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This means that you introduced a jar package or class, but failed to find it, so you reported an error.

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Generally speaking, there are two common situations:

1. When sealed: true has been marked in the list. MF of a jar file, all classes of java package in this jar must come from this jar package, which is a security measure of JVM. With digital signature, it can prevent tampering, so Microsoft signed its SQLServer driver. For example, JRE's rt.jar is sealed, so if you create a java.lang.MyClass class to run, the JVM will refuse.

Usually, if you use AOP based on dynamic proxy, such as Hibernate delayed loading or Spring AOP, it may be because the temporarily generated subclasses themselves are generated in memory, not from jar packages. At this time, if the jar package is sealed, it will not work. For example, the JDBC driver of Microsoft SQL Server is sealed, and the delayed loading using Hibernate will get an error, because the proxy class generated by Hibernate inherits a class in JDBC driver package, but this driver package is sealed, and the package name of the generated class will get an error in the jar of the driver name.

2. Maybe the class version is wrong. This error is that you said that the version of other class libraries referenced by your current class is different from the version used when compiling this class. For example, your class A uses a class in c_ 1.0.jar and compiles it on the server, but the only class on the server is c_ 1. 1.jar, and the version is the same as C _ 668.

Therefore, in the development of J2EE, we must make sure that we are programming for API or interface, not for implementation classes, otherwise different versions will have a great influence on you, and you must test on all possible servers to find problems.

What if you use something similar? Eclipse development tools, so when you want to use APIs provided by servers such as jstl.jar, JSP.jar, servlet.jar, J2EE.jar, we should not copy the jar on the server to WEB-INF/lib, but in Eclipse? Add a library to the Java build path of project properties. Server runtime or user library to avoid copying jars. Once copied, others may not know the difference between your class library and the server's class library after the server configuration changes.