Java Servlets 覆盖 init(ServletConfig 配置)

Question

我正在尝试覆盖 init(ServletConfig config) 方法。我的代码是:

 public void init(ServletConfig config) throws ServletException {
    ServletContext sc = getServletContext(); // ----- NullPointerException
}

这是给出 NullPointerException 。

如果我将其修改为:

   public void init(ServletConfig config) throws ServletException {
    ServletContext sc = config.getServletContext(); // ----- works fine
}

这很好用。 我知道我们应该覆盖 init() 方法而不是 init(ServletConfig config) 但是
谁能告诉我为什么会这样？

Answer 1

比较init(ServletConfig)的文档:

public void init(ServletConfig config)throws ServletExceptionCalled by the servlet container to indicate to a servlet that the servletis being placed into service.See Servlet#init. This implementation stores the ServletConfig objectit receives from the servlet container for later use. When overridingthis form of the method, call super.init(config).

And compare that with the documentation for init():

public void init() throws ServletExceptionA convenience method which can be overridden so that there's no need tocall super.init(config).Instead of overriding init(ServletConfig), simply override this methodand it will be called by GenericServlet.init(ServletConfig config). TheServletConfig object can still be retrieved via getServletConfig().

When overriding init(ServletConfig), the first thing that must be done is to call:

super.init(config);

如果您这样做，那么在您的方法中直接调用 getServletContext() 将不再导致 NPE。