在doget或dopost中直接获取ServletContext对象

getServletContext()方法是ServletConfig接口定义的方法，在servlet中可以直接调用getServletContext()来获取ServletContext对象

@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println("上下文参数："+getServletContext().getInitParameter("age"));
	}

如果Servlet重写了带有ServletConfig参数的init方法，那么调用getServletContext()时会引发空指针异常

@Override
	public void init(ServletConfig config) throws ServletException {
		//
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println(getServletContext());
	}

引发空指针异常是GenericServlet中的getServletConfig()为空引发的

@Override
    public ServletContext getServletContext() {
        return getServletConfig().getServletContext();
    }

那么也就意味着在servlet中获取ServletConfig对象就是空的

@Override
	public void init(ServletConfig config) throws ServletException {
		//
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println(getServletConfig());  //null
		//System.out.println(getServletContext());
	}

这是因为父类的init(ServletConfig config)里对ServletConfig对象进行了初始化

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

而我们重写了init方法，ServletConfig就是空的，所以我们要在重写的init方法中调用父类的init方法对ServletConfig赋值

@Override
	public void init(ServletConfig config) throws ServletException {
		super.init(config);
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println(getServletContext());
	}

或者通过request对象的getServletContext来获取ServletContex

@Override
	public void init(ServletConfig config) throws ServletException {
		//super.init(config);
	}
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		System.out.println(req.getServletContext());
	}

或者选择重写的是public void init()方法也行。

来源：CSDN

作者：a3226988

链接：https://blog.csdn.net/a3226988/article/details/103744413

标签

servlet