SpringBoot的reload加载器的方法

背景

springboot越来越多的被大家所使用SpringBoot DevTool实现热部署

出现了相同类castException

分析

首先确定出现相同类的castException比如是由于classloader不同造成的。

一个class是否相同取决于两个因素

1. classloader相同
2. class文件相同

即不同classloader解释出来的class是不同的class

我们在学习jdbc的时候常见的使用

/**
 * Returns the {@code Class} object associated with the class or
 * interface with the given string name. Invoking this method is
 * equivalent to:
 *
 * <blockquote>
 * {@code Class.forName(className, true, currentLoader)}
 * </blockquote>
 *
 * where {@code currentLoader} denotes the defining class loader of
 * the current class.
 *
 * <p> For example, the following code fragment returns the
 * runtime {@code Class} descriptor for the class named
 * {@code java.lang.Thread}:
 *
 * <blockquote>
 *  {@code Class t = Class.forName("java.lang.Thread")}
 * </blockquote>
 * <p>
 * A call to {@code forName("X")} causes the class named
 * {@code X} to be initialized.
 *
 * @param   className  the fully qualified name of the desired class.
 * @return   the {@code Class} object for the class with the
 *       specified name.
 * @exception LinkageError if the linkage fails
 * @exception ExceptionInInitializerError if the initialization provoked
 *      by this method fails
 * @exception ClassNotFoundException if the class cannot be located
 */
public static Class<?> forName(String className)
      throws ClassNotFoundException {
  return forName0(className, true, ClassLoader.getCallerClassLoader());
}

从上面我们可以了解不同的classloader解释的相同class也无法互相转换

这样我们把目标放在devtools上。

我们在springboot中引入了如下依赖

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-devtools</artifactId>
  <optional>true</optional>
</dependency>

那么如何排除devtool的依赖呢？

在application.properties中增加

spring.devtools.restart.enabled=false

发现启动时仍然可以看出使用的restartedMain

2018-03-19 22:04:37.641  INFO 53428 --- [restartedMain] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@7443f7a3: startup date [Mon Mar 19 22:03:34 CST 2018]; root of context hierarchy
2018-03-19 22:04:37.654  INFO 53428 --- [restartedMain] s.w.s.m.m.a.RequestMappingHandlerAdapter : Detected ResponseBodyAdvice bean in org.springframework.boot.actuate.autoconfigure.EndpointWebMvcHypermediaManagementContextConfiguration$ActuatorEndpointLinksAdvice
2018-03-19 22:04:37.956  INFO 53428 --- [restartedMain] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/swagger-ui.html] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-03-19 22:04:37.956  INFO 53428 --- [restartedMain] o.s.w.s.handler.SimpleUrlHandlerMapping  

这边线程名为restartedMain 为啥设置spring.devtools.restart.enabled 无效呢？

代码

在对应devtools的包中使用了ApplicationListener

private void onApplicationStartingEvent(ApplicationStartingEvent event) {
  // It's too early to use the Spring environment but we should still allow
  // users to disable restart using a System property.
  String enabled = System.getProperty(ENABLED_PROPERTY);
  if (enabled == null || Boolean.parseBoolean(enabled)) {
   String[] args = event.getArgs();
   DefaultRestartInitializer initializer = new DefaultRestartInitializer();
   boolean restartOnInitialize = !AgentReloader.isActive();
   Restarter.initialize(args, false, initializer, restartOnInitialize);
  }
  else {
   Restarter.disable();
  }
}

很明显其实restarter的开启是从系统变量中读取 而并非从spring的环境中读取 正如注释所说 其实此刻使用spring的property还太早

因此可以使用系统变量

因此我们可以使用jvm参数

-Dspring.devtools.restart.enabled=false

果然此时一切就OK了

2018-03-19 22:18:12.928  INFO 66260 --- [main] com.f6car.base.Application               : The following profiles are active: dev
2018-03-19 22:18:13.131  INFO 66260 --- [main] ationConfigEmbeddedWebApplicationContext : Refreshing org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext@2a4354cb: startup date [Mon Mar 19 22:18:13 CST 2018]; root of context hierarchy

那在Spring的配置文件中配置的目的是啥呢？

/**
 * Restart properties.
 */
public static class Restart {
 
  private static final String DEFAULT_RESTART_EXCLUDES = "META-INF/maven/**,"
     + "META-INF/resources/**,resources/**,static/**,public/**,templates/**,"
     + "**/*Test.class,**/*Tests.class,git.properties,META-INF/build-info.properties";
 
  private static final long DEFAULT_RESTART_POLL_INTERVAL = 1000;
 
  private static final long DEFAULT_RESTART_QUIET_PERIOD = 400;
 
  /**
  * Enable automatic restart.
  */
  private boolean enabled = true;
 
  /**
  * Patterns that should be excluded from triggering a full restart.
  */
  private String exclude = DEFAULT_RESTART_EXCLUDES;
 
  /**
  * Additional patterns that should be excluded from triggering a full restart.
  */
  private String additionalExclude;
 
  /**
  * Amount of time (in milliseconds) to wait between polling for classpath changes.
  */
  private long pollInterval = DEFAULT_RESTART_POLL_INTERVAL;
 
  /**
  * Amount of quiet time (in milliseconds) required without any classpath changes
  * before a restart is triggered.
  */
  private long quietPeriod = DEFAULT_RESTART_QUIET_PERIOD;
 
  /**
  * Name of a specific file that when changed will trigger the restart check. If
  * not specified any classpath file change will trigger the restart.
  */
  private String triggerFile;
 
  /**
  * Additional paths to watch for changes.
  */
  private List<File> additionalPaths = new ArrayList<File>();
 
  public boolean isEnabled() {
   return this.enabled;
  }
 
  public void setEnabled(boolean enabled) {
   this.enabled = enabled;
  }

从代码中看到似乎是用来配置是否监听能否自动重启

/**
  * Local Restart Configuration.
  */
  @ConditionalOnProperty(prefix = "spring.devtools.restart", name = "enabled", matchIfMissing = true)
  static class RestartConfiguration {
 
   @Autowired
   private DevToolsProperties properties;
 
   @EventListener
   public void onClassPathChanged(ClassPathChangedEvent event) {
     if (event.isRestartRequired()) {
      Restarter.getInstance().restart(
         new FileWatchingFailureHandler(fileSystemWatcherFactory()));
     }
   }
 
   @Bean
   @ConditionalOnMissingBean
   public ClassPathFileSystemWatcher classPathFileSystemWatcher() {
     URL[] urls = Restarter.getInstance().getInitialUrls();
     ClassPathFileSystemWatcher watcher = new ClassPathFileSystemWatcher(
        fileSystemWatcherFactory(), classPathRestartStrategy(), urls);
     watcher.setStopWatcherOnRestart(true);
     return watcher;
   }
 
   @Bean
   @ConditionalOnMissingBean
   public ClassPathRestartStrategy classPathRestartStrategy() {
     return new PatternClassPathRestartStrategy(
        this.properties.getRestart().getAllExclude());
   }
 
   @Bean
   public HateoasObjenesisCacheDisabler hateoasObjenesisCacheDisabler() {
     return new HateoasObjenesisCacheDisabler();
   }
 
   @Bean
   public FileSystemWatcherFactory fileSystemWatcherFactory() {
     return new FileSystemWatcherFactory() {
 
      @Override
      public FileSystemWatcher getFileSystemWatcher() {
        return newFileSystemWatcher();
      }
 
     };
   }
 
   private FileSystemWatcher newFileSystemWatcher() {
     Restart restartProperties = this.properties.getRestart();
     FileSystemWatcher watcher = new FileSystemWatcher(true,
        restartProperties.getPollInterval(),
        restartProperties.getQuietPeriod());
     String triggerFile = restartProperties.getTriggerFile();
     if (StringUtils.hasLength(triggerFile)) {
      watcher.setTriggerFilter(new TriggerFileFilter(triggerFile));
     }
     List<File> additionalPaths = restartProperties.getAdditionalPaths();
     for (File path : additionalPaths) {
      watcher.addSourceFolder(path.getAbsoluteFile());
     }
     return watcher;
   }
 
  }
 
}

整个根据该配置来返回是否注册对应的watchService

当然我们也可以移除该jar

需要注意的是 当将这一段代码注释时 需要重新

mvn clean

否则有可能无法自动排除该jar

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持呐喊教程。

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