无法从angular 6 httpclient向java servlet发送请求

如何从Angular连接到Javaservlet？ 这是我的角度服务代码

import { Injectable } from "@angular/core";
import { HttpClient } from "@angular/common/http";
import { Observable } from "rxjs";

@Injectable({ providedIn: "root" })
export class UploadService {
  url: string;
  constructor(private http: HttpClient) {
    this.url = "http://localhost:8081/ServletSample/";
  }

  uploadFile(data): Observable<any> {
    return this.http.post(this.url, data);
  }

  sendData(data): Observable<any> {
    return this.http.post(this.url + "DataServlet", data);
  }
}

下面是Javaservlet代码

package com.test.servlet;

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DataServlet extends HttpServlet{

      /**
     * 
     */
    private static final long serialVersionUID = 548987777;

    public DataServlet() {
            super();
            System.out.println("DataServlet constructor called");
        }

        /**
         * @see Servlet#init(ServletConfig)
         */
        public void init(ServletConfig config) throws ServletException {
            System.out.println("DataServlet \"Init\" method called"+ config.toString());
        }

        /**
         * @see Servlet#destroy()
         */
        public void destroy() {
            System.out.println("DataServlet \"Destroy\" method called");
        }


        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException,IOException
        {
            System.out.println("DataServlet doGet method called"+ request);
            doPost(request, response);

        }

        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            System.out.println("DataServlet doPost method called");
        }

}

但当我在浏览器控制台中发送请求时出错

CORS策略已阻止从源站“”访问“”处的XMLHttpRequest:对飞行前请求的响应未通过访问控制检查：请求的资源上不存在“访问控制允许源站”标头。

---

从您得到的错误判断，您必须在servlet上启用CORS以允许接受来自其他来源的调用

只需创建一个

private

方法，并从发送实际响应的方法调用它。您可以在此方法中设置

标题。大概是这样的：

private void setAccessControlHeaders(HttpServletResponse resp) {
  resp.setHeader("Access-Control-Allow-Origin", "http://localhost:9000");
  resp.setHeader("Access-Control-Allow-Methods", "GET");
}


资料来源：

您能用您尝试过的更新代码更新问题吗？我还建议你把源文章通读一遍。
package com.test.servlet;

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DataServlet extends HttpServlet{

      /**
     * 
     */
    private static final long serialVersionUID = 548987777;

    public DataServlet() {
            super();
            System.out.println("DataServlet constructor called");
        }

        /**
         * @see Servlet#init(ServletConfig)
         */
        public void init(ServletConfig config) throws ServletException {
            System.out.println("DataServlet \"Init\" method called"+ config.toString());
        }

        /**
         * @see Servlet#destroy()
         */
        public void destroy() {
            System.out.println("DataServlet \"Destroy\" method called");
        }


        protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException,IOException
        {
            System.out.println("DataServlet doGet method called"+ request);
            doPost(request, response);

        }

        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            System.out.println("DataServlet doPost method called");
        }

}

private void setAccessControlHeaders(HttpServletResponse resp) {
  resp.setHeader("Access-Control-Allow-Origin", "http://localhost:9000");
  resp.setHeader("Access-Control-Allow-Methods", "GET");
}