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。