RestTemplate 的拦截器 ClientHttpRequestInterceptor

迈不过友情╰ 2024-03-31 16:33 77阅读 0赞

## RestTemplate ##

RestTemplate 是 spring-web 模块提供的一个执行同步http请求的客户端，底层依赖的是 JDK HttpURLConnection, Apache HttpComponents 和 OkHttp3 等，在将请求提交给这些底层模块之前，提供了扩展点：通过ClientHttpRequestInterceptor接口的实现类对请求进行拦截处理。这篇文章是 Spring Cloud Loadbalancer 模块学习的前置文章。因为 Spring Cloud loadbalancer 是通过 ClientHttpRequestInterceptor 对 RestTemplate 进行负载均衡的。因此需要对 ClientHttpRequestInterceptor 有所了解。

>  *  需要注意的是，根据文档显示从Spring 5.0开始 RestTemplate 已经进入维护的阶段，目前主推的是org.springframework.web.reactive.client.WebClient，支持异步请求。

## 基本类 ##

ClientHttpRequestInterceptor 接口是比较简单

public interface ClientHttpRequestInterceptor {
        
    	ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution)
    			throws IOException;
    }

涉及的其他类如下，本质上是对HTTP协议的抽象。

// HttpRequest 代表了一个 http 请求体，包含了请求行（HttpMethod,URI),请求头
    public interface HttpRequest extends HttpMessage {
        
        @Nullable
        default HttpMethod getMethod() {
        
            return HttpMethod.resolve(getMethodValue());
        }
        String getMethodValue();
        URI getURI();
    }
    public interface HttpMessage {
        
        HttpHeaders getHeaders();
    }
    public class HttpHeaders implements MultiValueMap<String, String>, Serializable {
        
        // 省略
    }

ClientHttpResponse 代表了http请求的响应，也是包含了状态码，响应头，响应体。

public interface ClientHttpResponse extends HttpInputMessage, Closeable {
        
        HttpStatus getStatusCode() throws IOException;
        int getRawStatusCode() throws IOException;
        String getStatusText() throws IOException;
        @Override
        void close();
    }
    public interface HttpInputMessage extends HttpMessage {
        
        InputStream getBody() throws IOException;
    }
    
    public interface ClientHttpRequestExecution {
        
        ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException;
    }

ClientHttpRequestExecution 主要是用于构建拦截器调用链，并通过调用栈的形式执行拦截器。

@FunctionalInterface
    public interface ClientHttpRequestExecution {
        
    	ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException;
    }

实现类如下：

class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest {
        
        private final List<ClientHttpRequestInterceptor> interceptors;
        private final ClientHttpRequestFactory requestFactory;
    
        private class InterceptingRequestExecution implements ClientHttpRequestExecution {
        
    		private final Iterator<ClientHttpRequestInterceptor> iterator;
    		public InterceptingRequestExecution() {
        
    			this.iterator = interceptors.iterator();
    		}
    		@Override
    		public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
        
    			if (this.iterator.hasNext()) {
        
                    // 执行调用链
    				ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
    				return nextInterceptor.intercept(request, body, this);
    			}// 调用链结束，回到主流程，获取底层的 ClientHttpRequest，并执行。
    			else {
        
    				HttpMethod method = request.getMethod();
    				Assert.state(method != null, "No standard HTTP method");
    				ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), method);
    				request.getHeaders().forEach((key, value) -> delegate.getHeaders().addAll(key, value));
    				if (body.length > 0) {
        
    					if (delegate instanceof StreamingHttpOutputMessage) {
        
    						StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) delegate;
    						streamingOutputMessage.setBody(outputStream -> StreamUtils.copy(body, outputStream));
    					}
    					else {
        
    						StreamUtils.copy(body, delegate.getBody());
    					}
    				}
    				return delegate.execute();
    			}
    		}
    	}
        // 省略..
    }

## 拦截器应用 ##

拦截器的应用还是比较简单的，只需往 RestTemplate 实例添加拦截器即可。

public class InterceptorTest {
        
        public static void main(String[] args) {
        
            ClientHttpRequestInterceptor i1 = new ClientHttpRequestInterceptor() {
        
                @Override
                public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
        
                    System.out.println("拦截器1开始。。。");
                    ClientHttpResponse execute = execution.execute(request, body);
                    System.out.println("拦截器1结束。。。");
                    return execute;
                }
            };
            ClientHttpRequestInterceptor i2 = new ClientHttpRequestInterceptor() {
        
                @Override
                public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
        
                    System.out.println("拦截器2开始。。。");
                    ClientHttpResponse execute = execution.execute(request, body);
                    System.out.println("拦截器2结束。。。");
                    return execute;
                }
            };
            RestTemplate restTemplate = new RestTemplate();
            restTemplate.getInterceptors().add(i1);
            restTemplate.getInterceptors().add(i2);
            String forObject = restTemplate.getForObject("http://www.baidu.com", String.class);
            System.out.println(forObject);
        }
    }

执行以上代码，输出如下：

>  *  拦截器1开始。。。
>  *  拦截器2开始。。。
>  *  拦截器2结束。。。
>  *  拦截器1结束。。。

我们作出一点改变，通过拦截器来实现负载均衡。

public class InterceptorTest {
        
        static class Mywraper extends HttpRequestWrapper{
        
            private String url;
    
            public Mywraper(HttpRequest request,String url) {
        
                super(request);
                this.url = url;
            }
            @Override
            public URI getURI() {
        
                try {
        
                    return new URI(url);
                } catch (URISyntaxException e) {
        
                }
                return null;
            }
        }
        public static void main(String[] args) {
        
            String[] loabancerhost = {
        "https://www.baidu.com", "https://www.sina.com.cn"};
            AtomicInteger times = new AtomicInteger(0);
            ClientHttpRequestInterceptor i1 = new ClientHttpRequestInterceptor() {
        
                @Override
                public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
        
                    int length = loabancerhost.length;
                    int i = times.getAndIncrement() % length;
                    //因为 HttpRequest 和 URI 不提供修改功能，因此需要借助 HttpRequestWrapper 对request进行包装
                    Mywraper mywraper = new Mywraper(request, loabancerhost[i]);
                    ClientHttpResponse execute = execution.execute(mywraper, body);
                    return execute;
                }
            };
            ClientHttpRequestInterceptor i2 = new ClientHttpRequestInterceptor() {
        
                @Override
                public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
        
                    System.out.println("第"+times+"次请求的host为: "+request.getURI());
                    ClientHttpResponse execute = execution.execute(request, body);
                    return execute;
                }
            };
            RestTemplate restTemplate = new RestTemplate();
            restTemplate.getInterceptors().add(i1);
            restTemplate.getInterceptors().add(i2);
            for (int i = 10; i > 0; i--) {
        
                restTemplate.getForObject("http://www.baidu.com", String.class);
            }
        }
    }

>  *  第1次请求的host为: https://www.baidu.com
>  *  第2次请求的host为: https://www.sina.com.cn
>  *  第3次请求的host为: https://www.baidu.com
>  *  第4次请求的host为: https://www.sina.com.cn
>  *  第5次请求的host为: https://www.baidu.com
>  *  第6次请求的host为: https://www.sina.com.cn
>  *  第7次请求的host为: https://www.baidu.com
>  *  第8次请求的host为: https://www.sina.com.cn
>  *  第9次请求的host为: https://www.baidu.com
>  *  第10次请求的host为: https://www.sina.com.cn

至此，如何通过 InterceptingClientHttpRequest 进行 RestTemplate 负载均衡介绍完毕，本质上原理还是很简单的。只需要通过拦截器修改URI即可。