SpringCloud 系列 --3. 负载均衡 @LoadBalanced 注解原理

restTemplate本身不具有负载均衡原理,但是加入了@LoadBalanced注解后具有负载功能，得益于RestTemplate的拦截器功能.

我们模拟实现一个自定义的@LoadBalanced注解  @MyLoadBalanced

package org.crazyit.cloud;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.beans.factory.annotation.Qualifier;

@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface MyLoadBalanced {

}

然后实现org.springframework.http.HttpRequest 接口，将原先的URI进行重写，所有的请求转发到http://localhost:8080/hello这个地址. SpringCloud对RestTemplate进行拦截时候也做了同样的事情,只不过更加灵活的处理,并不是像我们返回固定的URI。

package org.crazyit.cloud;

import java.net.URI;

import org.springframework.http.HttpHeaders;
import org.springframework.http.HttpMethod;
import org.springframework.http.HttpRequest;

/**
 * 自定义的请求类，用于转换URI
 * @author 杨恩雄
 *
 */
public class MyHttpRequest implements HttpRequest {
	
	private HttpRequest sourceRequest;
	
	public MyHttpRequest(HttpRequest sourceRequest) {
		this.sourceRequest = sourceRequest;
	}

	public HttpHeaders getHeaders() {
		return sourceRequest.getHeaders();
	}

	public HttpMethod getMethod() {
		return sourceRequest.getMethod();
	}

	/**
	 * 将URI转换
	 */
	public URI getURI() {
		try {
			URI newUri = new URI("http://localhost:8080/hello");
			return newUri;
		} catch (Exception e) {
			e.printStackTrace();
		}
		return sourceRequest.getURI();
	}
}

然后我们自定义拦截器, 将原先请求使用我们刚才定义的 MyHttpRequest 进行转换.

package org.crazyit.cloud;

import java.io.IOException;

import org.springframework.http.HttpRequest;
import org.springframework.http.client.ClientHttpRequestExecution;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.http.client.ClientHttpResponse;

/**
 * 自定义拦截器
 * 
 * @author 杨恩雄
 *
 */
public class MyInterceptor implements ClientHttpRequestInterceptor {

	public ClientHttpResponse intercept(HttpRequest request, byte[] body,
			ClientHttpRequestExecution execution) throws IOException {
		System.out.println("=============  这是自定义拦截器实现");
		System.out.println("               原来的URI：" + request.getURI());
		// 换成新的请求对象（更换URI）
		MyHttpRequest newRequest = new MyHttpRequest(request);
		System.out.println("               拦截后新的URI：" + newRequest.getURI());
		return execution.execute(newRequest, body);
	}
}

配置在容器启动的时候将我们的自定义拦截器 MyInterceptor 加入到 RestTemplate 实例中

package org.crazyit.cloud;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.springframework.beans.factory.SmartInitializingSingleton;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.web.client.RestTemplate;


@Configuration
public class MyAutoConfiguration {

	@Autowired(required=false)
	@MyLoadBalanced
	private List<RestTemplate> myTemplates = Collections.emptyList();
	
	@Bean
	public SmartInitializingSingleton myLoadBalancedRestTemplateInitializer() {
		System.out.println("====  这个Bean将在容器初始化时创建    =====");
		return new SmartInitializingSingleton() {
			
			public void afterSingletonsInstantiated() {
				for(RestTemplate tpl : myTemplates) {
					// 创建一个自定义的拦截器实例
					MyInterceptor mi = new MyInterceptor();
					// 获取RestTemplate原来的拦截器
					List list = new ArrayList(tpl.getInterceptors());
					// 添加到拦截器集合
					list.add(mi);
					// 将新的拦截器集合设置到RestTemplate实例
					tpl.setInterceptors(list);
				}
			}
		};
	}
}

最后使用我们的注解 @MyLoadBalanced

package org.crazyit.cloud;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

@RestController
@Configuration
public class InvokerController {
	
	@Bean
	@MyLoadBalanced
	public RestTemplate getMyRestTemplate() {
		return new RestTemplate();
	}
	
	/**
	 * 浏览器访问的请求
	 */
	@RequestMapping(value = "/router", method = RequestMethod.GET, 
			produces = MediaType.APPLICATION_JSON_VALUE)
	public String router() {
		RestTemplate restTpl = getMyRestTemplate();
		// 根据名称来调用服务，这个URI会被拦截器所置换
		String json = restTpl.getForObject("http://my-server/hello", String.class);
		return json;
	}
	
	/**
	 * 最终的请求都会转到这个服务
	 */
	@RequestMapping(value = "/hello", method = RequestMethod.GET)
	@ResponseBody
	public String hello() {
		return "Hello World";
	}
}

启动类

package org.crazyit.cloud;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class TestInterceptorMain {

	public static void main(String[] args) {
		SpringApplication.run(TestInterceptorMain.class, args);
	}

}

最后访问 http://localhost:8080/router 本来访问 http://my-server/hello
但是现在不管 my-server 原先是哪个地址都会转发到 localhost:8080/hello 这里。