Spring Cloud Hystrix 源码系列：HystrixRequestContext

悠悠 2022-03-16 12:21 234阅读 0赞

在讲HystrixRequestContext之前，先了解下Hystrix 跨线程传递数据的知识，我们都知道ThreadLocal、InheritableThreadLocal跨父子线程传递数据。本文基于[hystrix-core][] 1.5.18（近年来几乎很少更新，建议升级）

**需求描述**

服务A 通过 Feign + Hystrix 调用服务B，服务间调用时需传递 JWT Token，希望在Feign发起的所有请求中都自动加上token以在各服务中传递环境信息，**如何将token从tomcat工作线程传递到hystrix线程池线程？**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk_size_16_color_FFFFFF_t_70][]

注：JWT Token 一是用于确保访问的安全，二是存储用户context信息。

我们都知道，当 Hystrix 隔离策略为线程池时（复习[Hystrix线程池的知识][Hystrix]），Hystrix 会专门创建线程池用于执行对 Service-B的调用。

**这时候你会想到，java中父子线程之间传递数据是这样的：**

每个Thread都有个有两个属性，

*   **threadLocals（**ThreadLocalMap类型**）**，它用于存储线程私有数据，ThreadLocal对象只是充当检索数据的Key，本身不存储任何信息。
 *   **inheritableThreadLocals （**ThreadLocalMap类型**）**，当使用 **InheritableThreadLocal**时，数据存储在它中，否则存储在threadLocals。**在创建子线程时，会将父线程的inheritableThreadLocals拷贝到子线程，从而达到跨线程传递数据的目的。**

private void init(ThreadGroup g, Runnable target, String name ...) {
        ...
        if (parent.inheritableThreadLocals != null)
            this.inheritableThreadLocals = ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
        ...
    }

不急，先看**Hystrix** 的实例代码吧，下面就实现了从 main线程传递到子subThread线程。

@Test
    public void test() throws InterruptedException {
        // 在当前线程下创建一个HystrixRequestContext对象
        HystrixRequestContext.initializeContext();
        // 创建HystrixRequestVariableDefault作为检索数据的key
        final HystrixRequestVariableDefault<String> variableDefault = new HystrixRequestVariableDefault<>();
        // 将<HystrixRequestVariableDefault,kitty>存储到当前线程的HystrixRequestContext中
        variableDefault.set("kitty");
        // HystrixContextRunnable 是核心, 下面将分析源码:
        HystrixContextRunnable runnable = new HystrixContextRunnable(() -> System.out.println(variableDefault.get()));
        // 用子线程执行任务
        new Thread(runnable, "subThread").start();
    }
    //输出: kitty

难道就是用的InheritableThreadLocal？那你就错了！接下来看源码吧！

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk_size_16_color_FFFFFF_t_70 1][]

// request level的属性
    public interface HystrixRequestVariable<T> extends HystrixRequestVariableLifecycle<T> {
        T get();
    }
    // HystrixRequestVariableDefault提供了get和set
    public class HystrixRequestVariableDefault<T> implements HystrixRequestVariable<T> {
        public T get() {
            // 拿到当前线程的存储结构, 以自己作为key, 来检索数据
            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.");
            } else {
                ConcurrentHashMap<HystrixRequestVariableDefault<?>, HystrixRequestVariableDefault.LazyInitializer<?>> variableMap = HystrixRequestContext.getContextForCurrentThread().state;
                HystrixRequestVariableDefault.LazyInitializer<?> v = (HystrixRequestVariableDefault.LazyInitializer)variableMap.get(this);
                if (v != null) {
                    return v.get();
                } else {
                    HystrixRequestVariableDefault.LazyInitializer<T> l = new HystrixRequestVariableDefault.LazyInitializer(this);
                    HystrixRequestVariableDefault.LazyInitializer<?> existing = (HystrixRequestVariableDefault.LazyInitializer)variableMap.putIfAbsent(this, l);
                    return existing == null ? l.get() : existing.get();
                }
            }
        }
        // 拿到当前线程的存储结构, 用自己作为key, 存储实际的数据
        public void set(T value) {
            HystrixRequestContext.getContextForCurrentThread().state.put(this, new HystrixRequestVariableDefault.LazyInitializer(this, value));
        }
    }
    // HystrixRequestContext是真正存储数据的
    public class HystrixRequestContext implements Closeable {
        // 每个线程各有一份HystrixRequestContext,当然,前提是调用了initializeContext()进行初始化
        private static ThreadLocal<HystrixRequestContext> requestVariables = new ThreadLocal();
        // 实际的存储结构(每个线程),key就是HystrixRequestVariableDefault
        ConcurrentHashMap<HystrixRequestVariableDefault<?>, LazyInitializer<?>> state = new ConcurrentHashMap();
        //当前线程是否有数据
        public static boolean isCurrentThreadInitialized() {
            HystrixRequestContext context = (HystrixRequestContext)requestVariables.get();
            return context != null && context.state != null;
        }
        // 获取当前线程的HystrixRequestContext
        public static HystrixRequestContext getContextForCurrentThread() {
            HystrixRequestContext context = (HystrixRequestContext)requestVariables.get();
            return context != null && context.state != null ? context : null;
        }
        // 覆盖当前线程的HystrixRequestContext
        public static void setContextOnCurrentThread(HystrixRequestContext state) {
            requestVariables.set(state);
        }
        // 当前线程创建一个HystrixRequestContext
        public static HystrixRequestContext initializeContext() {
            HystrixRequestContext state = new HystrixRequestContext();
            requestVariables.set(state);
            return state;
        }
        private HystrixRequestContext() {
        }
        public void shutdown() {
            if (this.state != null) {
                Iterator i$ = this.state.keySet().iterator();
    
                while(i$.hasNext()) {
                    HystrixRequestVariableDefault v = (HystrixRequestVariableDefault)i$.next();
    
                    try {
                        HystrixRequestVariableDefault.remove(this, v);
                    } catch (Throwable var4) {
                        HystrixRequestVariableDefault.logger.error("Error in shutdown, will continue with shutdown of other variables", var4);
                    }
                }
                this.state = null;
            }
        }
        public void close() {
            this.shutdown();
        }
    }

**这时你会问为何HystrixRequestContext 中requestVariables 的key要设计成HystrixRequestVariableDefault？**

感觉这样是普通码农和高级的区别，优秀的设计就是这样的，这样对使用者的而言代码更简洁更友好！接下来感受下吧

> 实例化v1,v2并set值相应的值，  
> *HystrixRequestVariableDefault<String> v1 = new HystrixRequestVariableDefault<>();  
> v1.set("1")  
> HystrixRequestVariableDefault<String> v2 = new HystrixRequestVariableDefault<>();  
> v2.set("2");*  
> 那当前线程的HystrixRequestContext存储的数据为：  
> *<v1, "1">  
> <v2, "2">*

**那Hystrix 如何实现request level context?**

//一个可执行的任务
    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) {
            // 将原始任务Runnable包装成Callable
            this.actual = concurrencyStrategy.wrapCallable(new Callable<Void>() {
                public Void call() throws Exception {
                    actual.run();
                    return null;
                }
            });
            // 获取并存储当前线程的HystrixRequestContext(如示例的main线程)
            this.parentThreadState = HystrixRequestContext.getContextForCurrentThread();
        }
        public void run() {
            //先保存子线程的HystrixRequestContext
            HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();
            try {
                //设置子线程的HystrixRequestContext为来自上一级线程HystrixRequestContext
                HystrixRequestContext.setContextOnCurrentThread(this.parentThreadState);
                try {
                    this.actual.call();
                } catch (Exception var6) {
                    throw new RuntimeException(var6);
                }
            } finally {
                // 还原子线程的HystrixRequestContext
                HystrixRequestContext.setContextOnCurrentThread(existingState);
            }
        }
    }
    public class HystrixContextCallable<K> implements Callable<K> {
        private final Callable<K> actual;
        private final HystrixRequestContext parentThreadState;
        public HystrixContextCallable(Callable<K> actual) {
            this(HystrixPlugins.getInstance().getConcurrencyStrategy(), actual);
        }
        public HystrixContextCallable(HystrixConcurrencyStrategy concurrencyStrategy, Callable<K> actual) {
            this.actual = concurrencyStrategy.wrapCallable(actual);
            this.parentThreadState = HystrixRequestContext.getContextForCurrentThread();
        }
        public K call() throws Exception {
            HystrixRequestContext existingState = HystrixRequestContext.getContextForCurrentThread();
    
            Object var2;
            try {
                HystrixRequestContext.setContextOnCurrentThread(this.parentThreadState);
                var2 = this.actual.call();
            } finally {
                HystrixRequestContext.setContextOnCurrentThread(existingState);
            }
            return var2;
        }
    }

重点关注下HystrixContextRunnable，而HystrixContextCallable与它原理类似，不难理解，在不同线程之间传递信息的载体是HystrixContextRunnable，也就是任务本身。

### HystrixRequestVariableDefault 和ThreadLocal的区别 ###

*  它使用前需要用 **HystrixRequestContext.initializeContext()** 进行初始化
 *  它结束时需使用 **HystrixRequestContext.shutdown() **进行清理
 *  它有一个生命周期方法 shutdown()用来清理资源
 *  它会以传引用的方式（线程之间使用的是相同的HystrixRequestVariables）拷贝到下一个线程，主要通过HystrixRequestContext.getContextForCurrentThread()和HystrixRequestContext.setContextOnCurrentThread()两个方法
 *  父线程调用shutdown时，子线程的HystrixRequestVariables也会被清理(因为就是一个对象，传的是引用)

而Thread的 threadLocals在创建线程时会初始化，HystrixRequestContext并不是每个线程都需要的，因此需要根据需要自行进行初始化。

总结，Hystrix 传递信息的思路是非常值得借鉴的，也许在某些场景下，我们也需要设计一个在特定范围内传递信息的模型。![88840491][]

[hystrix-core]: https://mvnrepository.com/artifact/com.netflix.hystrix/hystrix-core
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk_size_16_color_FFFFFF_t_70]: /images/20220316/d198acc358b5460895a0a697c43ef0df.png
[Hystrix]: https://blog.csdn.net/alex_xfboy/article/details/87990077
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2l0c29mdGNoZW5mZWk_size_16_color_FFFFFF_t_70 1]: /images/20220316/bc422810f0d74d50a453f65eb6a70b5d.png
[88840491]: /images/20220316/f7402814a19e435fb69c5c74376b1572.png