XMLHttpRequest无法加载在请求的资源上不存在“ Access-Control-Allow-Origin”标头

Question

我正在使用javaEE的Web项目中使用后端中的Servlet（我没有使用Spring），而在我的应用程序中使用Angular版本8作为前端框架。但是当我尝试访问端口8080（后端）中的服务器时从我的浏览器访问URL http：// localhost：8080 / openclassroom_test / test时 ，我收到以下错误消息：从源地址“ http：// localhost：4200 ”访问“ http：// localhost：8080 / openclassroom_test / test ”处的XMLHttpRequest已被CORS策略阻止：请求的资源上不存在“ Access-Control-Allow-Origin”标头。 我应该在servlet中添加什么说明？ 您能帮助我解决错误问题，并提前谢谢。 这是我在Servlet中的代码：Test.java

package com.coors.servlets;
import java.io.IOException;
import javax.jws.WebService;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebFilter;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import com.octest.beans.Utilisateur;
import com.octest.dao.DaoFactory;
import com.octest.dao.UtilisateurDao;
import com.octest.dao.UtilisateurDaoImpl;
import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/test")
public class Test extends HttpServlet {
private static final long serialVersionUID = 1L;
private UtilisateurDao utilisateurDao;
private DaoFactory daoFactory; 

public Test() {
    super();
} 

@Override
public void init() throws ServletException {
    daoFactory=DaoFactory.getInstance();
    utilisateurDao=daoFactory.getUtilisateurDao();
}

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

    //here am trying to send a string "hello" to Angular
    response.getWriter().write("hello");
}

public void doPost( HttpServletRequest request, HttpServletResponse response ) throws 
ServletException, IOException {
}

}