重新启动 Vaadin 应用程序时抛出 java.io.NotSerializedException

当我重新启动 Tomcat 时，出现以下错误：

2014;10;01; 15;49;47;055; WARN; com.vaadin.event.ListenerMethod;[localhost-startStop-2]; Error in serialization of the application: Class com.aaa.bbb.MyServlet$1 must implement serialization.
2014;10;01; 15;49;47;055; WARN; org.apache.catalina.core.ContainerBase.[Catalina].[localhost].[/my-vaadin];[localhost-startStop-2]; Cannot serialize session attribute com.vaadin.server.VaadinSession.MyServlet for session ...
java.io.NotSerializableException: org.apache.catalina.loader.WebappClassLoader
    at java.io.ObjectOutputStream.writeObject0(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeObject0(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeObject0(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeSerialData(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.writeObject0(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source) ~[na:1.6.0_31]
    at java.io.ObjectOutputStream.defaultWriteObject(Unknown Source) ~[na:1.6.0_31]
    at com.vaadin.event.ListenerMethod.writeObject(ListenerMethod.java:88) ~[vaadin-server-7.2.7.jar:7.2.7]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.6.0_31]
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[na:1.6.0_31]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:1.6.0_31]
    at java.lang.reflect.Method.invoke(Unknown Source) ~[na:1.6.0_31]
    ...

警告说匿名内部类（在MyServlet类）必须实现序列化。我的 servlet 类仅包含一个匿名内部类，并且它实现Serializable并有一个serialVersionUID:

public class MyServlet extends VaadinServlet implements SessionInitListener {
    //...

    @Override
    public void sessionInit(SessionInitEvent event) throws ServiceException {
        event.getSession().addBootstrapListener(new BootstrapListener() {
            private static final long serialVersionUID = 1L;

            @Override
            public void modifyBootstrapPage(BootstrapPageResponse response) {
                //...
            }

            @Override
            public void modifyBootstrapFragment(BootstrapFragmentResponse response) {
                //...
            }
        });
    }
}

经过一番研究，我找到了解决方案：匿名类上的 NotSerializedException https://stackoverflow.com/questions/17804704/notserializableexception-on-anonymous-class
所以我们要把匿名内部类改为静态嵌套类：

public class MyServlet extends VaadinServlet implements SessionInitListener {
    //...

    @Override
    public void sessionInit(SessionInitEvent event) throws ServiceException {
        event.getSession().addBootstrapListener(new MyBootstrapListener());
    }

    private static class MyBootstrapListener implements BootstrapListener {
        private static final long serialVersionUID = 1L;

        @Override
        public void modifyBootstrapPage(BootstrapPageResponse response) {
            //...
        }

        @Override
        public void modifyBootstrapFragment(BootstrapFragmentResponse response) {
            //...
        }
    });
}

或者，如果我们在反序列化后不想要多个对象：

public class MyServlet extends VaadinServlet implements SessionInitListener {
    //...

    @Override
    public void sessionInit(SessionInitEvent event) throws ServiceException {
        event.getSession().addBootstrapListener(MyBootstrapListener.INSTANCE);
    }

    private static class MyBootstrapListener implements BootstrapListener {
        private static final long serialVersionUID = 1L;
        private static final MyBootstrapListener INSTANCE = new MyBootstrapListener();

        @Override
        public void modifyBootstrapPage(BootstrapPageResponse response) {
            //...
        }

        @Override
        public void modifyBootstrapFragment(BootstrapFragmentResponse response) {
            //...
        }

        private Object readResolve() {
            return INSTANCE;
        }
    });
}