • If you really want to override init(ServletConfig), make sure you invoke super.init(ServletConfig);, usually as the first line, though it’s not strictly required.

Why Servlet Init() Method confusing?

You may ask, what is the difference between servlet init() and init(ServletConfig), and why is it so confusing?

When servlet was invented, there was only one init method, the one with parameter ServletConfig. The no-arg init method was added around Servlet 2.3 timeframe, mainly to address this common mistake. This solution applies a Template Design Pattern to outline what needs to be done in init method and also keeps it extensible for concrete servlet classes.

init(ServletConfig) is the template method, which stores ServletConfig, and then delegates to init(). This is what javax.servlet.GenericServlet.init(ServletConfig) looks like:

public void init(ServletConfig config)
 throws ServletException {
 this.config. = config;
 init();
}

The real work of initialization is supposed to be done in init() method, which can be overrid by servlet subclasses. Its default implementation in javax.servlet.GenericServlet is merely a no-op.

From the above code snippet, when init() in your servlet class is invoked by the container, an instance of ServletConfig has already been saved. So you can safely call getServletConfig() inside of init() method.

Servlet classes rarely need to know about init(ServletConfig ) method. Only web containers need to invoke it. Ideally, this method should be declared as final or even private in GenericServlet. But that would break backward compatibility.

Having two overloaded init methods may also add to the confusion. Unless developers read the Javadoc carefully, their relationship (one calling the other) is unclear. So it’s possible someone will override init() method like this:

@Override public void init()
 throws ServletException {
 //do some initialization work first,
 //then call init(ServletConfig)
 init(null);
}

It will cause infinite loop and java.lang.StackOverflowError during servlet initialization, and the servlet will never be put into service. I hope I didn’t make it more confusing.

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About Sanju
I am Software Programmer. I am working in JAVA/J2EE Technologies.

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