Fetch+SpringBoot跨域请求设置

两种方法从SpringBoot的方向解决跨域问题

今天搭建博客的时候，尝试性的传递数据，发现浏览器报了这个错误
…blocked by CORS policy: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled.

也就是跨域请求的错误，
首先 在运行过程中，通过各种测试，发现前端的设置并不影响跨域，只要后端配置了允许跨域就能进行跨域请求

解决跨域，SpringBoot2.x版本

这次知道了有些问题，直接在Spring官网搜索，好像要方便些
Spring官方关于配置跨域的引导

SpringBoot2.x版本对跨域的支持有了更好的完善

参考文档我们能知道两种方法设置跨域

1. Controller method CORS configuration（基于Controller的跨域配置）
2. Global CORS configuration（全局跨域配置）

基于Controller的跨域配置

使用@CrossOrigin注解，我们先码一下官方介绍

  @CrossOrigin(origins = "http://localhost:9000")
  @GetMapping("/greeting")
  public Greeting greeting(@RequestParam(required=false, defaultValue="World") String name) {
    System.out.println("==== in greeting ====");
    return new Greeting(counter.incrementAndGet(), String.format(template, name));
  }

This @CrossOrigin annotation enables cross-origin requests only for this specific method.
By default, its allows all origins, all headers, the HTTP methods specified in the @RequestMapping annotation and a maxAge of 30 minutes is used. You can customize this behavior by specifying the value of one of the annotation attributes: origins, methods, allowedHeaders, exposedHeaders, allowCredentials or maxAge. In this example, we only allow http://localhost:9000 to send cross-origin requests.
上面叽叽喳喳大致意思是，用@CrossOrigin注解在@Controller类上就能打开跨域，默认允许所有的访问，如果要定制的话，加属性即可，有

1. origins（允许跨域请求的域名），
2. methods（方法），
3. allowedHeaders
4. exposedHeaders
5. allowCredentials
6. maxAge（跨域允许的时间）

全局跨域配置

全局跨域配置使用的是SpringBoot的配置，重写WebMvcConfiger中的addCorsMappings方法，用Bean的方法注入即可达到开启多个跨域的效果，更多的配置，类似上面的6条，直接添加即可
下面是原文：
As an alternative to fine-grained annotation-based configuration, you can also define some global CORS configuration as well. This is similar to using a Filter based solution, but can be declared within Spring MVC and combined with fine-grained @CrossOrigin configuration. By default all origins and GET, HEAD and POST methods are allowed.

src/main/java/hello/GreetingController.java

  @GetMapping("/greeting-javaconfig")
  public Greeting greetingWithJavaconfig(@RequestParam(required=false, defaultValue="World") String name) {
    System.out.println("==== in greeting ====");
    return new Greeting(counter.incrementAndGet(), String.format(template, name));
  }
src/main/java/hello/Application.java

  @Bean
  public WebMvcConfigurer corsConfigurer() {
    return new WebMvcConfigurer() {
      @Override
      public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/greeting-javaconfig")
        .allowedOrigins("http://localhost:9000");
      }
    };
  }

You can easily change any properties (like the allowedOrigins one in the example), as well as only apply this CORS configuration to a specific path pattern. Global and controller level CORS configurations can also be combined.
除了上面这种添加Bean的配置方式，我们也可以用实现接口的方法来实现

@Configuration
@EnableWebMvc
public class CorsConfig implements WebMvcConfigurer {
 
    @Override
    public void addCorsMappings(CorsRegistry registry) {
        //设置允许跨域的路径
        registry.addMapping("/**")
                //设置允许跨域请求的域名
                .allowedOrigins("*")
                //是否允许证书 不再默认开启
                .allowCredentials(true)
                //设置允许的方法
                .allowedMethods("*")
                //跨域允许时间
                .maxAge(3600);
    }
}

效果是相同的，完全可行