SpringBoot class loader conflict causes LinkageError: Why is it a LinkageError instead of ClassCastException?

In-depth analysis of LinkageError: The root cause of class loader conflict in SpringBoot

This article analyzes the java.lang.LinkageError problem that occurs in a SpringBoot application. This problem occurs when integrating the log management system, due to the customization of LaunchedURLClassLoader 's delegate loading mechanism.

The problem stems from the developer trying to manage logs in a unified manner, modifying the delegation relationship of LaunchedURLClassLoader , and explicitly delegate log-related packages such as org.slf4j , ch.qos.logback.core to the parent class loader. However, the ch.qos.logback.classic.spi package is missed, causing the runtime to throw java.lang.LinkageError: loader constraint violation exception. The error message states that the ch.qos.logback.classic.spi.TurboFilterList class is loaded with different versions by LaunchedURLClassLoader and its parent class loader.

The developer wonders why it is LinkageError rather than ClassCastException . This is closely related to the Java class loading mechanism and the parent delegation model. When LaunchedURLClassLoader tries to load TurboFilterList , it is found that the parent class loader has loaded a different version of the class, violating the parent delegation model. The class loader attempts to define an existing class, which raises LinkageError . This is not a simple type conversion error, but a class of the same name loaded by different class loaders has conflicts of different definitions.

LinkageError is not because of the object type error returned getTurboFilterList() (although the object is indeed loaded by the parent class loader), but the JVM detects that TurboFilterList class has been loaded by another class loader during ClassLoader.defineClass phase, preventing LaunchedURLClassLoader from loading its own version. This reflects the strictness of the JVM for class loading, ensures the uniqueness of the class, and avoids runtime errors caused by conflicts in different versions of the class.

The solution is to ensure that all relevant log packages are correctly delegated to the parent class loader, or uniformly loaded by the same class loader to avoid class loader conflicts.

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