hibernate级联保存问题，出错not-null property references a null or transient value - JAVA

lina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)

at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:722)最近在和同学调代码，出现这个错误，半天才看明白出现这个错误原因:是因为not-null=true的限制，导致两个实体都不能保存。只要把b的一端得not-null设置成false就行了.

hibernate级联保存问题，出错not-null property references a null or transient value (三)