SpringApplicationRunListeners与GenericApplicationListener源码解析生命周期和区别

一，SpringApplicationRunListeners

1》SpringApplicationRunListeners负责在SpringBoot启动的不同阶段，广播出不同的消息， 传递给ApplicationListener监听器实现类。它的实例化和调用都在SpringApplication.run方法中。

2》我们看源码：在304行这里有一个SpringApplicationRunListeners的创建

 3》我们看看getRunListeners方法是怎么实现的，如下图

 PS:他去加载了两个class

1，com.nl.springapprunlistener.SpringApplicationRunListenerDemo（这个我是在META-INF/spring.factories自定定义的SpringApplicationRunListener）

org.springframework.boot.SpringApplicationRunListener=com.nl.springapprunlistener.SpringApplicationRunListenerDemo

2，org.springframework.boot.context.event.EventPublishingRunListener（而这个是系统默认加载的，这个跟GenericApplicationListener有关系，待会下面解释）

# Run Listeners
org.springframework.boot.SpringApplicationRunListener=
org.springframework.boot.context.event.EventPublishingRunListener

在spring-boot-2.3.6.RELEASE.jar 的 META-INF/spring.factories中能找到

4》这个时候我们拿到所有的监听器 ，那该怎么执行么，我们看回run方法

 我们可以看到整一个listeners的执行周期，

5》我们看下SpringApplicationRunListeners的listeners的实现方法

/*
 * Copyright 2012-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;

import org.apache.commons.logging.Log;

import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.util.ReflectionUtils;

/**
 * A collection of {@link SpringApplicationRunListener}.
 *
 * @author Phillip Webb
 */
class SpringApplicationRunListeners {

    private final Log log;

    private final List<SpringApplicationRunListener> listeners;

    SpringApplicationRunListeners(Log log, Collection<? extends SpringApplicationRunListener> listeners) {
        this.log = log;
        this.listeners = new ArrayList<>(listeners);
    }

    void starting() {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.starting();
        }
    }

    void environmentPrepared(ConfigurableEnvironment environment) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.environmentPrepared(environment);
        }
    }

    void contextPrepared(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.contextPrepared(context);
        }
    }

    void contextLoaded(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.contextLoaded(context);
        }
    }

    void started(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.started(context);
        }
    }

    void running(ConfigurableApplicationContext context) {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.running(context);
        }
    }

    void failed(ConfigurableApplicationContext context, Throwable exception) {
        for (SpringApplicationRunListener listener : this.listeners) {
            callFailedListener(listener, context, exception);
        }
    }

    private void callFailedListener(SpringApplicationRunListener listener, ConfigurableApplicationContext context,
            Throwable exception) {
        try {
            listener.failed(context, exception);
        }
        catch (Throwable ex) {
            if (exception == null) {
                ReflectionUtils.rethrowRuntimeException(ex);
            }
            if (this.log.isDebugEnabled()) {
                this.log.error("Error handling failed", ex);
            }
            else {
                String message = ex.getMessage();
                message = (message != null) ? message : "no error message";
                this.log.warn("Error handling failed (" + message + ")");
            }
        }
    }

}

ps:看源码的，这里就是将自己定义的SpringApplicationRunListenerDemo等等和系统定义的EventPublishingRunListener循环遍历实现他的方法，这样就知道他的周期和自定义的逻辑了，实现广播的模式

二，GenericApplicationListener这个又是怎么实现的呢？

1》事件触发机制，基于观察者模式

2》我们源码剖析下，如上SpringApplicationRunListeners 中执行了每个listeners的方法，listeners的类型是SpringApplicationRunListener是一个接口，我们看看源码

/*
 * Copyright 2012-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot;

import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.io.support.SpringFactoriesLoader;

/**
 * Listener for the {@link SpringApplication} {@code run} method.
 * {@link SpringApplicationRunListener}s are loaded via the {@link SpringFactoriesLoader}
 * and should declare a public constructor that accepts a {@link SpringApplication}
 * instance and a {@code String[]} of arguments. A new
 * {@link SpringApplicationRunListener} instance will be created for each run.
 *
 * @author Phillip Webb
 * @author Dave Syer
 * @author Andy Wilkinson
 * @since 1.0.0
 */
public interface SpringApplicationRunListener {

    /**
     * Called immediately when the run method has first started. Can be used for very
     * early initialization.
     */
    default void starting() {
    }

    /**
     * Called once the environment has been prepared, but before the
     * {@link ApplicationContext} has been created.
     * @param environment the environment
     */
    default void environmentPrepared(ConfigurableEnvironment environment) {
    }

    /**
     * Called once the {@link ApplicationContext} has been created and prepared, but
     * before sources have been loaded.
     * @param context the application context
     */
    default void contextPrepared(ConfigurableApplicationContext context) {
    }

    /**
     * Called once the application context has been loaded but before it has been
     * refreshed.
     * @param context the application context
     */
    default void contextLoaded(ConfigurableApplicationContext context) {
    }

    /**
     * The context has been refreshed and the application has started but
     * {@link CommandLineRunner CommandLineRunners} and {@link ApplicationRunner
     * ApplicationRunners} have not been called.
     * @param context the application context.
     * @since 2.0.0
     */
    default void started(ConfigurableApplicationContext context) {
    }

    /**
     * Called immediately before the run method finishes, when the application context has
     * been refreshed and all {@link CommandLineRunner CommandLineRunners} and
     * {@link ApplicationRunner ApplicationRunners} have been called.
     * @param context the application context.
     * @since 2.0.0
     */
    default void running(ConfigurableApplicationContext context) {
    }

    /**
     * Called when a failure occurs when running the application.
     * @param context the application context or {@code null} if a failure occurred before
     * the context was created
     * @param exception the failure
     * @since 2.0.0
     */
    default void failed(ConfigurableApplicationContext context, Throwable exception) {
    }

}

 2》我们看看SpringApplicationRunListener接口的实现，发现EventPublishingRunListener也是实现这个接口，我们看看EventPublishingRunListener的源码

/*
 * Copyright 2012-2020 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot.context.event;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.SpringApplicationRunListener;
import org.springframework.boot.availability.AvailabilityChangeEvent;
import org.springframework.boot.availability.LivenessState;
import org.springframework.boot.availability.ReadinessState;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationListener;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.event.ApplicationEventMulticaster;
import org.springframework.context.event.SimpleApplicationEventMulticaster;
import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.core.Ordered;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.util.ErrorHandler;

/**
 * {@link SpringApplicationRunListener} to publish {@link SpringApplicationEvent}s.
 * <p>
 * Uses an internal {@link ApplicationEventMulticaster} for the events that are fired
 * before the context is actually refreshed.
 *
 * @author Phillip Webb
 * @author Stephane Nicoll
 * @author Andy Wilkinson
 * @author Artsiom Yudovin
 * @author Brian Clozel
 * @since 1.0.0
 */
public class EventPublishingRunListener implements SpringApplicationRunListener, Ordered {

    private final SpringApplication application;

    private final String[] args;

    private final SimpleApplicationEventMulticaster initialMulticaster;

    public EventPublishingRunListener(SpringApplication application, String[] args) {
        this.application = application;
        this.args = args;
        this.initialMulticaster = new SimpleApplicationEventMulticaster();
        for (ApplicationListener<?> listener : application.getListeners()) {
            this.initialMulticaster.addApplicationListener(listener);
        }
    }

    @Override
    public int getOrder() {
        return 0;
    }

    @Override
    public void starting() {
        this.initialMulticaster.multicastEvent(new ApplicationStartingEvent(this.application, this.args));
    }

    @Override
    public void environmentPrepared(ConfigurableEnvironment environment) {
        this.initialMulticaster
                .multicastEvent(new ApplicationEnvironmentPreparedEvent(this.application, this.args, environment));
    }

    @Override
    public void contextPrepared(ConfigurableApplicationContext context) {
        this.initialMulticaster
                .multicastEvent(new ApplicationContextInitializedEvent(this.application, this.args, context));
    }

    @Override
    public void contextLoaded(ConfigurableApplicationContext context) {
        for (ApplicationListener<?> listener : this.application.getListeners()) {
            if (listener instanceof ApplicationContextAware) {
                ((ApplicationContextAware) listener).setApplicationContext(context);
            }
            context.addApplicationListener(listener);
        }
        this.initialMulticaster.multicastEvent(new ApplicationPreparedEvent(this.application, this.args, context));
    }

    @Override
    public void started(ConfigurableApplicationContext context) {
        context.publishEvent(new ApplicationStartedEvent(this.application, this.args, context));
        AvailabilityChangeEvent.publish(context, LivenessState.CORRECT);
    }

    @Override
    public void running(ConfigurableApplicationContext context) {
        context.publishEvent(new ApplicationReadyEvent(this.application, this.args, context));
        AvailabilityChangeEvent.publish(context, ReadinessState.ACCEPTING_TRAFFIC);
    }

    @Override
    public void failed(ConfigurableApplicationContext context, Throwable exception) {
        ApplicationFailedEvent event = new ApplicationFailedEvent(this.application, this.args, context, exception);
        if (context != null && context.isActive()) {
            // Listeners have been registered to the application context so we should
            // use it at this point if we can
            context.publishEvent(event);
        }
        else {
            // An inactive context may not have a multicaster so we use our multicaster to
            // call all of the context's listeners instead
            if (context instanceof AbstractApplicationContext) {
                for (ApplicationListener<?> listener : ((AbstractApplicationContext) context)
                        .getApplicationListeners()) {
                    this.initialMulticaster.addApplicationListener(listener);
                }
            }
            this.initialMulticaster.setErrorHandler(new LoggingErrorHandler());
            this.initialMulticaster.multicastEvent(event);
        }
    }

    private static class LoggingErrorHandler implements ErrorHandler {

        private static final Log logger = LogFactory.getLog(EventPublishingRunListener.class);

        @Override
        public void handleError(Throwable throwable) {
            logger.warn("Error calling ApplicationEventListener", throwable);
        }

    }

}

3》看到EventPublishingRunListener这个源码我们明白了，是在EventPublishingRunListener执行所有的ApplicationListener，而每一个GenericApplicationListener就是ApplicationListener，但是我们又有疑惑，EventPublishingRunListener的构造获取所有的ApplicationListener是什么时候初始化进去的呢？

public EventPublishingRunListener(SpringApplication application, String[] args) {
        this.application = application;
        this.args = args;
        this.initialMulticaster = new SimpleApplicationEventMulticaster();
        for (ApplicationListener<?> listener : application.getListeners()) {
            this.initialMulticaster.addApplicationListener(listener);
        }
    }

如下图，原来在run的方法的时候就已经初始化了所有的ApplicationListener（事件）

5》最终我们屡屡整个流程和关系

1，SpringApplicationRunListener

1）SpringApplicationRunListener执行的逻辑是在run获取所有的SpringApplicationRunListener类型，

2）然后遍历执行时生命周期的方法（包含了starting，environmentPrepared，contextPrepared，contextLoaded，started，running和failed）

2，GenericApplicationListener

1）GenericApplicationListener是继承ApplicationListener

2）ApplicationListener所有的执行是在EventPublishingRunListener（EventPublishingRunListener是SpringApplicationRunListener，这个是springboot框架自己初始化）

3）EventPublishingRunListener在自己的包含了starting，environmentPrepared，contextPrepared，contextLoaded，started，running和failed分别执行了ApplicationListener，

4）所以这是典型的观察者模式（GenericApplicationListener是使用SpringApplicationRunListener来实现事件的执行）