一、簡介
在微服務架構中，我們會有這樣的需求，A服務調用B服務，B服務調用C服務，ABC服務都需要用到當前用戶上下文信息（userId、orgId等），那么如何實現呢？
方案一： 攔截器加上ThreadLocal實現，但是如果在這次請求中創建了一個新的線程就拿不到了，也就是無法跨線程傳遞數據。
方案二： 使用攔截器加上 HystrixRequestContext 這個 request level 的 context實現，即保存到HystrixRequestContext中的數據在整個請求中都能訪問。
二、使用
2.1代碼示例
首先需要在pom文件引入依賴hystrix
<dependency>
<groupId>com.netflix.hystrix</groupId>
<artifactId>hystrix-core</artifactId>
<version>1.5.12</version>
</dependency>
保存上下文信息的對象ServiceContextHolder
package cn.sp.context;
import com.netflix.hystrix.strategy.concurrency.HystrixRequestContext;
import com.netflix.hystrix.strategy.concurrency.HystrixRequestVariableDefault;
/** * Created by 2YSP on 2019/7/28. */
public class ServiceContextHolder {
private static final HystrixRequestVariableDefault<ServiceContext> context = new HystrixRequestVariableDefault<>();
public static ServiceContext getServiceContext() {
initServiceContext();
return context.get();
}
public static void setServiceContext(ServiceContext serviceContext) {
initServiceContext();
context.set(serviceContext);
}
private static void initServiceContext() {
if (!HystrixRequestContext.isCurrentThreadInitialized()) {
HystrixRequestContext.initializeContext();
}
}
public static void destroy() {
if (HystrixRequestContext.isCurrentThreadInitialized()) {
HystrixRequestContext.getContextForCurrentThread().shutdown();
}
}
}
ServiceContextInterceptor的作用是將請求頭中的userId保存到上下文對象中。
@Slf4j
public class ServiceContextInterceptor extends HandlerInterceptorAdapter {
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
initServiceContext(request, request.getRequestURL().toString());
return true;
}
@Override
public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable ModelAndView modelAndView) throws Exception {
ServiceContextHolder.destroy();
}
private void initServiceContext(HttpServletRequest request, String url) {
ServiceContext serviceContext = new ServiceContext();
String userId = request.getHeader("userId");
serviceContext.setUserId(Long.valueOf(userId));
ServiceContextHolder.setServiceContext(serviceContext);
}
}
添加攔截器配置
@Configuration
@EnableWebMvc
@Import(value = {RestResponseBodyAdvice.class})
public class MvcConfig implements WebMvcConfigurer {
@Bean
public ServiceContextInterceptor getServiceContextInterceptor() {
return new ServiceContextInterceptor();
}
@Override
public void addInterceptors(InterceptorRegistry registry) {
registry.addInterceptor(getServiceContextInterceptor()).addPathPatterns("/request-context/**");
}
}
用於測試的RequestContextTestController
@RestController
@RequestMapping("request-context")
@Slf4j
public class RequestContextTestController {
@RequestMapping(value = "test", method = RequestMethod.GET)
public String test() {
System.out.println("請求的用戶id:" + ServiceContextHolder.getServiceContext().getUserId() + "");
HystrixContextRunnable runnable =
new HystrixContextRunnable(() -> {
//從新的線程中獲取當前用戶id
ServiceContext context = ServiceContextHolder.getServiceContext();
System.out.println("新線程的用戶id：" + context.getUserId());
context.setUserId(110L);
});
new Thread(runnable).start();
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
return ServiceContextHolder.getServiceContext().getUserId() + "";
}
}
注意： 只有使用HystrixContextRunnable或HystrixContextCallable創建線程才能在線程間傳遞數據，JDK自帶的是無效的。
2.2測試
使用postman發送請求
請求示例
請求頭中的userId是22，返回結果卻變成110，說明在新線程中改變了ServiceContextHolder中保存的userId。
控制台日志如下：
請求的用戶id:22
2019-08-31 14:25:29.787 [http-nio-80-exec-1] WARN c.n.c.sources.URLConfigurationSource - No URLs will be polled as dynamic configuration sources.
2019-08-31 14:25:29.787 [http-nio-80-exec-1] INFO c.n.c.sources.URLConfigurationSource - To enable URLs as dynamic configuration sources, define System property archaius.configurationSource.additionalUrls or make config.properties available on classpath.
2019-08-31 14:25:29.798 [http-nio-80-exec-1] INFO c.n.config.DynamicPropertyFactory - DynamicPropertyFactory is initialized with configuration sources: com.netflix.config.ConcurrentCompositeConfiguration@a6f6807
新線程的用戶id：22
說明新線程也能獲取到ServiceContextHolder中的數據，這種又是怎么實現的呢？下面介紹原理。
三、原理
上下文信息其實是保存在HystrixRequestVariableDefault類型的變量中，所以先看看這個類的源碼。
HystrixRequestVariableDefault是HystrixRequestVariable接口的實現類，HystrixRequestVariable接口表示request level的屬性，僅提供了get()來獲取屬性。
public interface HystrixRequestVariable<T> extends HystrixRequestVariableLifecycle<T> {
public T get();
}
HystrixRequestVariableDefault和ThreadLocal一樣，提供了 T get() 和 set(T value) 兩個工具方法。
public class HystrixRequestVariableDefault<T> implements HystrixRequestVariable<T> {
static final Logger logger = LoggerFactory.getLogger(HystrixRequestVariableDefault.class);
@SuppressWarnings("unchecked")
public T get() {
if (HystrixRequestContext.getContextForCurrentThread() == null) {
throw new IllegalStateException(HystrixRequestContext.class.getSimpleName() + ".initializeContext() must be called at the beginning of each request before RequestVariable functionality can be used.");
}
// 拿到當前線程的存儲結構，以自己為key索引數據
ConcurrentHashMap<HystrixRequestVariableDefault<?>, LazyInitializer<?>> variableMap = HystrixRequestContext.getContextForCurrentThread().state;
// short-circuit the synchronized path below if we already have the value in the ConcurrentHashMap
LazyInitializer<?> v = variableMap.get(this);
...
}
public void set(T value) {
// 拿到當前線程的存儲結構，以自己為key來存儲實際的數據。
HystrixRequestContext.getContextForCurrentThread().state.put(this, new LazyInitializer<T>(this, value));
}
}
set/get方法都調用了HystrixRequestContext的方法完成的，HystrixRequestContext的部分源碼如下：
public class HystrixRequestContext implements Closeable {
//每個線程的ThreadLocal將保存HystrixRequestVariableState
private static ThreadLocal<HystrixRequestContext> requestVariables = new ThreadLocal<HystrixRequestContext>();
// 當前線程是否初始化了HystrixRequestContext
public static boolean isCurrentThreadInitialized() {
HystrixRequestContext context = requestVariables.get();
return context != null && context.state != null;
}
// 從當前線程獲取HystrixRequestContext
public static HystrixRequestContext getContextForCurrentThread() {
HystrixRequestContext context = requestVariables.get();
if (context != null && context.state != null) {
return context;
} else {
return null;
}
}
public static void setContextOnCurrentThread(HystrixRequestContext state) {
requestVariables.set(state);
}
// 在每個請求開始的時候調用此方法，創建一個HystrixRequestContext,並與當前線程關聯
public static HystrixRequestContext initializeContext() {
HystrixRequestContext state = new HystrixRequestContext();
requestVariables.set(state);
return state;
}
ConcurrentHashMap<HystrixRequestVariableDefault<?>, HystrixRequestVariableDefault.LazyInitializer<?>> state = new ConcurrentHashMap<HystrixRequestVariableDefault<?>, HystrixRequestVariableDefault.LazyInitializer<?>>();
}
可以看出實際數據是存儲在state這個ConcurrentHashMap中的，每個線程關聯一個HystrixRequestContext，每個HystrixRequestContext有個Map結構存儲數據，key就是HystrixRequestVariableDefault。
如何實現request level context?
HystrixContextRunnable源碼如下：
// HystrixContextRunnable是個Runnable,一個可用於執行的任務
public class HystrixContextRunnable implements Runnable {
private final Callable<Void> actual;
private final HystrixRequestContext parentThreadState;
public HystrixContextRunnable(Runnable actual) {
this(HystrixPlugins.getInstance().getConcurrencyStrategy(), actual);
}
public HystrixContextRunnable(HystrixConcurrencyStrategy concurrencyStrategy, final Runnable actual) {
// 獲取當前線程的HystrixRequestContext
this(concurrencyStrategy, HystrixRequestContext.getContextForCurrentThread(), actual);
}
// 關鍵的構造器
public HystrixContextRunnable(final HystrixConcurrencyStrategy concurrencyStrategy, final HystrixRequestContext hystrixRequestContext, final Runnable actual) {
// 將原始任務Runnable包裝成Callable, 創建了一個新的callable
this.actual = concurrencyStrategy.wrapCallable(new Callable<Void>() {
@Override
public Void call() throws Exception {
actual.run();
return null;
}
});
// 存儲當前線程的hystrixRequestContext
this.parentThreadState = hystrixRequestContext;
}
@Override
public void run() {
// 運行實際的Runnable之前先保存當前線程已有的HystrixRequestContext
HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();
try {
// 設置當前線程的HystrixRequestContext,來自上一級線程,因此兩個線程是同一個HystrixRequestContext
HystrixRequestContext.setContextOnCurrentThread(parentThreadState);
try {
actual.call();
} catch (Exception e) {
throw new RuntimeException(e);
}
} finally {
// 還原當前線程的HystrixRequestContext
HystrixRequestContext.setContextOnCurrentThread(existingState);
}
}
}