原始碼分析

本文程式碼基於 Springboot 2.1.0

package org.springframework.boot.autoconfigure.logging;

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

import org.springframework.boot.autoconfigure.condition.ConditionEvaluationReport;
import org.springframework.
 
boot.context.event.ApplicationFailedEvent;
import org.springframework.boot.logging.LogLevel;
import org.springframework.context.ApplicationContextInitializer;
import org.springframework.context.ApplicationEvent;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.
 
context.event.ApplicationContextEvent;
import org.springframework.context.event.ContextRefreshedEvent;
import org.springframework.context.event.GenericApplicationListener;
import org.springframework.context.support.GenericApplicationContext;
import org.springframework.core.Ordered;
import org.springframework.
 
core.ResolvableType;
import org.springframework.util.Assert;

/**
 * ApplicationContextInitializer that writes the ConditionEvaluationReport
 * to the log. Reports are logged at the DEBUG level. A crash
 * report triggers an info output suggesting the user runs again with debug enabled to
 * display the report.
 * 雖然名字是一個Listener，但這實際上是一個ApplicationContextInitializer，其目的是將
 * ConditionEvaluationReport寫入到日誌，使用DEBUG級別輸出。程式崩潰報告會觸發一個訊息輸出，
 * 建議使用者使用除錯模式顯示報告。
 * 
 * 該ApplicationContextInitializer的具體做法是在應用初始化時繫結一個ConditionEvaluationReportListener
 * 事件監聽器，然後相應的事件發生時輸出ConditionEvaluationReport報告。
 *  
 * This initializer is not intended to be shared across multiple application context
 * instances.
 *
 * @author Greg Turnquist
 * @author Dave Syer
 * @author Phillip Webb
 * @author Andy Wilkinson
 * @author Madhura Bhave
 */
public class ConditionEvaluationReportLoggingListener
		implements ApplicationContextInitializer<ConfigurableApplicationContext> {

	private final Log logger = LogFactory.getLog(getClass());

	private ConfigurableApplicationContext applicationContext;

	private ConditionEvaluationReport report;

	private final LogLevel logLevelForReport;

	public ConditionEvaluationReportLoggingListener() {
		this(LogLevel.DEBUG);
	}

	public ConditionEvaluationReportLoggingListener(LogLevel logLevelForReport) {
		Assert.isTrue(isInfoOrDebug(logLevelForReport), "LogLevel must be INFO or DEBUG");
		this.logLevelForReport = logLevelForReport;
	}

	private boolean isInfoOrDebug(LogLevel logLevelForReport) {
		return LogLevel.INFO.equals(logLevelForReport)
				|| LogLevel.DEBUG.equals(logLevelForReport);
	}

	public LogLevel getLogLevelForReport() {
		return this.logLevelForReport;
	}

	@Override
	public void initialize(ConfigurableApplicationContext applicationContext) {
		this.applicationContext = applicationContext;
		// 向容器繫結一個ConditionEvaluationReportListener事件監聽器
		applicationContext
				.addApplicationListener(new ConditionEvaluationReportListener());
		if (applicationContext instanceof GenericApplicationContext) {
			// Get the report early in case the context fails to load
			this.report = ConditionEvaluationReport
					.get(this.applicationContext.getBeanFactory());
		}
	}

	protected void onApplicationEvent(ApplicationEvent event) {
		ConfigurableApplicationContext initializerApplicationContext = this.applicationContext;
		if (event instanceof ContextRefreshedEvent) {
			// 遇到ContextRefreshedEvent事件，根據繫結應用上下文是否活躍作相應報告
			if (((ApplicationContextEvent) event)
					.getApplicationContext() == initializerApplicationContext) {			
					// 事件應用上下文是該初始化器所繫結的應用上下文的話才報告		
				logAutoConfigurationReport();
			}
		}
		else if (event instanceof ApplicationFailedEvent
				&& ((ApplicationFailedEvent) event)
						.getApplicationContext() == initializerApplicationContext) {
			// 遇到ApplicationFailedEvent事件，說起應用程式啟動失敗，做應用崩潰報告		
			// 事件應用上下文是該初始化器所繫結的應用上下文的話才報告						
			logAutoConfigurationReport(true);
		}
	}

	private void logAutoConfigurationReport() {
		// 如果所繫結應用上下文不活躍則認為是崩潰，否則認為是正常
		logAutoConfigurationReport(!this.applicationContext.isActive());
	}

	public void logAutoConfigurationReport(boolean isCrashReport) {
		if (this.report == null) {
			if (this.applicationContext == null) {
				this.logger.info("Unable to provide the conditions report "
						+ "due to missing ApplicationContext");
				return;
			}
			this.report = ConditionEvaluationReport
					.get(this.applicationContext.getBeanFactory());
		}
		if (!this.report.getConditionAndOutcomesBySource().isEmpty()) {
			if (this.getLogLevelForReport().equals(LogLevel.INFO)) {
				if (this.logger.isInfoEnabled()) {
					this.logger.info(new ConditionEvaluationReportMessage(this.report));
				}
				else if (isCrashReport) {
					logMessage("info");
				}
			}
			else {
				if (this.logger.isDebugEnabled()) {
					this.logger.debug(new ConditionEvaluationReportMessage(this.report));
				}
				else if (isCrashReport) {
					logMessage("debug");
				}
			}
		}
	}

	private void logMessage(String logLevel) {
		this.logger.info(
				String.format("%n%nError starting ApplicationContext. To display the "
						+ "conditions report re-run your application with '" + logLevel
						+ "' enabled."));
	}

	private class ConditionEvaluationReportListener
			implements GenericApplicationListener {

		@Override
		public int getOrder() {
			return Ordered.LOWEST_PRECEDENCE;
		}

		@Override
		public boolean supportsEventType(ResolvableType resolvableType) {
			Class<?> type = resolvableType.getRawClass();
			if (type == null) {
				return false;
			}
			return ContextRefreshedEvent.class.isAssignableFrom(type)
					|| ApplicationFailedEvent.class.isAssignableFrom(type);
		}

		@Override
		public boolean supportsSourceType(Class<?> sourceType) {
			return true;
		}

		@Override
		public void onApplicationEvent(ApplicationEvent event) {
			ConditionEvaluationReportLoggingListener.this.onApplicationEvent(event);
		}

	}

}

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