在GWT编译期间出现以下NPE。它在编译器本身，而不是在应用程序代码中：
[ERROR] Exception in thread "pool-1-thread-96" java.lang.NullPointerException
[ERROR] at com.google.gwt.dev.util.DiskCache.transferToStream(DiskCache.java:185)
[ERROR] at com.google.gwt.dev.util.DiskCacheToken.writeObject(DiskCacheToken.java:73)
[ERROR] at sun.reflect.GeneratedMethodAccessor29.invoke(Unknown Source)
[ERROR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR] at java.lang.reflect.Method.invoke(Method.java:498)
[ERROR] at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
[ERROR] at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
[ERROR] at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
[ERROR] at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
[ERROR] at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1548)
[ERROR] at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:441)
[ERROR] at com.google.gwt.dev.javac.CachedCompilationUnit.writeObject(CachedCompilationUnit.java:222)
[ERROR] at sun.reflect.GeneratedMethodAccessor33.invoke(Unknown Source)
[ERROR] at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[ERROR] at java.lang.reflect.Method.invoke(Method.java:498)
[ERROR] at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:1028)
[ERROR] at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
[ERROR] at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
[ERROR] at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
[ERROR] at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
[ERROR] at com.google.gwt.dev.javac.PersistentUnitCache$6.run(PersistentUnitCache.java:492)
[ERROR] at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
[ERROR] at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
[ERROR] at java.lang.Thread.run(Thread.java:745)
GWT 2.6.1版
此错误通常发生在并非所有共享类（在服务器端和客户端都使用）都实现Serializable接口时。