在后端写一个java方法在前端调用时输出helloworld

好的，以下是一个简单的Java Servlet，当它被前端页面调用时，会向前端输出"helloworld"。

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloWorldServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        response.getWriter().println("helloworld");
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
}

当你在前端页面中向这个Servlet发送HTTP请求时，它会向前端输出"helloworld"。