CacheException: Error serializing object. Cause: java.io.NotSerializableException

错误堆栈信息如下：

	at org.apache.ibatis.cache.decorators.SerializedCache.serialize(SerializedCache.java:102)
	at org.apache.ibatis.cache.decorators.SerializedCache.putObject(SerializedCache.java:56)
	at org.apache.ibatis.cache.decorators.LoggingCache.putObject(LoggingCache.java:51)
	at org.apache.ibatis.cache.decorators.SynchronizedCache.putObject(SynchronizedCache.java:45)
	at org.apache.ibatis.cache.decorators.TransactionalCache.flushPendingEntries(TransactionalCache.java:122)
	at org.apache.ibatis.cache.decorators.TransactionalCache.commit(TransactionalCache.java:105)
	at org.apache.ibatis.cache.TransactionalCacheManager.commit(TransactionalCacheManager.java:44)
	at org.apache.ibatis.executor.CachingExecutor.close(CachingExecutor.java:61)
	at org.apache.ibatis.session.defaults.DefaultSqlSession.close(DefaultSqlSession.java:264)
	at TestSelectAll.testSelectAll(TestSelectAll.java:34)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
	at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
	at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:538)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:760)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:460)
	at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:206)
Caused by: java.io.NotSerializableException: org.entity.User
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1184)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
	at java.util.ArrayList.writeObject(ArrayList.java:747)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:483)
	at java.io.ObjectStreamClass.invokeWriteObject(ObjectStreamClass.java:988)
	at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1496)
	at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1432)
	at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1178)
	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:348)
	at org.apache.ibatis.cache.decorators.SerializedCache.serialize(SerializedCache.java:97)
	... 33 more

错误分析：

错误的提示信息翻译后的意思是mybatis缓存异常，正在序列化对象错误，原因是不能序列化异常。详细的堆栈信息如上，从堆栈信息中可以看出 SerializedCache.serialize(SerializedCache.java:102)表示是SerializedCache对象102行调用serialize()方法报错了，看看源码，

private byte[] serialize(Serializable value) {
    try {
      ByteArrayOutputStream bos = new ByteArrayOutputStream();
      ObjectOutputStream oos = new ObjectOutputStream(bos);
      oos.writeObject(value);
      oos.flush();
      oos.close();
      return bos.toByteArray();
    } catch (Exception e) {
      throw new CacheException("Error serializing object.  Cause: " + e, e);
    }
  }

上面代码的oos.writeObject(value);该行代码是写对象，源码如下:

/**
     * Write the specified object to the ObjectOutputStream.  The class of the
     * object, the signature of the class, and the values of the non-transient
     * and non-static fields of the class and all of its supertypes are
     * written.  Default serialization for a class can be overridden using the
     * writeObject and the readObject methods.  Objects referenced by this
     * object are written transitively so that a complete equivalent graph of
     * objects can be reconstructed by an ObjectInputStream.
     *
     * <p>Exceptions are thrown for problems with the OutputStream and for
     * classes that should not be serialized.  All exceptions are fatal to the
     * OutputStream, which is left in an indeterminate state, and it is up to
     * the caller to ignore or recover the stream state.
     *
     * @throws  InvalidClassException Something is wrong with a class used by
     *          serialization.
     * @throws  NotSerializableException Some object to be serialized does not
     *          implement the java.io.Serializable interface.
     * @throws  IOException Any exception thrown by the underlying
     *          OutputStream.
     */
    public final void writeObject(Object obj) throws IOException {
        if (enableOverride) {
            writeObjectOverride(obj);
            return;
        }
        try {
            writeObject0(obj, false);
        } catch (IOException ex) {
            if (depth == 0) {
                writeFatalException(ex);
            }
            throw ex;
        }
    }

方法说明中关于抛出异常的第二条说明中写：

NotSerializableException Some object to be serialized does not
     *          implement the java.io.Serializable interface.

因此当实体类没有实现该接口的时候会报该异常。

解决方案：给实体类实现java.io.Serializable接口

思考为什么要序列化：

    在MyBatis的二级缓存中我们分析了二级缓存的缓存装饰器，其中有一个SerializedCache,该类中对PerpetualCahce的装饰就是序列化，可以查看详细的博客信息https://blog.youkuaiyun.com/tony_java_2017/article/details/84321537