1.启动过程流程图

流程图详细地描述了各个作业细节的执行过程，看上去流程非常复杂，其主要的功能点为：判断作业是否可执行，判断作业是否分片执行，作业执行状态监听，作业失效转移等。下面我们结合代码一步步窥探他的执行过程。

2.核心源码分析

2.1 作业入口

/**
 * Elastic Job Lite提供的Quartz封装作业.
 *
 * @author zhangliang
 */
public class LiteJob implements Job {
    
    @Setter
    private ElasticJob elasticJob;
    
    @Override
    public void execute(final JobExecutionContext context) throws JobExecutionException {
        elasticJob.execute();
    }
}

LiteJob实现了Quartz的Job接口，并且持有elasticJob的实现类，通过代理的方式实现了ElasticJob与Quartz的无缝衔接；

【亮点】这是一种典型的代理模式，其好处在于体验上完全与Quartz的Job一致，并且遵循了代码的开闭原则，使得代码具有很好地拓展性：例如ElasticJob接口有SimpleJob，DataFlowJob或者用户自定义的多种实现类，因此具有很好地拓展性。

2.2 AbstractElasticJob抽象类及其原理

/**
 * 弹性化分布式作业的基类.
 * 
 * @author zhangliang
 * @author caohao
 */
@Slf4j
public abstract class AbstractElasticJob implements ElasticJob {
    
    //具体的业务实现放在jobFacade门面类中实现，简化代码复杂度
    private JobFacade jobFacade;
    
    @Override
    public final void execute() {
        log.trace("Elastic job: job execute begin.");
        //判断与注册中心时间差是否在允许范围内
        jobFacade.checkMaxTimeDiffSecondsTolerable();
        //获取分片上下文
        JobExecutionMultipleShardingContext shardingContext = jobFacade.getShardingContext();
        //若前面的任务仍在执行，则设置错过执行标记，延迟执行
        if (jobFacade.misfireIfNecessary(shardingContext.getShardingItems())) {
            log.debug("Elastic job: previous job is still running, new job will start after previous job completed. Misfired job had recorded.");
            return;
        }
        //清除作业上次执行的信息
        jobFacade.cleanPreviousExecutionInfo();
        try {
            //各监听器执行job执行前方法
            jobFacade.beforeJobExecuted(shardingContext);
            //CHECKSTYLE:OFF
        } catch (final Throwable cause) {
            //CHECKSTYLE:ON
            handleJobExecutionException(new JobException(cause));
        }
        //执行具体的job业务逻辑
        executeJobInternal(shardingContext);
        log.trace("Elastic job: execute normal completed, sharding context:{}.", shardingContext);
        while (jobFacade.isExecuteMisfired(shardingContext.getShardingItems())) {
            log.trace("Elastic job: execute misfired job, sharding context:{}.", shardingContext);
            jobFacade.clearMisfire(shardingContext.getShardingItems());
            executeJobInternal(shardingContext);
            log.trace("Elastic job: misfired job completed, sharding context:{}.", shardingContext);
        }
        //按需失效转移
        jobFacade.failoverIfNecessary();
        try {
            //执行监听后事件
            jobFacade.afterJobExecuted(shardingContext);
            //CHECKSTYLE:OFF
        } catch (final Throwable cause) {
            //CHECKSTYLE:ON
            handleJobExecutionException(new JobException(cause));
        }
        log.trace("Elastic job: execute all completed.");
    }
    
    private void executeJobInternal(final JobExecutionMultipleShardingContext shardingContext) {
        if (shardingContext.getShardingItems().isEmpty()) {
            log.trace("Elastic job: sharding item is empty, job execution context:{}.", shardingContext);
            return;
        }
        //注册任务执行信息
        jobFacade.registerJobBegin(shardingContext);
        try {
            executeJob(shardingContext);
        //CHECKSTYLE:OFF
        } catch (final Throwable cause) {
        //CHECKSTYLE:ON
            handleJobExecutionException(new JobException(cause));
        } finally {
            // TODO 考虑增加作业失败的状态，并且考虑如何处理作业失败的整体回路
            jobFacade.registerJobCompleted(shardingContext);
        }
    }
    
    protected abstract void executeJob(final JobExecutionMultipleShardingContext shardingContext);
    
    @Override
    public void handleJobExecutionException(final JobException jobException) {
        log.error("Elastic job: exception occur in job processing...", jobException.getCause());
    }
    
    @Override
    public final JobFacade getJobFacade() {
        return jobFacade;
    }
    
    @Override
    public final void setJobFacade(final JobFacade jobFacade) {
        this.jobFacade = jobFacade;
    }

【亮点】外观模式传送门
上面的代码中应用到了外观模式（Facade），AbstractElasticJob持有jobFacade对象，Elasticjob负责统筹整体的job执行流程但无需关注业务的具体实现，转而将复杂的业务处理逻辑交由jobFacade中的方法进行处理，从而将job与具体的业务逻辑抽离出来方便阅读和拓展。