Question

在使用AOP之前，该项目运行良好，但是在添加LoggingAspect类之后，出现了NullPointerException。我进行了项目调试，在出现NullPointerException的时候，没有对象可以调用。 defaultLogger对象嵌入在Runner对象的构造函数中。为什么会发生？ Link to the project

@Component
@Aspect
public class LoggingAspect {

    @Pointcut("execution(* *.logEvent(..))")
    public void allLogEventMethods() {}

    @Before("allLogEventMethods()")
    public void logBefore(JoinPoint joinPoint) {
        System.out.println("BEFORE: " +
                joinPoint.getTarget().getClass().getSimpleName() + " " +
                joinPoint.getSignature().getName());
    }

    @AfterReturning(pointcut="allLogEventMethods()",
            returning="retVal")
    public void logAfter(Object retVal) {
        System.out.println("AFTER_RET: " + retVal);
    }

    @AfterThrowing(pointcut="allLogEventMethods()",
            throwing="ex")
    public void logAfterThrow(Throwable ex) {
        System.out.println("AFTER_THR: " + ex);
    }
}

@Component
public class Runner {
    private Client client;
    private EventLogger defaultLogger;
    private Map<EventType, EventLogger> loggers;

    @Autowired
    public Runner(
            Client client,
            EventLogger defaultLogger,
            Map<EventType, EventLogger> loggerMap) {
        this.client = client;
        this.defaultLogger = defaultLogger;
        this.loggers = loggerMap;
    }

    public static void main(String[] args) {
        ConfigurableApplicationContext ac = 
            new ClassPathXmlApplicationContext("context.xml");
        Runner app = ac.getBean(Runner.class);
        Event event;
        int i = 0;
        while (i < 10) {
            event = ac.getBean(Event.class);
            event.setMessage("Some event for " + i++);
            app.logEvent(null, event);
        }
        while (i < 20) {
            event = ac.getBean(Event.class);
            event.setMessage("Some event for " + i++);
            app.logEvent(EventType.INFO, event);
        }
        while (i < 30) {
            event = ac.getBean(Event.class);
            event.setMessage("Some event for " + i++);
            app.logEvent(EventType.ERROR, event);
        }

        ac.close();
    }

    private void logEvent(EventType type, Event message) {
        EventLogger eventLogger;
        if (type == null) eventLogger = defaultLogger;
        else eventLogger = loggers.get(type);
        eventLogger.logEvent(message);                //NullPointerException
    }
}

相关对象用@Component ("defaultLogger")注释。

@Component("defaultLogger")
public class CacheFileLogger extends FileLogger {
    private int cacheSize;
    private List<Event> events;

    public CacheFileLogger(
            @Value("${filename}") String filename,
            @Value("${cacheSize}") int cacheSize
    ) {
        super(filename);
        this.cacheSize = cacheSize;
        events = new ArrayList<>(cacheSize);
    }

    @Override
    public void logEvent(Event event) {
        events.add(event);
        if (cacheSize == events.size()) {
            writeAndClearCache();
        }
    }

    private void writeAndClearCache() {
        for (Event event1 : events) {
            super.logEvent(event1);
        }
        events.clear();
    }

    @PreDestroy
    private void destroy() {
        if (!events.isEmpty()) {
            writeAndClearCache();
        }
    }
}

文件context.xml：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd">
    <context:annotation-config/>
    <context:component-scan base-package="by.babanin"/>
    <aop:aspectj-autoproxy/>
</beans>

Debug image

Answer 1

当您调用logEvent方法时，您的eventLogger对象将变为null。确保通过您的context.xml将没有空引用的对象传递给Runner类的构造函数。我在这里写的内容仅用于将eventLogger对象传递给Runner类的构造函数。您还必须对构造函数的其他两个参数执行相同的操作。确保与我们分享您的结果。

const parseInput = (input: string): void => {
   if (!convert(input)) {
       return;
   }

   persist(convert(input));
}

Answer 2

由于对此问题的评论，错误已更正。我将私有方法logEvent类Runner更改为公开！我会解释为什么这行得通，但我不太明白为什么。如果知道的话，我会在对此答案的评论中留下解释。

public void logEvent(EventType type, Event message) {
        EventLogger eventLogger;
        if (type == null) eventLogger = defaultLogger;
        else eventLogger = loggers.get(type);
        eventLogger.logEvent(message);
    }

引入方面后，Spring容器不会注入依赖项

2 个答案: