HystrixRequestContext

　　請求的上線文實現，內部定義了一個靜態變量ThreadLocal,每個線程可以獲取自己的HystrixRequestContext對象。一個請求往往由一個tomcat線程處理，所以在該tomcat線程中，HystrixRequestContext對象可以共享。

private static ThreadLocal<HystrixRequestContext> requestVariables = new ThreadLocal<HystrixRequestContext>();

　　HystrixRequestContext內部是一個ConcurrentHashMap存儲請求變量。

ConcurrentHashMap<HystrixRequestVariableDefault<?>, HystrixRequestVariableDefault.LazyInitializer<?>> state = new ConcurrentHashMap<HystrixRequestVariableDefault<?>, HystrixRequestVariableDefault.LazyInitializer<?>>();

HystrixRequestVariableLifecycle->HystrixRequestVariable->HystrixRequestVariableDefault->HystrixLifecycleForwardingRequestVariable

　　HystrixRequestVariableLifecycle和HystrixRequestVariable定義了一個請求變量，這個請求變量對象的生命周期為在一個請求內。

　　HystrixRequestVariableDefault為默認實現類。內部他把變量值存儲在HystrixRequestContext對象中。key為當前HystrixRequestVariableDefault對象，value為變量真正的值。

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.");
        }
        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);
        if (v != null) {
            return (T) v.get();
        }
....

　　HystrixLifecycleForwardingRequestVariable只是一個封裝對象，內部封裝了一個HystrixRequestVariableLifecycle對象。

private final HystrixRequestVariableLifecycle<T> lifecycle;

HystrixRequestVariableHolder

　　定義了一個靜態變量，存儲所有的HystrixRequestVariable對象，全局共享。

private static ConcurrentHashMap<RVCacheKey, HystrixRequestVariable<?>> requestVariableInstance = new ConcurrentHashMap<RVCacheKey, HystrixRequestVariable<?>>();

　　RVCacheKey由兩部分組成：當前HystrixRequestVariableHolder對象，指定HystrixConcurrencyStrategy對象。

public T get(HystrixConcurrencyStrategy concurrencyStrategy) { 
　　RVCacheKey key = new RVCacheKey(this, concurrencyStrategy);
　　...

　　如果沒有已經存在的HystrixRequestVariable對象，通過HystrixConcurrencyStrategy新建一個。

...
if (rvInstance == null) {
            requestVariableInstance.putIfAbsent(key, concurrencyStrategy.getRequestVariable(lifeCycleMethods));
｝
...

hystrix源碼之hystrix請求變量