上一篇文章說到，之前使用了@Async註解，子執行緒無法獲取到上下文資訊，導致流量無法打到灰度，然後改成 執行緒池的方式，每次呼叫非同步呼叫的時候都手動透傳 上下文（硬編碼）解決了問題。
後面查閱了資料，找到了方案不用每次硬編碼，來上下文透傳資料了。
方案一：
繼承執行緒池，重寫相應的方法，透傳上下文。
方案二：（推薦）
執行緒池ThreadPoolTaskExecutor，有一個TaskDecorator裝飾器，實現這個介面，透傳上下文。
方案一：繼承執行緒池，重寫相應的方法，透傳上下文。
1、ThreadPoolTaskExecutor spring封裝的執行緒池
ThreadPoolTaskExecutor 執行緒池程式碼如下：
@Bean(ExecutorConstant.simpleExecutor_3)
public Executor asyncExecutor3() {
MyThreadPoolTaskExecutor taskExecutor = new MyThreadPoolTaskExecutor();
taskExecutor.setCorePoolSize(corePoolSize);
taskExecutor.setMaxPoolSize(maxPoolSize);
taskExecutor.setQueueCapacity(queueCapacity);
taskExecutor.setThreadNamePrefix(threadNamePrefix_3);
taskExecutor.initialize();
return taskExecutor;
}
//------- 繼承父類 重寫對應的方法 start
class MyCallable<T> implements Callable<T> {
private Callable<T> task;
private RequestAttributes context;
public MyCallable(Callable<T> task, RequestAttributes context) {
this.task = task;
this.context = context;
}
@Override
public T call() throws Exception {
if (context != null) {
RequestContextHolder.setRequestAttributes(context);
}
try {
return task.call();
} finally {
RequestContextHolder.resetRequestAttributes();
}
}
}
class MyThreadPoolTaskExecutor extends ThreadPoolTaskExecutor{
@Override
public <T> Future<T> submit(Callable<T> task) {
return super.submit(new MyCallable(task, RequestContextHolder.currentRequestAttributes()));
}
@Override
public <T> ListenableFuture<T> submitListenable(Callable<T> task) {
return super.submitListenable(new MyCallable(task, RequestContextHolder.currentRequestAttributes()));
}
}
//------- 繼承父類 重寫對應的方法 end
1、MyCallable是繼承Callable，建立MyCallable物件的時候已經把Attributes物件賦值給屬性context了（建立MyCallable物件的時候因為實在當前主執行緒建立的，所以是能獲取到請求的Attributes），在執行call方法前，先執行了RequestContextHolder.setRequestAttributes(context); 【把這個MyCallable物件的屬性context 設定到setRequestAttributes中】 所以在執行具體業務時，當前執行緒（子執行緒）就能取得主執行緒的Attributes
2、MyThreadPoolTaskExecutor類是繼承了ThreadPoolTaskExecutor 重寫了submit和submitListenable方法
為什麼是重寫submit和submitListenable這兩個方法了？
@Async AOP原始碼的方法位置是在：AsyncExecutionInterceptor.invoke
doSubmit方法能看出來
無返回值呼叫的是執行緒池方法：submit()
有返回值，根據不同的返回型別也知道：
- 返回值型別是：Future.class 呼叫的是方法：submit()
- 返回值型別是：ListenableFuture.class 呼叫的方法是：submitListenable(task)
- 返回值型別是：CompletableFuture.class呼叫的是CompletableFuture.supplyAsync這個在非同步註解中暫時用不上的，就不考慮重寫了。
public Object invoke(final MethodInvocation invocation) throws Throwable {
Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);
Method specificMethod = ClassUtils.getMostSpecificMethod(invocation.getMethod(), targetClass);
final Method userDeclaredMethod = BridgeMethodResolver.findBridgedMethod(specificMethod);
AsyncTaskExecutor executor = determineAsyncExecutor(userDeclaredMethod);
if (executor == null) {
throw new IllegalStateException(
"No executor specified and no default executor set on AsyncExecutionInterceptor either");
}
Callable<Object> task = () -> {
try {
Object result = invocation.proceed();
if (result instanceof Future) {
return ((Future<?>) result).get();
}
}
catch (ExecutionException ex) {
handleError(ex.getCause(), userDeclaredMethod, invocation.getArguments());
}
catch (Throwable ex) {
handleError(ex, userDeclaredMethod, invocation.getArguments());
}
return null;
};
return doSubmit(task, executor, invocation.getMethod().getReturnType());
}
@Nullable
protected Object doSubmit(Callable<Object> task, AsyncTaskExecutor executor, Class<?> returnType) {
if (CompletableFuture.class.isAssignableFrom(returnType)) {
return CompletableFuture.supplyAsync(() -> {
try {
return task.call();
}
catch (Throwable ex) {
throw new CompletionException(ex);
}
}, executor);
}
else if (ListenableFuture.class.isAssignableFrom(returnType)) {
return ((AsyncListenableTaskExecutor) executor).submitListenable(task);
}
else if (Future.class.isAssignableFrom(returnType)) {
return executor.submit(task);
}
else {
executor.submit(task);
return null;
}
}
2、ThreadPoolExecutor 原生執行緒池
ThreadPoolExecutor執行緒池程式碼如下：
//------- ThreadPoolExecutor 繼承父類 重寫對應的方法 start
class MyRunnable implements Runnable {
private Runnable runnable;
private RequestAttributes context;
public MyRunnable(Runnable runnable, RequestAttributes context) {
this.runnable = runnable;
this.context = context;
}
@Override
public void run() {
if (context != null) {
RequestContextHolder.setRequestAttributes(context);
}
try {
runnable.run();
} finally {
RequestContextHolder.resetRequestAttributes();
}
}
}
class MyThreadPoolExecutor extends ThreadPoolExecutor{
@Override
public void execute(Runnable command) {
if(!(command instanceof MyRunnable)){
command = new MyRunnable(command,RequestContextHolder.currentRequestAttributes())
}
super.execute(command);
}
public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
}
public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
}
public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);
}
public MyThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) {
super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
}
}
//------- ThreadPoolExecutor 繼承父類 重寫對應的方法 end
像ThreadPoolExecutor主要重寫execute方法，在啟動新執行緒的時候先把Attributes取到放到MyRunnable物件的一個屬性中，MyRunnable在具體執行run方法的時候，把屬性Attributes賦值到子執行緒中，當run方法執行完了在把Attributes清空掉。
為什麼只要重寫了execute方法就可以了？
ThreadPoolExecutor大家都知道主要是由submit和execute方法來執行的。
看ThreadPoolExecutor類的submit具體執行方法是由父類AbstractExecutorService#submit來實現。
具體程式碼在下面貼出來了，可以看到submit實際上最後呼叫的還是execute方法，所以我們重寫execute方法就好了。
submit方法路徑及原始碼：
java.util.concurrent.AbstractExecutorService#submit(java.lang.Runnable)
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
/**
* @throws RejectedExecutionException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
方案二：（推薦）
ThreadPoolTaskExecutor執行緒池
實現TaskDecorator介面,把實現類設定到taskExecutor.setTaskDecorator(new MyTaskDecorator());
//------- 實現TaskDecorator 介面 start
@Bean(ExecutorConstant.simpleExecutor_4)
public Executor asyncExecutor4() {
MyThreadPoolTaskExecutor taskExecutor = new MyThreadPoolTaskExecutor();
taskExecutor.setCorePoolSize(corePoolSize);
taskExecutor.setMaxPoolSize(maxPoolSize);
taskExecutor.setQueueCapacity(queueCapacity);
taskExecutor.setThreadNamePrefix(threadNamePrefix_4);
taskExecutor.setTaskDecorator(new MyTaskDecorator());
taskExecutor.initialize();
return taskExecutor;
}
class MyTaskDecorator implements TaskDecorator{
@Override
public Runnable decorate(Runnable runnable) {
try {
RequestAttributes attributes = RequestContextHolder.getRequestAttributes();
return () -> {
try {
RequestContextHolder.setRequestAttributes(attributes);
runnable.run();
} finally {
RequestContextHolder.resetRequestAttributes();
}
};
} catch (IllegalStateException e) {
return runnable;
}
}
}
//------- 實現TaskDecorator 介面 end
為什麼設定了setTaskDecorator就能實現透傳資料了？
主要還是看taskExecutor.initialize()方法，主要是重寫了ThreadPoolExecutor的execute方法，用裝飾器模式 增強了Runnable介面，原始碼如下：
@Nullable
private ThreadPoolExecutor threadPoolExecutor;
//初始化方法
public void initialize() {
if (logger.isDebugEnabled()) {
logger.debug("Initializing ExecutorService" + (this.beanName != null ? " '" + this.beanName + "'" : ""));
}
if (!this.threadNamePrefixSet && this.beanName != null) {
setThreadNamePrefix(this.beanName + "-");
}
this.executor = initializeExecutor(this.threadFactory, this.rejectedExecutionHandler);
}
@Override
protected ExecutorService initializeExecutor(
ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {
BlockingQueue<Runnable> queue = createQueue(this.queueCapacity);
ThreadPoolExecutor executor;
//判斷是否設定了，taskDecorator裝飾器
if (this.taskDecorator != null) {
executor = new ThreadPoolExecutor(
this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
queue, threadFactory, rejectedExecutionHandler) {
@Override
public void execute(Runnable command) {
//執行裝飾器方法包裝Runnable介面
Runnable decorated = taskDecorator.decorate(command);
if (decorated != command) {
decoratedTaskMap.put(decorated, command);
}
super.execute(decorated);
}
};
}
else {
executor = new ThreadPoolExecutor(
this.corePoolSize, this.maxPoolSize, this.keepAliveSeconds, TimeUnit.SECONDS,
queue, threadFactory, rejectedExecutionHandler);
}
if (this.allowCoreThreadTimeOut) {
executor.allowCoreThreadTimeOut(true);
}
//把初始化好的ThreadPoolExecutor執行緒池賦值給 當前類屬性threadPoolExecutor
this.threadPoolExecutor = executor;
return executor;
}
總結
無論是方案1還是方案2，原理都是先在當前執行緒獲取到Attributes，然後把Attributes賦值到Runnable的一個屬性中，在起一個子執行緒後，具體執行run方法的時候，把Attributes設定給當子執行緒，當run方法執行完了，在清空Attributes。
方案2實現比較優雅，所以推薦使用它。
工作沒多久的時候覺得spring的使用很麻煩，但是工作久了慢慢發現spring一些小細節、設計模式運用的非常巧妙，很容易解決遇到的問題，只能說spring ??。
我已經把上述程式碼例子放到gitee了，大家感興趣可以clone 傳送門～