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源码分析
本文源代码基于 Springboot 2.1.0
package org.springframework.boot.autoconfigure;
import java.nio.charset.StandardCharsets;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicBoolean;
import javax.validation.Configuration;
import javax.validation.Validation;
import org.apache.catalina.mbeans.MBeanFactory;
import org.springframework.boot.context.event.ApplicationFailedEvent;
import org.springframework.boot.context.event.ApplicationReadyEvent;
import org.springframework.boot.context.event.ApplicationStartingEvent;
import org.springframework.boot.context.event.SpringApplicationEvent;
import org.springframework.boot.context.logging.LoggingApplicationListener;
import org.springframework.context.ApplicationListener;
import org.springframework.core.annotation.Order;
import org.springframework.format.support.DefaultFormattingConversionService;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
import org.springframework.http.converter.support.AllEncompassingFormHttpMessageConverter;
/**
* ApplicationListener to trigger early initialization in a background thread of
* time consuming tasks.
*
* 对于一些耗时的任务使用一个后台线程尽早触发它们开始执行初始化,这是Springboot的缺省行为。
* 这些初始化动作也可以叫做预初始化。
*
* 设置系统属性spring.backgroundpreinitializer.ignore为true可以禁用该机制。
* 该机制被禁用时,相应的初始化任务会发生在前台线程。
*
* Set the #IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME system property to
* true to disable this mechanism and let such initialization happen in the
* foreground.
*
* @author Phillip Webb
* @author Andy Wilkinson
* @author Artsiom Yudovin
* @since 1.3.0
*/
@Order(LoggingApplicationListener.DEFAULT_ORDER + 1)
public class BackgroundPreinitializer
implements ApplicationListener<SpringApplicationEvent> {
/**
* System property that instructs Spring Boot how to run pre initialization. When the
* property is set to true, no pre-initialization happens and each item is
* initialized in the foreground as it needs to. When the property is false
* (default), pre initialization runs in a separate thread in the background.
* @since 2.1.0
*/
public static final String IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME = "spring.backgroundpreinitializer.ignore";
// 记录预初始化任务是否已经在执行,初始值设置为false,表示未开始
private static final AtomicBoolean preinitializationStarted = new AtomicBoolean(
false);
// 记录预初始化任务是否已经完成,1表示未完成,0表示完成
private static final CountDownLatch preinitializationComplete = new CountDownLatch(1);
@Override
public void onApplicationEvent(SpringApplicationEvent event) {
if (!Boolean.getBoolean(IGNORE_BACKGROUNDPREINITIALIZER_PROPERTY_NAME)
&& event instanceof ApplicationStartingEvent
&& preinitializationStarted.compareAndSet(false, true)) {
//如果没有设置spring.backgroundpreinitializer.ignore为true,
//并且当前事件是ApplicationStartingEvent,
// 并且preinitializationStarted表明预初始化任务尚未执行
// 则 :
// 1.先将preinitializationStarted设置为预初始化任务开始执行;
// 2. 在1成功的情况下开始执行预初始化任务;
performPreinitialization();
}
if ((event instanceof ApplicationReadyEvent
|| event instanceof ApplicationFailedEvent)
&& preinitializationStarted.get()) {
try {
// 如果到了应用就绪或者已经失败的时候,预初始化已经开始并且可能尚未结束,
// 等到他结束,也就是要等到 preinitializationComplete 变成 0
preinitializationComplete.await();
}
catch (InterruptedException ex) {
Thread.currentThread().interrupt();
}
}
}
private void performPreinitialization() {
try {
// 使用一个新的线程执行预初始化任务,使用 preinitializationComplete
// 在前台主线程和该新背景线程之间协调任务的执行情况。
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
runSafely(new ConversionServiceInitializer());
runSafely(new ValidationInitializer());
runSafely(new MessageConverterInitializer());
runSafely(new MBeanFactoryInitializer());
runSafely(new JacksonInitializer());
runSafely(new CharsetInitializer());
// 预初始化任务执行完成,将preinitializationComplete减为0。
preinitializationComplete.countDown();
}
public void runSafely(Runnable runnable) {
try {
runnable.run();
}
catch (Throwable ex) {
// Ignore
}
}
}, "background-preinit");
thread.start();
}
catch (Exception ex) {
// 针对GAE的处理,GAE上创建线程是被禁止的。
// 这种情况下直接将preinitializationComplete减为0。
//
// 这种情况下我们可以安全地进行初始化,不会稍微慢一些因为预初始化任务会在前台主线程中完成。
// This will fail on GAE where creating threads is prohibited. We can safely
// continue but startup will be slightly slower as the initialization will now
// happen on the main thread.
preinitializationComplete.countDown();
}
}
/**
* Early initializer for Spring MessageConverters.
*/
private static class MessageConverterInitializer implements Runnable {
@Override
public void run() {
new AllEncompassingFormHttpMessageConverter();
}
}
/**
* Early initializer to load Tomcat MBean XML.
*/
private static class MBeanFactoryInitializer implements Runnable {
@Override
public void run() {
new MBeanFactory();
}
}
/**
* Early initializer for javax.validation.
*/
private static class ValidationInitializer implements Runnable {
@Override
public void run() {
Configuration<?> configuration = Validation.byDefaultProvider().configure();
configuration.buildValidatorFactory().getValidator();
}
}
/**
* Early initializer for Jackson.
*/
private static class JacksonInitializer implements Runnable {
@Override
public void run() {
Jackson2ObjectMapperBuilder.json().build();
}
}
/**
* Early initializer for Spring's ConversionService.
*/
private static class ConversionServiceInitializer implements Runnable {
@Override
public void run() {
new DefaultFormattingConversionService();
}
}
private static class CharsetInitializer implements Runnable {
@Override
public void run() {
StandardCharsets.UTF_8.name();
}
}
}
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