Spring Cloud Alibaba Sentinel对RestTemplate的支持

尹吉欢2019-05-050条评论 339人阅读

Sentinel

Spring Cloud Alibaba Sentinel 支持对 RestTemplate 的服务调用使用 Sentinel 进行保护，在构造 RestTemplate bean的时候需要加上 @SentinelRestTemplate 注解。

需要注意的是目前的版本spring-cloud-starter-alibaba-sentinel.0.2.1.RELEASE在配置RestTemplate的时候有个Bug，需要将配置放在Spring Boot的启动类中，也就是@SpringBootApplication注解所在的类。

如果单独放在@Configuration标记的类中目前是有问题的，当然后续版本中会进行修复，对应的问题描述：[https://github.com/spring-cloud-incubator/spring-cloud-alibaba/issues/227](https://github.com/spring-cloud-incubator/spring-cloud-alibaba/issues/227)

```
@Bean
@SentinelRestTemplate(fallback = "fallback", fallbackClass = ExceptionUtil.class, blockHandler="handleException",blockHandlerClass=ExceptionUtil.class)
public RestTemplate restTemplate() {
	return new RestTemplate();
}
```

- blockHandler
限流后处理的方法
- blockHandlerClass
限流后处理的类
- fallback
熔断后处理的方法
- fallbackClass
熔断后处理的类

异常处理类定义需要注意的是该方法的参数跟返回值跟 org.springframework.http.client.ClientHttpRequestInterceptor#interceptor 方法一致，其中参数多出了一个 BlockException 参数用于获取 Sentinel 捕获的异常。

```
public class ExceptionUtil {
	public static SentinelClientHttpResponse handleException(HttpRequest request,
			byte[] body, ClientHttpRequestExecution execution, BlockException ex) {
		System.err.println("Oops: " + ex.getClass().getCanonicalName());
		return new SentinelClientHttpResponse("custom block info");
	}
	
	public static SentinelClientHttpResponse fallback(HttpRequest request,
			byte[] body, ClientHttpRequestExecution execution, BlockException ex) {
		System.err.println("fallback: " + ex.getClass().getCanonicalName());
		return new SentinelClientHttpResponse("custom fallback info");
	}
}

```

### 原理剖析

核心代码在org.springframework.cloud.alibaba.sentinel.custom.SentinelBeanPostProcessor中，实现了MergedBeanDefinitionPostProcessor接口，MergedBeanDefinitionPostProcessor接口实现了BeanPostProcessor接口。

核心方法就是重写的postProcessMergedBeanDefinition和postProcessAfterInitialization。

### postProcessMergedBeanDefinition

```
private ConcurrentHashMap<String, SentinelRestTemplate> cache = new ConcurrentHashMap<>();

@Override
public void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition,
			Class<?> beanType, String beanName) {
	if (checkSentinelProtect(beanDefinition, beanType)) {
	    SentinelRestTemplate sentinelRestTemplate = ((StandardMethodMetadata) beanDefinition
					.getSource()).getIntrospectedMethod()
							.getAnnotation(SentinelRestTemplate.class);
        // 获取SentinelRestTemplate注解对象存储起来
		cache.put(beanName, sentinelRestTemplate);
	}
}
// 判断bean是否加了SentinelRestTemplate注解并且是RestTemplate
private boolean checkSentinelProtect(RootBeanDefinition beanDefinition,
			Class<?> beanType) {
	return beanType == RestTemplate.class
				&& beanDefinition.getSource() instanceof StandardMethodMetadata
				&& ((StandardMethodMetadata) beanDefinition.getSource())
						.isAnnotated(SentinelRestTemplate.class.getName());
}
```

### postProcessAfterInitialization

```
@Override
public Object postProcessAfterInitialization(Object bean, String beanName)
			throws BeansException {
	if (cache.containsKey(beanName)) {
		// add interceptor for each RestTemplate with @SentinelRestTemplate annotation
		StringBuilder interceptorBeanName = new StringBuilder();
        // 缓存中得到注解对象
		SentinelRestTemplate sentinelRestTemplate = cache.get(beanName);
        // 生成interceptorBeanName SentinelProtectInterceptor名称
		interceptorBeanName
					.append(StringUtils.uncapitalize(
							SentinelProtectInterceptor.class.getSimpleName()))
					.append("_")
					.append(sentinelRestTemplate.blockHandlerClass().getSimpleName())
					.append(sentinelRestTemplate.blockHandler()).append("_")
					.append(sentinelRestTemplate.fallbackClass().getSimpleName())
					.append(sentinelRestTemplate.fallback());
		RestTemplate restTemplate = (RestTemplate) bean;
        // 注册SentinelProtectInterceptor
		registerBean(interceptorBeanName.toString(), sentinelRestTemplate);
        // 获取SentinelProtectInterceptor
		SentinelProtectInterceptor sentinelProtectInterceptor = applicationContext
					.getBean(interceptorBeanName.toString(),
							SentinelProtectInterceptor.class);
        // 给restTemplate添加拦截器
		restTemplate.getInterceptors().add(sentinelProtectInterceptor);
	}
	return bean;
}
// 注册SentinelProtectInterceptor类
private void registerBean(String interceptorBeanName,
			SentinelRestTemplate sentinelRestTemplate) {
	// register SentinelProtectInterceptor bean
	DefaultListableBeanFactory beanFactory = (DefaultListableBeanFactory) applicationContext
				.getAutowireCapableBeanFactory();
    BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder
				.genericBeanDefinition(SentinelProtectInterceptor.class);
	beanDefinitionBuilder.addConstructorArgValue(sentinelRestTemplate);
	BeanDefinition interceptorBeanDefinition = beanDefinitionBuilder
				.getRawBeanDefinition();
	beanFactory.registerBeanDefinition(interceptorBeanName,
				interceptorBeanDefinition);
}
```

看到这边大家就明白了，其实就是给restTemplate添加拦截器来处理。跟Ribbon中的@LoadBalanced原理是一样的。

### SentinelProtectInterceptor

Sentinel RestTemplate 限流的资源规则提供两种粒度：

- schema://host:port/path：协议、主机、端口和路径
- schema://host:port：协议、主机和端口

这两种粒度从org.springframework.cloud.alibaba.sentinel.custom.SentinelProtectInterceptor.intercept(HttpRequest, byte[], ClientHttpRequestExecution)方法中可以看的出来

```
URI uri = request.getURI();
String hostResource = uri.getScheme() + "://" + uri.getHost()
			+ (uri.getPort() == -1 ? "" : ":" + uri.getPort());
String hostWithPathResource = hostResource + uri.getPath();
```

下面就是根据hostResource和hostWithPathResource进行限流

```
ContextUtil.enter(hostWithPathResource);
if (entryWithPath) {
	hostWithPathEntry = SphU.entry(hostWithPathResource);
}
hostEntry = SphU.entry(hostResource);
// 执行Http调用
response = execution.execute(request, body);
```
在后面就是释放资源，异常处理等代码，大家自己去了解下。

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