TransactionSynchronizationAdapter结合ThreadPoolExecutor实现事务后处理功能

         项目中有一个功能涉及多个业务流程，且在同一个事务中，每个业务流程结束后都需要进行RPC调用，考虑到RPC调用的代价，决定做成异步调用，但是由于每个调用都必须在业务流程结束后进行，这时候就想到TransactionSynchronizationManager和TransactionSynchronizationAdapter。

         TransactionSynchronizationManager这个类中由一系列的ThreadLocal ，我们需要关注的是synchronizations，在后面使用到的TransactionSynchronizationManager.isSynchronizationActive()、TransactionSynchronizationManager.registerSynchronization()和new TransactionSynchronizationAdapter()，都与它密切有关。

        在Spring在开启数据库事务（无论是使用@Transactional注解，还是用xml配置）时，都会向其中写入一个实例，用于自动处理Connection的获取、提交或回滚等操作。

       再看isSynchronizationActive()方法，判断了synchronizations中是否有数据（Set<TransactionSynchronization>非null即可，并不要求其中有TransactionSynchronization实例。

       再看registerSynchronization()方法，首先调用isSynchronizationActive()做一个校验；然后将入参synchronization添加到synchronizations 中。入参synchronization中的方法不会在这里执行，而是要等到事务执行到特定阶段时才会被调用。

         TransactionSynchronizationAdapter是一个适配器：它实现了TransactionSynchronization接口，并为每一个接口方法提供了一个空的实现。这类适配器的基本思想是：接口中定义了很多方法，然而业务代码往往只需要实现其中一小部分。利用这种“空实现”适配器，我们可以专注于业务上需要处理的回调方法，而不用在业务类中放大量而且重复的空方法。

         结合TransactionSynchronizationManager和TransactionSynchronizationAdapter利用ThreadPoolExecutor实现一个事务后多线程处理功能。

package com.*.module.spring.support;

import com.google.common.util.concurrent.ThreadFactoryBuilder;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.transaction.support.TransactionSynchronizationAdapter;
import org.springframework.transaction.support.TransactionSynchronizationManager;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

/**
 * 事务提交异步线程
 *
 * @author ly
 */
public class TransactionAfterCommitExecutor extends ThreadPoolExecutor {

    private static final Logger LOGGER = LoggerFactory.getLogger(TransactionAfterCommitExecutor.class);


    public TransactionAfterCommitExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
    }

    public TransactionAfterCommitExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);
    }

    public TransactionAfterCommitExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);
    }

    public TransactionAfterCommitExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);
    }


    private ThreadLocal<List<Runnable>> currentRunables = new ThreadLocal<List<Runnable>>(){
        @Override
        protected List<Runnable> initialValue() {
            return new ArrayList<>(5);
        }
    };

    private ThreadLocal<Boolean> registed = new ThreadLocal<Boolean>(){
        @Override
        protected Boolean initialValue() {
            return false;
        }
    };

    /**
     * 默认策略丢弃最老的数据
     */
    public TransactionAfterCommitExecutor() {
        this(
                50, 500,
                500L, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue<Runnable>(1024),
                new ThreadFactoryBuilder().setNameFormat("transaction-after-commit-call-pool-%d").build(),
                new ThreadPoolExecutor.DiscardOldestPolicy());
    }

    @Override
    public void execute(final Runnable runnable) {
        //如果事务同步未启用则认为事务已经提交，马上进行异步处理
        if (!TransactionSynchronizationManager.isSynchronizationActive()) {
            super.execute(runnable);
        } else {
            //同一个事务的合并到一起处理
            currentRunables.get().add(runnable);
            //如果存在事务则在事务结束后异步处理
            if(!registed.get()){
                TransactionSynchronizationManager.registerSynchronization(new AfterCommitTransactionSynchronizationAdapter());
                registed.set(true);
            }
        }
    }

    @Override
    public Future<?> submit(final Runnable runnable) {
        //如果事务同步未启用则认为事务已经提交，马上进行异步处理
        if (!TransactionSynchronizationManager.isSynchronizationActive()) {
            return super.submit(runnable);
        } else {
            final RunnableFuture<Void> ftask = newTaskFor(runnable, null);
            //如果存在事务则在事务结束后异步处理
            TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {
                @Override
                public void afterCommit() {
                    TransactionAfterCommitExecutor.super.submit(ftask);
                }
            });
            return ftask;
        }
    }

    private class AfterCommitTransactionSynchronizationAdapter extends TransactionSynchronizationAdapter{
        @Override
        public void afterCompletion(int status) {
            final List<Runnable> txRunables = new ArrayList<>(currentRunables.get());
            currentRunables.remove();
            registed.remove();
            if(status == STATUS_COMMITTED){
                TransactionAfterCommitExecutor.super.execute(new Runnable() {
                    @Override
                    public void run() {
                        for (Runnable runnable : txRunables) {
                            try {
                                runnable.run();
                            } catch (Exception e) {
                                LOGGER.error("ex:",e);
                            }
                        }
                    }
                });
            }
        }
    }
}