细说Tomcat如何打破双亲委派(有源码和图)

vlambda
2021-05-27

细说Tomcat如何打破双亲委派(有源码和图)

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一句话概况: 不考虑缓存, 那么装载路径就是,先Classpath, 然后War包,当前工程, 最后才是Tomcat相关目录. 不考虑缓存,是因为缓存只是用来加快速度,对外不调用展现出的逻辑效果是可以忽略的.

下图是一个概况的流程图, 简介起见,绘图过程中移除了缓存功能. (个了感觉还是有些逻辑还是有些复杂的,图画出来了,自己看着都觉得还是不清楚,但是也没发现可以改进的地方)

下面是关键的代码,已经添加了注释:

 @Override      public synchronized Class<?> loadClass(String name, boolean resolve)          throws ClassNotFoundException {          if (log.isDebugEnabled())              log.debug("loadClass(" + name + ", " + resolve + ")");          Class<?> clazz = null;          //检查当前ClassLoad是否已经停止了          // Log access to stopped classloader          if (!started) {              try {                  throw new IllegalStateException();              } catch (IllegalStateException e) {                  log.info(sm.getString("webappClassLoader.stopped", name), e);              }   } 
//检查缓存1,Class是否已经被当前Class"实例"装载过   // (0) Check our previously loaded local class cache          clazz = findLoadedClass0(name);          if (clazz != null) {              if (log.isDebugEnabled())                  log.debug(" Returning class from cache");              if (resolve)                  resolveClass(clazz);              return (clazz);          }  //ClassLoader检查缓存2,Class是否已经被父类"实例"装载过    // (0.1) Check our previously loaded class cache          clazz = findLoadedClass(name);          if (clazz != null) {              if (log.isDebugEnabled())                  log.debug(" Returning class from cache");              if (resolve)                  resolveClass(clazz);              return (clazz);          }   //ClassLoader装载系统classpath下面的类, 阻止webapp覆盖了J2SE的类   // (0.2) Try loading the class with the system class loader, to prevent   // the webapp from overriding J2SE classes          try {              clazz = system.loadClass(name);              if (clazz != null) {                  if (resolve)                      resolveClass(clazz);                  return (clazz);              }          } catch (ClassNotFoundException e) {              // Ignore          }   //安全检查          // (0.5) Permission to access this class when using a SecurityManager          if (securityManager != null) {              int i = name.lastIndexOf('.');              if (i >= 0) {                  try {                      securityManager.checkPackageAccess(name.substring(0,i));                  } catch (SecurityException se) {                      String error = "Security Violation, attempt to use " +                          "Restricted Class: " + name;                      log.info(error, se);                      throw new ClassNotFoundException(error, se);                  }              }          }  //默认的返回值为false,   //delegate = false  //filter(name) 通过一个List指定哪些package里面的Class使用双亲委派模式,但是默认这个List是空  //比如包名以Java开头的,就需要使用双亲委派模式,不过这一点"部分"由 (0.2) 在上面已经实现了          boolean delegateLoad = delegate || filter(name);  //针对一些明确指定的package,使用双亲委派模式加载          // (1) Delegate to our parent if requested          if (delegateLoad) {              if (log.isDebugEnabled())                  log.debug(" Delegating to parent classloader1 " + parent);              ClassLoader loader = parent;              if (loader == null)                  loader = system;              try {                  clazz = Class.forName(name, false, loader);                  if (clazz != null) {                      if (log.isDebugEnabled())                          log.debug(" Loading class from parent");                      if (resolve)                          resolveClass(clazz);                      return (clazz);                  }              } catch (ClassNotFoundException e) {                  // Ignore                 //如果(1)通过双亲委派加载发生异常, 忽略异常,              }   }         //尝试从本地加载Class( 如果(1)通过双亲委派加载发生异常, 忽略异常,也会走到这一步)          // (2) Search local repositories          if (log.isDebugEnabled())              log.debug(" Searching local repositories");          try {              clazz = findClass(name);              if (clazz != null) {                  if (log.isDebugEnabled())                      log.debug(" Loading class from local repository");                  if (resolve)                      resolveClass(clazz);                  return (clazz);              }          } catch (ClassNotFoundException e) {              // Ignore          }  //最后依旧没有成功, 忽略所有配置,再次尝试一下双亲加载模式加载 
        // (3) Delegate to parent unconditionally          if (!delegateLoad) {              if (log.isDebugEnabled())                  log.debug(" Delegating to parent classloader at end: " + parent);              ClassLoader loader = parent;              if (loader == null)                  loader = system;              try {                  clazz = Class.forName(name, false, loader);                  if (clazz != null) {   if (log.isDebugEnabled())                         log.debug(" Loading class from parent");                      if (resolve)                          resolveClass(clazz);                      return (clazz);                  }              } catch (ClassNotFoundException e) {                  // Ignore              }   } //最后如果依旧没有成功,则抛出异常          throw new ClassNotFoundException(name);   }