当Servlet容器启动时，它：

When the Servlet container starts, it:

1. 读取 web.xml ;


2. 在类路径中查找声明的Servlet;和


3. 加载并实例化每个Servlet 仅一次。

1. reads web.xml;
2. finds the declared Servlets in the classpath; and
3. loads and instantiates each Servlet only once.

粗略地，像这样：

String urlPattern = parseWebXmlAndRetrieveServletUrlPattern();
String servletClass = parseWebXmlAndRetrieveServletClass();
HttpServlet servlet = (HttpServlet) Class.forName(servletClass).newInstance();
servlet.init();
servlets.put(urlPattern, servlet); // Similar to a map interface.

这些Servlet存储在内存中，并在每次请求URL与Servlet关联的 URL模式。然后，servlet容器执行类似于以下的代码：

Those Servlets are stored in memory and reused every time the request URL matches the Servlet's associated url-pattern. The servlet container then executes code similar to:

for (Entry<String, HttpServlet> entry : servlets.entrySet()) {
    String urlPattern = entry.getKey();
    HttpServlet servlet = entry.getValue();
    if (request.getRequestURL().matches(urlPattern)) {
        servlet.service(request, response);
        break;
    }
}

GenericServlet #service（） 轮到决定 doGet（）， doPost（）等...基于 HttpServletRequest＃getMethod（） 。

The GenericServlet#service() on its turn decides which of the doGet(), doPost(), etc.. to invoke based on HttpServletRequest#getMethod().

您可以看到，servletcontainer为每个请求重用相同的servlet实例。换句话说：servlet在每个请求之间共享。这就是为什么以线程安全的方式编写servlet代码非常重要 - 这实际上很简单：只需要​​不将请求或会话范围数据分配为servlet实例变量，而只是作为方法局部变量。例如

You see, the servletcontainer reuses the same servlet instance for every request. In other words: the servlets are shared among every request. That's why it's extremely important to write servlet code the threadsafe manner --which is actually simple: just do not assign request or session scoped data as servlet instance variables, but just as method local variables. E.g.

public class MyServlet extends HttpServlet {

    private Object thisIsNOTThreadSafe;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        Object thisIsThreadSafe;

        thisIsNOTThreadSafe = request.getParameter("foo"); // BAD!! Shared among all requests!
        thisIsThreadSafe = request.getParameter("foo"); // OK, this is thread safe.
    } 
}