被SpringBoot难倒的第N天，为什么我的CommandLi

2020-03-07 本文已影响0人代码小当家

先看现象

假设一个工程中需要用到 3 个 CommandLineRunner，其中 2 个 runner 需要执行一个 while(true) 循环

代码如下：

@Component
public class Runner1 implements CommandLineRunner {
    public void run(String... args) throws Exception {
        log.info("runner 1 running...");
    }
}

@Component
public class Runner2 implements CommandLineRunner {
    public void run(String... args) throws Exception {
        log.info("runner 2 running...")
        while(true) {
            doSomething();
        }
    }
}

@Component
public class Runner3 implements CommandLineRunner {
    public void run(String... args) throws Exception {
        log.info("runner 3 running...")
        while(true) {
            doOtherThing();
        }
    }
}

程序运行之后你会发现 Runner2 和 Runnner3 总会有一个不运行，甚至 Runner1 也不运行，但是程序也不报错，怎么回事？

透过本质看现象

是的，先看 CommandLineRunner 是怎么执行的才能知道错在了哪里从而解决这个问题。

CommandLineRunner.java

/**
 * Interface used to indicate that a bean should <em>run</em> when it is contained within
 * a {@link SpringApplication}. Multiple {@link CommandLineRunner} beans can be defined
 * within the same application context and can be ordered using the {@link Ordered}
 * interface or {@link Order @Order} annotation.
 * <p>
 * If you need access to {@link ApplicationArguments} instead of the raw String array
 * consider using {@link ApplicationRunner}.
 *
 * @author Dave Syer
 * @since 1.0.0
 * @see ApplicationRunner
 */
@FunctionalInterface
public interface CommandLineRunner {

    /**
     * Callback used to run the bean.
     * @param args incoming main method arguments
     * @throws Exception on error
     */
    void run(String... args) throws Exception;

}

注释第一句话表示：实现了这个接口的一个 spring bean 在程序启动之后应该 run.

这里的 run 用了 <em> 标签进行了强调，为什么？不着急回答，再往下看看看 CommandLineRunner 是如何被Spring Boot 执行的...

SpringApplication.java

接下来看看 SpringApplication 这个类的注释信息:

* Class that can be used to bootstrap and launch a Spring application from a Java main
 * method. By default class will perform the following steps to bootstrap your
 * application:
 *
 * <ul>
 * <li>Create an appropriate {@link ApplicationContext} instance (depending on your
 * classpath)</li>
 * <li>Register a {@link CommandLinePropertySource} to expose command line arguments as
 * Spring properties</li>
 * <li>Refresh the application context, loading all singleton beans</li>
 * <li>Trigger any {@link CommandLineRunner} beans</li>
 * </ul>

从这个注释可以知道，SpringApplication 负责触发所有 CommandLineRunner 的运行。到代码部分，就是下面的几个方法:

callRunners()

private void callRunners(ApplicationContext context, ApplicationArguments args) {
        List<Object> runners = new ArrayList<>();
        runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
        runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
        AnnotationAwareOrderComparator.sort(runners);
        for (Object runner : new LinkedHashSet<>(runners)) {
            if (runner instanceof ApplicationRunner) {
                callRunner((ApplicationRunner) runner, args);
            }
            if (runner instanceof CommandLineRunner) {
                callRunner((CommandLineRunner) runner, args);
            }
        }
    }

callRunner()

private void callRunner(ApplicationRunner runner, ApplicationArguments args) {
        try {
            (runner).run(args);
        }
        catch (Exception ex) {
            throw new IllegalStateException("Failed to execute ApplicationRunner", ex);
        }
    }

    private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
        try {
            (runner).run(args.getSourceArgs());
        }
        catch (Exception ex) {
            throw new IllegalStateException("Failed to execute CommandLineRunner", ex);
        }
    }

callRunners 负责从上下文中获取到所有 CommandLineRunner 的实现类，循环遍历，对于每一个实现类对象，调用 callRunner 方法进行触发。而在 callRunner 方法中同步执行的是 runner 对象的 run 方法.

到此，答案已经很清晰了，CommandLineRunner 不 run ，或者有的 run 有的不 run，程序也不报错的原因就是:

你有多个 CommandLineRunner 的实现类
在某个实现类的 run 方法体中调用了同步阻塞的API或者是一个 while(true) 循环

这样就导致了 SpringApplication 在执行这个 run 方法时阻塞，无法继续执行循环中的下一个 runner 的 run 方法。

为了避免程序中犯这个错，一定要记住，

CommandLineRunner != Runnable

虽然这两个接口都有一个 run 方法，很容易让人理解成一个 CommandLineRunner 就会对应一个线程。NO!

知错就改

对于 Runner1 不需要做任何修改。对于 Runner2 和 Runner3，需要做以下修改，让 while(true) 循环不阻塞 run 方法的运行即可。

@Component
public class Runner2 implements CommandLineRunner {
    public void run(String... args) throws Exception {
        log.info("runner 2 running...")
        new Thread(() -> {
            while(true) {
                doSomething();
            }
        }).start();
    }
}

@Component
public class Runner3 implements CommandLineRunner {
    public void run(String... args) throws Exception {
        log.info("runner 3 running...")
        new Thread(() -> {
            while(true) {
                doOtherThing();
            }
        }).start();
    }
}

作者：jwenjian
链接：https://juejin.im/post/5e6205b651882549215ce3a9
来源：掘金

被SpringBoot难倒的第N天，为什么我的CommandLi

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先看现象

透过本质看现象

CommandLineRunner.java

SpringApplication.java

知错就改

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