Java. lang. ClassCastException Solution

DB4O has been used in recent JAVA projects. It is indeed a good database. For the first time, the database used to store objects is quite fresh. It is necessary to perform basic operations on an ObjectContainer, it is more intuitive to understand than relational databases. However, using such a database can easily produce data redundancy. Otherwise, it may be difficult to query data. In addition, you must be very careful about data updates. It is easy to write data into the database due to negligence.

In the process of using it for WEB development, we encountered a very strange problem: execute the following statement

ObjectSet <Ids> allIds = db. queryByExample (Ids. class );

This statement queries All Ids class objects in the database, but it is strange that the returned result is com. db4o. reflect. generic. GenericObject, which directly causes problems in subsequent values. Why is this problem? After studying for a long time, I finally found a problem after trying constantly, the problem is that the JAR package of the db4o-7.12.145.14409-all-java5 exists under % JAVA_HOME %/jre/lib/ext and under the WEB-INF \ lib directory of the WEB project, resulting in the generation of conflicts, deleting any one of them will solve the problem. This may not be a special case. In other cases, I don't know if such an error will occur.

However, from the perspective of web engineering, it is best to put the jar file in the corresponding project's WEB-INF \ lib directory, to facilitate the transfer of web engineering, it also makes the dependency structure of the project clear, reducing unnecessary troubles. Of course, if the servlet required by the web project needs to be directly generated under the project, then you need to place the required external JAR under the jre/lib/ext directory so that the compilation will pass.

We recommend that you use ECLIPSE and other tools to directly run and test on the server. If there is no problem, copy the content under WebContent to the server directory for troubleshooting and reduce errors similar to the above.

Remind yourself, and hope to help other friends 650) this. width = 650; "src =" http://www.bkjia.com/uploads/allimg/131228/15331R292-0.gif "type =" face "alt =" java. lang. classCastException solution "title =" java. lang. classCastException solution "/>

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